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Highly Networked Capsular Silica–Porphyrin Hybrid Nanostructures as Efficient Materials for Acetone Vapor Sensing Izabela Osica, Gaku Imamura, Kota Shiba, Qingmin Ji, Lok Kumar Shrestha, Jonathan P Hill, Krzysztof J. Kurzydlowski, Genki Yoshikawa, and Katsuhiko Ariga ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15680 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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ACS Applied Materials & Interfaces

Highly Networked Capsular Silica–Porphyrin Hybrid Nanostructures as Efficient Materials for Acetone Vapor Sensing Izabela Osica1,2, Gaku Imamura1,4, Kota Shiba1, Qingmin Ji1,3, Lok Kumar Shrestha1, Jonathan P. Hill*1, Krzysztof J. Kurzydłowski2, Genki Yoshikawa1,5, Katsuhiko Ariga*1 1. World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 2. Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland 3. Herbert Gleiter Institute for Nanoscience, Nanjing University of Science and Technology, 200 Xiaolingwei, Nanjing, 210094, China 4. International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan 5. Materials Science and Engineering, Graduate School of Pure and Applied Science, University of Tsukuba, Tennodai 1-1-1 Tsukuba, Ibaraki 305-8571, Japan Corresponding authors: Jonathan P. Hill, Katsuhiko Ariga E-mail: [email protected] [email protected] KEYWORDS Nanoflake-shell silica particles; Silica-porphyrin hybrid materials; Nanomechanical surface stress type sensor; Acetone sensor; Metalloporphyrin; Chemical sensor

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ABSTRACT The development of novel functional nanomaterials is critically important for the further evolution of advanced chemical sensor technology. For this purpose, metalloporphyrins offer unique binding properties as host molecules that can be tailored at the synthetic level and potentially improved by incorporation into inorganic materials. In this work, we present a novel hybrid

nanosystem

based

conjugated through covalent

on bonding

a

highly to

an

networked organic

silica functional

nanoarchitecture molecule,

a

tetraphenylporphyrin derivative, and its metal complexes. The sensing properties of the new hybrid materials were studied using a nanomechanical Membrane-type Surface stress Sensor (MSS) with acetone and nitric oxide as model analytes. This hybrid inorganic-organic MSSbased system exhibited excellent performance for acetone sensing at low operating temperatures (37 °C) making it available for diagnostic monitoring. The hybridization of an inorganic substrate of large surface area with organic molecules of various functionalities results in subppm detection of acetone vapors. Acetone is an important metabolite in lipid metabolism and can also be present in industrial environments at deleterious levels. Therefore, we believe that the analysis system presented by our work represents an excellent opportunity for the development of a portable, easy-to-use device for monitoring local acetone levels.


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1. INTRODUCTION Chemical sensing research is a fast-growing field of science with great potential to improve the quality of people’s lives. There is a burgeoning demand for effective sensing solutions in strategic applications including in human health diagnostics, food security, and environmental monitoring (both in the workplace and the wider environment).1-4 Although the development of chemical sensors is a multidisciplinary effort, it depends strongly on the characteristics of any prospective sensing materials.5 For example, materials capable of detecting some analyte at the nanoscale level may lead to rapid progress in the development of a sensing device and help determine its commercial success. Although inorganic and organic compounds have both been widely investigated for such applications, each are subject to particular advantages and drawbacks. In general, inorganic materials are characterized by high chemical stability and low-cost production while organic compounds feature richness of reactivity towards various target molecules but often lack the stability and low cost of their inorganic counterparts.68

In order to optimize sensing materials’ performance, a combination of inorganic and organic

substances is expected to provide access to the advantages of each type of material.9-11 Porphyrins are extraordinary molecules which possess a wide range of unique properties including intense colors and synthetic flexibility.12 They are also found widely in nature and play key roles in the basic mechanisms of living organisms. A prime example is the iron porphyrin complex heme that not only enables hemoglobin to bind, store and carry oxygen molecules in animals but also performs a variety of other functions in catalysis and electron transport.13 The unique ability of porphyrins to bind specific guest molecules has attracted the attention of workers in the chemical sensors field due to the potential for selectivity enhancement involving supramolecular chemistry concepts.12,14 Porphyrins can interact with an analyte by many


