Porphyrinoids for Chemical Sensor Applications - ACS Publications

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Porphyrinoids for Chemical Sensor Applications Roberto Paolesse,*,† Sara Nardis,† Donato Monti,† Manuela Stefanelli,† and Corrado Di Natale*,‡ †

Department of Chemical Science and Technologies, University of Rome Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy ‡ Department of Electronic Engineering, University of Rome Tor Vergata, via del Politecnico, 00133 Rome, Italy ABSTRACT: Porphyrins and related macrocycles have been intensively exploited as sensing materials in chemical sensors, since in these devices they mimic most of their biological functions, such as reversible binding, catalytic activation, and optical changes. Such a magnificent bouquet of properties allows applying porphyrin derivatives to different transducers, ranging from nanogravimetric to optical devices, also enabling the realization of multifunctional chemical sensors, in which multiple transduction mechanisms are applied to the same sensing layer. Potential applications are further expanded through sensor arrays, where cross-selective sensing layers can be applied for the analysis of complex chemical matrices. The possibility of finely tuning the macrocycle properties by synthetic modification of the different components of the porphyrin ring, such as peripheral substituents, molecular skeleton, coordinated metal, allows creating a vast library of porphyrinoid-based sensing layers. From among these, one can select optimal arrays for a particular application. This feature is particularly suitable for sensor array applications, where cross-selective receptors are required. This Review briefly describes chemical sensor principles. The main part of the Review is divided into two sections, describing the porphyrin-based devices devoted to the detection of gaseous or liquid samples, according to the corresponding transduction mechanism. Although most devices are based on porphyrin derivatives, seminal examples of the application of corroles or other porphyrin analogues are evidenced in dedicated sections.

CONTENTS 1. Introduction 2. General Definitions on Chemical Sensors and Sensor Arrays 3. Porphyrinoid-Based Chemical Sensors for Gaseous Analytes 3.1. Chemoresistors 3.1.1. Porphyrins and Carbon Nanotubes 3.1.2. Porphyrins and Polymers 3.1.3. Porphyrins and Metal Oxides 3.2. Work Function Based Sensors 3.3. Optical Sensors 3.3.1. NO2 and CO2 3.3.2. Volatile Organic Compounds 3.4. Surface Plasmon Resonance 3.5. Reflectance Anisotropy Spectroscopy 3.6. Optical Sensor Arrays 3.6.1. ″Smell Seeing” Arrays 3.6.2. Computer Screen Photoassisted Technique 3.7. Mass Transducers 3.7.1. Quartz Microbalances 3.7.2. Corrole-Based Sensing Layers 3.7.3. Other Mass Transducers 4. Porphyrin-Based Chemical Sensors for Liquid Phase Analytes 4.1. Electrochemical Sensors 4.1.1. Potentiometric Sensors

© 2016 American Chemical Society

4.1.2. Porphyrinoid-Based Ion Selective Electrodes 4.1.3. Volt-Amperometric Sensors 4.1.4. Amperometric Sensors: NO Detection 4.1.5. Amperometric Sensors: H2O2 Detection 4.1.6. Amperometric Sensors: Inorganic Analytes 4.1.7. Amperometric Sensors: Organic Analytes 4.1.8. Voltammetric Sensors: Dopamine and Neurotransmitters Detection 4.1.9. Voltammetric Sensors: Nitroaromatic Explosives 4.1.10. Voltammetric Sensors: Pollutants 4.1.11. Voltammetric Sensors: Pharmaceutical and Biological Analytes 4.1.12. Quasi-Stochastic Sensors 4.2. Photoelectrochemical Sensors 4.3. Optical Sensors 4.3.1. Ammonia and Amines 4.3.2. Explosives 4.3.3. Metal Ions 4.3.4. pH Sensors 4.3.5. Glucose

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: June 7, 2016 Published: October 19, 2016 2517

DOI: 10.1021/acs.chemrev.6b00361 Chem. Rev. 2017, 117, 2517−2583

Chemical Reviews 4.3.6. Porphyrinoid Receptors 4.3.7. Optical Sensor Arrays 5. Oxygen Sensors 5.1. Macrocycle Structure Optimization 5.2. Polymeric Membrane Sensors 5.2.1. Covalently Linked Luminophores 5.2.2. Noncovalently Linked Luminophores 5.3. In Vivo, Biological, And Medical Applications 5.4. Technical Applications. Coating and Paintings 6. Conclusions and Future Remarks Author Information Corresponding Authors Notes Biographies Acknowledgments Glossary References

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1. INTRODUCTION The recognition, quantification, and monitoring of chemical compounds in different environments has always been a major concern since it is an important issue in different fields, ranging from environmental control to clinical diagnosis. The technological evolution of analytical laboratory instruments, such as gas chromatography−mass spectrometry (GC/ MS), mass spectrometry (MS), atomic absorption spectroscopy (AAS) and others, enables the detection and the quantification of target analytes with high sensitivity and resolution. These features make these instruments irreplaceable for the accurate determination of target compounds, especially when they occur in complex matrices. Analytical instrumentations, however, suffer from some drawbacks that prevent their wide exploitation: the instruments are generally costly, need experienced operators, and cannot be used in the real field with a continuous monitoring of the target matrix. In this scenario, the development of simple devices able to satisfy these important requirements is becoming more and more urgent. Chemical sensors research and development aim at responding to these urgent needs. According to the IUPAC definition, a chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to overall composition analysis, into an analytically useful signal.1 Another general definition of chemical sensors is given by the so-called Cambridge definition: chemical sensors are miniaturized devices that can deliver real-time and online information on the presence of specific compounds or ions in complex samples.2 At the current state of the art, electronics is the most suitable technology for sensors development. Indeed, in electronics signals can be efficiently acquired, processed, stored, transmitted and used by final users. Therefore, modern sensors are electronic devices that create signals (analog or digital) that carry information about the sensed analytes. Such devices are logically made up of two main components: the sensing material (or receptor) and the transducer (Figure 1). The sensing material is in charge of interacting with the target analyte and, upon interaction, of changing one or more of its properties that the transducer can transform into an electric signal. Sensing materials can be of a different nature from metal oxides to polymers and molecular materials. In this scenario, porphyrins and related macrocycles represent a versatile platform

Figure 1. Logical structure of a chemical sensor. Analytes interact with the sensing material changing some of its physical properties (e.g., temperature, ΔT; mass, ΔM; conductivity, Δσ; work function, ΔΦ; refractive index, Δn, and permittivity, Δε). The transducer is an electric device that converts one of the above physical quantities into the variation of its electric parameter (here, capacitance, inductance, and resistance are mentioned). Finally, the circuit to which the sensor is connected gives rise to the sensing signal. It is worth reminding that electric signals are either current or voltage and for each we can measure magnitude, frequency, and phase.

to develop sensing materials. These molecules have in fact numerous properties that can be used to signal interaction with host molecules: for example, they are natural chromophores, an adaptable chelating framework, and they have a large π-aromatic system. All of these features allow their use with different transducers, thus developing different chemical sensors based on porphyrinoids. It is interesting to consider that in chemical sensors porphyrins mimic most of their biological functions: they can reversibly bind with gaseous compounds and can operate photophysical and/or redox processes mediated by the target analytes. It is also interesting to note that matching porphyrins and transducers is not univocal. In practice each porphyrin can be coupled with many different transducers and vice versa, resulting in many possibilities to assemble sensors. The variety of choices grows with the possibility of operating a wide range of synthetic modifications to the porphyrin ring, such as modifying the coordinated metal, the peripheral substituent, or even the molecular skeleton. Due to the strict structure−property relationships, one can tune the porphyrin properties, further mimicking what Nature does, using porphyrinoids to activate molecular oxygen or to catalyze photosynthetic processes. These features are generally extended to different porphyrinoids (i.e., macrocycles with skeletal modifications respect to the parent porphyrin framework), expanding the possibility of sensors design. All these features have been the basis for the development of a large number of porphyrinoid-based chemical sensors, devoted to detecting analytes both in the gaseous and liquid phases (Figure 2). In Scheme 1 the molecular structures of the 2518


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receptors, this confusion of terms became more pronounced since the development of supramolecular chemistry concepts led to the design of complex receptors specific for a particular application that are sometimes indicated as sensors. The term chemosensor for these molecular receptors can better evidence that it is a mere component of the complete device. In this review, we describe chemical sensors that use porphyrins and related macrocycles as sensing materials. This topic has been reviewed in the past,3,4 and to the best of our knowledge, a review paper was published in 2010;5 for this reason, here we focus mainly on articles published in the last five years (2010−2015). Considering the large number of different devices reported, we have organized the review introducing the basic features of chemical sensors and then describing the devices developed for gaseous and liquid phase analysis, according to the different transduction mechanisms. When examples are present, a separate section is dedicated to the exploitation of porphyrin analogs. Considering the intense research activity dedicated to the development of porphyrin-based molecular oxygen sensors, a dedicated section is devoted to these devices.

porphyrinoids more frequently used as sensing layers are reported.

2. GENERAL DEFINITIONS ON CHEMICAL SENSORS AND SENSOR ARRAYS The properties of chemical sensors are typically represented by a number of quantities that enable the comparison among different devices.6 The fundamental property is the response curve, namely, the plot of the sensor signal versus the concentration of the analyte. Response curves are generally drawn considering the sensor’s steady-state response. Actually, the steady state is reached through a dynamic process whose temporal length corresponds to the sensor’s response time. The response time is in general not a fixed quantity but depends on the working condition (e.g., analyte flow and sensor cell volume) and also on the concentration of the analyte.7 Response curves contain the

Figure 2. Examples of porphyrin-based chemical sensors.

It should however be noted that some confusion is present in the literature related to the term chemical sensors; it has frequently been used to indicate the simple receptor, identifying the molecular recognition as the sensor output. This confusion was driven by the fact that most of the device’s performances, such as sensitivity and selectivity, strictly depend on the characteristics of the sensing material. In the case of organic Scheme 1. Molecular Structures of the Common Porphyrinoids

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However, it is important to remark that sensor’s signals are the combination of sensitivity and concentration; therefore, even analytes with little sensitivity can give rise to large signals when their concentration is large. The very large variety of analytes thwarts the quest of selective sensors. Generally, sensors are actually designed to be sensitive to molecular motifs that could be recurrent even in molecules very different from one another.12 Porphyrins, as will be seen later, are a typical example of sensing molecules that can harbor multiple interactions. Molecular design can emphasize one interaction with respect to others, but the selectivity of the sensors based on these molecules remains limited. Obviously, low selective sensors give rise to ambiguous signals; they can only really be used when the compounds in a sample are known a priori. On the other hand, selectivity was found to be surprisingly absent in an important category of natural chemical sensors, namely the olfactory neurons that populate the olfactory epitheliums located at the top of each nostril.13 The chemical sensitivity of olfactory neurons originates from the olfactory receptors that are transmembrane proteins endowed with sites where the volatile compounds may interact. The number of receptors is species variable. For instance, 60 receptors are found in drosophilas, while mice count 1037 receptors in their genome. The human repertoire is limited to 388 receptors.14 Each olfactory receptor neuron selects, among the repertoire of genes, only one kind of receptor. In fact, each olfactory neuron produces signals that reflect the chemical sensitivity of the particular olfactory receptor expressed by that cell. Olfaction is highly redundant with thousands of replicas of similar olfactory neurons. The most important characteristic of olfaction is the combinatorial selectivity of the receptors. In practice, each receptor is sensitive to more molecules, and the same molecule may be sensed by many receptors. Therefore, the recognition of odorant molecules results from the pattern of signals produced by different receptors. This feature is ubiquitous: it is found in mammals, amphibians, insects, and fish.13 The observation that the partial selectivity of chemical sensors parallels the behavior of olfactory receptors gave rise to artificial olfaction systems, commonly known as electronic noses. Since then, a number of research has been carried out and several kinds of electronic noses have been developed.15 Three decades after the introduction of electronic noses, a standard electronic nose model common to the many available implementations of artificial olfaction can be identified. In this model, two major subsystems are found: the sensor array and the data analysis. The sensor array is the ensemble of chemical sensors made by sensors characterized by different sensitivity profiles. Data analysis is an important issue for sensor arrays. Arrays produce multivariate data that are treated with proper multivariate data analysis techniques.16 The use of these techniques is particularly important in chemistry, and it is substantiated by the discipline of chemometrics whose diffusion exceeds the boundaries of analytical chemistry. Thus, concepts such as principal component analysis (PCA) for picturing multidimensional data in representative planes are becoming popular even with a more general audience. The practice of combining nonselective sensors into arrays became a popular choice for sensor developers giving the false belief that the development of selective sensors can be avoided. Actually, selectivity is greatly demanded in many applications and is necessary when specific compounds, occurring in a

working principles of all the components of the sensor as suggested in Figure 1. Among them, it is worth mentioning the adsorption isotherm of the analyte onto the sensing material, the relationship between the number of the adsorbed molecules and the physical quantity, and finally the relationship between the physical quantity and the sensor signal. In most simple cases, the last two steps are linear and the response curve is qualitatively determined by the adsorption isotherm. This is the case for instance of mass transducers where the signal (typically the frequency of a voltage) is proportional to the mass and the mass is proportional to the number of adsorbed molecules.8 The sensitivity of the sensor is defined as the variation of the sensor signal with respect to a variation of the analyte concentration.9 It corresponds to the derivative of the response curve. In the case of a nonlinear response curve, sensitivity is a function of concentration. For a sensor ruled by a Langmuirian isotherm, the largest sensitivity is achieved at a small concentration and the saturation of the sensor signal corresponds to the condition of null sensitivity. The other important quantity of a sensor is resolution. It is defined as the smallest change of analyte concentration that can be measured.9 Resolution is driven by the sensor signal’s measurement error and is ultimately limited by the electronic noise of the sensor signal.10 The relationship between the response curve, sensitivity, and resolution is shown in Figure 3. It

Figure 3. Relationship between response curve, sensitivity, and resolution of chemical sensors. The uncertainty of the estimated concentration (ΔC) is proportional to the uncertainty in the measure of the signal (ΔV) and inversely proportional to the sensitivity. Better sensitivity enables the detection of the smallest concentration changes. The uncertainty in the measurement of the electric signal is due either to the electronic noise or to the limited accuracy of the instrument used (e.g., the number of bits of the analog to digital converter). When the response curve is nonlinear, sensitivity and resolution depend on concentration.

is clear that the resolution of the sensor depends on the electronic instrument that is applied to measure the sensor signal, and different resolutions can be obtained from the same sensor device. In practice, resolution is limited by the noise of the electronic circuit, the circuit linearity, and due to the ubiquity of digital processing, the accuracy of the analog-device conversion. Other important characteristics of sensors are reproducibility and reversibility (not applied to disposable sensors) and the drift that strongly impairs many sensor technologies.11 Besides the quantitative properties, the most crucial aspect of sensors is selectivity. This describes the capability of the sensor to detect the target analytes when it is presented in a mixture with other compounds. A selective sensor should be characterized by a prevalent sensitivity to one analyte with respect to all the others. 2520


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and phthalocyanines too, have different interaction mechanisms with the same molecule. Which of them is actually contributing to the sensor signal depends on the physical mechanisms ruling the transducer. However, among all interaction mechanisms binding volatile compounds onto the molecular film, those resulting in a charge donation dominate the sensor signal. As briefly mentioned above, two-terminal resistors made of pure porphyrins rather seldomly derive from the intrinsic low conductivity of the molecular aggregates. To solve this problem, a porphyrin dyad, where two tetraphenylporphyrin (H2TPP) units are linked by ester groups (Scheme 2) was proposed.23 The dyad was deposited by spincoating, obtaining very smooth and ordered thin films, which allowed high charge mobility. For the sensing experiments, these

mixture, have to be quantified. On the other hand, sensor arrays are effective when the sensitivity profiles of the individual sensor elements can be controlled in order to emphasize the detection of a relevant category of compounds.17 Besides the many positive and promising applications of sensor arrays,15 practitioners of this discipline are well aware of the cases where each specific sensor array fails to discriminate among samples, either because of the lack of sensitivity to key compounds or because of the overwhelming abundance of compounds, which are directly related to the scope.18 From this point of view, the synthetic flexibility of porphyrins provides a remarkable tool for the development of efficient sensor arrays.

3. PORPHYRINOID-BASED CHEMICAL SENSORS FOR GASEOUS ANALYTES As illustrated in the introduction, chemical sensors are typically made of two components: the sensitive material where the molecular recognition takes place and the transducer that allows the measurement of interaction events between the analyte in a gaseous phase and the solid-state sensitive layer. In comparison to the ample possibilities that chemistry offers to assemble diverse receptors, the available principles for electronic transducers are rather limited. They are restricted to four main categories related to the measurement of four physical quantities that are affected by molecular recognition events. These are electric impedance, mass, surface potential, and optical characteristics. Below is a brief description of each, including a critical survey of recent literature.

Scheme 2. Molecular Structure of the Bis-Porphyrin Dyad

3.1. Chemoresistors

Resistors are the simplest electronic components, and their integration in electronic systems is the easiest. Furthermore, their fabrication is simple: the sensing material is deposited across two parallel electrodes placed on an insulating substrate. Often, in order to adjust overall resistance, an interdigitated pair of electrodes is used. Chemoresistors made of metal-oxide semiconductors have been the most widely spread gas sensors since the seminal works on tin oxide in the sixties.19 The advances of conductive polymers and conductive molecular materials gave rise to the possibility of developing chemoresistors with organic chemistry.20 With respect to metal oxides, these materials offer two main advantages: they can operate at room temperature, and most importantly, the chemical sensitivity can be tailored taking advantage of the manifold chemical structures that can be synthesized. The nature of the electric contact is crucial for chemoresistors. This issue is beyond the scope of this review, yet it is important to consider that different electrode materials can give rise to different electric behavior ranging from pure ohmic to rectifying junctions.21 Among molecular materials, phthalocyanines have been widely developed. Phthalocyanines with respect to porphyrins show a more planar structure that leads to a more efficient charge transfer among the molecular units in the solid-state molecular aggregate. On the other hand, phthalocyanines and porphyrins share many chemical properties. In particular, the binding properties of the metal atom complexed in the aromatic ring are rather similar. To this regard, it has been demonstrated that the sensitivity of chemoresistors made of metallo-phthalocyanines is correlated with the electron donor character of the volatile compounds, measured as the enthalpy of formation of a complex with BF3.22 This result is in general valid also for porphyrin-based sensors. On the other hand, one must consider that porphyrins,

films were deposited onto gold electrodes and exposed to different gases, such as Cl2, NO2, H2S, and NH3, at a concentration of 500 ppb in air, following the corresponding change of the film’s conductance. At room temperature, the device was responding only to Cl2, showing an increase of the electrical conductance upon exposure to the gas. At this temperature, response time was however very long, therefore, measurements were carried out at different temperatures. For the other gases, the responses remained negligible, confirming the selectivity of the device toward Cl2, since for this gas sensor responses improved significantly with temperature, with the best conditions obtained at 170 °C. Sensor responses were highly reproducible and reversible up to 500 ppb of Cl2 concentration. At higher amounts, sensor responses decreased and they were not completely reversible. This feature was attributed to the porphyrin oxidation operated by Cl2, confirmed by the changes observed in the XPS and FT-IR characterization of the film before and after exposure to the gas. Later, the same group reported the exploitation of the same dyad, together with the corresponding Zn complex, for the development of chemoresistive sensors for NH3 detection.24 Both compounds were deposited as thin films using the spincoating technique; the AFM characterization of the resulting layers showed the amorphous character of the Zn complex layer, also showing a high porosity and a decreased conductance with respect to the free base layer. This feature was attributed to the presence of the coordinated Zn ion, which disturbed the close packing of the porphyrin dyad. The two sensors were exposed at room temperature to 5 ppm concentration of a series of gases (Cl2, H2S, NH3, CH4, CO, NO2, and NO). The films were sensible to Cl2, H2S, and NH3, but in the case of Cl2 and H2S, the 2521


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workers, a bundle of multiwalled carbon nanotubes (MWCNT) were deposited by CVD onto alumina substrates and then connected through thermal evaporation with Cr−Au electrodes.28 The surface of the MWCNT was functionalized with ZnTPP or Mn(TPP)Cl deposited by casting (Figure 4): the

responses were not reversible, thus precluding the use of the devices as chemical sensors for these gases. The reversible response to NH3 led to test the device with different analyte concentrations, up to 40 ppm. In this case, the Zn porphyrin dyad gave higher responses, with a Langmuir-like behavior of the adsorption isotherm. Even in this case, the interaction with NH3 led to an increase of the electrical conductance, as previously observed in the case of Cl2. This result is surprising, since ammonia should have a donor character, unlike chlorine, confirming that several factors should be considered for a correct interpretation of the response behavior of these species. An interesting approach was based on the development of a nanoelectronic chemosensor for the fast detection of vaporphase H2O2.25 Results show the formation of supramolecular nanostructures with different morphology by surfactant-assisted self-assembly (SAS) of three porphyrin derivatives differentiated by the metal ions coordinated. While the assembling of tetra(4pyridyl)porphyrin free base (H2T4PyP) and the corresponding zinc complex gave irregular nanostructures and short nanorods respectively, the oxo-titanium complex formed in the same conditions produced long yet straight 1D nanostructures with a diameter of ca. 70−200 nm and a length of ca. 30−60 μm. The assembled TiO(T4PyP) nanostructures were integrated into single nanofiber-based two-end nanoelectronics via an organic ribbon mask technique using Au as the electrode and the resulting device exposed to a flow of H2O2 vapor of ca. 0.25 ppm at 25 °C. The results reported indicated an acceptable selectivity toward H2O2 vapors with respect to other interfering gases (ethanol and ammonia), even if the authors evidenced that water electrolysis when a bias voltage is applied represents the main drawback of the device, which has yet to be overcome. Porphyrins are more frequently used as sensing materials either coupled with, typically, more conductive materials or in a three-terminal device such as field effect transistors. The concept of a gas sensor based on a nonconducting but chemically sensitive organic molecule matched with a conducting but nonsensitive inorganic structure was introduced some years ago by the group of N. Lewis at Caltech using carbon black nanostructures as the conductive sensor element.26 This approach was exploited to develop sensor arrays formed by several nonconductive polymers. The same approach was also demonstrated to be valid when polymers are replaced by porphyrins and phthalocyanines. An array formed by carbon black dispersed free base and complexes of Co (II), Cu(II), and Zn(II) of 2,3,7,8,12,13,17,18-octaethylporphyrin (H2OEP) and phthalocyanines were used to measure vapors of ammonia and 2,4,6-trinitrotoluene (TNT) with respect to common solvents.27 Porphyrins and carbon black filaments form a tridimensional thick film and can tunnel electrons from adjacent carbon particles while porphyrins act as a barrier and spacer among the carbon filaments. The absorption of airborne molecules modulates the barrier both because of a change in potential and because of the increased space between filaments. In a more straightforward approach, porphyrins are deposited as a solid layer directly onto conductive films. The underlying idea is that the absorption of molecules in the porphyrin layer can change the conductivity of the underlying conductive layer. This process is efficient because the absorption of volatile compounds modulates the interaction between the porphyrin and the conductive material. 3.1.1. Porphyrins and Carbon Nanotubes. This route has been successfully followed conjugating carbon nanotubes and porphyrins. In the first example, reported by Penza and co-

Figure 4. Structure of the porphyrin-MWCNT-functionalized chemoresistor. Reprinted with permission from ref 29. Copyright 2011 IOPScience.

devices were tested exposing them to different concentrations of model analytes of different classes of volatile organic compounds (VOCs), such as ethanol, triethylamine, and toluene. The interaction with analytes induced an increase of the sensor resistance for both the pristine and for porphyrin-functionalized MWCNTs, demonstrating that the working mechanism for the devices is always determined by the carbon nanotubes’ conductivity. The metalloporphyrins in general increased sensor sensitivities, with the Mn(TPP)Cl demonstrating a more pronounced effect. The same group later reported a more detailed study, functionalizing a layer of MWCNT grown on alumina with Co and Mn complexes of TPP. Porphyrins were simply castdeposited from a solvent solution and then left to freely aggregate among each other and noncovalently bound onto the meandrous MWCNT surface.29 Eventually, porphyrins formed small isolated nanometric aggregates, so that current flows only in the MWCNT layer. Results show a large increase of sensitivity toward VOCs (acetone, methanol, ethyl acetate, and THF) in the porphyrin-coated sensor with respect to the bare MWCNT. However, the enhancement of properties was observed only for Mn(TPP)Cl, while CoTPP was barely distinguished from pure MWCNT. This behavior confirms the key role of the metal ion in conductivity-based sensors where charge transfer is the transduced event. More recently, MWCNT was functionalized with CuTPP and CoTPP;30,31 in this case, the MWCNT were first oxidized by corona electrostatic discharge, with the aim of introducing OH groups that can improve the noncovalent functionalization with CoTPP, by grafting the macrocycles. The resulting devices were tested for detection of aromatic hydrocarbons, obtaining good responses, although in some cases affected by long response times. The authors interpreted the sensing mechanism based on the charge transfer interaction among analytes and nanotubes mediated by the grafted porphyrins, even if the results showed did not match the electronic character of the analyzed VOCs, suggesting a more complex mechanism for these sensors. Other than MWCNT, also single wall carbon nanotubes (SWCNT) have been functionalized with metalloporphyrins and tested as hybrid materials for chemoresistive sensors. For example, SWCNT were noncovalently functionalized with H2OEP, H2TPP, and a phthalocyanine by casting, and the sensing behavior of the resulting hybrid layers was studied by both resistive and mass transducer upon exposure to toluene vapors.32 The hybrid sensing material was prepared by mixing the SWCNT in a CHCl3 solution of the macrocycle and then 2522


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more than four times more sensitive than the corresponding pristine or noncovalently functionalized nanotubes. The devices showed stable performances for up to six months. An array made of SWCNT coated with eight H2TPPs with four metals in oxidation state III (Cr, Mn, Fe, and CO) and four in oxidation state II (Co, Ni, Cu, and Zn) was used to discriminate among various VOCs and demonstrated a clear segregation of amines.36 In this paper, PCA revealed a hierarchic discrimination of compounds with amines separated from the others along the first principal component, explaining 93% of variance. Aliphatic, alcohols, aromatic, and ketones are separated from each other in the plane of the second and third principal components where only 6% of the total variance is carried. A natural application of these sensors was the detection of organic tissue spoilage such as meat.37 For this purpose, Co complexes of tetrarylporphyrins were chosen to noncovalently functionalize SWCNTs, due to the known ability of this metal ion to axially bind amines in porphyrin complexes. The importance of the metalloporphyrin structure in sensor performance was studied in detail. Both the porphyrin and the oxidation state of the metal ion were changed to tune the electron density on the Co ion. The sensors were tested upon exposure to different concentrations of NH3 diluted in N2 and the importance of the charge density on the Co ion was confirmed by the highest responses obtained in the case of perchlorate Co(III) complex of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin [Co(TFPP)ClO4]-functionalized SWCNT sensor. The Co(TFPP)ClO4-SWCNT based device was also exposed to vapors of putrescine and cadaverine, as a model of biogenic amines, to test the device for meat spoilage monitoring. In this case, a good sensitivity was observed, but the device worked as a dosimeter, due to the absence of reversibility. The device was also selective for ammonia detection, since the response toward a series of other VOCs was negligible. The fabricated device was then tested to monitor the spoilage of different kinds of meats, stored at different temperatures, 4 and 22 °C. While no significant changes in the responses were observed for meat stored at 4 °C within a period of 4 days, a significant increase in the responses was observed for the samples stored at room temperature, indicating the evolution of amines as an indicator of meat spoilage. This result confirmed the potentialities of the chemoresistive sensor as a cheap and portable device to monitor meat spoilage. Porphyrins and CNTs were also tested as enrichment material for VOCs. A mixture of SWCNT and H2TPP embedded in a plasticized polymer achieved an enrichment factor of 32 for 1butanol, enabling the detection of subppm concentration of this alcohol with a quartz microbalance.38 Similar approaches were also applied to different porphyrinoids. Among these, corrole recently assumed a peculiar role that prompted different groups to test these analogs in different fields of application. For instance, 5,10,15-tritolylcorrole (Scheme 3) was used to functionalize SWCNT by a noncovalent method, taking advantage of the π−π interactions between the macrocycle and the nanotubes.39 The composite material was deposited onto copper electrodes and then exposed to NO2 diluted in dry air in a sealed measurement chamber. The adsorption of corrole onto the SWCNT surface induced a decrease of its electrical conductivity, due to the electron donor properties of corrole, which acts as a n-dopant. On the other hand, the interaction with NO2 gas induced an increase of the conductivity of the composite material, which maintains its ptype semiconductor characteristics. This sensor was tested down

sonicated to obtain a suspension that was then deposited onto transducer surfaces using a drop-casting method. The addition of both porphyrins and phthalocyanine-induced increased resistance. For both nanogravimetric and resistive transducers, the phthalocyanine induced the highest increase in sensitivity, although there was not a significant difference among the different macrocycles tested. The authors proposed a sensing mechanism for toluene detection, where the VOC is supposed to interact with both the adsorbed macrocycles and the CNT defects. The different interaction was evidenced in the quartz crystal microbalance (QMB) transducers, where two different desorption rates were present, attributed to the weak interaction (with defects) and strong interaction with macrocycles. However, the same feature was not observed in the case of the resistive device, and this was attributed to the negligible effect of macrocycles in catalyzing the electron transfer from the VOC to the CNT. In another study, SWCNT were aligned across interdigitated finger electrodes by dielectrophoresis and then functionalized with porphyrins via casting.33 The porphyrins tested were H2OEP, the corresponding Ru, Fe, and Mn complexes, and H2TPP free base with the Ru and Fe complexes. Although this paper reports Fe(II) and Mn(II) complexes, from the experimental conditions it seems more plausible that the Fe(III) and Mn(III) species were effectively used. The SWCNT surface functionalization with porphyrins induced a significant decrease of the electrical conductance; this feature can be attributed to the donor characteristics of porphyrins, which were reflected into the conductance decrease due to the p-type semiconductor character of the nanotubes. In addition, in this case, there was an increase of the resistance upon exposure to the different VOCs; the variation of the SWCNT sensitivity was different, but it was not possible to elucidate the relationships among the porphyrin structure and the corresponding variation. The investigation of the sensing mechanism showed a complex scheme, with influence depending on the porphyrin structure. RuOEP, for example, is governed by the electrostatic gating effect, while for SWCNT−FeTPP, the Schottky barrier modulation is the most important operating mechanism. The modulation of sensor properties operated by porphyrins demonstrated the possibility of building a sensor array with these porphyrin-SWCNT layers. The same group later reported the sensitivity to benzene of FeTPP-functionalized SWCNT.34 The influence of FeTPP on the I/V behavior of SWCNT was again a decrease in electrical conductivity, evidencing the donating effect of the metalloporphyrin. Also in this case, the authors reported the addition of Fe(II)TPP, but again it is reasonable that a corresponding Fe(III) complex was effectively used. The device showed a steep increase of resistance at a concentration of 1 ppm of benzene. The Fe complex was more effective than the H2TPP free base in increasing benzene sensitivity. With the aim of obtaining a more efficient SWCNT surface coverage and thus superior sensing characteristics of the hybrid layer, the same group later followed a different approach for SWCNT functionalization.35 In this case, the nanotubes were covalently modified by attaching a poly-H2TPP layer prepared using an electropolymerization technique. The deposition of the porphyrin polymeric film was obtained by anodic oxidation of H2TPP at a potential higher than 2 V (toward Ag/AgNO3). Different thicknesses of the porphyrin polymer were obtained by applying different charge densities, and the optimized sensing layers were tested by exposure to acetone vapors, chosen as model VOC. The covalently functionalized SWCNT layers were 2523


