Electroanalysis with Membrane Electrodes and LiquidâLiquid Interfaces

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Review

Electroanalysis with Membrane Electrodes and Liquid-Liquid Interfaces Eric Bakker Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04034 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 13, 2015

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Analytical Chemistry

Electroanalysis with Membrane Electrodes and Liquid-Liquid Interfaces Eric Bakker Department of Inorganic and Analytical Chemistry, University of Geneva, 1211 Geneva, Switzerland

ACS Paragon Plus Environment


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Table of Contents Table of Contents............................................................................................. 2 Introduction ...................................................................................................... 4 Potentiometry ................................................................................................... 6 Books and Reviews ...................................................................................... 6 Reference Electrodes ................................................................................... 7 Solid Contact ISEs ....................................................................................... 9 Solid Contacts Based on PEDOT–PSS .................................................... 9 Solid Contacts Based on Other Conducting Polymers ............................ 11 Solid Contacts Based on Molecular Redox Couples ............................... 12 Solid Contacts Based on other Materials ................................................ 13 Mechanistic Studies ................................................................................... 14 Analytical Applications and Methodologies ................................................ 16 Methodologies for Membrane Electrodes ............................................... 17 New Ion-Selective Readout Principles .................................................... 18 Galvanostatic Pre-Polarization................................................................ 19 Membrane Electrodes Based on Fluorous Phases ................................. 19 New Applications for Ion-Exchanger Based Membrane Electrodes ........ 20 Ion-Selective Nanopipettes and Microelectrodes .................................... 21 Paper-Based and Miniaturized Ion-Selective Electrodes ........................ 22 Ion-Selective Field Effect Transistors ..................................................... 24 Dynamic Electrochemistry ............................................................................. 25 Pulstrodes .................................................................................................. 25 Thin Layer Samples ................................................................................... 28 Thin Layer Sensing Films ........................................................................... 30 Nanoscale Ion Transfer Voltammetry ......................................................... 33 Sensor Materials ............................................................................................ 36 Imprinted Polymer Materials ....................................................................... 36 Chemically Modified Polymers ................................................................... 37 Ionic Liquid Based Membranes .................................................................. 37 Other Polymers and Additives for Ion Sensing ........................................... 38 Ion-Exchange Nanopore Membranes ......................................................... 39


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Modified Electrodes .................................................................................... 40 Ionophores ..................................................................................................... 41 Anion-selective Ionophores ........................................................................ 41 Cation-Selective Ionophores ...................................................................... 43 Ionophores for Hydrogen Ions, Alkali Metals and Ammonium ................ 43 Ionophores for Transition Metals ............................................................ 45 Recognition of Lipophilic Ions ................................................................. 47 About the Author ............................................................................................ 48 Acknowledgments .......................................................................................... 49 Literature Cited .............................................................................................. 49


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Introduction This review describes, with 200 references, progress with ion-selective electrodes, membrane electrodes, and ion transfer voltammetry in the period between January 2013 and September 2015. The selection of papers was done at the author’s discretion and should therefore be understood as an opinion piece and a current snapshot of the activity in the field. Contributions were sourced from modern search engines, primarily SciFinder, searching for relevant keywords and known author names. Some journal tables of contents were also screened manually. Please accept my apology if a reader feels that his or her important contribution has been left out. Perhaps I simply did not see it. Anyone who has attended recent conferences in the field, such as Matrafured 2014, Heyrovsky Discussions 2015 or dedicated sessions at Pittcon, may agree that the field is full of optimism and growth, and has dramatically broadened in scope in recent years. Researchers traditionally focused on potentiometry have fully embraced dynamic electrochemistry approaches to characterize their systems and to develop new attractive methodologies. The quality and the playfulness of the research have likely never been higher. The field has also developed a very strong materials focus that is driving new innovation in the field. Solid contact ion-selective electrodes are a very active field of research and researchers thrive to come up with spray coatable, inkjet printable stable and reliable materials. Research on reference electrodes is in a wonderful phase of rebirth, and truly innovative and fundamentally sound concepts are being put forward that may eventually retire the venerable but little loved liquid junction based elements. Dynamic electrochemistry drives the field to ultra-thin sensing films and thin sample layers for exhaustive and absolute measurement protocols and ultra-sensitive and truly multianalyte detection capabilities. The field is also boldly multidisciplinary, incorporating or interacting with bioassays, corrosion monitoring, environmental monitoring, clinical diagnostics, nanotechnology and engineering. In comparison with this progress, current efforts on the synthesis of new chemically selective receptors (ionophores) have been relatively muted. To


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make matters worse, a number of researchers still put forward ionophore molecules that are not sufficiently lipophilic, fabricate membranes with inadequate ion-exchanger salts, and follow experimental protocols that are no longer at the current state of the art. While the author understands the financial constraints of some research environments, he finds this unfortunate as quality research in this area is of great importance to keep driving the field forward. So please put long alkyl chains on your molecules or covalently attach them onto the polymer backbone, and use modern, meaningful characterization protocols. This is also a pledge to the plethora of organic chemists who struggle to make water soluble chemosensors to invest in this field, since ionophores are lipophilic. As this review focuses on electrochemistry, it does unfortunately not cover advances in the very fruitful field of optical ion sensors. The only exceptions include a selection of papers where phase boundary potentials are the direct focus of the optical readout. The author has aimed to give context to the cited works and to provide some judgment on the science. A reader should be aware that these types of comments represent the current opinion of the author, and that he may very well be wrong. Keep in mind that progress in science always depends on healthy discourse, and the reader should see these comments in that light. This review is loosely organized by grouping papers in methodology categories (potentiometry and dynamic electrochemistry), followed by sensor materials and ionophores. The author finds it easier to group manuscripts based on the underlying working principle, rather than by application. This means, however, that some manuscripts dealing with a similar target ion or application (such as the detection of polyions) are located in different parts of the review.


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Potentiometry Books and Reviews Lindner and Pendley wrote a tutorial text for practitioners that however incorporates the current state of the art and includes an important section on statistical data treatment and comparison of different methodologies.

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Mikhelson published a book on ion-selective electrodes, which incorporates the key modern advances but remains true to the breadth of the field.