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different mechanisms including Van der Waals forces, hydrogen bonding, π-π interactions and coordination to a central usually transition metal ion.15,16 The interaction of porphyrins and metalloporphyrins with gases induces a variety of changes in mass, density, work function or optical properties, which present opportunities to employ these molecules in different sensing systems.17-20 Paolesse, Di Natale et al. have made extensive investigations involving porphyrins for sensing applications utilizing quartz crystal microbalance (QCM)21, electrical signaling22 and potentiometric sensor arrays23. The influence of different porphyrin self-assemblies on their sensing properties can result in sensitivity enhancements especially in the case of optical sensors.18 However, it has been noted that porphyrin aggregation due to possible π-π interactions between the macrocycles can also cause attenuation of sensitivity since it obstructs the axial coordination of target analyte molecules to the central metal ion. This effect is often evident when solid-state gas sensors are applied.21 In order to overcome this drawback, the incorporation of porphyrins into silicate structures towards specific sensor development has been investigated. Tao et al. have introduced a porphyrin-doped silica nanofibrous membrane and metalloporphyrin-doped mesostructured silica films for optical sensing of trace amounts of 2,4,6-trinitrotoluene (TNT) vapor. In both cases, they concluded that the larger surface area and porous structure considerably enhanced sensitivities of the resulting materials.24,25 Another examples is the mercury-specific fluorimetric/colorimetric

sensor

using

porphyrin-functionalized

Au@SiO2

core/shell

nanoparticles presented by Cho et al.26 Although these probes can detect certain target analytes with high sensitivity, the optical working principle makes them hard to implement in small, mobile devices, which is a significant limitation for their further real-life application.


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In this work, we have applied a simple porphyrin and its metal complexes as a functional decoration of a highly networked silica structure, so called silica flake-shell (SFS) capsules. The unique morphology of SFS, characterized by its large specific surface area (BET) 620 m2g-1 and pore volume of 2.8 cm3g-1 and abundance of silanol functional groups, favor surface functionalization, and present an excellent opportunity for the development of sensor applications.27-29 Incorporation of porphyrins on SFS surfaces is expected to obstruct aggregation and thus enhance sensitivity. The sensing properties of the resulting hybrid materials towards acetone and nitric oxide as model analytes were studied. Exhaled breath analysis has attracted notable clinical and scientific interest due to the fact that some endogenous compounds, including inorganic gases (e.g., NO, CO) and volatile organic compounds (VOCs, e.g., acetone, isoprene), have been assigned to specific pathologies.30-32 Acetone is also a very commonly used industrial solvent whose detection is warranted in the 2.5 – 12.8 vol% range in air due its high flammability. In fact, some industrial applications involving dry nitrogen environments (such as exist in large scale pharmaceutical plants) would benefit from acetone detection for safety purposes. In order to study the sensing properties of fabricated materials we utilized a nanomechanical surface stress type sensor; a newly developed Membrane-type Surface stress Sensor (MSS) that is a new generation of piezoresistive nanomechanical sensors with significantly enhanced sensitivity in comparison with commonly used cantilever-type nanomechanical sensors with optical readout.33 An additional advantage of MSS chips lies in their compactness making possible incorporation of a real-time nanomechanical olfactory system into a mobile platform such as a smartphone.34 The MSS working principle, construction elements and fabrication procedure, which involve piezoresistor readout in a full Wheatstone bridge configuration, have been thoroughly described in the literature.33-36 Briefly, the MSS is


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composed of a silicon-on-insulator (SOI) substrate and a round membrane suspended by four sensing beams with integrated p-type piezoresistors, composing a full Wheatstone bridge. When coated with a receptor layer a deflection of the membrane is produced and accumulated mechanical stress is applied on the sensing beams due to molecular interactions with the target analyte.

Figure 1. Schematic representation of the SFS-porphyrin composite synthesis strategy. Synthesis route to 4 and its metal complexes with: cobalt (4Co), nickel (4Ni), copper (4Cu) and zinc (4Zn). The SFS capsules are produced from solid silica spherical particles through a self-


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templating approach during a hydrothermal synthesis process. SFS were functionalized with porphyrin

4

and

triphenylporphyrin;

its 2:

metal

complexes.