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dielectric (SiO2). The conductive polymer layer was further modified with a spin-coated film of Fe(TPP)Cl. The device was sensitive to a few parts per million of NO in air. An OTFT made of poly(3-hexylthiophene) (P3HT), CuTPP, and a copolymer of diethynyl-pentiptycene and dibenzylProDOT (ADB) was also shown to be sensitive to vapors of nitro-based explosives.42 The possibility of developing OTFT with a pure porphyrin layer was demonstrated with a spin-coated film of tetrabenzoporphyrin.43 The mobility of the porphyrin layer was of the same order of magnitude of vacuum-evaporated phthalocyanines. The sensitivity of the OTFT toward supposedly strong binders such as trimethylphosphate, isophorone, and dimethyl-methylphosphonate exceeded that of weak binders (acetonitrile and methanol), confirming that sensors are driven by the porphyrin chemistry. 3.1.3. Porphyrins and Metal Oxides. Semiconducting metal oxides are probably the most common sensing materials used to develop chemoresistive sensors, with SnO2 being the prototypical example. Since the sensing mechanism of these species involves chemisorbed oxygen at the oxide surface, the exploitation of porphyrinoids can be interesting, due to their catalytic activity. As an example, the addition of 5-(4carboxyphenyl),10,15,20 triphenylporphyrin-Co(II) (CoMCPP) to SnO2 resulted in a reduction of the sensors’ working temperature.44 The hybrid material was prepared with a sol−gel technique, adding a THF solution of CoMCPP to the sol obtained by the reaction of SnCl4 in a 2-propanol/water mixture. It was deposited onto interdigitated gold electrodes over alumina substrates and tested for methanol detection, chosen as model VOC. The measurements were carried out at different temperatures, obtaining the best results at 250 °C, while at higher temperatures the responses were similar to that of bare SnO2 substrates. The reflectance optical spectra of the hybrid material showed that the CoMCPP was stable up to 250 °C, while it disappeared at higher temperatures, indicating a thermal

Scheme 3. Molecular Structure of 5,10,15-Tritolylcorrole

to 60 ppb of NO2 and the detection limit estimated at 10 ppb. No experiments were reported on sensor selectivity, although it can be expected, considering the sensing mechanism, that other oxidizing gases or humidity should also give similar responses. It is also important to note that the device response was not reversible and subsequently suitable for dosimeter applications. 3.1.2. Porphyrins and Polymers. The sensitive properties of cocktails of porphyrins and conductive polymers were tested by sensors prepared by spinning over a couple of interdigitated electrodes, binary mixtures of poly(2-phenyl-1,4-xylylene), and each of the following porphyrins: free base, Zn, Ni, and Co complexes of H2TPP and a free base H2TFPP. The sensor array was demonstrated showing the discrimination of vapors of ethyl acetate, ethanol, propanone, and toluene in a PCA scores plot.40 The same kinds of materials were used in OTFT configuration using, for instance, a porphyrin coated self-assembled monolayer of a conductive polymer.41 The monolayer consisted of polysiloxane carrying on one side of the chain a semiconducting quinquethiophene core and an aliphatic spacer on the other, modified with a monofunctional anchoring group to the gate

Figure 5. Sensing mechanism in photoactivated porphyrins coated with ZnO. Charge transfer from a donor-absorbed molecule (triethylamine in this example) in darkness (left) is enhanced in light (b). In the latter last case, light activates a charge transfer from the porphyrin to the ZnO. The depletion of electrons favors the charge transfer from the donor-absorbed species. 2524


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Figure 6. (a) Growth of the ZnO nanorods onto the modified ITO glass and (b) preparation of the porphyrin-functionalized ZnO nanorods sensor.

Figure 7. (a) Resistance changes of the H2MCPP-ZnO nanorods upon exposures to ethanol and triethylamine vapors and (b) relative resistance changes in darkness and in light for pristine and porphyrin-functionalized ZnO nanorods. Reprinted from ref 52. Copyright 2012 American Chemical Society.

constituents. For instance, light absorbance can lead to improved catalytic efficiencies.51 Following this idea, the influence of light in the gas sensing mechanism of ZnO nanorods functionalized with porphyrins has been reported.52 ZnO is in fact a wide gap semiconductor that has been of interest for photovoltaic applications when functionalized with porphyrins,53 in a way similar to that of TiO2. In our case, we were interested in studying the eventual influence of volatiles in the porphyrin-ZnO charge transfer process, which can be used for sensing purposes (Figure 5). ZnO nanorods were obtained by hydrothermal growth, onto indium tin oxide (ITO) electrodes and then functionalized with H2MCPP by drop casting (Figure 6). The conductive behavior of the pristine and hybrid material was tested both in darkness and in white light; conductivity greatly increased in white light (obtained by LED illumination). The sensing properties of these devices were tested upon exposure to vapors of ethanol and triethylamine, both in darkness and in UV and white light illumination. In the dark, the resistance of both pristine and porphyrin-functionalized ZnO nanorods was not affected by VOC vapors. In UV light, we observed a comparable increase of the resistance for both devices; in this case, the response is due to ZnO, activated by UV illumination. In white light, a completely different behavior was observed (Figure 7): while ethanol vapors induced a small drop of resistance, a significant decrease was obtained in the case of triethylamine, where the response was dosimetric, since no reversibility was observed under light. The system had to be switched to darkness to allow triethylamine desorption. It is interesting to note that in this way it is possible to modulate the selectivity of the device toward these two analytes: while in darkness no appreciable

decomposition. A more detailed investigation of the influence of the Co porphyrin in the working mechanism of the hybrid material indicated that the addition of the Co porphyrin produced on one hand a more reactive SnO2 structure, due to the presence of more defects, and on the other, it had a catalytic effect on methanol oxidation.45 A similar decrease of the working temperature of chemoresistive ZnO sensors was also observed upon surface functionalization with freebase H2TPP, where the optimal temperature was fixed at 200 °C for the detection of ethanol, acetone, and benzene.46 However, a complex response behavior was observed, depending on both the VOC and the concentration range considered. Metal complexes of porphyrins (H2OEP and H2TPP) and etioporphyrin were less effective than H2TPP to improve the responses of ZnO hybrid materials, and these differences were interpreted in the different surface interaction with O2− species.47 Another interesting class of porphyrin-based conductometric sensors stems from the realm of the hybrid materials formed by organic dyes and wide bad-gap semiconductors. This combination of materials is very attractive mainly for their application as dye-sensitized solar cells.48 For sensing applications, porphyrins and ZnO are highly desirable materials in which the porphyrins’ properties can be complemented by the ZnO features that are exploited in several different applications, from gas sensing to optoelectronics. Furthermore, ZnO can be prepared in controlled nanosized structures.49 Porphyrins binding onto the ZnO surface can be adequately obtained adding a carboxylic group at the peripheral position of the macrocycle.50 The mutual interaction between porphyrins and ZnO may give rise to peculiar properties not exhibited by the individual 2525


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Scheme 4. Molecular Structure of CoT4PyP-Ru(bipy)2 and CoT3PyP-Ru(bipy)2 Conjugates

porphyrins radically change the structure. Interestingly, the sensing properties of the one-pot material are different from those of the material prepared growing the porphyrin onto the unformed ZnO nanorods, and they offer a further degree of freedom to design sensor arrays extending the properties and capabilities of porphyrin-based sensor arrays.60 The OTFT configuration was demonstrated with noncovalently coated porphyrin SWCNT.61 In this case, the bundle of SWCNT formed the conductive materials between source and drain contacts of a standard organic thin film transistor (OTFT) configuration.62 The porphyrin layer was grown onto 100 nm of SiO2 grown over a highly doped p-type silicon that acted as the transistor gate. Cobalt and iron complexes of H2TPP were tested. The sensors were tested with vapors of benzene, toluene, and xylenes (BTX). Both sensors showed a greater sensitivity to toluene, followed by p-xylene and benzene, the most sensitive of the tree was the Fe(TPP)Cl device.

differences were observed, in light there was a significant increase of the sensor’s selectivity toward the amines. Finally, a sensor devoted to the detection of water content in ethanol was developed by functionalization of Vanadium pentoxide xerogels with tetra(3- or 4-pyridyl) porphyrins conjugated with Ru(bipy)2+ groups [CoT(4-Py)P−Ru(bipy)2 and CoT(3-Py)P−Ru(bipy)2] (Scheme 4).54 In this case, the lamellar structure of the xerogel incorporated both porphyrins, and the macrocycle intercalation induced an increase of the interlamellar distance. The Vanadium pentoxide xerogels have semiconductor characteristics, which strongly depend on the amount of water molecules present in the interlamellar space. When porphyrins are intercalated, they can interact with analytes modifying the interlamellar distance and consequently conductivity. This effect was applied to sense the amount of water present in ethanol, exposing the functionalized xerogels to vapor of ethanol/water solution in the concentration range of 0−10%, an application case important for the use of ethanol as fuel. The relative change of resistance showed a Langmuir-like behavior, with saturation for concentrations above 10% of water, demonstrating the possibility to measure the amount of water in commercial ethanol destined to be used as fuel.

3.3. Optical Sensors

In optical chemical sensors, the analyte is recognized through the changes of the optical properties of a sensitive material. The most practical optical properties are absorbance and fluorescence. In conventional arrangements, the sensitive material is irradiated either with a monochromatic or with a polychromatic radiation, and the interaction is evaluated measuring either the attenuation of the incident radiation or the secondary radiation emitted from the sensitive materials. This technique requires that the sensitive material is immobilized in a liquid or solid phase to favor interaction with light. Typically, solid-phase matrices are used, and for this scope, the sensitive dyes are absorbed, covalently or ionically, or simply encapsulated into a solid matrix that is permeable to the analyte. The straightforward approach for developing chemical sensors based on the optical properties of porphyrins is to measure the absorbance spectra with a spectrophotometer. In many cases, the interaction with analytes can be observed at a single wavelength, typically involving the Soret band. For instance, the absorbance at 470 nm of H2T(4-Py)P coated glass is linearly correlated to the concentration of HCl in air,63 and the absorbance at 424 nm of 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin (H2TNP) is sensitive to the exposure to concentrations of tens of parts per billion of ammonia.64 Other than absorbance changes, fluorescence quenching or fluorescence “turn on” of porphyrin layers can also be successfully used to detect the analyte. Although HCl63,65−70 and NH364,71−73 have been widely used as model analytes to test porphyrin-based optical sensors, the scope of these devices is not limited to acids and bases detection, but they can successfully be

3.2. Work Function Based Sensors

The work function of a material is defined as energy required to transfer one electron from inside a material to the vacuum level. It is defined as the energy distance between the vacuum energy level and the Fermi level.55 The work function is a fundamental quantity in determining the electronic properties of junctions between materials. From the sensor’s point of view, the work function can be altered by both the adsorption of polar molecules and by the adsorption of either donor or acceptor electron molecules.56 The work function can be measured in a laboratory with the Kelvin probe, and it is exploited in OTFT sensors.57 The Kelvin probe was applied to study the interplay between photosensitivity and gas sensitivity in porphyrins coated with functionalized ZnO nanorods. Results show that adsorbed volatile compounds improve the photovoltaic properties of the dye−semiconductor system.58 Kelvin probe investigations were also used to study the sensing effects of alternative methods of fabrication of hybrid porphyrinZnO structures where porphyrins are directly added to the hydrothermal precursor solution to grow ZnO Nanorods.59 Porphyrins interfere with ZnO growth altering the morphology of the materials. The aspect depends on the metal complexed in the porphyrin ring. For instance, the columnar growth is retained in the presence of manganese porphyrins, while copper 2526


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Figure 8. ZnTPP-based gasochromic sensor: (a) cross section of the waveguide and (b) measurement chamber. Reprinted with permission from ref 77. Copyright 2012 Springer-Verlag.

PtTFPP emission and α-naphtholphthalein absorption, at around 650 nm, provided efficient energy transfer via the Förster Resonance Energy Transfer (FRET) mechanism. This decreased at higher concentrations of CO2 with an overall increase of phosphorescence intensity. The sensor, that retained its sensitivity to the gas for 21 days at 4 °C, a sufficient time for many packaged products, showed significant cross-sensitivity to O2, but this could be compensated with a tandem oxygen sensor. A water-soluble polymer−dye hybrid (PVP-H2PyTPOPP) was prepared by incorporating a 5-(4-pyridyl)-10,15,20-tris(4phenoxyphenyl)porphyrin (H2PyTPOPP) into a biocompatible polymer, namely polyvinylpyrrolidone (PVP). The interaction between CO2 and the hybrids was monitored through a continuous hypochromic response of the Soret band in direct relationship with increasing CO2 concentration. The visible color change, from yellow to red, is a good supplementary response to the presence of the toxic gas, even in trace amounts. It is worth mentioning that AFM and TEM analyses suggested that the mechanism of CO2 recognition is based on chemisorption phenomena instead of changes in the pH, as expected.79 The same group reported the characterization of another porphyrin, 5-(4-pyridyl)-10,15,20-tris(3,4-dimethoxyphenyl)porphyrin, for the preparation of CO2 sensitive nanomaterials. The introduction of a pyridyl substituent in the meso position of the macrocycle led to some degree of hydrophilicity which conferred novel self-assembly properties, useful for the sensing performances.80 3.3.2. Volatile Organic Compounds. Wang and coworkers81 reported the copolymerization of a Zn 5,10-bis(4aminophenyl)-15,20-diphenylporphyrin(cis-DADPP) into polyimide backbones with pyromellitic dianhydride and oxidianiline and its electrospinning and imidization to obtain a polymer nanofibrous membrane for rapid and reversible detection of trace amounts of pyridine vapor. The membrane displayed evident color change (Figure 9), as well as dramatic variation in absorption and fluorescent emission spectra upon exposure to pyridine vapor, with a detection limit of 0.041 ppm. The sensing membrane showed excellent selectivity for pyridine over other common amines and was recovered by repeated puffing with high purity nitrogen. A Cr(TPP)Cl·H2O embedded in a polydimethylsiloxane matrix with an atmospheric pressure dielectric barrier discharge method (AP-DBD) has been reported for amine sensing applications.82 In particular, the optical sensing capabilities of plasma polymerized coating were investigated by UV−vis spectrophotometry, exposing the sample to triethylamine vapors. Five nanometers hypsochromic shift of the Soret band, smaller than the solution spectral change (12 nm), indicated a good permeation of TEA through the siloxane matrix but suggested

exploited for a wider range of analytes, as described in the following sections. 3.3.1. NO2 and CO2. Devices sensitive to NO2 have been developed using composite films based on micro-structured columnar TiO2 films with three freebase porphyrins. The exposure of the films to NO2 elicited important changes in the UV−vis spectra revealing good sensing capabilities in all cases. 5,10,15,20-Tetrakis(3-carboxyphenyl)porphyrin (H2T3CPP) resulted in the most stable, showing less aggregation when compared to H2T4CPP and H2MCPP.74 5,10,15,20-Tetrakis [3,4-bis(2-ethylhexyloxy)phenyl]-21H, 23H-porphyrin (EHO) is known to be very sensitive to NO2 gas: LB mixed films of EHO and p-tert-butylcalix[8]arene75 demonstrated an improvement of the sensing properties of the porphyrin in the solid state. This study revealed an important thickness dependence of the film’s response kinetics, with an optimum at 20 layers for which both the speed of response and the surface roughness were at a maximum. The combination of EHO with calix[8]arene has also been investigated.76 In this case, the PMMA: calix[8]arene mixture was used as a barrier layer deposited on top of EHO LS films to act as an analyte size selector. Excellent gas sensing properties have also been reported for sensors based on copolyporphyrin methacrylates deposited on PMMA substrates.69 As for EHO/calixarene, H2−P(TEG−ME)3Acr-based sensors were reversible and versatile and able to recognize other analytes, such as TFA and HCl. Woellenstein and co-workers developed an optical NO2 gas sensor based on ZnTPP, embedded into a PVC matrix and deposited onto a planar optical waveguide, as a gas-reactive dye. A color change, due to the porphyrin’s interaction with the toxic analyte, was detected in the evanescent field of the waveguide using a high power LED coupled into one end of the guide and passing through it under condition of total internal reflection (TIR) with a photo detector made of two independent photo diodes applied. The use of two independent photodiodes allowed half of the waveguide surface to be functionalized with the sensing membrane while the other half, only coated with the polymer, was used as the reference channel. The reported gasochromic sensor demonstrated good selectivity, being sensitive to NH3 only at a high concentration, little influence from humidity, and good long-term stability. However, reversibility is rather limited (Figure 8).77 An optochemical sensor has been developed and optimized to measure CO2 in food packaged under modified atmosphere. For this purpose, the phosphorescent platinum H2TFPP and a colorimetric pH indicator such as α-naphtholphthalein were incorporated in a plastic matrix together with a phase transfer agent, namely tetraoctyl- or cetyltrimethyl- ammonium hydroxide, and applied on a Mylar foil.78 The good spectral overlap of 2527


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MgEHO LB films coated on a phototransistor were studied; the highly conjugated porphyrin was decidedly absorbing in the visible region of the spectrum and underwent a subtle shift upon exposure to specific VOCs. The detection of alcohol vapors (methanol, propanol, ethanol, octanol, and methyl butanol) was demonstrated with a commercial blue LED and a phototransistor as detector.76 A second approach considered the preparation of MgTPP thin films by spin coating onto microscope glass slides and their annealing at 280 °C in argon atmosphere. These sensors revealed significant responses to methanol compared to ethanol and isopropanol.85 The same porphyrin, but free-base, was used as a fluorescent probe in the development of an ethanol sensor. The fluorescence of H2TPP, dispersed in a PVC and plasticizer bis(2-ethyl-hexyl)phthalate (DOP) film, was quenched by ethanol, thus allowing a linear response of the sensor in the range from 1 to 75% of saturated pressure, with a detection limit of 0.05 vol %.86 This approach led to a miniaturized reflectance device for alcohol sensing.87 In this device, a photodiode, namely the breakout board TAOS TCS3200-DB (Parallax, Inc.), was used to monitor changes in reflectance characteristics upon interaction with alcohols, with five different porphyrins: 5,10,15,20-(4-carboxyphenyl)porphyrin (H2T4CPP), MCPP, meso-tri(4-sulfonatophenyl)mono(4-carboxyphenyl)porphyrin (H2M4CTPPS3), deuteroporphyrin IX bis ethylene glycol (DIX), and 5,10,15,20tetrakis(4-aminophenyl)porphyrin (H2TAPP). The sensor array, whose fingerprint-type response provided good specificity, can be combined with digital radio communication capabilities to obtain a chemical sensing network or can be utilized on unmanned air vehicle (UAV) platforms. Other alcohol colorimetric sensors have been realized by interfacial polymerization of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (H2THPP)-containing membranes. The films, deposited on glass slides, were metalated with Zn(II), Cu(II), and Co(II) and exposed to the vapor of several alcohols: the absorbance spectrum of the membranes in the range of 350−800 nm was collected prior to and after the introduction of the analyte to monitor absorbance at 425 and 550 nm. As a result, the Co(II) sensor suffered from irreversible responses, the copper complex was best at discriminating among the alcohols (methanol, isopropanol, and ethanol), while the Zn(II) film turned out to be an excellent methanol sensor.88 An interesting strategy to improve the selectivity of sensors is to use a kind of molecular sieve that can perform a geometric selection of the target molecules. As an example of this approach, selectivity for carboxylic acids was achieved by using a sieve layer made of calix[8]arene molecules incorporated in PMMA, on top of EHO as the optical sensing material.89 This material’s performance, in terms of selectivity, was compared to that of both PMMA- and PMMA/calix[8]arene-coated porphyrin films: a 100% PMMA barrier layer stopped 80% of vapor molecules entering the EHO layers dramatically restricting the sensor response, while the introduction of calix[8]arene, creating some pores, allowed sensitivity to be recovered. In these diffusionlimited sensors, acetic acid generated a greater response than butyric or hexanoic acids, probably due to the stronger response of EHO molecules to CH3COOH, but also due to the smaller size of the acid. Zn(T3CPP) and Zn(T4CPP), bound to microcolumnar TiO2 thin films, have been compared in terms of their different molecular structure and anchoring to the titania surface: the gassensing performances of the two systems were studied by analyzing the spectral changes of the porphyrins in the UV−vis

Figure 9. Molecular structure of ZPCPI and chromatic effect of pyridine interaction. Reprinted with permission from ref 81, which is an open access article distributed under the Creative Commons Attribution License (CC BY), MDPI.

that the analyte did not coordinate with all the porphyrins present in the layer, preventing detection by human eyes. Detection of amine through UV−vis spectroscopy has also been obtained by the group of Pereira83 using a tripodal porphyrin, 1,3-di[5-(3-hydroxyphenyl)-10,15,20-(triphenyl)porphyrin]-2-(5-(3-hydroxyphenyl)-10,15,20-(triphenyl) porphyrin)-2-methylpropaneZn(II) (ZnTriad) (Scheme 5), spinScheme 5. Molecular Structure of the ZnTriad

coated onto Menzel-Glazer microscope glass slides. A total of five primary amines were tested to understand if the geometry of ZnTriad could drive the detection regarding their size and shape: for each amine, the nonexposure spectrum was subtracted from the exposure spectrum at each wavelength and then normalized to the maximum absorbance of the nonexposure spectrum. This allowed discriminating amines according to size and/or shape. Mn(TPP)Cl and Fe(TPP)Cl were spin-coated onto quartz substrates and exposed to pyridine, ammonia, triethylamine, and dimethylamine vapors. A two-dimensional histogram of Soret band integrated area change rate when the two complexes are exposed to the different amines, allowed to identify and distinguish the analyzed organic gases.84 Two approaches were followed to exploit magnesium porphyrins as sensing material to detect alcohols. In the first 2528


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attributed to a switch between Co(II) and Co(III) resulting in the formation of either fewer or more axial ligands, respectively, with a large associate spectral change. Spectral changes displayed by Mn EHO were noticeably less significant, probably due to the lack of switching between two different oxidation states, even compared to those observed for Zn EHO as an amine sensor.

region upon their exposure to six different VOCs (acetone, acetonitrile, butylamine, chloroform, ethanol, and tetrahydrofuran). All samples featured important changes in their spectra during exposure to the analytes, confirming the abilities of these systems to detect VOCs, as already reported for NO2. However, the response magnitude, quantified through the creation of the change fraction parameter, was considerably higher for Zn(T3CPP) compared to Zn(T4CPP): meta derivatives offers a more selective response to the different analytes, paving the way for the preparation of multisensor arrays based on different metal derivatives of porphyrin.90 The change in fluorescence color of a porphyrin-modified CCBs array was used to distinguish different VOCs vapor (cyclohexane, ethyl acetate, acetic acid, acetone, ethanol, and methanol). The reflection peak of the CCBs was used as the encoding signal to distinguish between different sensors: the use of six different porphyrins, H2TPP, ZnTPP, SnTPPCl2, 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (H2MAPP), ZnMAPP, and H2T4CPP, allowed an excellent discrimination of compounds belonging to different classes, as well as a good differentiation of compounds from the same chemical family and the evaluation of the same compound at different concentrations.91 A multisensors approach was developed making use of ten free-base tetrarylporphyrins as VOC sensing materials.92 The macrocycles were deposited as LB films, and the authors investigated the dependence of their sensitivity on the functional groups attached to the peripheral phenyl rings on the basis of their electron donating/withdrawing strength. A comparison among different metal complexes (Mg, Sn, Zn, Au, Co, and Mn) of the same porphyrin (EHO) was made to determine the best compound to be used as solid state colorimetric gas sensors for a wide range of compounds.93 Porphyrins (Figure 10) that readily exchange or coordinate extra

3.4. Surface Plasmon Resonance

Surface Plasmon Resonance (SPR) is an important technique to study the binding of biomolecules to metal surfaces (gold or silver), as well as to monitor the adsorption of analytes onto a thin sensing layer deposited over a gold surface. SPR measures the change of the plasmon resonance in a thin metal layer due to the change of the refractive index at the surface of the metal.94 A gas-sensitive layer made of H2THPP embedded in a nanoporous silica matrix on top of a gold thin film was used as an NO2 sensor: the interaction with the gas induced changes in the refractive index of the sensing layer, leading to a modification of the dispersion curve of the surface plasmon polariton. Detection of NO2 gas in the 100 ppb range was experimentally achieved.95 Besides the standard SPR configuration, in recent years a magneto optic SPR was introduced. These systems take advantage of an increase of the Kerr effect observed in correspondence of the surface plasmon resonance. It results in an increase of the sensitivity with respect to the refractive index of the dielectric layer on the metal surface.96 As an example, a magneto-optic SPR using an Au/Co/Au transducer surface was used to measure the adsorption of aliphatic amines onto a Langmuir-Shafer film of ethane-bridged Zn porphyrin dimers.97 Plasmon technology has also been demonstrated to be an enhancer of fluorescent detection in core−shell gold nanorods coated with CuT4CPP.98 Plasmon resonance enables the detection with suspensions of functionalized nanorods of micromolar of pyrophosphate, an important anion involved in several metabolic processes. 3.5. Reflectance Anisotropy Spectroscopy

The adsorption of volatile compounds onto solid-state layers of porphyrins was also studied with a more sophisticated technique called reflectance anisotropy spectroscopy (RAS). In a RAS experiment, the sample is illuminated, at a near normal incidence, by a linearly polarized light, alternatively directed along two orthogonal directions, via a Photo-Elastic-Modulator. The reflected beam is collected and then focused at the detector.99 Usually, the orthogonal directions coincide with well-defined symmetry directions of the sample. Once applied to porphyrin layers, RAS evidence the optical anisotropy of large scale ordered structures such as those achieved by the Langmuir−Blodgett deposition. Besides the somewhat obvious sensitivity to the adsorption of alcohols and amines onto a solid layer of a zinc complex of 5,10,15,20-tetrakis(4-heptyloxyphenyl) porphyrin, the RAS signal was also found sensitive to the adsorption of hexane whose adsorption by dispersion interaction in the porphyrin layer is not expected to change the optical properties of the film.100 The sensitivity of RAS to hexane can be explained considering that its adsorption probably interferes with the forces holding the porphyrins together, leading to a distortion of the local order of the porphyrin film, thus affecting the optical anisotropy. Furthermore, the spectral feature elicited by adsorption of amine, alcohol, and alkane are sufficiently different to allow recognition of the adsorbed volatile compound.101