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Frost

and Meyerhoff described the challenges and promises of potentiometric and biosensors for direct in vivo detection in the blood stream. 3 The authors cover progress in the research literature with catheter type probes as well as commercially available systems, but appear to advocate the use of ex vivo sensing systems because they can be more easily calibrated. Bochenska and co-workers wrote a review on ionophores for Pb(II), giving an overview of progress of the past 15 to 20 years, with 81 references.

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It gives a valuable

overview of environmental requirements, speciation, as well as required and observed detection limits and selectivities, but notes that many contributions are severely detached from modern approaches in potentiometry. Ortuño published a book chapter on ionic liquids for ion-selective electrodes.

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They

considered their use as membrane matrix, ion-exchanger or a combination of both functions. Bakker started to write an ebook series on electroanalysis that includes the topic of electrochemical ion sensors. The first chapter is downloadable from the ibooks store (compatible with Apple devices) and introduces

electrochemical

potentials,

extraction

principles, ion activities and liquid junction potentials.

and 5

mass

transport

A solutions manual

accompanies the text. This author also contributed a book chapter on potentiometry in a text on environmental electrochemical sensors that also includes a section of the measurement of redox potential but then focuses mainly on ionophore-based ion-selective electrodes. 6


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Reference Electrodes After a relative lull of many decades, research on new types of reference electrodes has picked up rather dramatically in pace in recent years, partly driven by research directed to autonomous, mass fabricated chemical sensors that can no longer rely on traditional liquid junctions based reference elements. A dedicated handbook on the topic of reference electrodes appeared, with some 15 contributions by recognized authors, and edited by Inzelt, Lewenstam, and Scholz. 7 A new type of reference electrode was proposed by the group of Buhlmann, which works on the basis of an electrochemically triggered ion perturbation at the surface of an ion-selective membrane (see Fig. 1A).

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Specifically, a

polymeric ion-exchanger membrane containing tetrabutylammonium ions is galvanostatically pulsed to impose a cation flux of this ion, which keeps this concentration, and therefore the membrane potential, reasonably constant at a fixed measurement time. The methodology was validated in serum samples. The major advance in novel reference electrode technology is likely attributable to the group of Kakiuchi on their work on ionic liquid membranes (see Fig. 1B; for a mini review describing progress in the last 10 years, see 9). The reference element works on the basis of a partial self dissolution of the ionic liquid, the concentration of which is then potential determining. More recent work has mainly focused on showing that this type of reference element allows one to obtain activity coefficients of high accuracy in hydrobromic acid

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and sulfuric acid solutions.

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Buhlmann’s group has

combined this principle with a redox buffer at the inner membrane side, based on 1,10-phenanthroline complexes of a Co(II)/Co(III) couple, to bring these reference elements into the solid state.

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A separate paper by Stein and co-

workers reported on the underlying materials aspects of combining ionic liquids and three dimensionally ordered carbon materials as capacitive transducers.

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While the approach is promising, water uptake into the

membrane during initial exposure to aqueous solution gave rise to important potential drifts that could be minimized by preconditioning or exposure to humid air. This type of chemistry was subsequently used to realize a paperbased all solid state potentiometric sensor with very promising results. 14


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Diamond and co-workers also used an ionic liquid reference electrode approach,

entrapping

the

material

in

poly(butyl-co-decylmethacrylate)

membrane and using a backside contact of a conducting polymer, either PEDOT or POT.

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Relatively important potential drifts were noted, however,

which were attributed to spontaneous charging and discharging reactions at the conducting polymer. Doroodmand and co-workers appear to have obtained a membrane material that is remarkably insensitive to electrolyte concentration changes.

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While

the mechanism is unfortunately not clearly explained, the affinity of the membrane to calcium (through the incorporated casein) and chloride ions (supposedly by adsorption) appears to result in a membrane that undergoes Donnan failure. The authors report a ca. 5 min equilibration time with calcium solution, thereafter giving a ±5 mV potential variation upon exposure with different electrolytes. This is unfortunately not sufficiently precise for the practical demands of direct potentiometry. Buhlmann’s group studied liquid junction materials bearing ion-exchange groups, particularly Vicor.

17

They showed that they can give very large errors

up to 150 mV owing to the permselective properties of such junction materials in contact with dilute electrolyte solutions and are therefore not recommended for general use in electrochemistry. Sokalski and co-workers described a solid composite reference electrode containing an Ag/AgCl element and dried KCl.

18

While the underlying

electrochemistry is analogous to that of classical liquid junction based systems, it requires less maintenance. The authors quantified KCl leakage from this electrode and found it be much smaller than more classical systems, while tests showed no appreciable emf deviations from expected values. The same authors subsequently extended this approach to a system that can be fabricated by injection molding.

19

Shitanda et al. used a conceptually similar

approach to realize a paper-based reference electrode.

20

The KCl was

entrapped into a paper layer, giving a lifetime of the electrode of about 75 h. Andrade and co-workers employed a similar entrapment principle, but using polyvinyl butyral as matrix.

21

It appears somewhat curious that the authors

used NaCl as entrapment salt, yet require subsequent conditioning of the electrode in KCl. The focus of Merkoçi’s group was also on the Ag/AgCl


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element, showing that ink jet printed silver elements can be very simply and conveniently treated with bleach solution to chloridize them. 22 Martz and co-workers developed a pH probe for direct seawater analysis with a pH sensitive field effect transistor measured directly against a chloride ionselective electrode, thereby avoiding the need for a traditional reference electrode.

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This study used commercially available probes, so the key

innovation was their critical evaluation in the intended matrix. The bromide levels of seawater were found to give some interference, and a prolonged preconditioning step (2 weeks) was used to minimize this contribution. The realization of a fundamentally sound reference electrode concept marks a very important advance in the field of electrochemical sensor science that should have a wide ranging impact. Other scientists have adopted alternative methodologies to avoid the use of a high precision reference element altogether, see the section on dynamic electrochemistry below.

Solid Contact ISEs The development and characterization of ion-selective electrodes based on solid ion to electron transducers for the realization of all solid state potentiometric sensors has continued at a significant pace. The overall trend today is towards the incorporation of a material providing a high charge transfer capacitance (see Fig. 2A), thereby inducing only a small potential change upon any faradaic perturbation. Solid Contacts Based on PEDOT–PSS Lindner’s group took a careful look at the conditioning times required to stabilize the potential at such buried interfaces,

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using PEDOT–PSS on a

range of conductive substrates. Curiously, platinum substrates fared significantly worse in terms of conditioning times compared to gold or glassy carbon, with potential drifts of many hours. The results are explained by the need for a hydration step of the polymeric layer, so it is possible that more hydrophobic materials will not show these types of challenges. In view of introducing printable solid contact transducing layers, Bobacka and co-workers reported on the deposition of PEDOT–PSS, with ethylene glycol


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and quaternary ammonium salts as adhesive additives.