1:

5-(4-Methoxycarbonylphenyl)-10,15,20-

5-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin;

triphenylporphyrin-5-yl)benzoyl

chloride;

4:

3:

4-(10,15,20-

5-[4-(N-(3-triethoxysilylpropylbenzamido))]-

10,15,20-triphenylporphyrin

2. EXPERIMENTAL SECTION 2.1. Reagents and chemicals. All syntheses were carried out with reagent grade chemicals, which were used as received. Tetraethoxysilane (TEOS; Tokyo Chemical Industry Co., Ltd.), methanol (MeOH; Kanto Chemical Co., Inc.), ethanol (EtOH; Kanto Chemical Co., Inc.), isopropyl alcohol (IPA; Wako Pure Chemical Industries, Ltd.) and 28 %-aqueous ammonia solution (NH3(aq); Kanto Chemical Co., Inc.) were used for the synthesis of silica particles. Silica flake-shell capsules were produced from silica colloid solution with an average diameter of 450 ± 20 nm (Nissan Kagaku Co.). Silica particles were collected from this solution by centrifugation and then heated at 500 °C for 5 h. 5-(4-Methoxycarbonylphenyl)-10,15,20-triphenylporphyrin (1) was purchased from Tokyo Kasei Chemical (TCI) Co. Ltd. Other chemicals were of analytical grade and were used as received. All solutions were prepared with Milli-Q water purified in a Millipore system. 2.2. Synthesis of porphyrin 4. 5-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin

(2).

5-(4-Methoxycarbonylphenyl)-

10,15,20-triphenylporphyrin (1) (0.119 g, 0.177 mmol) was dissolved in tetrahydrofuran (20 mL) and a solution of KOH (0.986 g, 17.574 mmol) in water (17.5 mL) was added. The reaction mixture was diluted with methanol (5 mL). The mixture was refluxed at 70 °C during 24 h and


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cooled at room temperature. After cooling to room temperature, the solution was evaporated and aqueous HCl (0.5 M) was added until neutral then the solution partitioning with CHCl3. The organic layer was washed with water and brine, then dried over Na2SO4. Na2SO4 was removed by filtration and the solvents were removed under reduced pressure. 1H NMR and MALDI-TOF MS data were in agreement with those reported in the literature37 and the product was used in the next step without further purification. 5-[4-(N-(3-Triethoxysilylpropylbenzamido))]-10,15,20-triphenylporphyrin (4). 5-(4-Carboxy phenyl)-10,15,20-triphenylporphyrin (2) was dissolved in CHCl3 (5 mL), SOCl2 (500 µL, 6.884 mmol) was added, then the solution was refluxed at 65 °C for 3 h. After the reaction mixture was cooled to room temperature, the chloroform and excess SOCl2 were removed under reduced pressure. The solid green residue was dissolved in dry CH2Cl2 (20 mL) then triethylamine (TEA) (160 µL) and (3-aminopropyl)triethoxysilane (APTES) (80 µL) were added. The mixture was refluxed for 24 h and then cooled to room temperature. All volatiles were removed under reduced pressure and the crude product was purified by silica gel column chromatography eluting with CHCl3-5% MeOH. The material obtained was identical with that previously reported.26 UV-vis data for 4 in CH2Cl2: λmax: 418, 515, 549, 590, 645 nm. 1H-NMR (300 MHz, CDCl3): δ = -2.78 (s, 2H, NH), 0.87 (t, 2H, HSiCH2), 1.30 (t, 9H, HCH3), 1.94 (m, 2H, HCH2), 3.67 (q, 2H, HNCH2), 3.93 (q, 6H, HSiCH2), 6.92 (t, 1H, HNH), 7.74 – 7.80 (m, 9H, Hph), 8.17 – 8.25 (m, 8H, Hph), 8.30 – 8.32 (d, 2H, Hph), 8.87 (m, 8H, Hβ) ppm. 13C NMR (75 MHz, CDCl3) δ = 8.05, 18.37, 23.05, 42.49, 58.62, 76.59, 77.01, 77.22, 77.43, 118.69, 120.34, 120.49, 125.31, 126.72, 127.78, 131.26, 134.39, 134.54, 134.62, 142.09, 142.11, 145.37, 167.64 ppm. MS (MALDITOF): m/z = 862 [M + H]+ (calcd. for C54H51N5O4Si m/z = 862). 2.3. Metallation of porphyrin 4.