Figure 10. Response of porphyrins LB films with various analytes. Reprinted from ref 93. Copyright 2010 American Chemical Society.

ligands in solution are shown to be suitable materials to develop colorimetric detectors. Those porphyrins that already have strongly attached axial ligands only showed a sensor response to those analytes able to substitute the existing ligand, while Au(III) and Sn(IV) complexes showed no response to any of the analytes. In particular, Co EHO displayed a considerably better response than the other porphyrins investigated, which was 2529


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Figure 11. Smell-seeing array. (a) Linear array of colorimetric sensors and disposable cartridge. (b) Cartridge front view with the array of colorimetric indicators. (c) Colormap of the response to various explosives. The color differences between reference air and explosive vapors are scaled in an 8-bits range and displayed as a color. See the glossary for acronyms. Reprinted with permission from ref 112, which is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0), Royal Chemistry Society.

detectors.109 The uneven sensitivity of silicon devices in the three colors introduces a distortion factor that should be adequately considered to interpret sensor signals. 3.6.1. ″Smell Seeing” Arrays. The first demonstration of the analytical properties of consumer electronic devices was obtained with a flatbed scanner, a system devised by K. Suslick at the University of Illinois. The scanner was used to image an array of a set of metalloporphyrins immobilized onto reverse silica thin-layer-chromatography plates.110 The optical sensitivity of metalloporphyrins is rather limited to Lewis basicity (that is, electron-pair donation and metal-ion ligation). To extend the receptive field of the array, metalloporphyrins were complemented with pH indicators that respond to Brønsted acidity and basicity (that is, proton acidity and hydrogen bonding), dyes with large permanent dipoles (for example, vapochromic or solvatochromic dyes) that respond to local polarity, and metal salts that respond to redox reactions.111 The array formed gave rise to a universal device that has been used in several applications, including vapors of explosives,112 toxic industrial chemicals,113 and food applications (e.g., coffee aroma114). An interesting method to extend the set of sensed molecules including less reactive volatile compounds consists in the preoxidation of volatile compounds in a tube filled with chromic acid on silica (Figure 11).115 This approach was also found appropriate to detect volatile compounds relevant to diseases (such as lung cancer116) in vivo and to identify pathogenic microorganisms.117 The preparation of the sensitive spots was optimized along the years from the original porphyrins spotted on a silica gel layer to a mix with plasticizers and finally to nanoporous pigments created from the immobilization of dyes in ormosils.118 The different deposition methods and geometrical arrangement of the array were also compared revealing that the optimum immobilization matrix is highly dependent on the dye, its formulation, and the kind of substrate. In general, plasticizer formulations were preferred for polypropylene while ormosil is the optimal choice for polyvinylidene fluoride.119 The use of only a couple of indicators (Cresol Red and ZnTPP) was shown to be sufficient to detect changes in Brønsted acidity and Lewis acidity, respectively. The two indicators were used to characterize artificial saliva modified with the addition of alleged markers of stomach cancer such as NH3 and CO2.120 In the past few years, the method devised by Suslick has attracted the interest of several researchers, resulting in a number of applications of similar colorimetric arrays in different studies. Starting from ammonia detection and quantification,121 the majority of these articles are devoted to the quality assessment of

3.6. Optical Sensor Arrays

The multivariate nature of optical spectra can be exploited in combination with chemometrics data analysis in order to expand the selectivity of a single porphyrin. For instance, a neural network processing of visible spectra of Ru(OEP)CO allowed to quantify mixtures of acetic acid and ethanol102 or a principal component analysis of vis spectra of MgTPP allowed to discriminate among ethanol, methanol, isopropanol and their mixtures.103 Apart from the approach of using spectro/fluoro-photometers, or more complex instruments such as SPR, to record optical changes, an important research line is gaining importance. This is based on the expanded performance of consumer optoelectronic devices which gave rise to numerous examples of low-cost advanced optical equipment such as digital scanners and cameras embedded in ubiquitous devices like computers, tablets, and smartphones. The characteristics of these devices largely fit the requirements necessary to capture the optical changes occurring in the sensitive layers. The most interesting feature of these devices is that they are image detectors. Electronic components such as CCD chips, CMOS devices, and high-density integrated circuits provide the ability to deal with high density sensing arrays and to collect enormous amounts of data in short time scales.104 Image sensors segment the sensing layer into a number of elementary units that correspond to the pixels of the image. If pixels are independent of each other, they may correspond to an individual sensor. This assumption leads to a massive sensor array where the number of sensors may approach the number of receptors found in olfaction,105 giving the possibility of testing the computation models of olfaction in a realistic situation.106 Furthermore, image sensors enable to track optical signals over a large area. This concept was exploited measuring the optical changes of a layer of ZnTPP dissolved in PVC thanks to the interaction with chemicals like these diffused into the polymer layer. This approach was compared to the analogous mechanism of odorant molecule diffusion in the olfactory mucosa where the recognition of molecules is also determined by the different diffusion constants.107 In these devices, the optical spectra are decoded in three values (RGB) approximating the human eye spectral response. However, the codification is not trivial, and the correlation between the wavelength and the responsiveness of the silicon photodetectors has to be considered.108 The smallest sensitivity is found in the blue region of the spectrum where the Soret band occurs. The optimal measure of spectral changes in the blue region of chemical indicators requires a proper design of silicon 2530


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Figure 12. In a CSPT setup (a) the sensing layer is illuminated by a computer screen and imaged by a computer camera. (b) Array of indicators made by porphyrins, pH indicators, and different formulation of blends of one porphyrin and one pH indicator. As an example of results, the cluster analysis of fingeprints of the array shown in (c) exposed to various classes of volatile compounds. Reprinted with permission from ref 146. Copyright 2015 Springer.

food matrices, such as the determination of the freshness of meat,122,123 fish,124 and beverages such as tea,125,126 liquor,127 rice wine,128 and vinegar.129 Recently, the application of a colorimetric array to distinguish lung cancer biomarkers has also been reported.130,131 All these papers were supported by a generic illustration of the interaction mechanisms between the VOCs and the various indicators. An attempt to study the interactions between VOCs and metallo tetraphenylporphyrin was performed with density functional theory. Results are in fair agreement with the experiments, although they are still hardly exploitable for a practical sensor array design.132,133 In all the papers produced by Suslick’s group, the variation in optical absorbance is illustrated as a change of color. This captivating representation is inherent in the RGB codification of the absorbance spectrum of the dye. The color indeed is a synthesis of the whole spectrum, typically from 400 to 700 nm integrated over three filtering curves approximating the spectral response of three color receptors in humans. Suslick introduced a simple method to display the spectral changes calculating the absolute difference between the RGB colors of the dye during the exposure to the gaseous sample and before being exposed to a reference air. However, the representation as a color is slightly deceiving because colors are positively defined while the spectral changes can be either positive or negative. In particular, in the case of a spectral shift, if the Soret band (in the blue region) moves toward larger wavelengths then the intensity of blue channels decreases and the intensity of green channels increases. The color representation, being based on absolute values, cannot

discriminate the situation depicted above from the opposite case when the shift occurs toward smaller wavelengths. 3.6.2. Computer Screen Photoassisted Technique. The approach introduced by Suslick was extended, complementing the image sensor device with a computer controlled light source. The most obvious source is the computer screen, which can be programmed to deliver light controlling both position and color. This approach is called computer screen-photoassisted technique (CSPT), and the first demonstration of the concept, applied to (bio)chemically sensitive optical reporters, was given in 2003.134 Since the emission spectra of displays and the spectral response of digital cameras are known, the CSPT fingerprint can be theoretically calculated from the absorbance spectra and the overall method can be properly calibrated.135 In the end, it was demonstrated that CSPT may retain most of the spectroscopic information.136 CSPT captures specifically composed subjects (optically responsive chemical sensing devices) under conditioned illumination in order to enhance the analytical information. Just like in photography, every indicator is a unique subject, and the technique needs to be adapted to capture its character. The separation between illumination and detection enables the measurement of both absorbance and luminescence.137 Indeed, while absorbance is appreciated detecting the same color that is used in the illumination, the emission elicited by the illumination with blue light can be independently detected in the other camera channels giving rise to contemporaneous fluorometer and colorimeter detection. However, fluorophores on transparent surfaces emit most fluorescence within the 2531


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case supported the above assumption since alkanes are detected only by the blend of Mn(TPP)Cl and BCP. The use of common equipment such as webcams and scanners raises the question about the performance of such systems. Suslick demonstrated that many toxic compounds can be detected well below their allowed limits.147 In the case of CSPT, the detection threshold was numerically estimated measuring the optical fingerprint of a layer of ZnTPP deposited onto a quartz microbalance and exposed to butylamine vapors.148 The contemporaneous measurement of the CSPT fingerprint and the frequency shift of the quartz allowed one to estimate that the resolution of each pixel of the webcam was on the order of few femtomoles of absorbed molecules.

substrate and above the critical angle, confining most of the light within the substrate.138 To enhance the light collected by the camera one needs to shape the substrate in order to avoid light confinement. To this end, arrays of transparent SU8 micromachined pillar structures, with a height of a few tens of micrometers, were demonstrated to provide an efficient fluorescence collection.139 CSPT is frequently used to measure the changes of optical properties of porphyrins. The first paper documenting its use for a sensor array based on porphyrins appeared in 2006.140 In that paper, CSPT measured the optical properties of thin layers of ZnTPP Fe(TPP)Cl and 2,3,17,18-tetraethyl-7,8,12,13-tetraamethyl-a,c-biladiene dihydrobromide (BD) dispersed in a polyvinyl chloride (PVC) matrix. A rainbow of 50 colors (from purple to red) monitors the interaction between gas molecules (ammonia, amines, ethanol, CO, and NO) and receptors. For each illuminating color, the camera captured an image composed of red, green, and blue channels. Each measurement results in a fingeprinting vector of 150 elements, 50 illumination colors for each of the 3 camera channels. CSPT fingeprinting of spots of Mn(TPP)Cl, ZnTPP, and Fe(TPP)Cl was able to discriminate between different freshness levels of fishes.141 CSPT was used to study the optical properties of porphyrin nanotubes achieved by the ionic self-assembly of two oppositely charged porphyrins.142 In particular, Sn[T(4-Py)P]Cl2 and H2TPPS form J-aggregates, whose nonplanar shape induces the formation of nanotubular structures.143 Solid-state layers of porphyrin nanotubes deposited on glass were found sensitive mainly to triethylamine. However, the analysis of the CSPT fingerprint allowed the identification of other compounds such as acetic acid, ethanol, and NOx. The webcam’s optical resolution was also enough to detect color changes occurring in ZnTPP impregnated cotton threads elicited by exposure to butylamine vapors.144 Noteworthy in this paper is that the photographic technique of high dynamic range (HDR) was implemented in the CSPT setup. HDR is aimed at extending the dynamic range of a picture allowing for a correct exposure of luminous and dark portions of the image. HDR is typically performed with digital cameras processing images taken at different exposure setups, and in CSPT it was obtained changing the intensity of the light of the display. As demonstrated by Suslick, a practical use of colorimetric arrays cannot be exclusively based on porphyrins because only a restricted kind of interactions can modify porphyrin optical properties. However, interaction among porphyrins may promote additional sensing effects. Interactions among porphyrins in solid-state lead to a noteworthy broadening of the visible spectrum features. The absorption of weakly interacting molecules, driven by, for example, van der Waals forces, could then modulate the mutual interaction between contiguous porphyrins, eliciting a detectable change of the absorbance spectrum.145 A similar mechanism was recently found in blends of optically active molecules such as porphyrins and pH indicators (Figure 12).146 Observing these systems, one found that the adsorption of analytes, besides changing the spectra of the individual constituents (porphyrin and pH indicator), can also modulate the interaction between the constituents, resulting in an additional spectral change. The system was tested considering the sensitivity to VOCs of porphyrins (ZnTPP and Mn(TPP)Cl), pH indicators (bromocresol purple and phenol red), and ionic salts. Results show the correct identification of 13 VOCs belonging to different families including alkanes. This last

3.7. Mass Transducers

The change of mass is a straightforward consequence of absorption. On the other hand, to enable chemical sensing, the change of mass has to be measured with great sensitivity: the mass of a tetraphenylporphyrin increases by about 7% after adsorbing one molecule of ethanol. From a relative point of view, the change of mass due to adsorption is large. However, the detection limit of the mass detector fixes the minimum number of detectable molecules. It is important to remark that mass transducers are not the primary choice of transducers. Indeed, contrary to electric conductivity and optical properties where the alteration requires some sort of charge transfer, and in the case of mass, the transducer detects, in principle, any form of bindings of the volatile compound onto the sensing layer, even the weak and ubiquitous van der Waals interactions. For this reason, mass transducers are seldom used as a stand-alone sensor but rather as elements of sensor arrays. Piezoelectricity offers the grounds to the fabrication of mass detectors. Devices such as QMB and surface acoustic waves (SAW) are endowed with sufficient resolution for chemical sensing purposes. These sensors offer a surface that can be properly coated with the sensitive material. Then, the absorption of molecules into the sorbent layer produces a change of mass whose evaluation allows estimating the amount of adsorbed molecules. 3.7.1. Quartz Microbalances. In literature, quartz microbalances have been extensively indicated using the QCM acronym; however, since quartz is a crystal itself, we consider this denomination a tad redundant, and in this paper we refer to this device as QMB. QMBs are thin quartz plates cut along a particular crystalline direction that confers the device the thickness shear resonance mode. They thus belong to the more general class of thickness shear mode resonators.149 In this resonance mode, the acoustic waves propagate perpendicularly from one surface to the other. The range of the fundamental resonant frequency depends on the angle of the cut. Crystal cut may be either AT or BT. The AT cut is the most commonly used. It has an orientation of 35°15′ from the Z axis of the crystal, and it can achieve resonance frequencies on the order of tens of MHz. The resonant frequency depends on the thickness of the quartz plate, according to the relationship, f 0 = vs/2hq, where vs is the quartz sound velocity (3750 m/s for quartz) and hq is the thickness of the quartz. For instance, f 0 = 20 MHz is achieved by a plate thickness of about 37 μm. Due to the piezoelectricity of quartz, the mechanical resonance of the crystal is coupled with its electric resonance. Since the mechanical resonance of the crystal is characterized by very low 2532


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energy dissipation, the electric resonance exhibits a very large quality factor. This property is largely exploited in electronics to build stable oscillators such as clock references. For this scope, QMBs’ surfaces are coated with metal electrodes (e.g., gold on chromium). In 1959, Sauerbrey demonstrated that, in the low perturbation regime, the frequency of a QMB depends on the mass graviting onto the surface according to the following linear eq (eq 1):150 Δf = −

2f02 A μq ρq

Δm (1)

where f 0 is the fundamental resonance frequency of the quartz, A the active area, μq is the quartz shear module, and ρq is the quartz density. The Sauerbrey equation is strictly valid for rigid coatings that do not store elastic energy. In accordance with the Sauerbrey equation, the mass sensitivity of a QMB depends on the resonance frequency of the crystal and the resonant frequency depends on the thickness of the crystal. The thinner the quartz plate, the higher the resonant frequency and the mass sensitivity. However, in order to ensure mechanical rigidity, thickness is never too small. The diameter of commercially available AT-cut quartzes is between 1 and 3 cm, and the resonance frequency is between 5 and 50 MHz. Quartz crystals are typically connected to oscillator circuits so the change of mass on the quartz surface results in a shift of the output signal of the oscillator. The theoretical sensitivity (eq 1) of a QMB with a fundamental frequency of 20 MHz and a diameter of 1 cm is 1.15 Hz/ng. This means that if the frequency of the signal at the output of the oscillator is measured with an accuracy of 1 Hz, the detection limit is 1/0.58 = 0.8 ng. The wider use of QMBs is in thin film technology where they complement growing machines, such as sputtering and evaporator, providing a direct measurement of the amount of material deposited onto the substrate. The use of QMBs as chemical sensors began at the beginning of the sixties when the gas sensitivity of quartz crystals coated with gas chromatographic solid phase was demonstrated.151 The first use of porphyrins as coating of quartz microbalances dates back to half way through the nineties,152 where an array of four QMB functionalized with H2TPPs with different metals (rhodium, ruthenium, cobalt, and manganese) was investigated. Results show that sensors were cross selective to alcohols and amines but each with a distinct profile of sensitivities. These findings paved the way to the development of porphyrin-coated QMB arrays and to their applications in various fields.153 Figure 13 reports the latest version of the porphyrinoids (porphyrins and corroles)-based sensor array developed by the Paolesse and Di Natale group. Among the applications worthy of mention are those aimed at identifying diseases by measuring the released volatile compounds. In 2003, a porphyrin-based quartz microbalance provided the first evidence that lung cancer can be diagnosed measuring breath with an array of solid-state gas sensors.154 The finding was confirmed and extended in several successive papers and using different sensor technologies;155 among these, porphyrin-based colorimetric sensors were present.156 The role of porphyrins as sensitive materials for cancer detection is intriguing considering that oxidative stress is one of the major causes of production of cancer-related VOCs.157 Oxidative stress is the result of the increased activity of the cytochrome P450 enzyme whose prosthetic group is a iron porphyrin (the heme).158 Eventually, the molecule used by the

Figure 13. Array of 12 QMBs coated with different metal complexes of porphyrins and corroles. QMBs are characterized by a gold electrode pad that leaves an uncoated quartz corona. Porphyrinoid coating provides a colored hue to the otherwise transparent quartz. The Teflon lid of the sensors cell is visible on top and, in the background, the interface electronic board. (University of Rome Tor Vergata).

organism to produce the volatile compounds might also be able to detect them. Further studies indicated that arrays of porphyrin-coated QMBs could also distinguish among different stages of lung cancer.159 Furthermore, the same array was used to investigate the origin and the propagation of VOCs from lungs to breath.160 These results confirm that the most likely pathway of VOCs released by cancer cells is through blood, transferred to breath at the blood/air interface in the lungs.157 The diagnostic properties of such arrays were also demonstrated for other respiratory diseases such as asthma, skin cancer, and urinary tract diseases.161 The sensing properties of porphyrin solid films are strongly related to their tridimensional arrangement. For this reason, a number of efforts were made to study the sensing properties of solid films with different compositions. An example is offered by the combination of metal oxide semiconductors and porphyrins. In the previous sections, we have seen the progress made in optical and electric transduction. To this regard, microporous organic networks have shown unique physical properties such as a high surface area and a pore size smaller than 2 nm.162 Furthermore, they can be decorated with sensing moieties. For instance, a microporous (metal-free) porphyrin network (MP) was formed on the surface of ZIF-8 via the Sonogashira coupling of tetrakis(4-ethynylphenyl)porphyrin with 1,4-diiodobenzene.163 QMB coated with the microporous organic network demonstrated a sensitivity exceeding that of the single constituents, in particular with respect to ammonia with a limit of detection approaching 1 ppm. Another interesting opportunity is offered by GUMBO materials. GUMBO stands for “Group of Uniform Materials Based on Organic salts”, a collective term for solid phase ionic liquids and related organic salts that, contrary to the majority of these materials, are solid at room temperature.164 The gas sensitivity properties of the sodium salt of CuTpCPP reacted with trihexyl(tetradecyl)phosphonium chloride ([P66614][Cl]) to produce [P66614]4[CuTCPP]. The GUMBO of porphyrin was used to coat a QMB by cast dipping. The sensor was tested with several gases. The sensor is cross selective, and the smallest 2533


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Scheme 6. Molecular Structures of the Zn Porphyrins and an Example of the Proposed Binding Mechanism with Nicotine

matrices, as demonstrated by the adsorption isotherms.172,173 These results make these complexes promising as sensing layers, although no direct examples of their sensing applications was reported. The possibility of detecting CO by corrole layers has also been reported for tris-3,5 dihydroxyphenylcorrole-functionalized QMB, although in this case, the binding mechanism cannot be simply ascribed to metal coordination.174 This complex mechanism can explain the reports of Fe(III) porphyrin-based CO sensors, although these metalloporphyrins cannot coordinate CO. The possible exploitation of the different binding behaviors of porphyrins and corroles as sensing layers was recently investigated to prepare cross sensitive sensor arrays, by measuring the response of porphyrinoids-functionalized QMB toward model VOCs.175 The results obtained evidenced a great influence of the macrocycle structure on the sensing properties. In particular, free base corroles showed higher sensitivities than corresponding porphyrins. These results revealed the possibility of tuning sensing layer properties by structural modifications, and that porphyrin and corrole derivatives can positively cooperate to enhance the performance of sensor arrays. 3.7.3. Other Mass Transducers. Piezoelectric materials and micromachined silicon structures offer the possibility of fabricating more sophisticated mass transducers. The principle is the same as with QMBs, namely, the link between mass and frequency in mechanical resonators. Surface acoustic wave (SAW) devices are the natural development of QMBs. With respect to a QMB, an SAW is a planar device where an acoustic wave, traveling at the surface of a crystal, is launched and collected by pairs of interdigitated electrodes. The path between launcher and receiver is the place where a sensitive film can be applied. As a first approximation, a SAW is sensitive to the mass of the film, just as the QMB, even if,

detection limit was found for 3-methyl-1-butanol in the order of 0.2 μg/L.165 QMB is also particularly suitable for humidity sensing using different nano arrangements of porphyrins in order to enhance interaction with water vapor. A layer-by-layer growth structure of poly(diallyldimethylammonium chloride) and poly(sodium 4styrenesulfonate) provided the basis for the deposition of Mn(TPPS)Cl. The spontaneous deposition of each layer of the previous element gave rise to a multistack film that revealed a constant sensitivity in the whole relative humidity range.166 The same sensor was also used to measure the respiration rate in humans, collecting breath through a face mask.167 A similar approach was shown with a polylysinated substrate onto which porphyrins are spontaneously deposited. In this case too, an excellent and constant sensitivity in the whole relative humidity range was found with H2TPPS.168 An interesting variation of the QMB device working in solution is the electrochemical quartz crystal microbalance (EQMB), where mass measurement and electrochemistry can be performed simultaneously on the same film. The technique has been applied to measure the sensitive properties of two electropolymerizable zinc(II) porphyrins bearing the 2-phenoxyacetamide binding group (Scheme 6).169 The porphyrins were tested in the detection of alkaloids such as nicotine, cotinine, and myosmine with a detection limit of the gravimetric measurement below 1 mM. 3.7.2. Corrole-Based Sensing Layers. The exploitation of Mn corrole LB films as sensing layers of QMB sensors was probably one of the first applications of these macrocycles, since the peculiar chemistry of such a porphyrinoid immediately kindled interest for analyte detection.170 Guilard and co-workers later reported the high affinity of Co corroles for CO binding, evidencing also the selectivity toward O2 and N2.171 The same results were obtained when the complexes were grafted into silica 2534


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4. PORPHYRIN-BASED CHEMICAL SENSORS FOR LIQUID PHASE ANALYTES

in the case of a SAW, all mechanical properties, including rigidity and viscosity, are sensed.176 SAWs are operated at frequencies in the range of 100−1000 MHz. They are in fact supposed to be more sensitive than QMBs whose use is devised for applications requiring low detection limits. On the other hand, it is important to remember that the resolution of a sensor, namely the minimum measurable change of analytes, is the ratio between the noise of the sensor signal and its sensitivity.6 SAWs are operated at high frequencies, a range where signal fluctuation is definitely larger than that in a QMB. (ZnTMPyP)Cl4-coated diamond nanoparticles were used to functionalize a 433 MHz SAW.177 The device was found sensitive enough to detect tens of parts per billion of dinitrotoluene with a scarce interference from ethanol and humidity. The highest frequencies, therefore the greatest sensitivities, can be achieved with resonating structures made with materials with superior piezoelectric properties such as AlN. A typical configuration is the so-called film bulk acoustic resonator (FBAR) where an AlN layer is sandwiched between two metal electrodes. The resonance frequency of such a structure can be on the order of GHz. Thanks to their superior sensitivity they have mainly been used as biosensors.178 Their use as gas sensors is still not fully exploited. This is chiefly because the complexity of their fabrication and the sophisticated high-frequency electronics needed hinder their competitiveness with low-frequency devices such as QMBs and SAWs. H2OEP was used as a coating of a 4.4 GHz FBAR as a part of an array composed of other sensing molecules such as cavitands and cyclodextrins.179 All coatings were deposited as very thin Langmuir−Blodgett films. The array of four sensors could identify several volatile compounds at concentrations not below 10% of the saturated pressure. It is important to consider that in FBAR devices as sensitivity to mass increases greatly, the overall dimension of the detector is smaller with respect to a less sensitive QMB. On the other hand, the sensor’s sensitivity is proportional to the number of sensing molecules immobilized onto its surface. Eventually, the greatest sensitivity to mass is not paralleled by a similar improvement of the sensitivity with respect to the concentration of the analyte. Cantilevers are another important category of mass transducers. These devices stem from the development of atomic force microscopy tips. They can be adequately coated with a sensitive film in order to measure the adsorption of volatile compounds. The variation of mass can be measured either as a change of resonance frequency or as a change of the cantilever strain. An example of porphyrin-coated cantilever was based on a SU8 micromachined cantilever functionalized 5,10,15,20-tetrakis (4,5-dimethoxyphenyl) porphyrin iron(III) chloride and aimed at the detection of carbon monoxide.180 The cantilever length, width, and thickness were 200, 50, and 3.5 μm, respectively. The adsorption of mass was evaluated measuring the strain of the cantilever. For the scope, the nonconductive SU8 was turned into a conductor by a dispersion of carbon black nanoparticles in SU8. The porphyrin-coated cantilever was sensitive to few parts per million of CO with a good rejection of interferents such as CO2, O2, and N2O. Although Zn complex and free-base porphyrin were reported not to be responding, since they have no coordinating site for CO, it should be noted that also Fe(III)porphyrin complexes do not bind CO, so the sensing mechanism should be more complex than the metal coordination.