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The best

formulations exhibited less than 0.02 mV potential drift per hour. Unfortunately, the detection limits for the electrodes selective for Pb(II) using this approach were rather modest (>0.1 µM). The group of Bochenska subsequently reported on a multisensor array on the basis the same conducting polymer (PEDOT–PSS), which was drop cast as a dispersion in water.

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The focus here was on the application to the measurement of the

electrolytes of sweat. The multisensor was based on numerous encapsulated carbon rods coated with the appropriate membranes, while a pseudo-solidstate reference electrode based on PVA and KCl completed the cells. The groups of Lindfors and Höfler provided a study correlating hydrophobicity of the transduction layer with potentiometric response of the resulting electrodes, using electrosynthesized polyazulene as a polymeric material with a higher hydrophobicity than PEDOT–PSS.

27

The hydrophobization was achieved by

an electrochemical pretreatment at cathodic potentials. Unfortunately, it was difficult to distinguish between water uptake and materials hydrophobicity as evidenced by FTIR–ATR spectroscopy. In an unconventional approach, the electrode intercepts of different PEDOT–PSS based solid contact ISEs were found to approach each other upon short circuiting them one another.

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The

authors argued that this step may equalize the oxidation state of the conducting polymer, hence resulting in a much better matched electrode intercept. While this short circuiting protocol took up to three days, the variation of the E0 values was reduced to a few mV. These values were found to slowly drift back after use. This type of protocol may be most attractive for single use sensors that should work correctly right out of the box. The same authors also explored an electrochemical treatment to adjust the standard potential of solid-state ion-selective electrodes.

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When a potential increment

of 100 mV relative to open circuit was applied for 24 h, the electrode intercept was found to shift by approximately this amount. Longer potential treatments gave smaller deviations of the resulting standard potential from the applied value, but any additional gain was too small to be practical.


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Solid Contacts Based on Other Conducting Polymers Polyoctylthiophene (POT) continues to be used as inner transduction layer with good results, see in particular the result of Chumbimuni-Torres on paper based solid state ISEs with low detection limits discussed further below.

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Taryba and Lamaka fabricated miniaturized all-solid-state pH electrodes using a Pt/Ir tip, a layer of POT, and a self plasticized methacrylic co-polymer doped with a commercial hydrogen ion ionophore.

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While this detailed study

described a range of important analytical characteristics, the aim of the work was to develop a probe for corrosion monitoring. Scanning potentiometric microscopy in conjunction with scanning probe microscopy for localized current density monitoring was used to map pH changes and current at a cut edge of a coated steel sheet, with good results. The group of Qin found that 10% POT can be freely dissolved in the membrane to fabricate ion-selective electrodes for silver ions, with attractive detection limits.

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The key drawback of this material is the known sensitivity

to light. The old puzzle of why drop cast, electrically neutral POT layers can give such excellent behavior as intermediate transducing layers in potentiometric was unravelled by the groups of De Marco and Bakker. 33 They showed by a range of surface sensitive techniques that the outermost layer of POT is easily oxidized with tetraphenylborate doped ion-selective membranes while the buried POT remains electrically neutral. This appears to give a chemically stable redox buffer, similar to the soluble redox couples discussed above, while providing a lipophilic transducing layer. More recent work by Bakker on anion-selective electrodes indeed showed that a POT underlayer does not translate into potential stability, seemingly because the any surface oxidation of the POT layer must now be accompanied by the extraction of a hydrophilic anion into the membrane, which is energetically no longer favored. 34

In the same paper, the group subsequently advocated the use of a purely

capacitive, lipophilic transducing layer based on modified multiwalled carbon nanotubes to fabricate a wide range of all solid-state anion-selective electrodes. Ying Ye’s group used electropolymerized polyaniline as inner transducing layer of ammonium-selective electrodes.

35

The authors emphasize that the


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preparation of the polyaniline films is best performed by cyclic voltammetry, giving excellent adhesion. This simple architecture was compared with one containing 2,5-dimethoxyaniline and aniline (drop cast from THF) within the polyaniline pores. This choice was not sufficiently motivated, but appeared to improve the resulting potentiometric stabilities to some extent. Polyaniline was also used by Mohamed K. Abd El-Rahman et al., but in nanoparticulate form that was drop cast onto the glassy carbon electrode before coating with ionselective membrane.

36

While the general approach was originally introduced

by Lindfors, the electrodes were here doped with a calix[8]arene to detect a cationic drug molecule, distigmine. The so-called water layer test showed no measurable drift, and the authors find an excellent inter-electrode potential variability of less than 1 mV. Solid Contacts Based on Molecular Redox Couples This principle is shown in Figure 2B. The Buhlmann group doped the overlaying ion-selective membrane with a Co(II)/Co(III) complex acting as redox buffer, realizing ion-selective electrodes with excellent inter-electrode potential reproducibilities,

37

although the system was not usable with

ionophore-based membranes because spontaneous exchange with cationic analytes was observed. The authors therefore introduced more a lipophilic redox buffer based on cobalt complexes that were compatible with ionophorebased membranes. 38 The authors found that they are sufficiently reproducible (ca. 0.7 mV) to realize calibration-free ion-selective electrodes. The same group subsequently built on its earlier work on high surface area ordered carbon nanostructures as purely capacitive transducers by reporting on colloid imprinted mesoporous carbon materials that also incorporated the same type of cobalt complex based redox cople.

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The incoporation of the redox couple

reduced the inter-electrode potential variability about 10-fold to less than 1 mV. Paczosa-Bator et al. described transducing layers based on either the pure form

or

the

salt

of

the

radical

form

of

TCNQ

(7,7,8,8-

40

Since this

tetracyanoquinodimethane) as intermediate transducing layer.

compound is lipophilic, it forms a water resistive layer. While the results are convincing, it remains unclear why the authors have not explored a redox


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buffer composition consisting of TCNQ and its radical ion similar to the cobalt redox couple discussed above. Characterizing ferrocene tagged PVC as ion to electron transducing layer, the groups of De Marco and Bakker found by Cyclic voltammetry (CV), synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption fine structure (NEXAFS) that electrochemical polarization results in a dramatic accumulation of iron at the underlying electrode surface.