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5-[4-(N-(3-Triethoxysilylpropylbenzamido))]-10,15,20-triphenylporphinato

cobalt(II)

(4Co). Porphyrin 4 (50 mg, 0.058 mmol) was dissolved in chloroform (20 mL) then Co(OAc)2 · 4 H2O (667 mg, 2.678 mmol) dissolved in methanol (10 mL) was added. The mixture was stirred at 65 °C for 6 h. After completion of the reaction, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3 as eluent. MS (MALDI-TOF): m/z = 918 [M + H]+ (calcd. for C54H49N5O4SiCo m/z = 918). UV-vis data for 4Co in CH2Cl2: λmax: 410, 527 nm. 5-[4-(N-(3-Triethoxysilylpropylbenzamido))]-10,15,20-triphenylporphinatonickel(II) (4Ni). Porphyrin 4 (50 mg, 0.058 mmol) was dissolved in chloroform (20 mL). Then Ni(OAc)2 · 4 H2O (721 mg, 2.897 mmol) dissolved in methanol (10 mL) was added. The mixture was stirred at 65 °C for 6 h. After completion of the reaction, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel and CHCl3 as eluent. UV-vis data for 4Ni in CH2Cl2: λmax: 414, 527 nm. 1H-NMR (300 MHz, CDCl3): δ = 0.84 (t, 2H, HSiCH2), 1.28 (t, 9H, HCH3), 1.90 (m, 2H, HCH2), 3.63 (m, 2H, H, HNCH2), 3.89 – 3.94 (q, 6H, HSiCH2), 6.85 (t, 1H, HNH), 7.65 – 7.75 (m, 9H, Hph), 8.03 – 7.99 (m, 6H, Hph), 8.11 (s, 4H, Hph), 8.70 – 8.77 (d, 8H, Hβ) ppm.

13

C NMR (75 MHz, CDCl3) δ = 7.89, 18.23, 22.91, 30.81, 42.38, 58.55, 117.73,

119.21, 125.55, 126.95, 127.86, 132.51, 133.76, 134.52, 140.91, 142.87, 144.2, 167.68, 207.05 ppm. MS (MALDI-TOF): m/z = 917 [M + H]+ (calcd. for C54H49N5O4SiNi m/z = 917). 5-[4-(N-(3-Triethoxysilylpropylbenzamido))]-10,15,20-triphenylporphinatocopper(II) (4Cu). Porphyrin 4 (50 mg, 0.058 mmol) was dissolved in dichloromethane (20 mL) then Cu(OAc)2 (210 mg, 1.156 mmol) was added. The mixture was stirred at room temperature for 12 h. After completion of the reaction, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3/5% MeOH as eluent.


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MS (MALDI-TOF): m/z = 922 [M + H]+ (calcd. for C54H49N5O4SiCu m/z = 922). UV-vis data for 4Cu in CH2Cl2: λmax: 414, 508, 548, 586 nm. 5-[4-(N-(3-Triethoxysilylpropylbenzamido))]-10,15,20-triphenylporphinatozinc(II)

(4Zn).

Porphyrin 4 (50 mg, 0.058 mmol) was dissolved in dichloromethane (20 mL) then Zn(OAc)2 (230 mg, 1.254 mmol) was added. The mixture was refluxed for 3 h. After completion of the reaction, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel with CHCl3/5% MeOH as eluent. MS (MALDI-TOF): m/z = 923 [M + H]+ (calcd. for C54H49N5O4SiZn m/z = 923). UV-vis data for 4Zn in CH2Cl2: λmax: 419, 547, 587 nm. 2.4. Synthesis of silica nanoflake-shell capsules. The synthesis protocol of SFS capsules has been described in our previous work.27,28 Briefly, spherical solid silica particles (150 mg) were first well dispersed by ultrasonication in Milli-Q water (5 mL) and heated at 75 °C in the presence of NaBH4 (500 mg) in a Teflon-lined autoclave for 24 h. Subsequently, the SFS capsules were collected by centrifugation, washed several times with water until the solution reached neutral pH. The resulting flake-shell capsules were then freeze-dried. 2.5. Synthesis of silica particles. Monodispersed silica particles of different sizes (29 nm, 376 nm and 556 nm) were synthesized by the Stöber method38,39 combined with a modified microfluidic approach.40,41 Briefly, two starting solutions (solutions A and B) were prepared. The detailed composition of each solution is summarized in Table 1 of the Supporting Information. Solutions A and B were individually passed through perfluoroalkoxyalkane (PFA: 1.0 mm inner diameter, 1/16 inch outer diameter,