4.1. Electrochemical Sensors

Electrochemical methods have often been used for sensor analysis of liquid samples, relying on the direct transformation of an electrical signal into the target analyte concentration, based on known theoretical principles of electrode processes. Unlike other transduction mechanisms, which usually consider the homogeneous solution, the electrochemical processes occur at the electrode−solution surface. Two main groups of electrochemical processes can be used for sensor development: potentiometric and volt-amperometric methods. In the first case, no current flows in the electrochemical cell and the electrode potential is measured. In the second case, the interesting phenomenon is the current flowing in the cell due to the oxidation and reduction processes at the electrodes. Both methods require dedicated electronic setups, making use of high input impedance amplifiers. In accordance with this schematic separation, we illustrate the different devices in the following sections. 4.1.1. Potentiometric Sensors. Porphyrins and their metal complexes have been amply exploited as ionophores in potentiometric ion selective electrodes (ISEs).181 These ISEs are usually composed of a plasticized polymeric membrane, where the ionophore is dispersed together with ionic additives, which are needed to maintain the membrane’s electroneutrality. The working mechanism of these devices has been previously described in literature.182 The versatile character of porphyrins as ionophores is due to their coordination chemistry, since they can act as ligand for cations as free bases, or ion carriers for anion binding, when used as metal complexes. It should be noted that porphyrin-based ISEs have also been used to detect organic species in aqueous solutions. The four nitrogen atoms present in the porphyrin macrocyclic core are probably one of the most versatile chelating systems known, and the vast majority of the elements of the periodic table have been coordinated to a porphyrin. For this reason, porphyrins are ideal ionophores to detect cations, where their selectivities can be finely tuned by changing the peripheral substituents. Since chemical sensors require a reversible interaction, in the case of porphyrins the coordination of the metal ions is supposed to occur through the so-called sitting-atop complex, where the ion is not completely inserted in the porphyrin plane (Scheme 7). Scheme 7. Porphyrin Sitting-Atop the Binding Mechanism of Cations

Following this approach, H2TAPP was tested as an ionophore in a plasticized PVC membrane to detect Hg(II) ion.183 The developed ISE showed a good linear response in the concentration range of 10−8∼10−3 mol/L, with a Nernstianlike slope. The influence of pH was also studied, obtaining the best result at pH = 2.5, citrate buffer. Selectivity was studied for different cations, such as Na(I), K(I), Ba(II), Mg(II), Ca(II), and Cr(III), with no significant interferences observed. The 2535


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Scheme 8. Molecular Structure of Porphyrins Tested as Fe(III) Ionophores in Ref 184

Scheme 9. Molecular Structures of Pt Porphyrins

and Fe(III), etc. The ISE can be used in the pH range of 2−8, with a near-Nernstian response, with a six-week lifetime here too. The ionophore was also tested in removing Cu(II) ions from solutions, showing that this porphyrin can be used as an efficient adsorbent for the target ion. 5,10,15,20-Tetraferrocenyl porphyrin (H2TFcP) was used as an ionophore in a dual mode potentiometric-optical CSPT platform.185 The simultaneously measured optical and potentiometric responses of solvent polymeric membranes based on H2TFcP permitted the detection of lead ions in sample solutions, in the concentration range from 2.7 × 10−7 to 3.0 × 10−3 mol/L. The detection limit of lead determination was 0.27 μmol/L, low enough to perform the direct analysis of Pb(II) ion in natural waters. Metalloporphyrins have been exploited on a large scale for the development of anion selective electrodes, since they can bind different anions thanks to their axial coordination chemistry. In this case, the selectivity of the resulting ISE can be modulated by changing the porphyrin coordinated metal ion. For this reason, the selectivity pattern of metalloporphyrin-based ISEs differs from the Hofmeister series. Selectivity might also be influenced by the peripheral substitution of the macrocycle or by the addition of ionic additives to the polymeric membrane. These different ways of modulating the properties of the corresponding ISE make metalloporphyrins very promising materials; for this reason they have been extensively studied for anion detection.5 One of the major drawbacks of metalloporphyrin-based ISEs is represented by the frequent super-Nernstian behavior exhibited

developed ISE demonstrated a good stability and reproducibility, and its performances were tested for the detection of Hg(II) ion in the environmental wastewater of the Xiang River. The ISE showed accuracy comparable to that of AAS, suggesting its potential application in the real field. Modified tetraphenylporphyrins (Scheme 8) were used to prepare PVC-based ISEs, and the influence of the peripheral porphyrin functionalization on the detection of Fe(III) ion was reported.184 The best responses were obtained with the porphyrin bearing a meso-4-carboxyphenyl group, which showed a near Nernstian response in the 10−7−10−1 mol/L concentration range, with a good selectivity toward different alkaline and heavy metal ions. The influence of pH in the ISE response was also studied, showing the possible use in the pH range of 2.0−3.8, while for higher pH values the precipitation of iron hydroxide occurs, thus hampering the ISE response. The ISE was stable during a working period of 6 weeks, and it was tested for the detection of Fe(III) ion both in tap water and in synthetic solutions mimicking those of spent lithium ion batteries, presenting good results. The same group studied the performances of a tetrakis(4allylphenyl)porphyrin-based ISE in a similar application, to detect and remove Cu(II) ion in synthetic solutions, similar to those of spent lithium ion batteries. The influence of the plasticizer used for the membrane’s realization was studied, and the best results were obtained using dioctyl phthalate. The resulting ISE showed good selectivity toward the ions potentially present in the target leach liquors, such as Co(II), Li(I), Al(III), 2536


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binding. Both complexes functioned as neutral carriers and needed cationic additives to demonstrate a good selectivity of iodides, with PtOEP displaying a better performance. The Nernstian slope of the ISE responses suggested the possibility of preventing ionophore dimerization by using Pt porphyrins. This feature was also confirmed by the absence of variations in the UV−vis spectra of the macrocycles in the PVC membranes. Mn tetraarylporphyrins have been used as ionophores to develop ISEs devoted to the detection of organic anions of pharmaceutical interest.189,190 Diclofenac is a drug used on a large scale with analgesic, anti-inflammatory, and antipyretic properties. Its universal diffusion makes it one of the emerging pollutants of wastewaters. Monitoring its concentration is becoming urgent. Fagadar-Cosma and co-workers investigated the performances of PVC based ISEs to detect diclofenac, using two Mn(III) tetraarylporphyrin complexes as ionophores. The influences of the different components of the PVC membrane, such as plasticizers and cationic and anionic additives, on sensor responses were investigated. Among the two different Mn complexes, Mn(TPP)Cl was chosen as the ionophore, since manganese(III)-tetrakis(3-hydroxyphenyl)porphyrin chloride [Mn(T3HPP)Cl] showed a super-Nernstian slope, indicating its dimerization in the membrane. An anionic additive was necessary to obtain the best results, with a linear almost Nernstian slope in the 5 × 10−4 to 10−2 mol/L concentration range. The sensor also showed good diclofenac detection, with SCN− being the strongest interferent. The developed sensor can work in the 5.5−11.5 pH range, and its performances in detecting diclofenac were compared with those of the HPLC method, presenting substantial agreement. Mn(TPP)Cl has also been tested in PVC-based ISEs to detect sulfadiazine, a sulfonamide antibacterial drug.190 Also in this case, the influence of ionic additives on sensor responses was investigated and performances were compared with those of ISEs using cyclodextrins as ionophores. Without additives, Mn(TPP)Cl-based membranes showed a near Nernstian slope, with a narrow 1.3 × 10−4 to 1.0 × 10−2 mol L−1 linear concentration range. The addition of cationic additives increased the response slope to a value closer to the Nernstian behavior, while the linear range was not changed. If anionic additives were added, a decreased sensitivity was observed, but with a good selectivity pattern, different from the Hofmeister series; on the other hand, these Mn(TPP)Cl-based ISEs were strongly influenced by the solution’s pH, and they effectively worked as a pH sensor. ZnTPP was recently exploited as an ionophore to develop a cyanide selective electrode.191 Cyanide contamination is of great concern for environmental or terroristic poisoning, due to its high toxicity. Metalloporphyrins are ideal binding agents for cyanide ions, since they are a good ligand for metal ions. For this reason, different metalloporphyrins, with coordinated Co(II), Co(III), Cu(II), Fe(III), Zn(II), and Ni(II) ions, were tested as ionophores in PVC membranes to develop cyanide selective electrodes. Among them, ZnTPP-based ISEs exhibited the highest selectivity for cyanide and were as a result investigated in more detail. TDDMA+ was added as a cationic additive, since ZnTPP acts as a noncharged ionophore. KCN was used in a 1 mM NaOH background solution, to avoid the formation of HCN vapors. The developed ISE showed a Nernstian linear response in the 0.1−10−5 mol/L concentration range, with a 10−5.5 mol/L as a lower detection limit. The ISE showed a high selectivity toward different ions, in particular those that can find application in the mining industry; moreover, the ISE was highly selective

by these electrodes, that is, when the slopes of the sensor responses to ion exposure are much higher than those expected by the theoretical Nernstian law.186 The origin of this superNernstian response was demonstrated to be ascribed to the formation of metalloporphyrin hydroxo-bridged dimers, which also induces a pH cross response and the ionophore crystallization inside the membrane.187 While different approaches have been proposed in literature to overcome this problem, we are interested in exploiting platinum porphyrins as ionophores.188 In fact, platinum has a reduced oxophilicity. For this reason the hydroxy induced metalloporphyrin dimerization in the membrane could be avoided, together with the pH cross influence. Pt porphyrins have been used extensively in polymeric membranes to detect molecular oxygen, but they have not been investigated as ionophores, probably due to the reduced axial chemistry of such square planar complexes. Three Pt complexes were studied (Scheme 9), with PtTPPCl2 as the first, since we hypothesized that the Pt(IV) oxidation state could allow an active axial chemistry useful for the target sensing mechanism. DFT studies indicated a potential selectivity toward the iodide ion, also confirming a reduced affinity in terms of binding energies toward oxoanions. However, the preparation of a membrane with PtTPPCl2 showed a “mixed mode” response, with the complex working both as charged and neutral ionophore. This behavior was made explicit by controlling the optical feature of the membrane, showing a variation of the UV− vis spectrum over time (Figure 14).

Figure 14. Changes of the UV−vis spectra of the Pt(IV)TPPCl2 containing membranes in dichloromethane solutions. The spectra of Pt(IV) and Pt(II) tetraphenylporphyrins are given for comparison. Adapted with permission from ref 188. Copyright 2011 Royal Society of Chemistry.

PtTPPCl2 was not stable, and it was reduced to the corresponding PtTPP by the residual THF present in the membrane, confirmed by analyzing the oxidation product via GC/MS. The presence of both complexes induced the mixed mode response of the resulting membrane. While the reduction of PtTPPCl2 precluded its exploitation as an ionophore, the results obtained revealed the unexpected properties of PtTPP as an ionophore. For this reason, we investigated the preparation of membranes using PtTPP and PtOEP as ionophores for iodide 2537


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The exploitation of metalloporphyrins grafted on graphene oxide (GO) as ionophores was proposed almost contemporaneously by the same group.196,197 In the first article, the hybrid NbTPP-GO material is proposed to detect fluoride.196 In this case too, the exact nature of the porphyrin complex was not reported; it was a simple bare complex, without the necessary axial ligands. The hybrid material was dissolved in a PVC membrane and used to develop a coated wire ISE, tested for fluoride detection. The sensor showed a Nernstian linear response, with a good selectivity in the 10−7 to 0.1 mol/L range. In the second article, a CuTPP was grafted on GO and the hybrid material was used as an ionophore to detect the salicylate ion in a PVC membrane-coated wire electrode.197 Although this second article reported that the GO-CuTPP functionalized ISE was superior for salicylate detection with respect to a similar ISE doped with CuTPP, the influence of GO on the sensing properties of the metalloporphyrins as binding units is unclear in both articles.197 Mn(TPP)Cl198 and Zr(TPP)Cl2199 based ISEs have been integrated in potentiometric sensor arrays to monitor complex chemical matrices, such as soft cheese and methane fermentation, as selective ionophores for chloride [Mn(TPP)Cl] and acetate ions [Zr(TPP)Cl2]. 4.1.2. Porphyrinoid-Based Ion Selective Electrodes. The possibility of tuning the binding properties of the macrocyclic ionophore by skeletal modification led to studying the exploitation of porphyrinoids as binding units of ISEs. This opportunity has been one of the seminal applications of corrole in the chemical sensor field, first using free base corroles a s i o n o p h o r e s . I n an i n i t i al w o r k , 5 , 10 , 1 5 -t r i s (pentafluorophenyl)corrole was used as an ionophore for Ag(I)-selective PVC polymeric membrane electrode, showing a close to Nernstian potentiometric response slope in the 5.1 × 10−6 to 1.0 × 10−1 mol/L concentration range, with a working pH range between 4.0 and 8.0, and a fast response time.200 At almost the same time, Dehaen and co-workers reported the exploitation of free base corroles in PVC membranes for the potentiometric detection of salicylic acid.201 In this work, the authors studied the influence of pH on the ISE response, since corrole is more acidic than porphyrin and it can be both protonated and deprotonated in water solution, presenting the pH depending equilibria reported in Scheme 10. The most

with respect to the hydroxide ion, without showing a super Nernstian response even at pH = 11, ruling out the formation of hydroxide-bridged dimers. The most interfering anions were thiocyanate and iodide, but only the first can be present in applications of interest to the mining industry. The lifetime of the developed ISE can be improved by using freshly distilled THF for the membrane preparation and by storing the membrane in the dark. All these features indicated an oxidative decomposition of the ionophore due to the formation of peroxide species. ZnTPP was also used as ionophore to develop SO2 selective electrodes to analyze wine.192 The sensing mechanism follows the same chemistry adopted for a colorimetric sensor based on the indicator displacement assay principle:193 SO2 reacts with a diethylamine (DEA) complex of ZnTPP, inducing the displacement of the amine due to the formation of the SO2:DEA adduct. The consequent change of the ZnTPP color allows the determination of SO2. This mechanism is difficult to operate with colored matrices, such as wines. Potentiometry can exploit the same chemistry without this drawback. In this case, the PVC membrane containing ZnTPP was conditioned in a solution containing DEA before its use, to allow the formation of the ZnTPP-DEA complex, which can be evidenced by the red to green color change of the membrane. The influence of the DEA concentration in the conditioning solution and the addition of ionic additives to the membrane on sensing performances were evaluated, to optimize SO2 detection. It was found that the membrane containing ZnTPP and conditioned with a 2 mol/L solution of DEA gave the best performances, with a Nernstian slope in the 10−6 to 10−3 mol/L concentration range and a lower detection limit of 7.1 × 10−6 mol/L. The pH was fixed at 1.6 for the best results. When the pH value was increased to 5, no response was observed, since all the SO2 was converted to a bisulfite anion, not useful for the membrane sensing mechanism. The selectivity of the developed ISE was tested toward the ions potentially present in wine samples and toward the compounds present in wines that bear amine groups, such amino acids or biogenic amines. For all these species, no interferences were observed at the contents expected in wine samples. The developed ISE was tested for SO2 analysis in wines, and the results obtained compared with those of a standard method for SO2 quantification. The results were comparable with those of the reference method. Selenite ISEs were developed using the Co(II) complex of 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin (CoTMPP) as ionophore.194 Three different kinds of sensors were prepared, such as classic PVC membrane, modified carbon paste, and coated wire electrodes. The optimal configuration for each electrode was studied, in terms of membrane composition and plasticizer used, obtaining similar performances for the different electrodes, with a linear Nernstian-like behavior in the 10−5 to 10−3 mol/L concentration range. The pH influenced the slope of the ISE responses, which at pH 11 was half of that observed at pH 6.47, due to the diprotic character of the selenous acid. The developed ISEs showed a good selectivity to a series of anions, and the PVC membrane electrode was tested to detect the selenite ion in selected applications, delivering satisfactory performances. The detection of perchlorate anion in hazardous materials, such as fireworks and propellants, was made using a PVC membrane ISE using InTPP as an ionophore.195 No indications of the InTPP counterion were reported in the article, with the developed ISE tested with acceptable results also for the flow injection analysis of perchlorate present in firework samples.

Scheme 10. pH Depending Equilibria of Corrole

positive responses were obtained at pH 2, where neutral salicylic acid is present. The response improved with the presence of a cationic exchanger. High super Nernstian response slopes were observed in these conditions, indicating a complex mechanism for the potentiometric response. At a higher pH, the response slope decreased, while the ISE was not sensitive to the salicylate ion at a basic pH. The strong pH influence on the corrole-based ISE was later confirmed, studying the behavior of triphenylcorrole-based PVC membrane electrodes toward different anions.202 The influence 2538


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obtaining devices with a Nernstian response slope in the concentration range of 10−6 to 0.1 mol/L, with a short response time. The electrode performances were not affected by pH in the 4.0−8.4 range; an excellent selectivity toward e series of cations was observed, with an 8-week-lifetime. The electrode that was developed was tested to determine Mg content in real matrices, such as baby foods and soft drinks, with satisfactory results that make this Mg selective electrode a good alternative to the existing sensors for Mg quantification. Octamethylcalix[4]pyrrole has been tested as ionophore in ISEs designed for the selective detection of Ti(III) ions in industrial wastewaters.205 Also in this case, different membrane compositions were tested to optimize device performances, allowing a Nernstian response slope in the 1−3 pH range, where the Ti(OH)2+ ion is present. The working concentration range was 10−6 to 10−2 mol/L, with a fast response time. The selectivity of the resulting membrane was excellent toward a series of mono-, di-, and trivalent cations tested. The electrode lifetime was satisfying (i.e., three months). The developed ISE was successfully exploited for the quantitative detection of Ti(III) ions in industrial wastewater samples and in tap water. A significant limitation of potential applications of such a Ti(III) ISE is represented by the narrow and acidic working range of the developed electrode, where the targeted Ti(OH)2+ ion is present. 4.1.3. Volt-Amperometric Sensors. Different techniques have been developed based on the observation of the current flowing in the cell, but all mainly deal with metallic electrodes and observe the situation when the current flows and the analyte concentration changes as a result of an electron-transfer red-ox reaction on the electrode surface. The corresponding sensors can be named amperometric or voltammetric, depending on the form of the applied potential (amperometry uses a constant applied potential, while voltammetry varies the applied potential). Porphyrinoids are applied to functionalize the working electrodes. The following sections describe recent examples in literature, according to the different techniques exploited and the target analytes. 4.1.4. Amperometric Sensors: NO Detection. Porphyrinbased electrodes to detect nitric oxide produced by cancer cells are rarely reported. This analyte is electrochemically active, and its detection involves oxidation on the electrode surface in the NO+ specie that produces a Faradaic redox current. The modification of the electrode surface with metalloporphyrins, carbon based materials, or permselective membranes is crucial to increase the analytical performances in terms of selectivity and sensitivity. Lang and co-workers reported the detection of NO produced by cervical cancer cells (HeLa) by a GC electrode surface modified with H2TMPP.206 The fabricated H2TMPPmodified GC introduced into a three-electrode system was used to quantify the extracellular NO released upon activation of HeLa cells promoted by phorbol myristate acetate (PMA). The NO sensor registered an amperometric current sensitivity of 0.0138 nA/μL with a linear correlation coefficient of 0.99571. Another example of microelectrochemical sensor for NO detection based on 3-aminophenylboronic acid (APBA) and metalloporphyrin cofunctionalized reduced graphene oxide (rGO) was reported by Huang and co-workers.207 The developed hybrid system merged the excellent catalytic oxidative activity of iron porphyrin for NO together with the nanostructural and conductive properties of graphene to give the selfassembling nanosheets of Fe(T4CPP)Cl and rGO, which are deposited onto an ITO microelectrode array via electrophoretic deposition. The further covalent functionalization of these

of the plasticizer polarity and the lipophilic additives was also studied to gain information on the working mechanism of the corrole receptor. The results obtained highlighted the fact that triphenylcorrole’s selectivity pattern is different from the Hoffmeister series. Creating an ISE is quite problematic, due to the strong influence of the background pH of the analyzed solution. The exploitation of metal complexes of corrole is also interesting for anion detection, since the peculiar coordination chemistry of this macrocycle offers the opportunity to tune the selectivity pattern of the developed ISE. The same article investigated the use of Cu, Mn, and Fe complexes of corroles as ionophores. 202 The Cu complex of 5,10,15-tris(4-tertbutylphenyl)corrole (Cut-BuPC) was dispersed in PVC membranes, and cationic additives were added to some, to check if the Cu corrole worked as a neutral carrier. The addition of cationic additives improved the response behavior of the corresponding electrodes, which showed an enhanced selectivity toward carbonate and monohydrogen phosphate anion, with a response slope close to the Nernstian value. UV−vis measurements demonstrated no dimerization of the Cu corrole when the membrane was immersed into the NaH2PO4 solution. The observed selectivity toward hydrophilic anions is quite interesting, since it is difficult to obtain with analogous porphyrin complexes. Mn(III) 5,10,15-triphenylcorrole chloride [Mn(TPC)Cl] has been tested as an ionophore, preparing a membrane with a composition similar to that used for the analogous Mn(TPP)Cl complex, which is applied for commercial chloride ISE. The developed ISE showed a low pH response and a good selectivity toward common anions, such as nitrate, nitrite, bromide, and perchlorate ions. Furthermore, optical investigations revealed that Mn(TPC)Cl did not suffer from dimerization in the membrane, a drawback that usually affects metalloporphyrins. The influence of the peripheral substituents on the sensing behavior was investigated for iron complexes of corrole.202 Fe(TPC)Cl and Fe(III) 2,3,17,18-tetraethyl,7,8,12,13-tetramethylcorrole chloride [Fe(Et4Me4)CCl] were tested as ionophores in PVC membranes, and both showed a selectivity pattern opposite to the well-known Hofmeister series, that is high selectivity toward hydrophilic anions, such as carbonate or HPO42−. The response slope for this latter anion was close to the theoretical Nernstian behavior, while for carbonate a supernernstian slope probably indicated a partial corrole dimerization. It is interesting to note that the β-alkylcorrole complex gave better results than the corresponding meso-triaryl species, suggesting the possibility of tuning sensing performances by synthetic modifications. More recently, Yang and Meyerhoff reported the exploitation of Co(t-BuPC)PPh3 to develop nitrite PVC membrane selective electrodes.203 The developed ISE displayed a selectivity decidedly unlike the Hofmeister series, and the addition of lipophilic cationic additives strongly enhanced the nitrite selectivity toward lipophilic anions, such as perchlorate or thiocyanate. The PVC membranes doped with Co(III) corrole showed a Nernstian behavior, with fast responses and recovery times. A modest increase of selectivity toward nitrate ion was also obtained by using tributylphosphate as plasticizer. Among other porphyrinoids, only sparse examples of their exploitation as ionophores have been reported in the past decade. 5,10,15,20-Tetrakis(2-furyl)-21,23-dithiaporphyrin was used dispersed in a PVC membrane to develop Mg(II) selective electrodes.204 The choice of the membrane components allowed 2539


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H2O2 and nitrite ions. These electrocatalytic activities were exploited to construct an amperometric sensor constituted by a GCE modified with the MWCNTs/Cu-CoT4CPP composite material. The fabricated sensor can be used to determine H2O2 and NO2− working at the proper potential: −0.25 V was chosen to obtain good performance for the reductive detection of the first analyte, and +0.85 V was used for the oxidative detection of nitrite. The sensor exhibited adequate performances, in line with or even superior to many other electrochemical sensors for the same analytes based on different sensing materials, such as noble metals, nanotube composites, and cobalt compounds. The interference of potential coexisting analytes such as uric acid, glucose, or K(I) and Zn(II) cations was also evaluated, evidencing a good selectivity except for the serious disturbance of dopamine to detect nitrite. The bifunctional determination of NO2− and H2O2 with porphyrin-based sensing materials was also reported by Zou and co-workers. They prepared the donor− acceptor dyad ZnP-C60, where ZnTMPP and fullerene units were linked by a flexible methylene chain.212 The ditopic system was incorporated into a tetraoctylammonium bromide (TOAB) film and then spread on the surface of a GCE. The TOAB/ZnP-C60/ GCE system produced an excellent nonenzymatic sensing ability, with a wide linear range and low LODs comparable to those of other H2O2 and nitrite sensors. Notably, the merger of the spatial three-dimensional bent conformation of the dyad, the excellent uniformity of the film on GCE, and the remarkable electrochemical properties influenced the sensor’s sensitivity. It was 2 orders of magnitude greater than those reported for other systems. Furthermore, the sensor’s practical applicability was demonstrated by determining the nitrite concentration in real samples of river water and rainwater. Electrochemical nitrite sensing was also recently performed by using different types of electroactive porphyrin-based materials, such as the tetraruthenated porphyrin complexes and MOF thin films built from H2T4CPP linkers and hexa-zirconium nodes.213,214 Furthermore, the amperometric detection of NO2− also occurred with citrate-gold nanoparticles decorated on Co(II)TAPP selfassembled GCEs.215 Co porphyrin was chosen as a suitable linker to immobilize AuNPs on the electrode, since it is able both to self-assemble on the GC surface and bind the cit-AuNPs by the four amino groups, which are in the self-assembling monolayer (SAM). The composite material shifted the NO2− oxidation potential toward less positive and demonstrated a high electrocatalytic activity thanks to the high coverage of citAuNPs besides the cobalt ion of the porphyrin. The orto−NH2 groups on a porphyrin macrocycle were also exploited to prepare a porphyrin-based electrosynthesized molecularly imprinted polymer (MIP), using the 4-(2,4-dichlorophenoxy)butyric acid as template.216 The MIP was then successfully applied for the amperometric determination of this herbicide, even if other interfering pollutants like chlorophenols were present, confirming the remarkable imprinting effect of the polymeric matrix which is not suitable for smaller analytes. An analogous compound, the herbicide 2,4-dichlorphenoxyacetic acid (2,4D), was successfully quantified by a biomimetic sensor based on a carbon paste modified with Fe(TFPP)Cl and multiwalled carbon nanotubes, whose addition increased the sensor response by 10fold when compared to that of the biomimetic sensor without MWCNT.217 The developed system allowed a sensitive and selective quantification, as revealed by the studies of selectivity reported toward 10 pesticides other than 2,4-D. Moreover, the practical applicability of this sensor was tested to analyze soil samples, with an absence of matrix effects. The same porphyrin

nanostructures with a small cell-adhesive molecule (APBA) also provides the sensor with excellent cytocompatibility and practicable reusability. The authors also fabricated a patterned ITO microelectrode for the sensitive and selective real-time monitoring of NO molecules released from attached human umbilical vein endothelial cells cultured directly on the sensor surface. The developed device featured fast responses (about 400 ms) and a calculated LOD of about 55 pM in PBS and 90 pM in cell medium, which are lower than those previously reported. 4.1.5. Amperometric Sensors: H2O2 Detection. Monitoring amounts of H2O2 is of great importance in various fields, ranging from food control, pharmaceutical, clinical, and environmental protection. The amperometric detection of this analyte is usually accomplished based on its reduction rather than oxidation, since the latter is more affected by many other electroactive interferents in the biological fluids. Although the use of biosensors is rather common, the development of nonenzymatic H2O2 sensors is particularly appealing, being economic and more stable in comparison. An amperometric sensor for H2O2 detection in beverages that uses a nanocomposite porphyrin material was reported by Chen and coworkers.208 In this work, the sensing material is constituted by the water-insoluble [Fe(III)-5,10,15,20-tetrakis(α,α,α,α-2-pivalamidophenyl)porphyrin] bromide [Fe(TpivPP)Br] assembled on MWNTs by noncovalent interactions. The functionalization of MWNTs by an iron porphyrin complex led to enhanced electron transfer by synergic effect that produces a highly sensitive amperometric sensor for the reduction of hydrogen peroxide with a low overpotential. The developed MWNTs/ FeTpivPP/GCE system presented excellent performances: rapid response, wide linear range, low detection limit, good fabrication reproducibility, high sensitivity, and selectivity toward the tested interferents (sugars and organic acids). The last feature enables the practical use of the developed amperometric sensor to detect H2O2 in different commercial beverages without any sample pretreatment. The analytical results obtained with this system were in satisfactory agreement with those determined by the classical titration method with KMnO4. A similar system for hydrogen peroxide detection was developed by Jeong and co-workers.209 In this work, nanowires of poly-[Co(II)5,10,15,20-tetrakis(2-aminophenyl)porphyrin] (PCoT2APPNW) were electropolymerized in a very uniform and controlled way by the CV method using anodic aluminum oxide membranes as a template and then mixed with SWNT and deposited on a GCE, which is the working electrode for the determination of H2O2. The increased surface area of the modified electrode with the nanostructures enhanced the rate of electron transfer between the target analyte and the nanocomposite material thanks to their synergistic effect. The sensitivity of this sensor was 194 A mM−1 cm−2, which is comparable with those reported in literature. A similar system that uses the corresponding free base aminoporphyrin derivative was also mentioned by the same group for the selective and sensitive determination of serotonin in the presence of ascorbic acid and dopamine.210 Jiang and co-workers developed the first example of electrochemical sensor for H2O2 and NO2− based on a bimetallic metalloporphyrinic framework Cu-CoT4CPP, where the Co(II)T4CPP monomers are bound by Cu2(COO)4 paddle-wheels with an interlayer spacing of 1.0 nm.211 The electrochemical investigation of the prepared porphyrin material evidenced that copper’s catalytic activity is toward the reduction of hydrogen peroxide, while cobalt is active toward the oxidation of both 2540


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Figure 15. (a) Schematic representation of (Co−Fe-LDH/MnTSPP)n film formation using the MFA technique. (b) Chemical detection of glucose at the electrode surface reported by Duan and co-workers. Reproduced from ref 225. Copyright 2011 American Chemical Society.