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Subsequent studies confirmed this behavior and

suggested an irreversible reaction of the ferrocene species, which however does not appear to inhibit electrochemical kinetics. 42 The results suggest that ferrocene should only cautiously be used as ion to electron transducing material. Solid Contacts Based on other Materials Wei Qin and co-workers also proposed to eliminate conducting polymers or high surface area carbon materials by simply using a nanoporous gold film as intermediate layer between the membrane and the underlying gold electrode. 43

While the analytical characteristics are promising, the approach is mainly

attractive for its convenience, since the nanoporous layer can be formed rather simply by electrochemical control. Gold (nanoparticle) supports were also used by Michalska’s group in the fabrication of Cu(II)-selective electrodes.

44

Here, the gold surface was

modified with dithizone,which exhibits a chemical affinity for copper. Different chemical pretreatments (mainly with Zn(II) or Cu(II) salts) resulted in a change of the potentiometric behavior at low concentrations, showing that this layer is chemically reactive and can influence the resulting transmembrane ion fluxes for an optimization of the lower detection limit down to about 10 nM. Paczosa-Bator et al. proposed the use of carbon black on a platinum nanoparticle film as capactivive intermediate layer in ion-selective electrodes. 45

The layer was formed by drop casting. The membranes were carefully

characterized and electrodes for the detection of nitrate and potassium were described as model systems. Of particular note were the calibration curves after one 1 h and one month exposure, which are perfectly superimposed on each other. A conceptually similar approach was taken by the group of Wei


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Qin, who introduced carbon nanomaterials dispersed in an ionic liquid and using a lipophilic salt as binder as a intermediate capacitive transducing layer. 46

In contrast to most other work in this area, this group demonstrated

nanomolar detection limits for potentiometric sensors responsive to Pb(II) and Cu(II). Wei Qin’s group showed that the use of a capacitive carbon material based on porous, crystalline C60 could be used for the fabrication of electrodes for the potentiometric detection of Pb(II) with excellent detection limits.

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The porous

C60 layer was deposited under electrochemical control (constant applied potential of 5 V). Despite the convincing and attractive results, the potential drifts within a calibration run appeared to be remarkably high, even at high analyte concentration. Khun et al. showed that the ionophore tetrathia-12-crown-4 can be used to 48

decorate nanostructured CuO for the potentiometric detection of Cd(II).

While the response slope is found to be nernstian to 1 nM, the observed selectivity coefficients are rather poor, which may point to a bias in the data. The mechanism of response, whether by electron or ion transfer, has unfortunately not been elucidated in this work.

Mechanistic Studies It is well known that selectivity coefficients of highly selective polymeric ionselective membrane electrodes are difficult to quantify without experimental caution, such as conditioning the membrane with a discriminated ion prior to the measurement. The group of Egorov has put forward an elegant approach to obtain unbiased selectivity coefficients with ready to use electrodes, which could be useful for practically minded scientists who want to properly characterize any given ISE prior to use (see Fig. 3).

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The time dependent

potential change in a solution containing only the interfering ion of interest is monitored over time and analyzed with diffusion theory, giving a time dependent potential change that can be easily extrapolated to the unbiased selectivity coefficients. This elegant approach was further evaluated and confirmed by Bakker with numerical simulation, where the protocol was further tested for membranes exhibiting strong inward and outward fluxes as well.


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For inward ion fluxes, a modification of the protocol by extrapolating to logarithmic selectivity coefficients was proposed for best results. Egorov’s group then evaluated the influence of the membrane composition on the diffusion parameter q (called by other authors the permeability ratio), which has direct bearing on the determination of selectivity coefficients by extrapolation.

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Here, the parameter was obtained by a formula that

compares the unbiased selectivity coefficient with the experimentally observed one, along with the membrane composition. As the diffusion layer in the membrane expands with time, so does the diffusion parameter. This is a very detailed paper with rich experimental data. Jasielec et al. described a theoretical model based on the Nernst Planck Poisson model that is more complete than previous approaches, including ones the phase boundary potential model.

52

In this particular case, time

dependent complexation/decomplexation reactions are also taken into account so that systems not at interfacial equilibrium can be considered. The behavior of a calcium-selective electrode was used as practical example with good results, but the work would have been even stronger with independent evidence of the reaction rate constants. In particular, it is not clear why the phase boundary potential model should not give equally adequate results for situations where chemical kinetics are slow. In a conceptually similar approach, Osakai and co-workers further developed a mixed potential method, which borrows its concepts from ion transfer voltammetry but assumes zero current conditions.

53

In principle, this model

allows one to consider cases that deviate from local equilibrium at the sample–membrane interface, and therefore could be an alternative to the phase boundary potential model. Unfortunately, the authors do not appear to have chosen a system where this distinction was considered, which appears to be unfortunate. Lindfors and co-workers quantified the water uptake of polymeric ion-selective membranes by the Karl Fischer technique as well as with in situ ATR–FTIR. 55 The authors compared the water uptake of two different membrane matrices (SR and PVC). Calcium-selective PVC based membranes exhibited a significant water uptake in contact with deionized water (0.16 wt%), while a higher calcium electrolyte concentration resulted in a lower water loading.


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These types of studies are valuable for the realization of stable solid contact ion-selective electrodes and for mechanistic purposes. ATR–FTIR was also employed by Bakker’s group to monitor ligand exchange reactions during initial contact of ion-selective membrane with aqueous electrolyte.

54

This was

performed with a nitrite-selective Co(III)salophene ionophore that was initially in the acetate form. De Marco and his group found that the resistivities of self plasticized ionselective membranes made from the co-polymer methylmethacrylate– decylmethacrylate abruptly increase with decreasing film thickness down to molecular dimensions.

55

The authors used electrochemical impedance

spectroscopy and X-Ray reflectometry to find whether the effect is due to ballistic transportation or a quantum confinement effect. They came to the cautious conclusion that surface scattering of charge carriers may explain the findings.

Analytical Applications and Methodologies While the measurement of redox potential is sometimes thought to be a trivial experiment, kinetically hindered electron transfer kinetics at the electrode surface can introduce severe errors. The group of Mandler reported on a careful study where the reversibility of the potentiometric response was improved by the surface coating of platinum electrode substrates with Au and Pt nanoparticles (see Fig. 4).