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product of YMC Co., Ltd.) tubes using a syringe pump (CXN1070, product of ISIS, Co., Ltd.) at 10 mL/min. The two solutions were mixed in a polytetrafluoroethylene (PTFE) fluidic channel with a “Y” shaped junction (the channel cross section of ca. 1 mm2, KeyChem mixer, product of YMC Co., Ltd.). The mixture was passed through a PFA tube of 70 cm length. The reaction solution was then collected in a glass vial and aged at r.t. for 24 h. For further modification, silica particles were collected by centrifugation and dried under reduced pressure. 2.6. Preparation of porphyrin functionalized SFS and silica particle composites. Porphyrin 4 and its metal complexes 4Co, 4Ni, 4Cu, 4Zn (5 mg) were separately dissolved in anhydrous toluene (5 mL) to which silica flake shell (SFS) capsules (10 mg) were then added. The mixtures were well dispersed using ultrasound for no more than one minute and allowed to stand with frequent shaking at room temperature under a nitrogen atmosphere for a period of 24 h. After centrifugation the collected solid was washed several times with toluene to rinse away any excess porphyrin and then dried. Porphyrin functionalization of silica particles was achieved in an identical fashion. 2.7. Materials characterization. 1

H NMR spectra were obtained on a JEOL AL 300 FT-NMR spectrometer with

tetramethylsilane (TMS) as internal reference. Matrix Assisted Laser Desorption/Ionization Mass Spectroscopy (MALDI-TOF-MS) was performed on an AXIMA-CFR＋MALDI-TOF mass spectrometer (Shimadzu Co.). The morphological characteristics of the SFS-porphyrin materials were observed using a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) operated at 30 kV and equipped with an EDX detection column. In the case of solid silica particles, samples were coated with a few nanometers of platinum prior to each observation. The


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average particle size (D) and coefficient of variation (CV) values were derived from the SEM images by counting 100 particles. Solid UV-vis absorption spectra in the spectral range 350–800 nm were recorded using a JASCO V-7200 spectrophotometer. Fourier Transform Infrared (FTIR) spectra of powdered samples were measured from KBr pellets on a Thermo Scientific Nicolet 4700 FTIR instrument with nitrogen purge. ζ-Potential measurements of SFS4M suspensions in water were made using a DelsaNano C particle analyzer (Beckman Coulter, USA). The Smoluchowski equation was used to convert the electrophoretic mobility to the ζpotential. X-ray photoelectron spectroscopy (XPS) was carried out on a Theta Probe spectrometer (ThermoElectron Co., Germany) using monochromated Al Ka radiation (photon energy 15 keV, maximum energy resolution #0.47 eV, maximum space resolution #15 mm). 2.8. Fabrication and Measurement of Gas Sensor. The gas-sensing property measurements of the silica-porphyrin hybrids were performed using a nanomechanical Membrane-type Surface stress Sensor (MSS). The construction of the sensor and its working principle have been previously reported.37 Coating of the sensor membrane was carried out using an inkjet spotting unit. This instrument selectively spots only onto the upper surface of the membrane, using a dispensing system. In this study, we have used MSS chips with four channels consisting of round-shaped membranes with a diameter of 300 µm and thickness of 2.5 µm. The working membranes were coated with receptor layers: SFS, SFS4, SFS4Co, SFS4Ni, SFS4Cu, SFS4Zn and porphyrin only 4, 4Ni, 4Cu, 4Zn. The injection parameters were adjusted to maintain identical droplet volumes in order to deposit similar amounts of each material. The concentration of SFS-porphyrin in DMF solution was 1 mg/mL. The receptor layers of SFS-porphyrin hybrids formed a rough, thick and mostly uniform coating. The MSS chip was settled into a Teflon chamber and placed in an incubator with controlled


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temperature. The chamber was connected to a gas system consisting of two mass flow controllers (MFC), a target gas cylinder, and a purge gas line. The specimens were exposed to 50 ppm of acetone or nitric oxide in pure nitrogen carrier gas with a flow rate of 30 mL/min. Since oxygen and nitrogen are both rather inert at room temperature with nonpolar characteristics, it was found that the difference in these carrier gases does not have a large influence relative to the signal observed for acetone vapor. Three cycles of 10-min target gas injection and 30-min nitrogen purging were executed. Measurements were conducted at a temperature of 37 °C. Sensitivity was calculated based on signal-to-noise ratio. The presence of interfering humidity in the system was checked for by evaluating the base humidity level in the gas flow line using a cooled mirror hygrometer. The observed dew point indicates that contamination of humidity is at a much lower level than the concentration of the target gases. In order to avoid the effects of H2O vapor on response signals, we used high grade gases for this investigation which guarantees that the dew point of the H2O contamination is below -80℃ corresponding to a water concentration of

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