conjugation of the recognition ability of the GR-5 oligonucleotide toward Pb(II), with the peroxidase-like performance of the iron porphyrin MOF material resulted in a highly selective and sensitive sensor, of low cost and simple fabrication, with a LOD value of 0.034 nM, much lower than those of other reported electrochemical methods. 4.1.7. Amperometric Sensors: Organic Analytes. The same group reported the excellent electrocatalytic behavior toward oxygen reduction at −0.28 V of a modified CGE with reduced graphene nanoribbons functionalized with water-soluble FeTMPyP nanocomposite film.223 The coupling of this system with glucose oxidase enabled the amperometric biosensing of glucose based on dissolved O2 consumption during the enzymatic reaction. The biosensor had satisfactory analytical performances, a LOD value of 0.2 nM, and a linear concentration range from 0.5 to 10.0 nM, suitable to quantify glucose in blood where the sugar-level is normally 4.0−6.0 mM. The same Feporphyrin was used as the active component of a supramolecular film deposited on a gold electrode employed for dissolved oxygen determination.224 The organic/metal NP multilayers were constructed using the LBL technique, exploiting the hosting ability of cyclodextrin molecules toward porphyrin guest molecules. In detail, the SAM used in the sensor’s fabrication derived from the combination of mono-(6-deoxy-6-mercapto)β-cyclodextrin, the iron porphyrin complex, and cyclodextrinfunctionalized AuNPs as components. The amperometric measurements carried out at −0.15 V versus Ag/AgCl evidenced the high sensitivity of the developed sensor, ascribable to a very efficient electron transfer between O2 and the catalytic supramolecular Fe-porphyrin films at the electrode surface. Furthermore, the strong binding between cyclodextrin molecules and porphyrin complexes at the surface accounted for the good repeatability of sensor preparation and the stability of the modified electrode. Sensor performances were excellent, yielding the O2 determination with LOD and LOQ values of 0.05 and 0.2 mg L−1, respectively. The practicability of the sensor was demonstrated by interference studies evidencing the high tolerable concentrations of several foreign compounds as well as by its application to the analysis of pond and tap water samples, producing results consistent with those measured with the dissolved oxygen meter. Another example of LBL films containing the negative Mn(TPPS)Cl alternatively deposited on positively charged CoFe-Layered double hydroxide (LDH) nanoplates was reported by Duan and co-workers for glucose detection. 225 The ordered ultrathin films (UTFs) were assembled on ITO substrates with the assistance of an external magnetic field that allows a controlled and fine-tuned deposition

coordinated with a manganese ion was used with gold nanoparticles to modify fluorine tin oxide glasses for the electrochemical sensing of cysteine at pH 7.0.218 The hybrid AuNP/Mn porphyrin system developed produced LOD and LOQ values of 2.40 and 8.15 μmol L−1 respectively, in accordance with other modified electrodes used for L-cysteine detection. Alternatively, this analyte can be electrochemically determined using a Co(II) 1,8,15,22-(NH2)4phthalocyanine SAM-modified GC electrode as reported by S. A. John and coworkers.219 4.1.6. Amperometric Sensors: Inorganic Analytes. Sporadic examples of porphyrin-based amperometric sensors for some inorganic species detection have been reported. The use of nanoparticles of conducting polymer of NiTPPS (PNi-TPPS-NPs) as electrocatalyst for hydrazine oxidation was recently reported by Zakavi and co-workers.220 The deposition of the sensing nanomaterial onto the GCE was accomplished in two steps, first by electrodepositing Ni NPs at −1.3 V potential onto the electrode surface and then by dipping the modified substrate into an alkaline solution containing the porphyrin monomer, to form the PNi-TPPS-NPs by cyclic voltammetry. The electrooxidation of hydrazine occurred at the electrode surface through a catalytic mechanism involving the Ni(II)/Ni(III) redox center. The detection of the target analyte was performed by hydrodynamic amperometry in 0.1 M NaOH and at a potential step of +0.45 V vs Ag/AgCl, with a LOD of 0.11 μM. The analytical performances of the fabricated sensor were comparable with other reported electrochemical methods. Furthermore, the influence of interfering compounds that typically exist in medical samples was negligible. A GCE modified with a stable conducting polymer containing porphyrin units was used for the amperometric determination of S(IV) oxoanions in both artificial and real wine samples.221 The authors of this work studied the electrochemical behavior of three different electrodeposited polymeric films, consisting of tetraruthenated porphyrins, where the Ni(II), Zn(II), and the free base H2T4PyP coordinate four [Ru(5-NO2-phen)2Cl]+ units. The nickel-based material possessed the best electrocatalytic properties, presenting more available active sites on the electrode surface, where the Ni(II) ions easily coordinate SO2 and HSO3− species. The amperometric method developed for sulphite determination in a white wine sample was carried out in the reduction process at pH 1.0 and was precise and reliable with a LOD value of 1.40 mg L−1. With regard to metal cation detection, an efficient electrochemical sensor for Pb2+ was recently reported by Ju and coworkers, using a DNA functionalized FeT4CPP-MOF deposited on the surface of a screen-printed carbon electrode.222 The 2541


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Figure 16. Stepwise fabrication of Hemin/PAMAM/MWCNT/GCE system for L-tyrosine detection. Reproduced with permission from ref 226. Copyright 2010 Elsevier.

process. The schematic representation for film formation is reported in Figure 15a. The (Co−Fe−LDH/MnTPPS)n (n= 1−12) UTF’s regular formation under magnetic fields of different densities was monitored by UV−vis spectroscopy, showing the linear correlation between the intensity of the Soret band at 474 nm and the increase of bilayer number n, together with an increased packing density of the building blocks for a higher value of the applied magnetic field. The best amperometric performance was obtained by UFTs-modified ITOs with n = 6 formed at a 0.5 T magnetic field. Applying the potential of 0.6 V, the Co(II) species of the electrode is oxidized to Co(III): the glucose contained in the alkaline solution reaching the electrode surface is oxidized to glucolacton by Co(III) species that simultaneously generates the Co(II) species (Figure 15b). These electrodes exhibited a suitable sensing performance in the linear range of 0.1−15 nm, LOD of 0.79 μM, and a high sensitivity determined by the deposition method that afforded a dense packing of LDH nanoplates, resulting in a high number of active species and an easy electron transfer process on the electrode. Other strong points of this nonenzymatic glucose sensor are the selectivity observed for glucose oxidation also in the presence of uric and ascorbic acid, the lack of the Cl− poisoning effect on the electrocatalyst and the long electrode lifetime. An amperometric sensor for L-tyrosine detection using a nanocomposite material containing hemin was reported by Tiantian Tang and co-workers.226 The iron porphyrins were covalently immobilized onto nanostructured LBL film consisting of poly(amidoamine)/multiwalled carbon nanotubes (PAMAM/MWCNT), as can be observed in Figure 16. PAMAM dendrimers have positive surface charges in neutral aqueous solutions that can be electrostatically combined with the carboxyl groups on carbon nanotubes deposited on a GCE surface, giving a stable conducting film. The −COOH functionalities on hemin periphery can be activated by EDC, producing a covalent linkage with the exposed amino groups of the LBL film. The active species O = Fe(IV)-porphyrins oxidize conspicuously with the L-tyrosine, enabling its amperometric determination with the Hemin/PAMAM/MWCNT/GCE system at 0.8 V and 0.2 M pH 7.0 PBS. Although this sensor generated a low value of LOD, it was demonstrated that the system could similarly oxidize other interferent species, such as uric acid and L-cysteine, influencing the selectivity of the proposed method. Finally, the amperometric detection of a series of phenolic endocrine compounds (EDCs) such as bisphenol A, nonyphenol, and ethinylestradiol was reported by Wong and co-

workers.227 The electrode was fabricated by electropolymerizing NiTSPP on CNT-modified-GCE and was coupled with FIA analysis to quantify low concentrations of EDCs in water samples. The results obtained were satisfactory in terms of sensitivity, stability, and detection limit, the latter ranging from 15 nmol L−1 to 260 nm L−1. 4.1.8. Voltammetric Sensors: Dopamine and Neurotransmitters Detection. The determination of dopamine (DA) is of great importance in clinical analysis, since it is related to neurological diseases, such as Parkinson’s. Different porphyrin-based voltammetric sensors were recently developed for its electrochemical detection, mainly using iron porphyrins. Barrio and co-workers, for example, reported the fabrication of microsensors made of carbon fiber microelectrodes covered with nonconducting molecular imprinted poly[iron(III) tetra(2aminophenyl)porphyrin], electrosynthesized using dopamine as the model analyte.228 The stable deposition of the MIP film onto the electrode surface was performed by cyclic voltammetry in a potential range between −0.15 and 1.0 V, using the optimal template concentration that allows the selective recognition of the target analyte even if other interfering compounds with similar structures are present. The electroanalytical determination of DA was carried out in an acetate buffer solution by square wave voltammetry (SWV), in a linear range between 10−6 and 10−4 M and with a detection limit of 3.89 × 10−7 M. The selectivity of the MIP-based microsensors was also excellent against other catecholamines, showing good analytical performances also when applied to real samples of rat brain tissue. Dopamine was also electrochemically detected by using hybrid systems composed of porphyrins and graphene. Qu and coworkers reported the use of a glassy carbon electrode modified with a porphyrin-functionalized graphene for the selective determination of DA in the presence of common interfering compounds, like uric and ascorbic acids (UA and AA, respectively).229 The noncovalent assembling of the anionic H2T4CPP onto reduced graphene offered a negatively charged electrode surface able to electrostatically select the target analyte among the two interferents: the positively charged DA molecules were attracted by the sensing material, accelerating the electron transfer process, whereas the organic acids, which are both in the anionic form in neutral aqueous solutions, unfavorably interacted with the surface, giving lower oxidation peaks than that of DA at the electrode. Moreover, the use of the differential pulse voltammetry technique (DPV) permitted the resolution of the overlapped oxidation voltammetric peak of the three analytes in three well-defined peaks, enabling the selective determination of 2542


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The simultaneous determination of these compounds was performed using a carbon paste electrode modified with TiO2 nanoparticles and Co(II) 5,10,15,20-tetrakis(3-methylphenyl)porphyrin as a mediator.233 The DPV method was used to determine the concentrations of LD and CD, showing welldefined and separate oxidation peaks at potentials of 450 and 680 mV for the two compounds, respectively. These analytes were also detected in water, urine, and human blood serum samples with satisfactory results in terms of recovery and reproducibility. Yuan and co-workers reported the use of a ternary hybrid nanomaterial composed of H2TPPS-functionalized graphene, SnO2, and Au nanoparticles deposited on a GCE to detect catecholamine epinephrine (EP) in the presence of UA.234 The developed electrode displayed improved electrocatalytic activity toward the oxidation of EP at pH 7.0, thanks to the synergic effects of favorable electrostatic interactions between the positively charged EP and the negative charges on porphyrinfunctionalized grapheme, and thanks to the increased specific surface of the composite material conferred by both tin oxide and gold nanoparticles. CV responses of the electrode in a mixture containing equimolar amounts of EP and UA at pH 7.0 evidenced the good separation of 132 mV of the oxidation peaks of the two analytes, which made their simultaneous determination feasible in standard solutions as well as in human urine. UA was also quantified in the presence of AA and nitrite ions using a GCE modified with a porphyrin-based composite material, prepared using Fe(TPP)Cl and a natural Cameroonian smectite clay.235 The electrochemical response toward AA and UA investigated with the SWV technique showed the resolution of the merged SWV peaks into two well-defined oxidation peaks. These were separated by 170 mV at PB pH 6.0 for the individual determination of AA and UA, decreasing to 155 mV when both analytes coexist. The slight variation on the separation peaks suggested the possibility of analyzing binary mixtures of these acids without cross interference. Since this system was able to efficiently electrooxidize the NO2− ion at +0.930 V, the analysis of a ternary solution containing AA, UA, and nitrite ions was also carried out via SWV, producing three separate peaks at potentials of ca. +0.272 V, +0.410 V, and +0.930 V, respectively. This allowed the simultaneous determination of these three analytes. The use of this electrode to detect AA in vitamin C syrup with good results proved the validity of the proposed method. 4.1.9. Voltammetric Sensors: Nitroaromatic Explosives. The electrochemical detection of explosive nitroaromatic compounds was performed using different nanomaterials such as varied carbon-based materials or mesoporous silica. Displaying a good affinity toward electron-deficient analytes, such as nitroaromatic compounds, porphyrins were also used for their detection, mostly in combination with other electroactive materials. Li and co-workers reported the detection of ultratrace explosives using a porphyrin/graphene sensor fabricated by modifying a GCE with an H2T4PyP-graphene conjugate.236 2,4Dinitrotoluene (DNT) was chosen as the model analyte to study the electrochemical reduction of explosives on the prepared sensor, using NaCl as the supporting electrolyte. The CVs showed two well-defined reduction peaks, corresponding to the conversion of one −NO2 to −NHOH group, followed by the partial or complete transformation of the latter to −NH2. The described reactions involved protons, so the pH effect was also investigated by the authors, indicating pH 6.0 as the optimal detection value. However, the experiments were performed at pH 7.0, which is the typical pH value of environmental samples, since the sensor’s response was not significantly different at these

DA even in the presence of 10000 times more AA and 2000 times more UA than DA. Similar results were recently reported by Xingquan He’s group, using a composite hybrid material consisting of poly[Fe(II)TPP] and poly(sodium-p-styrenesulfonate) (PSS)-modified graphene.230 In this case, the anionic feature of PSS contributed to adsorb the positively charged DA in acidic conditions, while polyporphyrins enhanced the sensor’s reversibility and reduced the overpotentials for DA oxidation. The fabricated sensor exhibited analytical performances in line with those previously reported, allowing the selective detection of DA when a large excess of UA and AA was used, both in standard DA solutions and in real hydrochloride injection. The binding properties of corroles with dihydroxybenzene derivatives by hydrogen bond interactions was exploited to develop functionalized gold electrodes for DA detection.231 In this case, a thiol-substituted triarylcorrole (Scheme 11) was Scheme 11. Molecular Structure of Thiol-Substituted Corrole

chemisorbed onto a gold electrode, together with hydroxyalkyl thiols, to form so-called “ion-channel” electrodes. The functionalized electrode is neutrally charged in the measuring conditions (pH = 7) and is permeable to a cationic redox marker {[Ru(NH3)6]3+}. At neutral pH, DA is positively charged (pKa = 8.9) and when added to the functionalized electrode, the binding to corrole leads to a positively charged SAM, which repels [Ru(NH3)6]3+, reducing the electron transfer between the redox marker and the electrode through the SAM. This signal was measured by OSWV and electrochemical impedance spectroscopy. The oxidation of the Ru complex does not interfere with DA or AA, and the corrole binding toward dopamine gives the selectivity of the sensor, via the mechanism described. The developed sensor was highly sensitive, with an LOD around 10−12 mol/L. Selectivity was tested toward analytes that can be found in real matrices, such as ascorbic acid, creatinine, creatine, and uric acid. The favorable results obtained were also tested by measuring DA levels in human plasma, demonstrating the potential application of the developed corrole-functionalized electrode in clinical analysis. Huang and co-workers also reported the use of a hybrid porphyrin-graphene nanocomposite material for DA determination.232 They deposited CuTPP on chemically reduced graphene oxide to generate a delocalized electron-rich environment on a graphene surface suitable to adsorb the positively charged DA. The fabricated sensor was highly sensitive to and selective for DA oxidation in the presence of 100-fold higher concentrations of AA and UA. Its practical applicability in biological samples like human urine and saliva was also demonstrated. L-DOPA (LD) is the DA precursor, used in association with carbidopa (CD) for the medical treatment of neural disorders. 2543


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Figure 17. Stepwise fabrication of fullerenols/PANI-mixed-thiol/GCE electrode reported by Zhou and co-workers: (a) bare GCE, (b) PANI/GCE, (c) PANI-mixed-thiol/GCE, and (d) fullerenols/PANI-mixed-thiol/GCE. Reproduced with permission from ref 238. Copyright 2013 Elsevier.

Figure 18. Schematic representation of the preparation of mercury IIP nanoparticles. Reproduced with permission from ref 242. Copyright 2013 Elsevier.

located on H2THPP to form the hydrogen bond with the explosive nitro groups together with an enhanced π-donor− acceptor interaction. In a following publication,238 the detection of DBN was carried out with a DPV technique using a polymeric nanocomposite material composed of polyaniline (PANI), thiolporphyrin based monolayer, the alkyl thiol C10H21SH, and fullerenols C60(OH)x. Their assembling on a glassy carbon electrode is illustrated in Figure 17. Intercalating alkyl thiol between two porphyrins was crucial to fill up the potential space on the PANI surface between the tetrapyrroles but also to provide space for the subsequent fullerenols’ insertion. The electrochemical response of C60(OH)x/PANI-mixed thiol/GCE system was excellent, thanks to the ability of the hydroxyl groups of fullerenols to concentrate the explosive at the surface, giving acid-based pairing with the nitro groups of DNB, followed by the formation of strong EDA complexes on the hybrid film. The excellent detection limit of 9.72 pmol L−1 and the linear range were both better than those from other methods previously reported. 4.1.10. Voltammetric Sensors: Pollutants. The electrochemical detection of some organophosphorous compounds largely used in agriculture with porphyrin-based materials was recently reported by the Chen and Fa groups independently. The first fabricated a GCE modified with graphene functionalized with MnTMPy to detect the insecticide dimethoate at pH 3.0 in a

two values. The linear sweep voltammetry (LSV) technique was chosen as the detection technique for 2,4-DNT using solutions from 0 to 500 ppb. The sensor gave an excellent linear range up to 500 ppb and high sensitivity, being able to detect 1 ppb of this analyte with ease. The ultratrace determination of other explosives, namely 2,4,6- trinitrotoluene (TNT), 1,3,5- trinitrobenzene (TNB), and 1,3- dinitrobenzene (DNB), was feasible with this system, with reduction peaks occurring at −0.64, −0.58, and −0.60 V, respectively. Similarly, the detection of DNB was reported by Xiaoquan Liu and co-workers using two different porphyrin-functionalized sensitive materials. In a first work,237 the use of H2THPP or 5-(4hydroxyphenyl)-10,15,20-triphenylporphyrin (H 2MHTPP) electropolymerized onto CNTs was exploited to efficiently detect the explosive, thanks to the combination of the preconcentration ability of CNTs to accumulate explosives and the multiple interactions between porphyrin macrocycle and the analyte. In detail, the electronic features of the hydroxyporphyrin and DNB formed instantaneous acid-based interactions, and the benzene ring became planar to the porphyrin macrocycle, giving a strong π donor−acceptor complex (EDA) accountable for explosive detection. The electrochemical response of H2THPPCNTs film was dramatically higher than that of the analogous H2MHPP-based material. This result was ascribed to the increased possibilities offered by the additional −OH groups 2544


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B-R buffer solution by DPV,239 and the second developed a sensor to determine herbicide methyl parathion (MP) using a CoT4CPP-Co3O4-GO composite material.240 This sensor had good analytical performances, a wide linear range (4.0 × 10−7 to 2.0 × 10−5 M), LOD of 1.1× 10−8 M, acceptable stability and repeatability, and good selectivity when some inorganic ions as well as other pesticides (omethoate and dipterex) were tested as interferences. Furthermore, quality recoveries were obtained using this system to quantify MP in both river and tap water. In recent years, the proven negative health effect of phenolic estrogenic compounds on human and animal endocrine systems required extensive monitoring of the concentration level of these compounds in an aquatic environment. The major drawback of the direct oxidation of phenolic groups on an electrode surface is the severe surface passivation by phenoxonium or quinine compounds formed during the oxidation process, thus the surface modification with different materials to minimize electrode fouling was pursued. An example in this field was recently given by de Oliveira’s group, that proposed an electrochemical method for the detection of 17β-estradiol using a GCE covered with a composite material of Cu(II) 5,10,15,20-tetrakis(thien-2-yl)porphyrin (CuTthPP) supported on reduced graphene oxide.241 On the CuTthPP/rGO/GC electrode, the analyte oxidation to the ketone derivative occurred at +0.54 V, being well-separated by the oxidation peaks of other pesticides carbendazim (+0.93 V) and carbaryl (+ 1.1 V) that have been valued in the interference studies. The method gave an excellent LOD value of 1.9 μg L−1, which is even lower than the values required to detect this hormone in natural water. When applied to the analysis of river water samples, the results obtained were similar to those obtained by the HPLC procedure, offering the possibility to use the developed sensor in situ for real environmental monitoring. Highly hazardous mercury ions were selectively determined by DPV using GCE modified with ion imprinted polymeric nanobeads (IIP) and MWCNTs.242 The Hg2+ ion selective imprinted polymeric nanoparticles were prepared with the thermal precipitation polymerization technique using H2T3HPP in the presence of HgCl2 and methacrylic acid, ethylene glycoldimethacrylate, and 2,2-azobis(isobutyronitrile) compounds as functional, cross-linker monomer and initiator species, respectively (Figure 18). The leaching of the coordinated ions from the porphyrin units of the polymeric material was accomplished by using concentrated HCl, leaving empty cavities of the right shape to selectively adsorb the target Hg2+ ions, excluding substances larger than the imprinted template and minimizing the effect of other metals that typically affect the voltammetric determination of such an analyte, as demonstrated in the selectivity studies reported. The optimization of some experimental parameters such as washing and preconcentration times and pH value, enabled the analytical testing of the GC−IIP−MWCNTs modified electrode via the differential pulse anodic stripping voltammetric technique toward mercury ion solutions at different concentrations. These resulted in a linear response in the range from 1 × 10−8 to 7.0 × 10−4 M and a LOD of 5.0 nM. The application of this sensor in ground and wastewater samples pointed to the possibility of achieving a precise and accurate detection of mercury in real samples, giving satisfactory percentage recoveries and acceptable standard errors. The detection of aflatoxins in both pet and human foods is essential, for their known poisoning and cancer-causing effects. Only very recently, Tang and co-workers reported a highly efficient electrochemical immunoassay to detect the hazardous

aflatoxin B1 (AFB1) using a peroxidase mimetic system based on hybrid nanostructures of PtNPs/CoTPP/rGO.243 This composite system was used as the recognition element to label monoclonal rabbit anti-AFB1 antibody and exploited to develop a competitive-type electrochemical immunoassay at the sensing interface. The electrochemical signal derives from the catalytic reduction of hydrogen peroxide by the organic−inorganic hybrid structures. The results reported satisfactory electrochemical responses for the target compound, detected at a concentration as low as 5.0 ppt. The advantages of the described methodology lie in the simple instruments required and the possible extension to detecting other small biotoxins by varying the corresponding antigen or antibody. 4.1.11. Voltammetric Sensors: Pharmaceutical and Biological Analytes. Dinelli and co-workers reported the electrochemical determination of several analytes of pharmaceutical preparations by using the class of supramolecular tetraruthenated porphyrin derivatives {M-T4PyP[RuCl3(dppb)4} (where M is a transition metal) electropolymerized on GCE by cyclic voltammetry. The electropolymerization of these compounds occurs with the formation of binuclear mixed-valence complexes (RuII/RuIII) between the peripheral groups “RuCl3(dppb)” by chloride bridges that link the porphyrin macrocycles to form the polymeric film {MTPyP[Ru(dppb)4 (Cl3)2}2n8n+. The metal ion coordinated into porphyrin rings influences the electrocatalytic behavior of the formed material, thus its sensing ability toward a specific class of analytes. In particular, this group showed that the electrodeposition of the corresponding manganese derivative gave a stable film suitable for the identification and detection of acetaminophen in commercial drugs, with satisfactory results that are comparable to those obtained with the HPLC method.244 Conversely, the modification of a GCE surface with the polymeric film formed by the analogous oxovanadium porphyrin complex was exploited as a voltammetric sensor for the single and simultaneous determination of catechol (CC) and hydroquinone (HQ) isomers, with peak-to-peak potential separation of 129 mV at pH 4.57 and with detection limits of 0.41 and 0.55 μmol L−1 for CC and HQ, respectively.245 A promethazine (PMZ) sensor based on nanostructured films of amphiphilic cardanol-porphyrin derivatives was reported by Sandrino and co-workers (Scheme 12).246 The amphiphilic character of porphyrin monomer made this molecule suitable to Scheme 12. Structure of the Porphyrin-Cardanol Derivative Reported by Sandrino and Co-workers

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fabricate Langmuir and LB films on a quartz substrate. In detail, 5 layers of LB films deposited on glass coated with Au at pH 6.0 were used to determine PMZ concentrations based on the oxidation peak at 0.68 V corresponding to the formation of sulfoxide promethazine. A linear range from 2.00 to 36.25 μM was obtained, in line with other methods that sometimes reported even lower LOD values. However, one must highlight that in this case, good results were obtained without the use of a conductor material and with a simple and environmentally friendly methodology that used a natural phenol to modify the electrochemical electrode surface. The use of FeT4CPP-MOFs materials conjugated with streptavidin used as a recognition element was recently reported by Ju’s group for electrochemical DNA sensing. In these works, the integration of the mimetic electrocatalytic activity of iron porphyrinic MOFs with the molecular switch of hairpin DNA247,248 generated biosensors with a significantly improved sensitivity toward the target DNA, with detection limits in the femtomolar range. Similarly, the same group reported the use of a biomimetic catalyst for electrochemical biosensing obtained by the noncovalent assembly of FeT4CPP on porous carbon produced by direct carbonization of a zeolite-type MOF.249 The obtained nanocomposite showed excellent electrocatalytic activity toward O2 reduction, thus allowing the fabrication of a glucose biosensor with a linear range from 0.5 to 18 mM and a LOD of 0.08 nM at a signal-to-noise ratio of 3. The practical use of this biosensor was validated in real-life serum samples with no treatment, producing accurate glucose determination in real samples. An electrochemical genosensor using cobalt porphyrin conjugated with DNA was reported by Radecki and coworkers.250 The active probe synthesized was stably deposited on an Au electrode and embedded very close to the electrode surface in a mercaptohexanol SAM. The Osteryoung square wave voltammetry (OSWV) technique was applied to determine the concentration of the target ssDNA sequence, evidencing a linear trend between the redox Faradaic current and the logarithm of the 20-mer ssDNA concentration, ranging from 10 to 80 fM. It was further reported that the signal changes strictly depended on the lipophilicity of the supporting electrolyte used. Voltammetric sensors to detect protein of clinical interest have also been reported. Dong and co-workers developed an electrochemical label-free aptasensor to detect thrombin with improved sensitivity thanks to synergistic effects deriving from the combined use of graphene, 5,10,15,20-tetrakis(4-methoxyl-3sulfonatophenyl)porphyrin and aptamer, the latter conferring the high affinity and specificity for the target analyte.251 The decrease in the DPV signals was observed after the target analyte binding, as a consequence of the hindered electron transfer process and was found to be linearly related with thrombin concentrations. The good analytical performance of the developed aptasensor, with a wide linear range of 5−1500 nM and a LOD of 0.2 nM, and negligible responses to other proteins tested (BSA and HSA), together with its easy preparation and low cost, makes it a promising device for bioanalytical applications. Furthermore, worthy of mention is the label-free detection of cyclin A2 with a porphyrin functionalized graphene-modified electrode reported by Qu and co-workers.252 This analyte is a prognostic indicator in early stage cancers, thus its ultra sensitive detection in cancer cells is of great importance for clinical diagnosis. In such a context, the authors realized label-free electrochemical impedance detection of this protein in cancer cell extracts by using a graphene noncovalently functionalized

with H2T4CPP porphyrin deposited on GCE, coupled with a specific hexapeptide as a detection probe and Tween 20 to prevent the nonspecific binding events of target biomolecules. Moreover, the graphene surface generated in this work presented no obvious cytotoxicity under the experimental conditions used, opening the possibility of using this system in further biomedical applications. 4.1.12. Quasi-Stochastic Sensors. Stochastic sensors are a particular kind of resistive pulse devices, which measure the ionic current flowing through a single pore embedded in a nonpermeable membrane.253 When the size of the pore is in the nanometer range, the sensors can detect single molecules passing through the pore, since they can perturb the pore’s conductivity. The modulation of the ionic current depends on the kind of molecules, and the frequency of the events can be related to the concentration of molecules. Porphyrin’s ability to form molecular aggregates in varied water mixtures can be exploited for the fabrication of sensors inspired by this principle. These sensors cannot be strictly defined stochastic because of the large number of pores; however, they preserve the property of molecular identification and quantification. In order to preserve the inspiration of these devices, hereinafter, they are defined quasi-stochastic. To this regard, the group of van Staden reported the development of disposable dot sensors based on different meso-phenyl-functionalized porphyrins and modified carbon or diamond pastes for the electrochemical assay of ascorbic acid in pharmaceutical, beverage, and biological samples.254 The tOFF value obtained varied with the porphyrin, but it was independent of the carbon matrix used in the sensor. The pH value and possible interferences such as dopamine or vitamin B did not affect the sensor’s response. The same group reported the use of similar amperometric dot microsensors based on different porphyrinoids to determine analytes of clinical interest by employing differential pulse voltammetry.255 Three types of Zn porphyrins (ZnTPP, ZnTPPS, and Zn tetranaphtaloporphyrin) were used to modify carbon or diamond pastes, and the resulting sensors were exploited to detect the sildenafil citrate in its pharmaceutical formulation known as Viagra as well as in biological samples. The lowest detection limit of 2.46 pmol L−1 was obtained using the diamond paste dot-sensor with the water-soluble ZnTPPS, whose deposition on the diamond surface was more efficient and endowed with higher numbers of active centers compared to that of the deposition with the other two Zn complexes. More recently, the same authors reported the application of similar microsensors using graphite or graphene modified with different porphyrin and phthalocyanine metal complexes for the analysis of folic acid in pharmaceutical tablets and urine samples.256 Under optimized conditions, corresponding to pH = 7.0 and 0.1 M with KCl as electrolyte, the greatest sensitivity was obtained when the tetraamino cobalt(II) phthalocyanine was used to modify the graphite surface, thanks to the dual contribution of Co metal center and the extended π system of the aminofunctionalized macrocycle. Very recently, the same group reported the performance of porphyrin-based nanostructured materials for the pattern recognition of clinical target analytes, such as biomarkers (NSE and CEA)257 and growth factor receptors (HER-1).258 The detection of biomarkers was accomplished with the multipores sensor structure fabricated using nanoporous Au microspheres, diamond, graphite, or graphene pastes and H2TPP and α-cyclodextrins. The sensor based on the H2TPP/graphene system gave the best performance to simultaneously detect NSE and CEA and represents a 2546