56

While a solution containing different ratios of

hexacyanoferrate(II/III) always gave agreement with the Nernst equation, only appropriately modified electrodes showed good results for solutions containing l-dopa and catechol chosen as model systems. Bakker's group developed a potentiometric sensing array, including a self contained electrochemical multichannel module, concentrations in freshwater lakes.

58

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to measure electrolyte

The sensor array was mounted on a

floating platform and lake water was pumped at varying depths for analysis. The potentiometric data correlated adequately to concentrations obtained with benchtop methods.


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Methodologies for Membrane Electrodes Xie et al. showed for the first time that a pH electrode measured against an ionophore-based carbonate selective electrode based on a tweezer type carbonate ionophore containing trifluoroacetyl groups gives a direct potentiometric response to PCO2 (see Fig. 5).

59

The key advantage over

Severinghaus probes is the lack of a gas permeable membrane, giving dramatically faster response times. Moreover, other permeating neutral species such as H2S were found not to interfere with this sensing principle. The groups of Granado-Focil, McGraw and Radu showed that the detection limit of solid state and self plasticized carbonate-selective electrodes based on diamide N,N1-bis(2,4-dinitrophenyl)- isophthalohydrazide as ionophore can be dramatically improved by 4 orders of magnitude, by only doping the sample side of the membrane with ionophore from a THF–water solution.

60

The authors explain the improvement by the outer region of the membrane acting as a chemical sink for carbonate, thereby inhibiting outward ion fluxes. Lisak and co-workers put forward a multicalibration procedure to render potentiometric measurements at low concentrations more reliable.

61

The

approach was found to be especially attractive with time dependent potentiometric signals, as they are sometimes found near the detection limits. Amorim and co-workers described membranes containing a Zn-porphyrin, 5,10,15,20-tetraphenyl(porphyrinate) zinc(II), for the detection of SO2 in wine. 62

The electrode needed to be conditioned extensively with 2 M diethylamine

before use, and the mechanism was explained on the basis of a competitive ligand exchange between monoprotic diethylamine extracted together with hydroxide on one hand, and SO2 on the other. In this work, the SO2 concentration in wine was measured, with good correlation to values found with the Ripper method. The group of Ortuño used principal component analysis to evaluate the response of transient ion-selective membranes for a wide range of anions, which they coined differential dynamic potentiometry.

63

In this methodology,

the time dependent response of the electrodes was recorded and analyzed, even in mixtures of two anions. While this is a refreshing contribution, the


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Page 18 of 76

method will likely be difficult to implement in complex samples of unknown composition. New Ion-Selective Readout Principles Bobacka’s group reported on an interesting new way to read out ion-selective membranes, by taking advantage of an inner transducing layer acting as an ideal capacitor.

64

A constant potential experiment forces the capacitive layer

to react to any changes of the phase boundary potential at the membran– sample interface, thereby resulting in a transient current pulse whose charge acts as the analytical signal. The work was only preliminary and one would hope to see further studies to more fully characterize and develop this new methodology. Xie et al. introduced a manner to visualize the phase boundary potential by the use of ionic solvatochromic dyes.

65

The principle works with ion-selective

nanosphere emulsions that have the same basic compositions as ionselective membranes. Owing to the short diffusion distances in the emulsion, the ionic solvatochromic dye rapidly distributes between the aqueous and organic phases according to the Galvani potential, thereby allowing one to use fluorescence to read out ion concentration fluctuations. Also using ion optodes, the group of Mikhelson used a lipophilic electrolyte to impose a constant phase boundary potential across the sample–membrane interface to make ion optodes responsive to a single ion only.

66

The authors

demonstrate the response to only pH, but one may ask the question why the optode shows an ion response at all if the phase boundary potential does not change. Perhaps the lipophilic electrolyte functions in some analogy to the solvatochromic dyes described above: since its sample concentration is constant, it controls the ion-exchange (or co-extraction) equilibrium with hydrogen ions and therefore serves as reference ion. Erkang Wang’s group showed that cell potentials from ion-selective electrodes can be directly converted to a luminescent output by integrating a light emitting diode to indicate the potential change.

67

While this idea is not

chemically innovative, using off-the-shelf electronics, it does allow for the naked eye detection of ion activity changes with low cost instrumentation. The authors emphasize their difficulty of using miniature reference electrode


18

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element and preferred a differential arrangement using two ion-selective electrodes measured against each other. Galvanostatic Pre-Polarization Lewenstam and co-workers showed that a galvanostatic pre-polarization is useful to lower detection limits of precipitate membranes based on PbS/Ag2S for the detection of Pb(II).

68

The authors demonstrated that unprocessed

environmental samples could be measured at concentrations of 18 ppb, giving a quantitative correlation with ICP–MS. Galvanostatic prepolarization was also described by the group of Michalska on ISEs containing a capacitive intermediate

layer

consisting

of

carboxy-functionalized

graphene

or

multiwalled carbon nanotubes to fabricate disposable potentiometric sensors. 69

Potassium-selective electrodes served as model systems, exhibiting the

same selectivities as their aqueous inner solution counterparts. The authors showed that a mild galvanostatic pre-polarization for a 3-min period can give lower detection limits. A galvanostatic pretreatment was also proposed by Mikhelson’s group, but this time to increase the upper detection limit of ion-selective electrodes.

70

The applied current pulse has the purpose to conveniently alter the concentration gradients in the ion-selective membranes to modulate the upper detection limit during a zero-current period immediately after electrochemical perturbation. The authors emphasized that the current–potential behavior obeys Ohm’s law, but every sample composition requires an optimized current amplitude. This contribution would have benefited from an example that eventually satisfies a real-world need, as Cd(II) measurements at 1 M concentrations do not appear to be practical.

Membrane Electrodes Based on Fluorous Phases Buhlmann's group used the fluorous phase ion-selective electrodes developed in his group to study silver ion dissolution from nanoparticles.

71

The main argument in favor of this membrane material is that it should resist contamination from lipophilic species such as lipids. The electrodes were indeed successfully used to correlate dissolved silver concentration and


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Page 20 of 76

bacteria viability, demonstrating the technology for the assessment of toxicity. A related paper by the same group studied the affinity of silver ions to natural organic matter, again using fluorous phase based ion-selective electrodes.

72

While this work likely could have been equally performed with more traditionally membrane formulations, it is a nice example how potentiometry with ion-selective electrodes provides valuable speciation information in complex samples. Fluorous phase ion-selective membranes containing anion-exchangers do not exhibit a selectivity pattern that strictly follows the Hofmeister sequence, but favor fluorous ions relative to lipophilic or hydrophilic ones. This was exploited by the Buhlmann group to devise and characterize an electrode for the detection of anionic perfluorocarbon surfactants, which are important environmental pollutants.

73

Indeed, chloride ions are discriminated by 7 to 10

orders of magnitude, depending on the perfluorocarbon anion of interest. Detection limits were on the order of 100 nM.

New Applications for Ion-Exchanger Based Membrane Electrodes Wei Qin’s group introduced a biosensor principle based on the natural response of anion-exchanger based ion-selective membranes to oligomeric phenols.

74

The potentiometric bioassay for G-quadruplex/hemin DNAzyme

involved the biocatalytic conversion of the monomeric phenol, for which the membrane is not responsive, to the oligomeric form. Sensitivities were reported to be superior to colorimetric and fluorimetric methods. The same group found that anion-exchanger based ion-selective membranes respond to a range of boronic acids, see Fig. 6.

75

Supported by NMR evidence, the

authors argue that the boronic acid extracts as a neutral species into the membrane phase where it forms the hydrolyzed anionic form, thereby expelling HCl from the membrane and giving a potentiometric response to the extracted species. This approach was subsequently shown to be useful for the detection of fructose, on the basis of the reduced concentration of boronic acid upon binding with the sugar. Again using ion-exchangers, but this time for cations, the same authors make ion-selective electrodes responsive for oxidized 3,3′,5,5′-tetramethylbenzidine, which is an important marker in


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bioassays involving peroxidases.

76

This provides a simple yet powerful

alternative methodology for such assays. In another approach, the affinity of a DNA aptamer probe to Ag(I) was used in conjunction

with a

concentration.

77

silver ion-selective electrode to detect aptamer

The aptamer conformation that binds to Ag(I) changes in the

presence of ATP, and so this bioassay was demonstrated to detect ATP with attractive detection limits (0.37 µM). The same group also utilized the natural affinity of protamine with DNA aptamers in conjunction with a protamineselective electrode to detect the surface charge of aptamer modified magnetic beads involved in bioassays.

78

Using an endocrine disruptor as model

system, a detection limit of just 80 pM was achieved. In a very similar approach, the concept was used to detect Listeria monocytogenes via aptamer recognition, and protamine again serving as the indicator that could be detected potentiometrically.

79

Despite the promising results, it is not

entirely clear why the authors did not prefer to use a controlled delivery of protamine for these assays as they have done with other conceptually similar systems (see Pulstrodes section below). Ion-Selective Nanopipettes and Microelectrodes Egorov’s group fabricated chloride and sodium-selective microcapillary-based electrodes, filled with ion-selective cocktail and doped with a range of known ionophores, and applied them as scanning probes in corrosion studies.

80

Of

particular emphasis in this study was the resistance of the electrode responses to widely changing pH, as often observed in corrosion, along with excellent suppression of the metal ions that make up the corroding steels and alloys. The final system succeeded in meeting these demands and was successfully applied to the scanning potentiometric microscopy of Al–Zn alloys. Mauzeroll’s group also reported on a microelectrode for corrosion studies, but this time the goal was to detect Mg Alloy corrosion and an electrode selective for magnesium was developed.

81

The electrode was of

the filled micropipette type with a ca. 500 nm tip diameter, incorporating the well known ionophore ETH 7025, and exhibited a micromolar detection limit for Mg(II). The selectivity over calcium was surprisingly poor, but of little significance for the intended study. The authors found distinct steps in the


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Page 22 of 76

corrosion process, involving a fast initial release, a subsequent decrease due to formation of insoluble Mg(OH)2, and a later stage given by Mg2+ diffusion through the porous Mg(OH)2 layer. Yamada et al. fabricated potassium-selective nanopipettes for scanning electrochemical microscopy, with inner tip diameters of about 500 nm.

82

To

mechanically stabilize the aqueous/organic solvent interface, it was slightly recessed into the capillary by removing the outer hydrophobic layer of the pipette. The device was read out by a current response, but the analytical figures of merit were unfortunately not described in detail. All-solid-state pH electrodes were fabricated by Unwin’s group to achieve scanning probes for pH profiling in tandem with a scanning ion conductance probe.

83

The small size was achieved by the in situ carbon filling of a pulled

pipette barrel following by pyrolytic decomposition of butane before electrodepositing a thin layer of hydrous iridium oxide that served as the pH sensing element. The reference electrode was integrated into the second barrel of the dual pipette configuration. As with other electrodes of this type, a super-Nernstian response slope of ca. 80 mV was observed, and one tends to worry about matrix effects on the electrode slope. Paper-Based and Miniaturized Ion-Selective Electrodes The groups of Whitesides and Buhlmann reported on a fully integrated paperbased potentiometric sensor, using printed wax channels to guide the sample and to form the appropriate solution compartments.

84

In this early work, the

reference electrode is of a conventional Ag/AgCl based liquid junction time, where the time of diffusive mixing of sample and reference compartment are sufficiently slow in the time scale of the measurement. Paper based sensors for calcium, potassium and sodium were reported that exhibited measuring ranges compatible with the requirements for clinical diagnostics. Bobacka’s group also described on a paper-based potentiometric sensing system, using macro-scale indicator and reference electrodes pressed against a piece of porous paper used for sampling.

85

This first paper introduced the

concept with potassium as a model analyte ion and explored various filter papers and a range of shapes for optimal sampling and response behavior. Detection limits were found to be dramatically higher (3 orders of magnitude)


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than with solution-based experiments. In subsequent work, a range of electrodes selective for different ions were explored, including Cd(II) and pH, thereby evaluating the inertness of various paper substrates to the ion of interest.

86

Among the many samples evaluated, poorly buffered solutions

such as rainwater gave the largest deviations between solution and paper based measurements. Chumbimuni-Torres and her group presented the realization of paper-based ion-selective electrodes for the detection of Cd(II), Ag(I) and K(I) that exhibit detection limits down to ca. 10 nM.