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valid screening test for the early detection of lung cancer. Conversely, the multipore detection of HER-1 performed better with the diamond paste modified with α-cyclodextrin instead of porphyrin, recording a LOD of 10−12 mg mL−1. The same group also studied the application of multipore sensors for chiral discrimination, a challenging task in the chemical field. In a first work, carbon and/or diamond paste microsensors were functionalized with the water-soluble H2TPPS or its Zn complex or polymeric surfactants.259 The resulting electrodes were used both as potentiometric electrodes or in the quasi-stochastic mode for the discrimination of pipecolic acid, which is an important biomarker for different pathologies. While the quasi-stochastic sensor was used to identify the enantiomer, the potentiometric sensor quantified the target analyte in the biological fluid tested. The microsensor based on ZnTPPS/diamond paste electrodes simultaneously determined both enantiomers of pipecolic acid. The sensor’s selectivity was tested against creatine, creatinine, and dopamine, as model analytes potentially present in real samples; the results revealed a good selectivity both in the quasistochastic and in the potentiometric mode. The sensors were successfully applied for the enantioanalysis of PA in whole blood and urine samples, first using the sensors in the quasi-stochastic mode to identify the enantiomer and then in the potentiometric mode to quantify it. A similar approach was recently used for the chiral analysis of fucose, a sugar important as a malignant tumor marker when the L-form is present in high levels in urine.260

Scheme 13. Molecular Structure of PtP used by Li and Coworkers

The sensor’s features were excellent in terms of stability, response time, and selectivity with other interfering molecules (sugars, amino acids, and vitamins) present. The practical application of such a device for the direct analysis of quercetin in beverages, for example tea and apple juice, was also tested and afforded fine results, comparable with those of the standard HPLC method. The use of porphyrin-sensitized titanium dioxide in PEC sensors was also reported by Tang and co-workers for the development of a photoelectrochemical immunoassay protocol to quantify biomarkers at low potential.262 The water-soluble H2TPPS was bound onto TiO2 nanoparticles giving a stable photoactive material that was then deposited onto an ITO slide. The use of a proper proportion of H2TPPS with respect to TiO2 nanoparticles (1:10) was needed to obtain the maximum photocurrent intensity and the stability of the sensing material at the same time. The deposited H2TPPS-TiO2 structure served as the affinity support to fabricate the PEC immunoassay, developed using carcinoembryonic antigen (CEA) as the model analyte. The schematic illustration of PEC immunoassay working principles is reported in Figure 19. The enzymatic catalysis performed by glucose oxidase allowed the in situ generation of H2O2 from glucose, which constituted the sacrificial electron donor that amplified the photocurrent eight times more than in the absence of H2O2. Under optimized conditions (e.g., pH, incubation time, applied potential, and enzymatic catalytic time), this immunosensor demonstrated at 0 V potential a satisfactory linear relationship ranging from 0.02 to 40 ng mL−1, with a LOD of 6 pg mL−1 for CEA. Both the reproducibility and the precision of the developed sensor were reasonable, while the selectivity and long-term stability were excellent. The detection of the target CEA in biological fluids was also tackled, giving results in good agreement with those from the commercially available CEA ELISA kit. PEC immunodetection of CEA was also reported by Yu and co-workers, who used a supramolecular approach to assemble the photoactive material onto an ITO electrode.263 The linear bicontinuous donor−acceptor (D-A) arrays were composed of porphyrin and fullerene complexes held together by means of π−π interactions. In particular, the 5,15-bis(4-carboxyphenyl),10,20-bis(mesityl)porphyrin was first self-assembled on the ITO surface by ester linkages between the peripheral −COOH groups of the macrocycle and the hydroxyl groups of the

4.2. Photoelectrochemical Sensors

Photoelectrochemical (PEC) detection is a rather new technology for sensing applications where light is used as a source of excitation for active species that efficiently transfer the excited electrons to the electrode, producing current as a detection signal. This method exhibits higher sensitivity and specificity than both optical and electrochemical methods thanks to the separation of the irradiation source and electrochemical detection. Furthermore, the equipment is simple, nonexpensive, and easy to miniaturize. Different semiconducting materials such as TiO2, ZnO, and quantum dots were used in PEC sensing. Some drawbacks limited their use in this technique. The exploitation of porphyrins as photosensitizers to enhance photocurrent intensity is related to their photoelectronic properties, like their wide photoresponse range in visible and near-infrared regions and their quick time of the charge of recombination between the HOMO orbital and the hole of oxidized porphyrin. Moreover, the ample possibilities of assembling these macrocycles onto different substrates favor the formation of very stable composite materials, reflecting the sensor’s performances. Li and co-workers developed a photoelectrochemical sensor based on porphyrins to detect quercetin, a bioactive polyphenol compound used to prevent different human diseases, like cancer and diabetes.261 The developed device consisted of a composite film containing titanium dioxide and a platinum porphyrin complex PtP (Scheme 13) deposited on a glassy carbon electrode by drop casting. Different parameters of the prepared electrode TiO2/PtP/GCE were studied and optimized, such as the optimal mass of TiO2, the illumination time and the applied potential. Under optimal conditions, the increase of the oxidation peak of quercetin at +0.38 V was in line with the analyte concentration, allowing its quantification with a detection limit of 0.8 μg L−1. 2547


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Figure 19. Schematic illustration of PEC immunoassay toward target CEA. Reproduced from ref 262. Copyright 2015 American Chemical Society.

Figure 20. Stepwise fabrication of linear bicontinuous D−A arrays on an ITO surface reported by Yu and co-workers. Reproduced with permission from ref 263. Copyright 2015 Elsevier.

dispersion of the carbon nanomaterial to increase its contact with the porphyrin and of immobilizing the tetrapyrrolic macrocycle by hydrogen bond interactions between the −NH2 groups of the biopolymer and the −COOH functionalities of the porphyrin complex. To gain an optimal photoelectrochemical sensor response, the right proportion of the three components of the hybrid film was determined and the sensor built this way was used to measure HNBAA, as illustrated in Figure 21. The PEC sensor determines the target analyte in concentrations ranging

inorganic substrate. Then, the monolayer formed served as a template for the further self-assembling of porphyrin monomers through hydrogen bonding. The favorable π−π interactions between the porphyrin macrocycle and the C60 unit enabled the infiltration of the carbon material amid the porphyrin stripes, producing ordered porphyrin-C60 arrays (Figure 20). The following covalent immobilization of the antibody Ab into the supramolecular D−A arrays generated the competitive PEC immunosensor used for the analysis of solutions containing CEA at different concentrations. The quantification of the target analyte was based on the decrease in the photocurrent intensity of the arrays, due to the competitive interaction of anti-CEA between CEA and CEA-CdTe, which were different in size. The developed PEC immunosensor afforded a LOD value of 3.4 pg mL−1 for CEA, which was much lower than those of other previously reported immunosensors. The exploitation of a ternary hybrid film made of hematin adsorbed on carbon nanohorns, and a poly-L-lysine complex support was reported by Lin and co-workers for the PEC detection of 4-hydroxy-3-nitrobenzene arsenic acid (HNBAA), a harmful substance for humans and environments frequently used as an additive for feed.264 The hematin used as a photosensitizer was immobilized on carbon nanohorn superstructures which possess attractive properties for photoelectrochemical sensing, namely a large specific surface area, porosity, and especially excellent conductivity which could efficiently enhance the enrichment of a photoelectrochemical material and prolong electron lifetime by decreasing recombination between excited electron and hole. The use of poly(L-lysine) to form the organic− inorganic hybrid film has the dual role of improving the

Figure 21. PEC sensing mechanism of HNBAA on a carbon nanohornshematin hybrid film developed by Lin and co-workers. Reproduced with permission from ref 264. Copyright 2015 Elsevier. 2548


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from 0.5 nM to 5 μM, with a LOD of at least 0.5 nM, lower than other methods used for the same analyte. The interference of other biomolecules including saccharides, amino acids, and proteins were also evaluated, yielding important sensor performances in terms of selectivity and anti-interfering capability. Another example of a ternary hybrid film used in PEC detection was reported by Ai and co-workers, where protoporphyrin IX, WO3, and rGO were efficiently combined and deposited on an ITO electrode.265 As illustrated in Figure 22,

was used as the working electrode in a three electrodes amperometric setup, and the change of the current due to the L-cysteine oxidation was measured in the dark and under visible light illumination, scanning the potential from 0 to 1.1 V. Illumination results in a depletion of the electron density in the porphyrin layer that may favor interaction with the target analyte. The sensor’s selectivity toward L-cysteine in the presence of other amino acids or organic acids was also tested and found to increase by UV irradiation, except for the L-arginine, which is, in any case, a strong interferent, possessing multiple amine groups, which can coordinate the porphyrin macrocycle, thus enhancing the donating character of the molecule. 4.3. Optical Sensors

4.3.1. Ammonia and Amines. Sonkusale and co-workers268 reported aqueous phase ammonia sensing with a composite material and a microfluidic optoelectronic platform with spectroscopic detection. ZnTPP dye was impregnated in ionexchange polymer microbeads arranged in a microfluidic array, allowing direct contact with the aqueous environment, while preventing the undesirable washout of the dye. A white lightemitting diode was used as a source of irradiation to study the optical transmission in a visible range of the spectrum (instead of measurement at a single wavelength), with an optical fiber as a light guide and a portable miniaturized spectrometer as a detector (Figure 23). The sensor proved to be sensitive to parts per million level concentrations of dissolved NH3 and had enhanced sensitivity to ammonia compared to other strong and weak tested bases, with a detection limit for dissolved ammonia of 420 ppb. This allowed the research group to evaluate the suitability of the sensor for medical diagnostic applications such as the subppm level detection of NH3 in human saliva for the Helicobacter pylori infection. A melamine-bridged bis-Zn(II) porphyrin dyad was reported by Carofiglio269 for biogenic diamine sensing in food safety and quality control. The described porphyrin tweezer (Scheme 14) featured a ditopic binding interaction with α,ω-aliphatic diamines, such as cadaverine, with three times greater stability constants compared to a monodentate reference amine. The immobilization of the sensing material onto aminated solid supports TentaGel resin and controlled pore glass allowed the

Figure 22. Schematic fabrication steps of photoelectrode reported by Ai and co-workers. Reproduced with permission from ref 265. Copyright 2013 Elsevier.

rGO was first electrodeposited onto an ITO slice and then the WO3 hydrates were electrostatically bound onto this substrate by consecutive drop casting and annealing. The anchoring of the photosensitizer on the annealed nanohybrid structures took place through the peripheral carboxylic groups on the porphyrin macrocycle in the final stage. The sensor’s characterization evidenced the synergistic effect of the three components on the photocurrent response, which was enhanced using 380 nm as the excitation wavelength. The developed sensor was used to detect cysteine in aqueous solution, with a linear range of 0.1 to 100 mM in 0.1 M PBS (pH 7.0) and a LOD of 25 nM (3σ). The PEC detection of cysteine displayed satisfactory analytical performance, such as short response, low applied potential, wide linear range, low detection limit, and good stability and selectivity toward cysteine from other amino acids. The determination of this analyte was also reported using a hybrid material composed of ZnO nanorods coated with Cu or Mn complexes of H2TPPS, prepared by one-pot hydrothermal synthesis directly on the ITO slide.266 The use of porphyrins with metals possessing different coordination properties led to a different morphology of the final material, also affecting the sensing behavior.267 The ensemble ITO-ZnO-metalloporphyrin

Figure 23. (a) Schematic representation of the optical fiber-based device, (b) array of ZnTPP-doped microbeads embedded in the microfluidic device, and (c) a micrograph of ZnTPP-doped microbeads trapped inside microwells. Reproduced with permission from ref 268. Copyright 2014 Elsevier. 2549


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flowing ultrapure water on the film or by heating the active layer to 50 °C (Figure 24). 4.3.2. Explosives. A diethylpyrrole-bridged bisporphyrin (H4DEP) (Figure 25) was employed to encapsulate highly explosive picric acid (PA) in its cleft.271 Such an efficient complexation, which resulted in a substantial quenching of the emission intensity of the bisporphyrin, was due to the perfect match of the host−guest size, hydrogen bonding, and strong π−π interactions between the host and guest, which collectively made the binding of PA rapid and highly selective with a detection limit of 2.4 ppm. The host H4DEP was also able to discriminate the highly explosive PA from other nitroaromatic compounds. Fluorescence titrations of H4DEP were performed with different nitroaromatics, namely, nitromethane, PA, DNP, 4nitrophenol, TNB, TNT, 4-nitrotoluene, DNB, and nitrobenzene (NB). Among these nitro explosives, only PA showed a substantial quenching of the H4DEP emission. The initial fluorescence intensity of H4DEP was attenuated by 95% by the addition of only 1.6 equiv of PA. This allowed detecting PA by a contact mode analysis: Whatman filter paper strips, impregnated with a dichloromethane solution of the designed porphyrin, entered into contact with a drop of PA solution or directly with solid PA. In both cases, the paper turned black under UV light (Figure 25).271 Liu and co-workers272 reported an electrospun nanofibrous film sensor based on H2TMPP and polystyrene for nitroaromatic explosives using fluorescence spectroscopy and fluorescence microscopy to characterize the performances of the nanofiber. The emission spectrum of the film, between 620 and 700 nm, was recorded using an excitation wavelength of 424 nm with several explosive solutions at different concentrations. The quenching efficiencies of fluorescence nanofibrous films increased with an increased concentration of all of the four explosives used. TNT and PA revealed quite similar quenching curves, similar to DNT and DNP, due to electron-deficiency factors. TNT and PA both have three nitro-groups, more than DNT and DNP (two), thus being more electron-deficient led to higher quenching sensitivities (Figure 26). Adding dodecylamine to the precursor electrospun mixture, the authors further improved the sensitivity since the quenching of the sensing layer toward DNT and TNT was enhanced by 1.4-fold and 2.4-fold, respectively, probably due to the proton transfer from the methyl group of explosive to the amino groups of the dodecylamine, strengthening the attachment of the explosive to the nanofibrous film.

Scheme 14. Zn(II) Porphyrin Tweezers As Optical Sensors for Diamines

construction of a recyclable optical sensor and its eventual miniaturization. Valli and co-workers270 focused on another porphyrin dyad. In particular, they reported the conformational switching of a bis(ZnOEP) in a Langmuir−Schaefer film on a quartz slide. As a consequence of appropriate host−guest interactions, the system proved to be an effective tool for selectively sensing aromatic amines. They studied both LS and spin-coating methods, discovering that the deposition technique strongly influences the molecular organization and consequently the sensing properties of the film: a bis(ZnOEP) LS film showed an absorption maximum center at 397 nm, indicating a syn conformation, while as a result of the spin-coating deposition the bis(ZnOEP) preferentially assumed a structural arrangement in anti conformation, probably due to the presence of ethyl alcohol in the solvent used. Aliphatic and aromatic amine aqueous solutions were fluxed onto the sensing film and the spectral changes monitored by UV−vis spectroscopy; no noticeable changes were induced by adding aliphatic amines, while a rapid transformation from syn- to anticonformer was observed when aniline entered into contact with the LS film, probably due to the π−π stacking interaction between the injected aromatic amine and the bis(ZnOEP) molecules reducing the intramolecular stacking interaction of the two porphyrin moieties. The lowest concentration of aromatic amine able to induce the spectral changes was 10−7 M (20 ppb), and the original absorption spectrum was completely recovered by

Figure 24. Conformational switching in bis(zinc porphyrin) LS film. Reprinted with permission from ref 270. Copyright 2012 Elsevier. 2550


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Figure 25. H4DEP interaction with PA (on the right) and paper strips for contact mode detection: (A) blank, (B) coated with H4DEP, (C) with PA solution and (D) with solid PA. Reprinted with permission from ref 271. Copyright 2015 Wiley-VCH.

Figure 26. Quenching efficiencies of nitroaromatic explosives and common interferents toward the fluorescence emission of the electrospun nanofibrous films with secondary porous structures. Reprinted with permission from ref 272. Copyright 2011 Royal Society of Chemistry.

Figure 27. Fluorescent porous thin film fabrication on the surface of U-bent PPMA optical fiber (U-bent POF) for vapor phase sensing of explosives. Reprinted from ref 274. Copyright 2015 American Chemical Society.

Another porphyrinated polymer based on Zn(II) 5,10-bis(4aminophenyl)-15,20-diphenylporphyrin273 was electrospun from a dimethylacetamide solution to form a highly permeable filamentous structure where the fluorescence emission was quenched by exposure to hundreds of parts per billion of trinitrotoluene and dinitrotoluene.

More recently, a sensor for trace amounts of TNT (10 ppb) and DNT (180 ppb) was developed by Zhang and co-workers:274 a fluorescent porous thin film was fabricated on the surface of Ubent PMMA optical fiber (U-bent POF) via “click” polymerization. The precursors, vinylfunctionalized polyhedral oligomeric silsesquioxanes, allyl-functionalized porphyrin, and alkane 2551


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Scheme 15. Benzoporphyrins and Porphyrin-2-ylpyridines Studied as Sensing Material for Soft Metal Ions

the H2T4CPP as the recognition site into the robust PCN-224 network. These allowed chemical and thermal stability due to the strong interaction between the Zr−O clusters of the particles and the carboxylate groups in the porphyrin molecules.276 The measurements, performed in solution, confirmed that the specificity of the sensor for TNT is not affected by possible interfering species such as anions, cations, aromatic compounds, and structurally similar nitroaromatic explosives. An alternative route consists in H2T4CPP covalently grafted onto the surface of Hexagonal SBA-15, Spherical SBA-15, and commercial SiO2.277 The substrates were opportunely functionalized to get large surface areas, high porosity, fast molecule/ion transport kinetics, and thermal and mechanical stability. The fluorescence quenching of the porphyrin-doped silica films showed rapid response upon exposure to TNT. 4.3.3. Metal Ions. A fluorimetric/colorimetric mercury sensor based on porphyrin-immobilized Au@SiO2 core/shell nanoparticles was reported.278 The porphyrin derivative was attached to the nanoparticles by covalent bonds displaying, in absence of metal ions, a red color and strong fluorescent properties. The addition of Hg(II) ions resulted in a fast color change, from red to green, within 10 s, and weak fluorescence (detection limit lower than 2 ppb). No significant changes in fluorescence

dithiols were reacted upon irradiation of evanescent UV light transmitting within POF under ambient condition. This represents a simple method to implement sensitive porous films on a surface of POF (Figure 27). The fiber probe thus obtained has two main characteristics: the fluorescent active molecule is covalently bonded into the network and the pore-size distribution of thin film can be easily adjusted by using alkane dithiols with different chain lengths. Fluorescent signals of PTCF/U-bent POF probes exhibited high fluorescence quenching toward trace TNT (10 ppb, 37.5% for 30 s), DNT (180 ppb, 36.1% for 30s), and NB (3 Å ∼ 105 ppb, 8.8% for 30 s). The detection of nitroaromatics was also obtained by a fluorescent probe based on phosphonate-functionalized porphyrin derivatives.275 It demonstrated that molecular recognition was based on cooperative hydrogen bonding and π−π stacking interactions with electron-deficient molecules, leading to superior detection toward TNT (with a 10 ppb detection limit): porphyrin-doped hybrid PMMA polymer films demonstrated the reversibility of the fluorescence behavior and high photostability. Two studies investigated the properties of H2T4CPP as sensing material for TNT detection. The first of them was concerned with the development of porphyrin-based luminescent metal−organic frameworks (LMOFs) by incorporation of 2552


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emission or color were observed in the parallel experiment with Li(I), Na(I), Ca(II), Cu(II), Cd(II), Mn(II), Co(II), Ag(I), and Pb(II) ions. Treatment with an ethylenediaminetetraacetic acid (EDTA) solution restored the color and fluorescence of the functionalized nanoparticles. The fluorescence intensity and the color change displayed by the sensor were constant between pH 4 and 10. Another fluorescent probe for mercury(II) ions was obtained with the electrostatic multilayer self-assembly technique, alternating absorption of H2TPPS with polyelectrolytes.279 The use of PSS in the poly(diallyldimethylammonium chloride) (PDDA)/PSS/PDDA/H2TPPS films reduced the formation of weakly fluorescent aggregates between adjacent porphyrins, resulting in higher fluorescent films that were efficiently quenched upon interaction with mercury in aqueous solutions. A colorimetric Hg2+ sensor based on a porphyrin-functionalized polyacrylonitrile fiber (CTAPP−PANAF) was prepared and investigated by Zhang and co-workers.280 The sensor was able to selectively detect Hg(II) ions, even in the presence of competing ions, with a detection limit of 10−7 mol L−1 (20 ppb) based on naked-eye observation, in a wide range of pH values. The metal ions were desorbed by dipping the complexed CTAPP−PANAF in HCl solutions, thus making the sensor reusable. High specificity and good reversibility were also obtained by Zargari’s group281 with an optode recently prepared by nanocomposites, based on H2THPP and GO nanosheets. Changes in the UV−vis absorbance of the sensor allowed a detection limit of 3.2 × 10−9 mol L−1 in buffer solutions at pH 7.5. Another optode was developed based on the fluorescence quenching of 5,10,15,20-tetrakis(2-hydroxynaphthyl)porphyrin in a PVC/DOS membrane:282 the system showed high selectivity to Hg2+ even in the presence of some alkali, alkaline earth, and heavy metal ions. An amphiphilic acrylamide-based polymer with porphyrin pendants was obtained by Wu and Wang:283 the copolymerization of 5-(4-acryloyloxyphenyl)-10,15,20-triphenylporphyrin and N,N-dimethylacrylamide resulted in a highly selective sensor for Hg(II) ion. Upon addition of Hg2+, a red shift of the Soret band and a new absorption peak at 456 nm were observed together with a dramatic color change of the copolymer solution from brown-red to green. Moreover, Hg2+ presented the most noticeable fluorescence quenching in aqueous solution among several metal ions studied, thus indicating an excellent selectivity and allowing the quantitative analysis of Hg (II) with a low detection limit of 50 nM. Lodeiro and co-workers284 investigated, by absorption and fluorescence spectroscopy, the sensing ability of two series of porphyrins, benzoporphyrins, and porphyrin-2-ylpyridines (Scheme 15), toward metal ions in solution, gas phase, and solid-supported polymers (PMMA). Changes in the spectra were observed only in the presence of Zn2+, Hg2+, Cu2+, and Cd2+; in particular, porphyrin-2ylpyridines manifested higher stability constants than the other series, probably due to a better host−guest interaction with the substituent in the 2-position, while the benzoporphyrin derivatives presented an unexpected increase in emission intensity in the presence of Hg2+ resulting in the best candidate to detect this metal (32 ppb) also in PMMA. The same group reported the use of a 3,5-bis(5,10,15,20tetraphenylporphyrin-2-ylmethyl)pyridine as the sensing material to distinguish Hg2+ and Zn2+ by absorption and fluorescence spectroscopy.285 After characterizing the dye in liquid and gas phases, a film of bisporphyrinylpyridine in PMMA was sprayed with aqueous solutions of the target ions at room temperature: the interaction with the analytes led to a drastic

Figure 28. Fluorescent sensing of metal ions by H2T4PyP-functionalized CdSe QDs. Reprinted with permission from ref 286. Copyright 2010 Springer-Verlag.

color change in both cases, but a quenching of the fluorescence emission was observed with Zn2+, whereas the presence of Hg2+ resulted in an enhancement of the same. A hybrid structure to directly sense metal ions was prepared based on the use of inorganic CdSe QDs functionalized with an organic ionophore, namely a H2T4PyP (Figure 28).286 The resulting fluorescent nanosensor was used for the selective detection of Zn(II) ion in an organic medium. Silica monolith doped with H2T4CPP shown sensitivity to various metal ions.287 This sensor was based on the change in the Q-band of the porphyrin spectrum, to detect Cu2+ (543 nm), Zn2+ (522, 559, and 596 nm), Pb2+ (531 and 559 nm), and Ni2+ (522 and 551 nm). The same porphyrin, H2T4CPP, was used to determine trace levels of uranyl ion.288 In particular, H2T4CPP terminated poly(N-isopropylacrylamide) (PNIPAM) was synthesized by controlled free radical polymerization to get a fluorescent system sensitive to pH in the 1−5 range and highly selective to UO22+ over several competing ions. Sorption and visual/spectrophotometric detection of Cd(II) ions in aqueous solutions were obtained by Zhang and coworkers289 by immobilizing through electrostatic interaction a H2TMPyP onto a poly(sodium 4-styrenesulfonate) (PNaSS) grafted on a microporous membrane of chloromethylated polysulfone (CMPSF). The common Ca2+ or Mg2+ ions had no significant interference, while detection and sorption performances were heavily influenced by low pH values. Reversibility was obtained by treatment with EDTA and HCl as a stripping agent. Selective removal and detection of the toxic Cd2+ ions in aqueous samples were also obtained using an aluminosilica mesocage sensor functionalized with H2TMPyP.290 This enabled the construction of probe surface-mounted naked-eye ion-sensor strips for highly sensitive responses to the selective removal and sensitive recognition of Cd(II) target ions down to nanomolar concentrations: the mesocollector demonstrated long-term stability, reversibility, and selectivity. The same porphyrin was used in a multifunctional membrane for visual warning and enhanced absorptive removal of cadmium ions in aqueous solutions:291 H2TMPyP was immobilized onto a chitosan/cellulose acetate membrane through polymer brushes, grafted on the membrane surface from 3-sulfopropyl methacrylate potassium salt via a surface initiated atom transfer radial polymerization method. The resulting functionalized membrane exhibited a rapid colorimetric change when in contact with cadmium ions in solutions, while no significant change was observed in the presence of interfering cations. The solution pH value, due to the protonation and deprotonation of the porphyrin moiety, influenced the chromatic changes: the binding of H2TMPyP to Cd(II) ions may be enhanced in alkaline solutions and reduced at lower pH values. Qiu and co-workers employed 2553


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Figure 29. Catalytic effect of NGQDs toward the metalloporphyrin formation. Reprinted from ref 292. Copyright 2015 American Chemical Society.

Scheme 16. Chemical Structure of Cationic Porphyrin-Based pH Sensors

the same porphyrin to coordinate Cd2+ using nitrogen-doped graphene quantum dots (NGQDs) to accelerate metalloporphyrin formation.292 The optical sensor obtained displayed rapid, sensitive, and selective responses toward the Cd2+ ions based on the fluorescence evolution of both the reagents involved (Figure 29): in the absence of Cd2+, the addition of TMPyP basically led to a fluorescence quenching of NGQDs due to the inner filter effect, resulting in a split of the fluorescence spectrum of H2TMPyP/NGQDs with a valley at about 430 nm; instead, in the presence of Cd2+ the formation of the porphyrin complex led

to a large bathochromic shift of the porphyrin Soret band, an integral overlap spectrum of NGQDs and Cd−H2TMPyP and a valley at around 460 nm. A porphyrin-functionalized sensor, based on a holographic grating, was developed by Yetisen and Qasim groups,293 demonstrating its reversible colorimetric tuneability, in response to variation in concentrations of organic solvents and metal cations (Cu2+ and Fe2+). A new porphyrin, namely a 5,10,15,20tetrakis[4″-(3“-(acryloyloxy)-propoxy)phenyl-4′carboxyphenyl]porphyrin, was prepared, with methacrylate groups, to crosslink with a 2-hydroxyethyl methacrylate-based 2554


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Scheme 17. Chemical Structure of PP1

Figure 30. Cross sections of the sensor system (left) and of the catheter with integrated optical sensor system (right). Reprinted with permission from ref 299. Copyright 2015 Elsevier.