30

A single walled carbon nanotube

suspension served to render filter paper conductive. A spot of gold was sputtered, which was coated with poly(octylthiophene) as ion to electron transducer before casting the membrane after applying a mask. Tuntulani and co-workers described a paper-based ion-selective electrode for Ag(I) that made use of conductive films of silver nanoparticles, which were screen printed.

87

This layer was then overcoated with a plasticized PVC

membrane containing benzothiazolyl calix[4]arene as ionophore. It remains unclear whether the approach is restricted to membranes selective to Ag(I) or has wider applicability, since such experiments were not reported. Michalska and co-workers used spray coating techniques to fabricate all solid state ion-selective electrodes.

88

Here, the carbon nanotube inner transducer

layer was spray coated first, followed by the application of the PVC based sensing layer and insulating layer by the same technique. The analytical figures of merit appear to be excellent, showing that this technique may be an attractive alternative to screen printing in the mass fabrication of such sensors. Lewenstam and co-workers described an attractive potentiometric multisensor probe of 1 cm diameter using a conventional inner solution design but making room for the reference electrode and indicator electrodes for the direct detection of pH, potassium, sodium, and chloride.

68

The system was

demonstrated in the measurement of ion transport across epithelial cells. The groups of Wang and Andrade described a potentiometric sensor for the analysis of ammonium in sweat that can be directly worn on the skin and is therefore coined tattoo sensor.

89

The main drawback of the approach


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Page 24 of 76

appears to be the reference electrode, which seems to be inherently sensitive to salt concentration. Ion-Selective Field Effect Transistors The group of List-Kratochvil reported on electrolyte gated organic field effect transistors that incorporated a state of the art sodium-selective membrane.

90

A poly(3-hexylthiophene) thin film transistor was contacted with an electrolyte and overcoated with an ion-selective membrane. This appears to alleviate the old problem of signal drift with traditional ISFETs that was caused by CO2 partitioning and localized pH change at the interface between the membrane and FET surface. Despite the conceptually convincing approach reported here, signal drifts were unfortunately still quite dramatically large. Sessolo and co-workers also described electrolyte gated organic field effect transistors, but using PEDOT–PSS as substrate coating and a gelified inner solution behind a potassium-selective membrane.

91

The sensitivities were promising and the

signal drifts appear well behaved, but the study did not focus on long term signal stability. Melzer et al. simplified this approach further and bonded the ion-selective membrane directly on the electrolyte gate, a platinum element, without any intermediate layer.

92

Unfortunately, the resulting ion-selective

OFETs for calcium and potassium exhibited selectivities that are not entirely convincing. The group of de Smet fabricated field effect transistors based on silicon nanowires that were coated with ion-selective siloprene membranes for potassium and sodium ions.

93

The devices compared favorably with

traditional ion-selective membranes of the same composition, with the sodium FET giving an even higher sensitivity. The authors emphasized the importance of keeping the boundary layer potential between the membrane and the underlying substrate constant. Myers et al. fabricated nitrate-selective ISFETs by using an AlGaN/GaN heterostructure overcoated with plasticized PVC membrane containing an anion-exchanger.

94

While the device appears to not require a traditional

reference electrode, signal drifts and day to day reproducibilities appear to be much worse than state of the art potentiometric sensor systems.


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Frisbie’s group implemented a floating gate into a high capacitance electrolyte gated FET to directly detect DNA based affinity reactions, which essentially results in a signal offset.

95

The principle was demonstrated by the detection

of DNA hybridization reactions, giving potential changes on the order of 100 mV. While background levels were shown to be low (ca. 10 mV) and single base pair mismatches gave smaller signals that depended on location, the detection of complex samples was unfortunately not attempted.

Dynamic Electrochemistry Crespo et al. described a range of emerging protocols for the dynamic electrochemistry interrogation of ion-selective membranes.

96

The authors

distinguish the ideally polarizable membranes used more traditionally in ion transfer

voltammetry

from

initially

non-polarized

ones

that

exhibit

permselective properties. The review describes stripping voltammetry, chronopotentiometry (pulstrodes) and thin layer coulometry as readout protocols. Writing to a more general audience, Bakker wrote a shorter review detailing these recent advances, emphasizing how these protocols may result in chemical sensors and electroanalytical tools with much improved characteristics. 97

Pulstrodes Ion-selective membranes controlled by a constant current period, so-called pulstrodes, combine the instrumental control of ion transfer voltammetry while giving the possibility for a readout in analogy to zero current potentiometry. Alternatively, one may observe a potential change (inflection point) at a transition time, which is analyzed by the Sand equation to give information on the concentration. These two readout options are illustrated in Fig. 7. Fundamentally, pulstrodes provide a control of the ion flux at the sample– membrane interface. As already mentioned above, Buhlmann's group used


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Page 26 of 76

this principle to induce a defined flux of a lipophilic cation, thereby forming a new type of reference electrodes. 8 Yawar Abbas et al. revisited chronopotentiometry at a Ag/AgCl element for the detection of chloride, and emphasized that this technique does not require a well defined reference electrode potential because the signal depends only on the transition time. 98 This work was ultimately targeted to the monitoring of chloride in concrete. Bakker's group developed chronopotentiometric sensors based on a ferrocene ion to electron transducers that are either covalently attached to the polymeric backbone of the membrane or freely dissolved. 99 Maximum current densities given by the turnover rate of the redox species were characterized and the chronopotentiometric detection of common anions was reported. While the potential reading gave information on the lipophilicity of the detected species, the calibration curves from the observed transition times and the established Sand equation reflected the mobility of the anionic species and were very similar. The investigators showed later with the same type of inner transducing

element

that

chronopotentiometrically.

100

carbonate

may

also

be

detected

The membranes were formulated with the

tweezer-type trifluoroacetyl modified ionophore based on a steroid backbone originally reported by Nam and were shown to provide valuable speciation information on total carbonate (chronopotentiometry) vs. free carbonate (potentiometry) in the course of a volumetric titration. Unfortunately, carbonate could not be reliably detected at environmental pH. More traditional aqueous inner solutions were used to show that pH responsive membranes operated by chronopotentiometry can give direct information on total acidity and alkalinity. 101 Solutions containing a wide range of bases were characterized by this technique and were shown to give direct information on total alkalinity. The key drawback of the technique is the dependence of the calibration slope (Sand equation) on the diffusion coefficient of the species, and coulometric methods appear to be more attractive in that regard. If the response of pulstrodes is based on a localized depletion at a transition time, measurement cells can be fabricated that no longer require a traditional reference electrode. Kofler et al. took advantage of this strategy with a two-


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electrode paper based sensing configuration, using two solid contact ionselective membranes with PEDOT–PSS as intermediate layer.