Scheme 18. Molecular Structures of H3TFPC and the Functionalized Ga Complex Exploited in Polymeric Membranes

different signal intensity ratios, and different fluorescent excitation wavelengths, sensitive pH-sensing can be achieved in the range of 2.1−8.0. The proposed probes were successfully applied in pH-sensing inside living cells. Low Q-factor nanocavities, defined in a silicon-nitride membrane, were used as pH sensors: the cavities were coated with a thin layer of a copolymer (PP1) of H2TAPP and diaminodurene with the fluorinated dianhydride (Scheme 17), thus introducing additional benefits to the sensing ability of fluorinated-polyimide polymers both from the porphyrin ring itself and its amine substituents. Amines are readily protonated by acidic materials, such as TFA, and the repulsion between the resultant ionized species caused the polymer to swell and increase its refractive index.297 4.3.5. Glucose. H2T4CPP-functionalized chainlike Co3O4 nanocomposites were fabricated as a mimic peroxidase resulting in a catalytic activity significantly higher than that of pure Co3O4 nanoparticles. The intrinsic peroxidase activity was used to develop a simple, highly sensitive, selective, visual, and colorimetric method to detect glucose.298

polymer matrix, and laser light was used to create the multilayer diffraction gratings, operating in the visible region of the spectrum and instantaneously useable as a sensor. A new complex based on H2MCPP and freshly synthesized silver colloid was obtained and used a new optical sensor for Ag0 detection in very small concentrations (2.5 × 10−9 to 0.82 × 10−7 mol/L) broadening the known detectable concentration range of rare metals and improving their recovery.294 4.3.4. pH Sensors. Free-base porphyrin self-assembled monolayers deposited on tapered optical fibers was demonstrated to be sensitive in acidic medium between 0.6 and 3.8 pH values.295 Mono- and diprotonation were observed, by studying absorption and emission spectra, while immersing the functionalized fiber into water solutions of hydrochloric acid in a pH range from 0.05 to 4. The recovery of the sensor was possible down to a pH value of 0.6, and at higher concentrations of acid, the sensor degraded. Five water-soluble cationic porphyrin derivatives296 (Scheme 16) were synthesized and demonstrated to be promising bimodal ratiometric probes for both colorimetric and fluorescent pH sensing. By selecting different porphyrins, 2555


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A continuous glucose monitoring, instead, for insulin infusion was developed by combining two optical sensors based on phosphorescent porphyrins, namely Pt(II)-5,10,15,20-tetrakis(4-fluorophenyl)tetrabenzoporphyrin (PtTPTBPF) and Pt(II)6-aza-13,20,27-triphenyltetra(tert-butylbenzo)-porphyrin (tBuPtNTBP). The surface of a polytetrafluoroethylene (PTFE)-tube simulating the insulin infusion catheter was functionalized with the glucose sensor (PtTPTBPF based) and with the reference oxygen sensor (tButPtNTBP based) and measurements performed both in vitro and in vivo (Figure 30).299 4.3.6. Porphyrinoid Receptors. The exploitation of porphyrin analogs as chemical sensors is obvious, since there is a strict relationship between molecular structure and optical properties of the corresponding macrocycle. Nature teaches us how important this feature is, and a simple saturation of a peripheral double bond affords chlorins from porphyrins, optimized for photosynthetic processes. 5,10,15-Tris(pentafluorophenyl)corrole (H3TFPC) and various Ga complex derivatives were tested as fluorophores for anion and amine binding (Scheme 18).300 The binding properties in solutions were tested dissolving the free base corrole in polymeric membranes, such as PMMA and 10% poly acrylamide. While the PMMA membrane did not change its fluorescence emission when immersed in anion solutions, the pristine poly acrylamide membrane was not fluorescent but became emissive when immersed in solutions containing fluoride or cyanide anions. The saturation limit for cyanide binding was 1 ppm, with a LOD of 70 ppb, which makes this system promising for practical applications. Co(t-BuPC)PPh3 was tested as an ionophore in a plasticized PVC membrane to develop optodes for nitrite determination.301 The sensing mechanism involved the presence of a chromoionophore, which was responsible for the membrane’s optical change upon nitrite binding. The extraction of the nitrite ion into the PVC membrane operated by Co(t-BuPC)PPh3 should be accompanied by a hydrogen ion, which was detected by a protonsensitive chromoionophore. Co(t-BuPC)PPh3 is a neutral ionophore, and in this case, the chromoionophore could be neutral or negatively charged; if neutral, the nitrite binding made the ionophore negatively charged and the chromoionophore positively charged, maintaining the membrane’s electroneutrality. In the case of a charged chromoionophore, lipophilic additives should be added to keep the membrane neutral. Both cases were tested with Co(t-BuPC)PPh3, obtaining similar satisfactory results. The selectivity pattern differed from the Hofmeister series, and it agreed with the pattern observed in the case of a potentiometric ISEe developed with the same ionophore.200 The Rh(III) chloride complex of 5,10,15,20tetra(p-tert-butylphenyl)porphyrin was also tested as the charged ionophore in similar optodes, obtaining comparable results with those of Co(t-BuPC)PPh3. A trifluoroacetyl-substituted bacteriochlorin (Scheme 19) was used as fluorophore in a plasticized PVC membrane for the detection of alcohols in aqueous solutions.302 The sensing mechanism in this case was based on the strong electronwithdrawing character of the trifluoacetyl group, which induced the linked carbonyl group to react with nucleophiles, such as alcohols, to form the corresponding hemiacetal, leading to a hypsochromic shift of the emission band of the macrocycle. The developed membrane was tested to sense ethanol in water with the sensor response fully reversible up to 25% v/v of ethanol concentration in water. The optimized composition of the

Scheme 19. Molecular Structure of the Oxobacteriochlorin

membrane, which needed the addition of TDMACl as a lipophilic additive for the sensing mechanism, also allowed a good stability over time. Although tested for alcohols, the sensing mechanism could be applied to other nucleophiles too, such as amines or carboxylate anions. An oxoporphyrinogen (Scheme 20) mixed with a LDH was exploited for the colorimetric discrimination of methanol and Scheme 20. Molecular Structure of the Oxoporphyrinogen: (a) Planar and (b) Side Viewa

a

Tert-butyl groups were omitted for clarity.

ethanol.303 LDH is a synthetic clay that consists of alternate layers, of opposite electronic charge, where the anions can be exchanged by anion-exchange reactions. In this case, oxoporphyrinogen was inserted in the interlayer space to obtain a homogeneous film, which did not leak the macrocycle upon washing with different solvents. Oxoporphyrinogens are doubly oxidized porphyrin, which can change the π-electron conjugation pattern by tautomeric equilibria and, consequently, their optical features, upon binding of guest analytes. The obtained film has a magenta color, which is the color of the oxoporphyrinogen when dissolved in ethanol or methanol; the film assumed a color change when immersed in methanol, while no changes were observed in ethanol. It is interesting to note that the color was persistent even after removal of methanol under reduced pressure, while the pristine color was recovered after washing with THF. The sensing mechanism was elucidated by solid-state 2556


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13

5. OXYGEN SENSORS The present, commonly used analytical methodologies for the laboratory evaluation of gaseous (pO2) or dissolved oxygen (DO concentration) are mainly based on electrochemical or on optical methods. Electrochemical methods are based on the Clark electrode (CE)314 that relies on the amperometric reduction of DO to water on a platinum electrode at a typical reduction potential of −0.7 V. The CE is commonly used to assess water quality. It is constituted by a Pt working electrode, an Ag/AgCl reference electrode, and a gas-permeable Teflon membrane. Notwithstanding its simplicity and accuracy, and the possibility of miniaturization, it suffers from several disadvantages: (i) the Teflon membrane has low oxygen permeability; (ii) the Pt device is quite expensive; (iii) it causes consumption of oxygen, which can be detrimental in the case of in vivo or small samples; (iv) the need of water (or at least traces of it); (v) the difficulty of application over large areas; and (vi) damage of tissues if employed in real biological samples. Biofouling of the systems can also be a serious problem. Improvements were achieved by replacing the Pt component by cheaper and robust adsorbed hemin derivatives.315,316 A system made of a PANI matrix with an embedded Fe(III)TPPS derivative with improved potentiometric and optic sensitivity was reported recently.317 An emerging and appealing alternative to potentiometric tools is the oxygen optical sensor (OOS), based on the dynamic (collisional) quenching of luminescence of a given probe by O2. These devices are essentially based on three key elements: (i) the sensing material (i.e., a luminophore) (in solution or embedded in suitable polymeric matrices); (ii) an optical system; and (iii) the electronic system to handle, process, and display the acquired data. OOS presents some advantages over electrochemical means, including: (a) full reversibility of the physicochemical interactions; (b) negligible consumption of O2; and (c) wide range of oxygen level detection, from parts per billion to high partial pressures. Moreover, they can be miniaturized on a micrometric scale or applied over large areas (oxygen imaging), also via remote sensing using optical fibers or guides. The OOS are negligibly influenced by hostile chemical and environmental conditions, such as humidity, electromagnetic fields, and radioactive sources. Several papers overviewing some fundamental general aspects and commercial applications of OOS have recently been published.318,319 The sensing materials are mainly based on inorganic or organic dyes. The detection mechanism uses the collisional quenching of their luminescence (fluorescence and phosphorescence) by molecular oxygen as a basis. Either the intensity of the luminescence or the excited state lifetimes can be monitored as a function of O2 activity (pO2 or DO). The quenching function is usually accounted for by the Stern− Volmer equation (SV) (eq 2)

C−CP/MAS NMR spectroscopy, which revealed that methanol was able to cause an acetate anion transfer from LDH to the oxoporphyrinogen, which led to the color change. The acetate remained linked to the oxoporphyrinogen even after the removal of methanol, maintaining the color variation, while THF led to the original color recovery, by removing the coordinated anion. The composite layer is highly selective for methanol detection in ethanol, which can represent a simple and advantageous method for the analysis of liquids suspected of containing this toxic species. Brückner and co-workers reported the optical pH sensing of various 5,10,15,20-tetrakis(pentafluorophenyl)porpholactone derivatives.304 In these species, the base-induced formation of a hemiacetal oxide derivative induces optical changes that can be used for pH determination in strong alkaline solutions. To make the porpholactone water-soluble, the macrocycles were PEGylated. To develop the optode membrane, the water-soluble PEGylated porpholactone was dispersed in Nafion membranes.305 While in solution, porpholactone derivatives produced very sensitive and short response times upon base addition and the membranes that developed had slower responses and reduced sensitivities. The materials developed are promising for application in fields that need to measure high hydroxide concentrations. 4.3.7. Optical Sensor Arrays. The colorimetric array developed by Suslick has also been applied for liquid phase analyses. For this application, porphyrins and the other chromophores should be fixed in a hydrophobic support, to avoid interferences from the water solvent. In this case too, the array exposed characteristic fingerprints for each analyte tested and a discriminatory power toward the identification of different beverages.306 The potential application of the colorimetric array for quality control and quality assurance of beers307 and soft drinks308 was studied, demonstrating the potential exploitation of this simple device in the real field. Other groups studied the application of similar arrays in the liquid phase, to discriminate different kinds of green tea,309 or to detect melamine adulteration of milk.310 The complexity of the interactions occurring in the colorimetric array were also exploited to identify proteins.311 The CSPT platform was used to develop an integrated opticalpotentiometric sensor array, based on porphyrin and corrole derivatives as chromophores, dispersed in PVC membranes deposited onto an ITO conductive substrate.312 In this system, optical and potentiometric measurements can be simultaneous. The array was first tested to detect model analytes, characteristics of vegetable oils, and then used to analyze real olive and seed oil samples. The dual mode response of the array allowed a significant improvement of device performances respecting individual transduction systems. To improve the stability of the polymeric membranes used in this dual mode array and to avoid the use of PVC membranes, porphyrin thin films were obtained from pyrrole peripherally substituted tetraphenylporphyrins via electropolymerization and deposited onto ITO substrates.313 After the optimization of the deposition conditions, potentiometric and optical responses were independently characterized toward model analytes and then both transduction methods were exploited on the same sensing film by using the CSPT platform. The multifunctional array displayed a significant improvement in the information and classification of analyzed samples with respect to the individual methods, confirming the promising potentialities for application in the real field.

I0/I = τ0/τ = 1 + KSV[O2 ]

(2)

where I0 and I represent the emission intensities in the absence and in the presence of the quencher, τ0 and τ are the corresponding lifetime values, KSV (i.e., the Stern−Volmer constant) is KSV = τ0qd, with qd as the bimolecular kinetic quenching constant.320 The typical amount of dissolved oxygen in water, at ambient temperature and pressure, is about 8 ppm (w:w). A good linearity over a large analyte (e.g., O2) concentration range facilitates the calibration of the sensors in practical applications. In the case of probes embedded in 2557


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dendrimeric structures by efficient click-chemistry. These nanostructures are successfully employed to map histological ex-vivo skin tissues. Naphthalene-Pt(II)-porphyrin derivatives were also considered as putative sensors.324 However, the covalent linking of naphthyl groups in meso-position did not result in the modification of the UV−vis and luminescence properties of the macrocycles because of the poor electronic communication between the porphyrin and the linked groups. This was confirmed by time-dependent density functional theory calculations (DT-DFT). A new class of Pd(II) and Pt(II) azatetrabenzoporphyrins were prepared and studied by Borisov and co-workers.325 The extended aromatic conjugation allowed for excitation in the red region, easily achievable by commercial laser sources and intense phosphorescence emission in the NIR region. In a toluene solution, luminescence is effectively quenched by oxygen, showing linear SV plots up to pO2 of 0.15 kPa. Decay times are longer for Pd(II) complexes, resulting in a more efficient quenching process for these dyes. In polystyrene matrices, usually employed for their high oxygen permeability, nonlinear SVs are obtained. The investigated dyes had appreciable features, although somewhat affected by a tendency to self-aggregation of the porphyrin luminophores. The different nature of the coordinated central metal ion can have a profound impact on the physicochemical properties of the macrocycles. The ligation of an octaethyl-porphyrin derivative by an Ir(III) ion, for example, yielded a substrate characterized by a bathochromically shifted, broader Soret band, with respect to the Pt(II) analogues.326 Additionally, interesting features such as water solubility or the possibility of covalent linking to biological or synthetic polymer matrices can be introduced by changing the nature of the Ir axial ligands. Either in buffered solution, embedded in polystyrene, or linked to SiO2, the probes showed surprisingly linear Stern−Volmer plots, indicating a high uniformity of the O2 binding sites. However, once linked to bovine serum albumin (BSA), the macrocycles had a lower quenching efficiency and nonlinear behavior, probably arising from the high variability of the local microenvironments, due to different linking positions of the porphyrin probe. Lantanide-porphyrin complexes were prepared and tested for O2 sensing at ambient temperature.327 In particular, Lu(III) and Gd(III) TPP derivatives were prepared. Both the macrocycles displayed both fluorescence and phosphorescence emission, with higher quantum yields in the case of the Lu(III) species. The macrocycles can be incorporated into different polymeric matrices (i.e., modified sil-sequioxane propyl methacrylate copolymers) and methylcellulose. The polymers were filmed onto inert substrates via spin coating. In all cases investigated, nonlinear SV plots were obtained, most likely due to the nonhomogeneity of the matrices, and hence different quencher accessibility to the dyes. In the low oxygen range, however, the dependence is linear, indicating a possible application of the systems as sensors. A Gd(III)-hematoporphyrin derivative spotted on paper was recently used as a quenchometric reporter for oxygen in methanol solution. The system demonstrated, along with a linear SV plot in the range from 10 to 100% oxygen concentration, fast response and recovery times and high photostability.328 Doubly emissive Lu(III) and Gd(III) derivatives (i.e., alkyl-, tolyl-, and fused benzoporphyrins), displaying both fluorescence and phosphorescence emission, were employed for “intra-

matrices, or in the case of real samples, SV plots are often nonlinear, due to a heterogeneous environment or differently accessible luminophores. They can be described by a multiterm equation (eq 3) I /I0 = (f1 /1 + K1SV[O2 ]) + (f2 /1 + K 2 SV[O2 ])

(3)

where K1SV and K2SV are different SV quenching constants and f1 and f 2 the corresponding contributions (f1 + f 2 = 1). A low I0/I ratio (e.g., 100 indicates a high quenching efficiency. In this case, the probes are useful for the detection of traces of oxygen as low as parts per billion. Intensity-based sensing is usually performed in ratiometric mode, by using so-called dual emitters. These systems, based on fluorescent and phosphorescent moieties, feature two different emission bands, one characterized by a short τ in the nanosecond range (not dynamically quenched by O2) and used as a “reference” intensity and the second with a more durable (typically in the microseconds range) quenchable emission. Commonly, detection is performed following decay time τ, a parameter that is independent of the probe’s concentration and of the detrimental effect of degradation by singlet oxygen, which can be formed in the presence of photosensitizers. New techniques to evaluate reporter lifetime are based on a “phase modulation technique”, or “time-gated fluorometry”, under pulsed excitation. Metal-complex organic dyes were widely employed as probes, due to their high quantum yields, strong emission intensities, and long excited-state lifetimes. Among them, metallo-porphyrin derivatives were the subject of intensive investigations. The most widely employed are Pt or Pd porphyrin derivatives, whose phosphorescence emission, with long τ0 in the range of microseconds, can be efficiently quenched by molecular oxygen. The phosphorescence arises from metal-induced (spin−orbit coupling) intersystem crossing to a long-lived triplet state. Molecular oxygen, with a ground-state triplet, efficiently deactivates the luminophore-excited state, via collisional quenching. In addition to their good photostability, their use is related to the possibility of being excited by visible light and their significant Stokes shift, as well as to their chemical and electronic properties that can be easily modified by functionalization of the periphery of the macrocycles, allowing for useful tuning, for example, of their binding attitude onto polymeric matrices or other surfaces, which favorably affects the sensor’s stability. Moreover, these probes can be either covalently or noncovalently bound to dendrimers or nanoparticles, obtaining nanostructured systems that can be employed at the cellular level. This issue is of great importance in cancer diagnosis and therapy, since the O2 level (e.g., hypoxia) is among the key markers of tumor growth. 5.1. Macrocycle Structure Optimization

Pd(II) and Pt(II) porphyrin derivatives were widely exploited as oxygen reporters, thanks to their phosphorescence emission that can be affected, both in terms of intensity and of lifetime, by O2.321,322 Brightly emissive benzoporphyrins, to detect oxygen under ambient light, were recently prepared. 323 The ad hoc modification of the periphery (i.e. expanded aromaticity) strongly affected the electronic π-system of the obtained luminophores, allowed to overlap their Soret bands with the wavelength of common commercial laser sources. Moreover, the presence of pivaloyl moieties allows for the formation of 2558


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Figure 31. (a) Chemical structures of the active materials used in the work. (b) SEM image of the sensing film cast on a silicon wafer (scale bar = 2 μm). Reprinted from ref 336. Copyright 2015 American Chemical Society.

ratiometric” oxygen sensing.329,330 The fluorescence emission, which is not affected by the presence of oxygen, serves as an “internal reference” toward the variation of the phosphorescence signal upon O2 quenching. The quantum yields of the Lu(III)benzo-derivatives are higher, as are their phosphorescence τ0, with respect to the Gd(III) analogues, indicating a better suitability of the former as sensor sensitive materials. The nature of the supporting matrices has a profound effect on the features of the dye-based sensors. Their general characteristics can be summarized in (i) ease and convenience of the synthetic procedures; (ii) negligible leakage of the embedded or linked dyes; (iii) inertness toward the photophysical and photochemical properties of the probes; (iv) high transparency with respect to the excitation and emission wavelengths; and finally (v) good oxygen permeability. The most used protocols to immobilize the dyes are physisorption (adsorption), embedding, and covalent attachment to organic or polymeric substrates. The embedding protocol is the most suitable, whereas covalent binding procedures are the method to choose in the case of dyes with poor solubility, in order to overcome self-association and consequent changes of the photophysical properties of the probes. Another appealing, well-established method to prepare sensing films is based on the inorganic (silica-based) or hybrid (ormosil) sol−gel domain. Some advantages are the ease of fabrication and manipulation even at room temperature, the robustness at high temperature, the inertness, and the porosity of the substrate thus guaranteeing higher uniformity of dye dispersion, resulting in a better linearity of SV plots within higher oxygen pressure ranges and reducing measurement errors. Some examples will be discussed.

temperature. In the case of the conjugated copolymer, the partition ratios (f) are independent of temperature, whereas in the case of the PdTPP/IBM-co-TFEM mixture, a change of the f coefficients is revealed, indicating a temperature-dependent oxygen-diffusion process in the latter polymeric matrix. Along the same lines, PtTFPP derivative, cross-linked to a hydrophilic methacrylate-co-polyacrylamide copolymer (PHEMA) and to a hydrophobic polystyrene film (PS), respectively, were also investigated.332 The polymers were thermally formed onto a quartz substrate, functionalized by an acrylate-modified silane. The grafted surfaces were tested as OOS by simple dipping into buffered solutions, at different DO values. Both systems were compared to the physically incorporated PtTFPP precursor into PHEMA copolymer and PS. The results produced better performance for copolymerized systems, in terms of photostability, leaching of the luminescent probes and shelf life, with respect to the mixed counterparts. Higher sensitivity is generally observed for the physical-prepared systems, a likely consequence of the restricted mobility of the porphyrin platforms within the membrane. In all cases, the SV plots were well-aligned, indicating an optimal degree of microdispersion of the PtTFPP probes. Moreover, the PHEMA films had higher quality properties with respect to the PS analogues, in terms of increased sensitivity (i.e., larger KSV quenching constant and higher O2 permeability) and faster response to DO. However, at higher PtTFPP/copolymer ratios, a decreased efficiency was observed, due to the probes’ selfquenching. Higher temperatures resulted in faster quenching responses, although with a decrease in emission intensities. In a different report, the author demonstrated that an analogous system, based on a PtTFPP-PHEMA copolymer, was unable to detect oxygen in air, as a consequence of the scarce permeability of the matrix in dry conditions.333 The work was subsequently extended to study a dual oxygen and temperature sensor for DO,334 consisting of a random copolymer of commercially available N-isopropylacrylamide (NIPAAm) and PtTPP units. The NIPAAm part serves as a thermosensitive unit, whereas the PtTPP groups serve as an O2 sensible probe. The detection mechanism is based on the different swelling ability of the polymer in water at a temperature (low critical solution temperature), which, for the polyNIPAAm, is about 32 °C. At higher temperatures, a contraction of the chain occurs, with the swollen matrix collapsing to a globular morphology. This results

5.2. Polymeric Membrane Sensors

5.2.1. Covalently Linked Luminophores. The conjugated luminescent polymer via radical copolymerization of isobutyl methacrylate (IBM), 2,2,2-trifluoroethyl methacrylate (TFEM), and a methacrylate monomer functionalized by PdTPP moiety was proposed as a oxygen sensor.331 The sensor performance of the system was compared to that of a mixture of PdTPP and IBM-co-TFEM copolymer. In both cases, the SV plots showed a two-site downward curvature, indicating a microheterogeneity of the luminophores within the matrices. Temperature-dependent experiments indicated the occurrence of two distinct microheterogeneities, one dependent and one independent of 2559


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homopolymers. The PtTPTBPStyr-co-PSMA was used in the form of dispersed nanoparticles in water (Figure 33d). The best performances were obtained in the case of fluorinated copolymer, in terms of sensitivity and suppressed migration of the porphyrin derivative through the polymer support. In all cases, nonlinear SV plots were obtained and higher Ksv for the PtTPTBPStyr-co-PHFIPMA system. The preparation of core−shell nanoparticles, with covalently linked O2 sensitive PtTFPP in the hydrophobic core, was reported by Tian and co-workers.338 The nanoprobe was prepared via a radical microemulsion/copolymerization approach of water-soluble (hydrophilic) 2,2′-N-isopropylacrylamide precursor (NIPAM) above its critical low-temperature transition. Monomers of styrene-functionalized PtTFPP, styrene, acrylonitrile (AN), acrylic acid (AC), and divinylbenzene (DVB, a cross-linker) were covalently included during the micellization process, forming the hydrophobic core. The pursued covalent binding strategy ensured leak-free features of the sensor, a key factor for application in vivo. “Clickable” fluorinated Pd(II)- and Pt(II)-porphyrin derivatives were proven to be excellent substrates for covalent linking (i.e., grafting) to a styrene-co-pentafluorostyrene matrix.339 Depending on the reaction conditions, the sensor materials can be obtained in either soluble form or insoluble cross-linked microparticles, following a suitable choice of reactant mole ratios. The soluble sensing materials were coated onto solid supports, such as PET or fibers of PMMA, whereas the microparticles were easily dispersed in silicone rubber. The systems produced adequate photostability and no leaching when exposed to organic solvents. The Pd(II)-porphyrin dye was more efficient, as a result of a larger KSV and longer τ0 with respect to the Pt(II) counterparts. Moreover, the macrocycles could be further modified to be efficiently coupled to organic-modified silicas (Ormosils), obtained by reaction of tetraethoxysilane (TEOS), and phenyltrimethoxysilane (PTMS), creating robust oxygen indicators, once finely dispersed in silicone rubber. The performances of the latter system were shown to be superior to those of the organic polymer-based analogues. In all cases, the corresponding physically included dyes exhibited poorer characteristics, in terms of stability, efficiency, and leaching of the porphyrin probes. The same authors reported on the studies of new Pt(II)- and Pd(II)-tetrabenzo-tetraphenylporphyrin derivatives, modified with styryl moieties in the four meso-phenyl rings.340 These dyes, featuring emission in the NIR region, were cross-linked to different polysiloxane prepolymers, obtaining an oxygen probe with high stability, high efficiency, fast response times, and linear SV plots with high KSV, larger in the case of the Pd(II) derivative, owing to the longer τ0 (Figure 34).

in a tuning of the PtTPP aggregation status and, consequently, of the quenching efficiency of the whole system. A further possibility based on a Pt(II)- and Pd(II)-TFPP derivative on amino-functionalized silica-gel nanoparticles (SiNPs) was proposed.335 The doped Si-NPs, once dispersed in silicone rubber matrices, featured high photostability, rapid response times, linear SV plots, and are suitable to detect trace oxygen. Fluorinated polymers were employed in the preparation of superhydrophobic surfaces, with selective optimal permeability to oxygen gas in aqueous solution.336 The sensible layer was prepared by dropcasting a mixture of a fluorinated poly methacrylamide solution in AK-225 (a hydrochlorofluorocarbon solvent), with an acetic acid solution of a copolymer of dodecylacrylamide and a PtTPP derivative functionalized by an appended methacryloyloxyethoxycarbonyl moiety (Figure 31a). SEM analyses showed that the film is composed of particles with 100−500 nm diameters (Figure 31b). The photophysical studies revealed excellent oxygen sensitivity in air as well as in water, with linear SV plots (Figure 32). This is due to the high hydrophobicity of the polymer mixture, which permits a free diffusion of O2 within the matrix.

Figure 32. (a) Luminescence spectra of porous sensor film in water as a function of oxygen concentration (top, 0.1 mg L−1; bottom, 40 mg L−1). (b) SV plots of the bare porous film (red) and cast film (blue). Reprinted from ref 336. Copyright 2015 American Chemical Society.

A sensor system based on a π-extended styryl-Pt(II)tetraphenyl-tetrabenzoporphyrin derivative (PtTPTBPStyr; Figure 33) covalently linked to different polymeric matrices was reported by the group headed by Borisov.337 The macrocycle features an intense luminescence in the NIR region (λem = 780 nm; τ0 = 46 μs). The dye was covalently linked via copolymerization with styrene, hexafluoroisopropyl methacrylate, or maleic anhydride and styrene (PS; PHFIPMA; PSMA, respectively), and the performances were compared to those of physically bound macrocycles within the correspondent

Figure 33. (a) Benzoporphyrin dye used in the work. Structural drawings of the dye-co-polymer sensitive materials: (b) styrene, (c) hexafluoroisopropyl-methacrylate, and (d) maleic anhydride and styrene, which can be used for water dispersive nanoparticles. Reprinted with permission from ref 337, which is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0), Royal Chemistry Society. 2560


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Figure 34. (a) Preparation of the O2 sensitive material with covalently bound porphyrin dye. (b) Response of 30 μm thick sensor foil to alternating atmosphere from 0 to 213 hPa of O2. Reprinted with permission from ref 340. Copyright 2015 Elsevier.

Figure 35. (a) Stern−Volmer plots for a dual sensor at different temperatures (24.8 to 65.6 °C). (b) Variation of the luminescence intensity decay of the dual sensor as a function of temperature and oxygen concentration. Reprinted with permission from ref 344. Copyright 2014 Elsevier.

of leaching and overall stability. Some notable examples are reported in this section. Ormosils with embedded cationic PtTMPyP or PtOEP derivatives were reported by Nogami.342 The hybrid matrices were composed of spin-coated xerogels of pentafluorophenylpropyltrimethoxysilane/n-octyltrimethoxysilane/tetramethoxysilane (PFTMOS/C8TMOS/TMOS) and trifluoropropyltrimethoxysilane/n-propyltrimethoxysilane (C3TMOS/TFTMOS). The studies revealed a generally high sensitivity, fast response, fine stability, and linear SV plots, in the quenchometric detection of gaseous O2, with the exception of the PtTMP/PFTMOS/ C8TMOS/TMOS gel. The sensitivities can be tuned using the xerogel and luminophore composition. The best performances were found with the PtOEP dye for PFTMOS/C8TMOS/ TMOS at a 10:45:45 ratio with IN2/IO2 = 330. More recently, Chu reported on an analogous Pd(II)TFPP derivative embedded in an ormosil matrix used for coating the ends of optical fiber. The system obtained displayed satisfactory homogeneity and sensitivity (IN2/IO2 = 260), high stability, and a fast response time.343 Another interesting system was developed later by the same author, by coincluding a PtTFPP and carboxyfluorescein (CF) into an ormosil gel of tetraethoxysilane/n-octyltriethoxysilane (TEOS/Octyl-triEOS).344 The xerogel obtained was coated onto the tips of a suitable optical fiber. Very interestingly, the presence of two distinct luminophores, PtTFPP and CF, permitted the simultaneous detection of both O2 and temperature, respectively, as reported in Figure 35.

Moreover, the sensitivity can be effectively tuned by changing the nature of the polymeric backbone, which influences the oxygen permeability of the matrices. The systems described were proven to be potentially interesting to detect trace oxygen in real samples, such as colored bottles of wine. NIR-emitting indicators are important for the high penetration and low light scattering, enabling oxygen detection in biological systems, where other conventional UV−vis luminophores fail. Polyethylene glycol diamine was cross-linked with a Pd(II)T4CPP derivative to give a biocompatible hydrogel with effective O2-responsive phosphorescence, suitable for in vivo subcutaneal applications.341 Very interestingly, the amount of dye that can be covalently linked is exceptionally high, reaching concentrations of up to 5 mM without the self-quenching effect, as a consequence of the spatial segregation of the macrocycles. The phosphorescence emission is 705 nm, and the SV analysis features a linear behavior, indicating a vast free diffusion of the molecular oxygen throughout the matrix, as well as a homogeneous distribution of the macrocycles. The in vivo imaging implanted in mice was also performed via intraratiometric evaluation, in the presence of different amounts of nonresponsive free-base- and Cu(II)-porphyrin analogues acting as reference dyes. 5.2.2. Noncovalently Linked Luminophores. The physical entrapment of sensitive dyes into inorganic, organic, or hybrid matrices is a procedure that is currently pursued, owing to the feasibility of the synthesis and the versatility of the materials obtained. In some cases, they overcome the drawback 2561


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Figure 36. (a) S−V plots of intensity emission quenching at different loading of PtTCBPyP/SBA-15 in the presence of oxygen. (b) Response time and relative intensity change and reproducibility for PtTCBPyP/SBA-15 (20 mg/g) on switching between (a) 100% N2 and (b) 100% O2. Reprinted with permission from ref 347. Copyright 2014 Elsevier.

Figure 37. (a) TEM image of the Ag-coated silica nanoparticles at a resolution of 50 nm. (b) S−V plots for a Pt(II)-doped sensor with metal-coated silica nanoparticles (⧫), and Pt(II)-doped sensor (●). Reprinted with permission from ref 349. Copyright 2013 Elsevier.

The use of a Pd(II)T4CPP derivative in the same ormosil gel allowed for the buildup of a fiber-optic-based sensor with good linearity and efficiency, with IN2/IO2 = 150.345 The system presented some improvements with respect to a similar PtTFPP/ TEOS/Octyl-triEOS optode, that achieved a IN2/IO2 of about 120, in the range of 0−40 mg/L of dissolved oxygen.346 A sensor featuring a very high sensitivity was reported.347 A novel dye, a Pt(II)-tetra-N-carbazylporphyrin derivative, PtTCBPyP (Figure 36), was embedded in different mesoporous ormosil matrices, such as SBA-15 and MCM-41. The best performances were obtained by the PtTCBPyP/SBA-15 sensor, which featured an exceptional sensitivity for gaseous oxygen, with IN2/IO2 > 8700. The best dye/matrix ratio was found to be 20 mg/g. Increasing the porphyrin content decreased the efficiency, due to the self-quenching of the luminophores. Silica nanoparticles with physically linked porphyrin dyes constitute interesting systems for oxygen detection. An early paper reported on the use of core−shell PtTFPP-entrapped SiNPs, embedded in an octyl-triEOS/TEOS ormosil matrix.348 The xerogel obtained was applied to the tip of glass optical fibers and employed as a gaseous oxygen sensor. The system presented better features, with respect to those of PtTFPP/Si-NPs or PtTFPP alone, with a high sensitivity (IN2/IO2 about 170), and a short response time, as a consequence of the increased surface area per unit mass and increased porosity of the matrix. Similar enhanced sensitivity was obtained in the case of PtTFPP and silver-coated Si-NPs, embedded in the same ormosil matrices.349

The Ag-coating is achieved by a straightforward reduction reaction of the Ag+-triethanolamine complex, in the presence of a suspension of Si-NPs (Figure 37). The enhancement effect is due to both the increased surface area per unit mass and to the heavy-atom effect on the emission of the porphyrin triplet state. The same approach was pursued to construct an O2/ temperature dual sensor, by using the same dye with CdSe QDs/SiO2 core−shell nanoparticles, embedded in n-propyltrimethoxysilane/3,3,3-trifluoropropyltrimethoxysilane xerogel (Prop-TriMOS/TFP-TriMOS) (Figure 38).350

Figure 38. (a) TEM images of core−shell CdSe/SiO2 nanoparticles (scale bar 20 nm). (b) High-magnification SEM image of PtTFPPdoped CdSe/SiO2 nanoparticles coated on the tip of an optical fiber (scale bar 100 nm). Reprinted with permission from ref 350. Copyright 2012 Elsevier. 2562


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The matrix can be used for real-time monitoring of interstitial oxygen of neural population networks of active brain tissues. A supramolecular assembly of QD-Pd(II)-pyridyl-porphyrin conjugates was prepared, and the application of the system for O2 sensing in the 0 to 160 Torr range was reported.352 The macrocycles bear several pyridyl groups in meso positions, allowing for an efficient coordinative binding to the QDs’ surface. The QDs act as a two-photon antenna upon NIR excitation (700−1000 nm). The QDs’ emission is quenched by (FRET) mechanism, upon one- and two-photon excitation, to the surfacebound macrocycles, enhancing their phosphorescence intensities. The QDs emission is insensitive to the presence of oxygen and can be used as an internal reference in the ratiometric O2 detection (Figure 41). A nanocomposite, comprising long-wavelength commercial QDs emitting near 800 nm (Qdot 800 ITK), embedded in a polydimethylsiloxane matrix (PDMS) with SiO2 microbeads with covalently bound Pt(II)T4CPP derivative (λem = 670 nm), was shown to represent an excellent ratiometric oxygen sensor in buffered aqueous solutions.353 The luminophores, that can be excited by a green LED source at ca. 530 nm, had distinct and slightly overlapping emission intensities, with those of the QDs unaffected by the presence of oxygen. The system showed excellent sensitivity below 300 μM of DO, good reversibility, and reasonably high stability. The dimension control of PDMS beads is a crucial factor in the efficiency of the above-described sensors. One way to achieve PDMS microbeads with a high degree of monodispersion was reported by Raghavan and co-workers, by following a “microfluiding” approach (Figure 42). A microfluid of the PDMS precursor, with codissolved dye (PtTFPP), was flow-focused in a continuous aqueous solution of surfactant (SDS) with proper viscosity.354 All the parts of the device that are in contact with the fluids were coated by hydrophilic PMMA to prevent adhesion and coalescence of the PDMS droplets. The droplets were then collected and thermally cured to give microbeads with an average of 80 ± 2 μm of diameter that could be directly used for the analytical purposes with good O2 detection performances, along with nontoxicity, biocompatibility, and chemical inertness for possible application in biological and medical fields. Modeling of the size and polydispersity of magnetic hybrid nanoparticles for oxygen detection was also reported.355 By varying the molecular weights of poly(styrene-co-maleic anhydride) polymers, the maleic contents and the reaction conditions, magnetic particles of mean diameter of 200 nm with very low polydispersity index (i.e., < 0.2) could be obtained. The inclusion of both magnetic particles (Fe2O3) and PtTFPP

The temperature affects both the emission intensity and the wavelength of the CdSe QDs at around 530 nm, with a sensitivity of ≈0.095 nm/°C. The PtTFPP phosphorescence emission at 648 nm is quenched by O2, with an IN2/IO2 of about 20 (Figure 39).

Figure 39. Variation of luminescence intensity with oxygen concentration (0−100%) at given temperature from 30 to 100 °C. Reprinted with permission from ref 350. Copyright 2012 Elsevier.

It was demonstrated that the proposed system, endowed with good reversibility and stability, could be suitable to applications in microbiological and medical fields. FRET sensing is a promising technique for the quantification of DO in solution. It is based on the transfer of singlet−singlet excitation energy from a suitable fluorophore (usually a quantum dot, QD, whose emission intensity is not quenched by O2) to a phosphorescent porphyrin dye. The fluorescence intensity of the QDs constitutes a reference signal, allowing the system to be employed in ratiometric mode, and enhances the intensity emission of the dye. Fluorescent QDs are excellent candidates to construct biosensors, owing to their high quantum yields, photostability, broad excitation profiles, and color tuneability. The toxicity of most of the inorganic QDs can be easily circumvented by entrapment in biocompatible, oxygen permeable PVC, or other related matrices. Further developments of this concept led to a FRET-based oxygen probe, suitable for application in living tissues (Figure 40).351 The optode matrix was prepared by codissolution of a PVC/DOS mixture with the proper amount of Pt(II)octaethylporphyrinketone (PtOEPK) and CdSe/ZnS QD nanocrystals. The solution was filmed on microscope glass coverslips by spin coating, to give a ca. 40 nm layer of oxygen sensible matrix.

Figure 40. Scheme of FRET transfer between NQDs and dye PtOEPK. (b) Emission response of NQDs and PtOEPK at various O2 concentrations. (c) SV plot of FRET matrix at 34 °C. Inset: reversibility diagram of sensor response at varying O2 concentrations. Reprinted with permission from ref 351. Copyright 2013 Elsevier. 2563


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Figure 41. Schematic drawing of the FRET-based sensing transduction mechanism of Pd-porphyrin/QD conjugates. Reprinted from ref 352. Copyright 2013 American Chemical Society.

Figure 42. (a) Scheme of the device used to produce the PDMS microbeads employed in constructing the microsensors. (b) SEM image of cured PDMS microbeads, with average size of 80 μm. Reprinted with permission from ref 354. Copyright 2012 Royal Society of Chemistry.

Figure 43. (a) SEM images of sputter-coated CCNPs at 20000× magnification (scale bar 500 nm). (b) Schematic representation of cross-sectional view of nanocapsule containing Fluo-Sphere (FS) and PfTCPP. Reprinted from ref 357. Copyright 2014 American Chemical Society.

luminophore as dopants marginally affected the properties of the obtained polymers. The same group prepared a nanostructured aluminum oxide-hydroxide solid matrix (AP200/19) incorporated with the standard PtTFPP as an oxygen sensitive dye.356 The nanocomposites were highly sensitive, allowing detection at low (0−10) and ultralow (0−1) O2 %, by means of phase-shift (i.e., frequency domain) luminescence lifetime measurements. Nanosized ratiometric luminescent sensors for oxygen optical imaging were fabricated from calcium carbonate (vaterite) nanoparticles (CCNPs), stabilized by poly(vinylsulfonic acid) (PVSA).357 The CCNPs were subsequently coated layer-by-layer with multilayers of opposite charged polyelectrolytes (anionic PSS and cationic PDDA). The proper choice of experimental conditions (i.e., pH, buffer systems, and PVSA concentration) allowed for the best encapsulation of the luminophores, namely

PdT4CPP, and a commercial reference oxygen insensitive dye (Fluo-Sphere; FS) (Figure 43). The encapsulation was achieved by coprecipitation of the dyes with the growing CCNPs from CaCl2/PVSA in a buffered alkaline aqueous solution. The final removal of the inorganic template gave the desired nanocapsules. Different nanostructured oxygen sensors were prepared by noncovalent conjugation of surfactants and hydrophobic Pt(II)or Pd(II)-porphyrin derivatives. In an earlier paper, Tian and coworkers reported on the inclusion of the usual PtTFPP in micelle formed from aqueous solutions of poly(ε-caprolactone)-blockpoly(ethylene glycol) (PCL-b-PEG) (Figure 44).358 The included dye showed increased quantum efficiency and photostability with respect to common organic solvents (i.e., THF, dichloromethane) or in water suspension. The system was 2564


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Figure 44. (a) Schematic drawing of the formation of PtTFPP-micelle supramolecular conjugates. (b) AFM image of the dried PtTFPP-micelle conjugates. Reprinted with permission from ref 358, which is an open access article distributed under the Creative Commons Attribution License (CC BY), PLOS.

proven to be suitable to use in biological environments, owing to its relatively high sensitivity (I0/I ca. 20, at 40 mg L−1 of O2) and short response time. A miniaturized phase fluorimeter has been developed for oxygen determination at nanomolar level.359 The luminescence measuring oxygen sensor device (LUMOS) is a miniaturized system optimized for high signal-to-noise ratio. The sensor material is based on the PdTFPP embedded in a Hyfion AD 60 polymer matrix. The luminophore presents KSV = 6.25 × 10−3 ppm v−1, the level of application is in the nanomolar range, with a detection limit of 0.5 nM, making the system suitable to detect oxygen contamination in sample containers and microbial or enzymatic oxygen consumption. Protein-based sensors, in which the heme cofactor was substituted by an unnatural Ru(II)-CO mesoporphyrin IX (RuMP) was reported.360 The protein scaffold chosen was myoglobin (Mb) and the heme nitric oxide/oxygen binding domain from thermophilic bacterium Thermoanaerobacter tengcongensis (Th H-NOX), which is stable at high temperatures (>70 °C). Both the Ru-(Mb) and Ru-(Th H-NOX) were stable under physiological conditions and displayed phosphorescence in the 600−800 nm region. However, Ru-(Mb) featured blueshifted emission with a reduced quantum yield because of partial exposure to the solvent of the RuMP moiety. Photophysical and quenchometric studies evidenced a linear τ0/τ dependence in a 0 to 300 μM DO concentration, indicating a possible application in biological relevant fields. With regard to the dual sensing of dissolved oxygen and other analytes, Papkovsky reported on the synthesis and application of two alkylporphyrin platforms (luminophore unit) linked to a Schiff-base (pH reversible site) (PTOEP-SB; PdCP-SB; Figure 45).361 The macrocycles, with differing peripheral decorations, were embedded in a PVC polymer, an optimal matrix for its proton permeability and inertness toward the quenching of the phosphors. A phase-transfer additive [i.e., tetrakis(4chlorophenyl)borate; TCPB] was added to the “cocktail” to allow proton transfer. The components were drop-casted from THF or CHCl3 solution, onto a polyester (Mylar) substrate, to give a film of ca. 5 μm thickness. Both sensors worked well in the physiological O2 range of 0−250 μM, with better response of PdCP-SB in the low OD range, thanks to its longer τ0 in the unprotonated form, with respect to that of PTOEP-SB (340 μs vs 84 μs at 30 °C). The intensity emission of both luminophores was drastically quenched at low pH, with the τ0 remaining

Figure 45. (a) Structure of PtOEP-SB (Me = Pt(II); R1-R8 = CH2CH3) and PdCP-SB (Me = Pd(II); R1,R3,R5,R7 = CH3; R2,R4,R6 = CH2CH2CO2CH3), highlighting the interaction site for protons and O2. (b) Response of a PtOEP-SB sensor in ratiometric absorbance (▲) and phosphorescence intensity (■) in a 0.1 M acetate buffer, 24 and 30 °C, respectively. Reprinted from ref 361. Copyright 2011 American Chemical Society.

unchanged, making the system suitable for simultaneous sensing of pH in the physiological range (5−8 units). Another dual sensor, for the simultaneous detection of O2 and Cu(II) ions, consisting of CdSe QDs and a porphyrin oxygen indicator (standard PdTFPP), coated on the tip of a plastic optical fiber, was reported.362 The CdSe QDs emission is linearly dependent on Cu2+ concentration in the range 0−10 μM, due to charge transfer from QDs to the copper ion363 but is independent of the oxygen concentration. The luminescence of the PdTFPP is efficiently quenched by oxygen, showing a linear SV plot, and by the Cu2+ ion to a different extent (Figure 46). Both indicators are embedded in a TEOS/Octyl-triEOS supporting matrix, and the xerogel is coated onto the tips of optical fiber and excited by a single 405 nm wavelength. The two nonoverlapping emission wavelengths are detected separately, computer elaborated, and reported as a function of concentration of both Cu2+ and O2 analytes (Figure 47). The reported system is favorable for application in biological, medical, and environmental fields. 5.3. In Vivo, Biological, And Medical Applications

The sensing and imaging of molecular oxygen in living cells and tissues is of crucial importance in biology, medicine, and tissue engineering. The principal parameters that need to be monitored are (i) in situ oxygenation and gradients and (ii) oxygen consumption rate (OCR). As an example, deviations of the OCR from normal physiological values indicated a perturbed metabolism or disease state. Variation of cell or tissue oxygenation, in particular, is associated with neurologic and metabolic disorders, cancer, or ischemia. A review by Papkovsky 2565


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Figure 46. Luminescence intensity at (a) 0 mM Cu2+, (b) 0 mg/L oxygen, and (c) 38 mg/L oxygen. Reprinted with permission from ref 362. Copyright 2010 Elsevier.

Figure 47. (a) QD emission intensity (589 nm) as a function of Cu2+ concentration, at different DO. (b) PdTFPP emission intensity (670 nm) as a function of DO, at different Cu2+ concentration. Reprinted with permission from ref 362. Copyright 2010 Elsevier.

excellently illustrates the progress made in this field in the recent past.364 Mayr developed a luminescent sensor to monitor DO in microfluidic devices, featuring fast response times and high signals.365 The intense brightness of the sensitive material permitted a thinner film to be integrated into a chip. Readout was accomplished off-chip by an epifluorescence microscope with lifetime or ratiometric imaging, by using the color channel of a CCD camera. The sensing system was composed of a mixture of fluorophore PtTFPP and a reference emitter Macrolex Yellow (MY), embedded in PS. The mixture was coated onto TiO2 nanoparticles (ca. 1 μm of film thickness). The ratiometric response displayed somewhat nonlinear SV plots. The microfluidic system was inoculated with microorganisms (Candida albicans), obtaining an overlay of the transmitted light image and lifetime image recorded on a camera (Figure 48). The system was used to measure the respiratory activity of human cell cultures (HeLa carcinoma cells and dermal fibroplasts).366 Oxygen sensitive microwells were constructed by embossing a thin film of PS doped with Pt(II)-octaethylporphyrin ketone on a PDMS microstamp.367 The microwells were charged by living cells (Madin-Darby canine kidney cells) at different density populations. The metabolic respiration was monitored by following the changes of the porphyrin luminescence. A similar approach was followed by Ichiki, who prepared flexible sheet-type sensors, to measure oxygen metabolism on a culture dish.368 The device was composed of transparent doublelayered polymer films (ethylene-vinyl alcohol, EVOH, and

Figure 48. (a) Ratiometric imaging of the phosphors in different conditions of oxygenation. (b) Resulting imaging of pO2 in the inoculated sample of Candida albicans. Reprinted with permission from ref 365, which is an open access article distributed under the Creative Commons Attribution License (CC BY), Elsevier.

PDMS), with an array of microholes (of 90 μm diameter and 50 μm depth) on the surface. The bottom part of the microholes was covered by 1 μm of porphyrin sensing material (Figure 49). The sensing layer was made of polystyrene doped with Pt(II)OEP. The device was applied slightly above the culture dish, to form a closed microspace. The oxygen consumption during the metabolic processes was directly evaluated by following the phosphorescence lifetime of the Pt-luminophores. The system was successfully applied to the culture of human breast cancer (MCF7), as well as to map O2 consumption rates on different rat brain slices. Luminescent thin-film sensors were employed for oxygen measurements in paper-based cell cultures.369 Paper-based scaffolds represent a useful engineered mimic for diffuseddominated environments in spheroids or solid tumors, such as 2566


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Figure 49. (a) Schematic drawing for the preparation of the hot-embossed device. (b) Schematic diagram of the flexible sensor sheet. Reprinted with permission from ref 368, which is an open access article distributed under the Creative Commons Attribution License (CC BY), PLOS.

nanoparticles.371 The PFO unit acts as a two-photon antennae and ratiometric partner for PtTFPP phosphor via a FRET mechanism. The sensor was included in MEF cells and the localization and stability evaluated by several means, including a time-resolved fluorescence plate reader and microsecond fluorescence/phosphorescence lifetime microscopy (FLIM). Because of the FRET mechanism, the porphyrin luminophore presented increased brightness with respect to other reported systems,361 as well as high photostability and compatibility with different detection devices. Water-soluble porphyrin derivatives, covalently conjugated to nanoparticles of poly(acrylamide) were prepared and employed for oxygen determination in living tissues (Figure 51).372 The

oxygen tension. This factor is extremely important in cancer cell metabolism, as it directly regulates cellular phenotype and invasiveness, through hypoxia-inducible transcription factors. The system comprises polystyrene films doped with the standard PdTFPP dye, which allow for a linear detection of O2 in the 0− 160 Torr range. The sensor can be effectively used, in conjunction with a fluorescence microscope, for the evaluation of oxygen gradient and consumption in paper-based scaffolds containing M231-eGFP cells. Phosphorescent cell-penetrating NPs were employed for intracellular oxygen sensing.370 NPs of a cationic polymer (Eudragit RL-100), containing physically included hydrophobic PtTFPP, feature uniform size (ca. 35 nm), high affinity for the anionic residues of the membrane cells, and are commonly used in drug-delivery systems. The PtTFPP-NPs probe, once included in the cells, features high photostability, with no significant leaching over long periods of time in cell growth conditions, no detectable cytotoxicity or changes in cell morphology, and high reproducibility in the O2 sensing experiments. The system was demonstrated to be widely and generally applicable in different human cell lines, such as human epithelial carcinoma (HeLa), hepatocellular liver carcinoma (HepG2), as well as mouse embryo fibroblast (MEF) and rat pheochromocytoma (PC12) (Figure 50).

Figure 51. (a) Schematic structure of the amino-functionalized PAM nanoparticles with a covalently linked porphyrin luminophore. (b) Experimental setting for dissolved O2 sensing in compressed collagen samples. Reprinted with permission from ref 372. Copyright 2014 Royal Society of Chemistry.

presence of cationic or anionic groups (i.e., pyridyl or sulfonate, respectively) permitted the choice of the most appropriate sensor for the nature of the matrix under investigation. Moreover, the presence of a carboxylic moiety allowed for efficient covalent linking to properly functionalized polymeric nanoparticles. The NP-porphyrin conjugates evidenced linear SV behavior throughout 0−3 × 10−4 M of oxygen in aqueous solutions. The ligation caused a 10-fold decrease of the quenching efficiency, probably by a reduced accessibility of the luminophores. The applicability of the systems to the detection of oxygen in compressed collagen gel was studied in detail, indicating the possibility of its use in tissue engineering. QD-porphyrin assemblies, included in micelle, were employed for in vivo two-photon oxygen sensing and mapping in vasculature.373 This issue is essential in chemotherapy for cancer since “vascular normalization” is one of the key factors for efficient drug delivery to target cells.374 The sensor is based on

Figure 50. (a) SEM (top) and AFM images of PtTFPP-RL 100 NPs. (b) Fluorescence imaging of MEF cells, loaded with 5 mg/mL of PtTFPPRL 100 NPs. Reprinted from ref 370. Copyright 2011 American Chemical Society.

The intracellular uptake and the imaging of oxygen concentration (cell oxygenation and metabolic response) were accomplished by measuring the phosphorescence emission lifetime with a commercial time-resolved fluorescence device and confocal microscopy. A new probe formulation makes use of the same PtTFPP and poly(9,9-dioctylfluorene) (PFO) embedded in cationic hydrogel 2567


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Figure 52. (a) Molecular structure of the carboxy-pyridylporphyrin dye. (b) Schematic representation of the oxygen sensing method developed in the work. Reprinted from ref 373. Copyright 2015 American Chemical Society.

illustrated in Figure 53, which reports a 3D projection of brain vasculature of an SCID mouse. A similar approach was pursued by Zhao and Huang, who reported the construction of dual emissive polyelectrolyte polymer dots.375 In the case in question, the amphiphilic character of the polymer, due to the presence of a hydrophobic backbone (a conjugated polyfluorene-co-Pt(II)porphyrin derivative) as well as cationic alkylammonium groups, can self-assemble in buffered aqueous solution to form small dots (Figure 54). The probe featured high stability and excellent biocompatibility once inoculated into the target tumoral cells, and its practical application was confirmed in the luminescence imaging of tumor hypoxia in vivo. The system described represents a development of a related fluorene/PtTFPP copolymer to detect O2 in 1,2-dichloethane.376 The presence of both the fluorescent fluorene groups (whose emission is unaffected by oxygen), and the porphyrin phosphors, resulted in an excellent ratiometric probe for O2. Interestingly, a full wearable system to monitor oxygen concentration in breath, with high resolution and sensitivity up to the parts per billion level, was developed.377 The prototype was based on an optical luminescent sensor (a PtOEP porphyrin dye) embedded in a PS matrix directly layered onto a color detector in the RGB color space, in which the red coordinate is directly related to the partial pressure of oxygen in the breath mixture. The sensor was found to be insensitive to humidity, and the effect of temperature was evaluated and implemented in an appropriate analysis program. The acquired colorimetric signal can be sent to remote devices, such as a smartphone or a tablet via a Bluetooth link.

the inclusion of a QD-Pd(II)porphyrin assembly (QD-PdP) in phospholipid surfactant (Figure 52). The nanosensor was assembled under mild sonication at room temperature and was templated by QDs. The QD-PdP was irradiated under two-photon excitation in NIR region (700− 1000 nm). Porphyrin emission, which can be quenched by molecular oxygen, was prompted by a QD-PdP FRET mechanism. The residual emission of QDs is insensitive to the presence of O2 and acts as a reference signal for ratiometric quantification. The sensing analysis revealed that the quenching of porphyrin emission occurs in the biological relevant 0−160 Torr range of pO2 with a linear SV plot (Figure 53).

Figure 53. (a) S−V plot for the sensor system at 25 °C (gray ■) and 37 °C (red ●). (b) Three-dimensional imaging of brain vasculature under two-photon excitation of an SCID mouse. Reprinted from ref 373. Copyright 2015 American Chemical Society.

Importantly, the system demonstrated high stability, remaining in circulation for several hours after the injection, and was successfully used as a probe for blood vessel oxygenation, as

Figure 54. Chemical structure of the fluorescent/phosphorescent conjugated polyelectrolyte and schematic drawing of the self-assembled FP-Pdots. (b) TEM image of self-assembled FP-Pdots in aqueous solution. Reprinted with permission from ref 375, which is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0), Royal Chemistry Society. 2568


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5.4. Technical Applications. Coating and Paintings

A fast response-time probe, based on a dye-adsorbed silica nanoparticle (SNP) film, was presented.379 The thin film of the oxygen molecular reporter (ca. 5 μm thickness) was easily obtained by simple spray coating of a toluene slurry of SNP (typically of 10 nm diameter) and the selected dye, namely the common PtTFPP derivative, at 0.3 mM concentration. The concentration of the dye in the solid phase was estimated to be ca. 130 nm/cm−2. The luminescence intensity, obtained upon excitation at 405 nm, was recorded by a CCD camera and featured linear SV plots (Figure 56). The probe also evidenced a dependence on temperature (1.5%/°C), so an efficient temperature regulation was required. The layered material, due to its high porosity allowing for a extensive gas permeability and exchange, featured a short response-time (0.1 ms), somewhat dependent on the layer’s thickness, as reported in Figure 56c. Sakamura recently reported on a pressure-sensitive coating based on self-assembled monolayers of a Pt(II)-porphyrin derivative.380 The porphyrin layer was covalently linked to an ITO surface, previously activated by 3-aminopropyltrimethoxysilane (APTMS), via straightforward coupling methods (Figure 57a). The excitation source was provided by a Xe lamp, and the luminescence signal was recorded by a CCD camera, as pictured in Figure 57b. The system, with its satisfactory performances for O2 detection in the range from 5 to 120 kPa and featuring good stability and temperature dependence, may represent a valuable tool for practical application in microscale flow devices. Nishide and Watanabe presented a system, composed of a poly(1-trimethylsilyl-1-propyne) (PMSP) matrix with embedded PtTFPP as a luminophore oxygen reporter.381 The system featured high sensitivity, with a linear SV plot up to 20 kPa, and was employed for oxygen partial pressure visualization inside a polymer-electrolyte fuel cell (PEFC) (Figure 58). The PtTFPP/ PMSP sensor was coated on the surface of a cathode gas diffusion layer on the airflow channel inside PEFC. The sensor was subjected to harsh working conditions, such as high relative humidity (RH), high temperature (above 80 °C), the presence of strong acids or methanol, demonstrating remarkable stability and adhesion to the coated surface. A fast response, pressure-sensitive sensor, based on biluminophore coating, was reported.377 The sensor was

Optical oxygen sensors are of fundamental importance also in many fields of applied sciences, for example, engineering and aerodynamics. In these specific fields, an important factor is the imaging of oxygen distribution on large surface areas and not only its concentration. Within this issue, optical sensor coatings or paintings offer the possibility of monitoring, often by the simple use of a CCD camera, the oxygen two-dimensional distribution with high spatial resolution, and fast response time. A surface pressure measurement technique using pressuresensitive paint (PSP), applicable in a low-density wind tunnel was recently reported.378 This technique is aimed at developing aircrafts for rarefied atmospheres, such as that of Mars (ca. 0.7 kPa). The experiments were conducted in the Mars Wind Tunnel (MWT) at Tohoku University (Figure 55).

Figure 55. (a) Mars Wind Tunnel (MWT) at Tohoku University. (b) Schematic setup for PSP measurements in the MWT. Reprinted with permission from ref 378, which is an open access article distributed under the Creative Commons Attribution License, Springer.

The typical PSP is composed of an oxygen permeable polymeric matrix, poly(1-trimethylsilyl-propyne (polyTMSP), embedded with the luminophore dye, the widely employed PdTFPP. The sample is an aluminum foil, coated with the PSP, placed in the pressure chamber of MWR, and excited by a UVemitting LED (395 nm). Luminescence is captured by a CCD camera, with an optical band-pass filter (

Porphyrinoids for Chemical Sensor Applications - ACS Publications

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