102

In analogy

to previous pulstrodes approaches, a 20-fold sensitivity enhancement relative to zero current potentiometry and excellent reproducibilities and lifetimes were observed. The group of Meyerhoff showed that pulstrodes selective for polyionic species can operate successfully in a flow injection analysis mode.

103

This work did

not use transition times, and the direct potential readout gave sigmoidal calibration curves. A follow-up work from the same group showed that such electrodes can serve as useful detectors after separation of polyion fractions in ion chromatography.

104

The resulting potential peaks were integrated, and

while the reason for doing so was not well motivated, the resulting calibration curves for one chosen polycation was found to be reasonably linear. These results show clearly that pulstrodes methodology is reproducible and does not suffer from memory effects, which is in clear contrast to polyion-selective electrodes based on zero current potentiometry. Wei Qin and co-workers used controlled current pulses to deliver polycationic protamine into the sample, where it was allowed to react with anticoagulant heparin for a pseudo-reagentless detection of heparin.

105

This is a very

elegant approach and the authors came very far in demonstrating the detection of heparin in sheep blood. Unfortunately, the authors did not explain clearly why one should expect a large protamine response at a protamine releasing membrane, since the high polycation charge should give small sensitivities under such conditions. The unusual sign of the response peaks in contact with whole blood also points to challenges related to the high protamine level in the vicinity of the membrane. In an expansion of this approach, protamine was electrochemically released in order to act as a signal reporter in the detection of E. coli.

106

The bioassay involves a DNA

aptamer selective for the E. coli, but which binds to protamine in the absence of target. The assay was integrated into a flow injection system using 1 µM concentrations of DNA aptamer, while non-specific targets did not show a response. Wei Qin's group developed this elegant concept where a current pulse is used to deliver a known quantity of p-nitrophenyl phosphate across the membrane,


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where it is brought in contact with an enzyme labeled sandwich immunoassay confined to the membrane surface by covalent linkage to carboxylic functionalities of modified PVC.

107

As the ion-selective membrane is made

responsive to the delivered marker ion, an increasing abundance of bound enzyme label (proportional to the analyte concentration) results in a larger signal. While the calibration curves were not quite linear, the concept operates in an apparently label-free manner with full electrochemical control of label delivery and measurement.

Thin Layer Samples Sample layers that are thinner than the diffusion layer give the possibility for an exhaustive conversion of the sample by electrochemical control. The key promise of this methodology is the resulting robustness of the readout: if the reaction is selective and the sample volume constant, the coulometric signal should be independent of temperature and mass transport kinetics and may form a sound fundamental basis for re-calibration free sensing devices. This well established concept is relatively new in combination with ion transfer voltammetry, with the main early breakthrough coming from the group of Kihara. Continuing this work, Yoshida, Maeda and co-workers back side contacted these organic phases with a conducting polymer (PEDOT) to realize thin films for the exhaustive detection of ions.

108

They described the

fabrication and fundamental characteristics of the materials and showed how this thin organic sensing layer is combined with a thin layer sample for the coulometric flow injection analysis of tetraethylammonium ion as a model system. Unfortunately, the organic film was based on a doped porous PTFE membrane of 30 µm thickness. This is much thicker than in subsequent studies by the groups of Amemiya and Bakker and likely resulted in nonoptimal mass transport kinetics and recovery of the experiments. The group of Bakker focused on the use of ionophore-doped permselective membranes for coulometric ion transfer analysis. In one direction, the previously reported tubular cell was further miniaturized for the detection of calcium as a model example.

109

In contrast to established ion transfer

voltammetry, the membranes in this work contained ion-exchanger and


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exhibited permselective properties. Their role was to electrochemically shuttle the ion of interest from the thin layer sample to the outer solution. In subsequent work, the tubular membranes were doped with appropriate organic anion salts for the selective transfer of protamine, resulting in the coulometric detection of protamine.

110

While the bare Ag/AgCl element in

direct contact with undiluted human blood samples resulted in a decrease of sensitivity, the coulometric detection of protamine in blood could successfully be demonstrated. Thin layer coulometry was also used by the Bakker group to exhaustively plate halide ions from the sample to a silver element, resulting in the direct coulometric sensing of iodide, bromide and chloride in the same sample.

111

With traditional ion-selective electrodes based on silver halide salts, any sample ion giving less soluble salts with silver would normlally poison the electrode and result in erroneous signals. The sample and outer solution compartments were here separated by a cation permselective membrane for the removal of the counter ions of the plated halides. This work was subsequently expanded to a paper-based format, where porous paper of a defined thickness served to transport the sample and outer solution to the detection compartment and defined the sample layer thickness.

112

A

magnetic latching mechanism completed the cell. The absolute detection of chloride (600 mM) and bromide in unmodified seawater was demonstrated with this device. This same chemical principle was also used to effectively desalinate seawater samples, resulting in a 100-fold reduction of the NaCl concentration.

113

The system was used in conjunction with a potentiometric

nitrate-selective electrode placed downstream as initial example, allowing one to reach detection limits in the micromolar range. The methodology was also used to demonstrate the coulometric detection of nitrite, using a cobalt tert-butyl salophene ionophore in the membrane. 114 This work further reduced the non-zero intercept of the calibration curve by identifying the potential dependent charge as a capacitive contribution and subtracting its value from the observed charge to identify the faradaic contribution. Measurements in physiological samples were demonstrated, with detection limits on the order of 10 µM.


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Thin layer samples were also essential in the design of a potentiometric ion detector used in combination with an electrochemically actuated ion pump, as recently put forward by the Bakker group for the direct detection of alkalinity, see Fig. 8. 115 Here, two pH responsive membrane electrodes were spaced on opposite ends of a thin layer gap, using paper of defined thickness (removed for measurement) to delimit the gap. An electrochemical perturbation at the proton pump results in an acidification of the sample while the resulting pH change is detected by the pH probe. The methodology was demonstrated to give direct alkalinity information in environmental samples, at concentrations on the order of 20 mM, which is much higher than the upper detection limits observed with chronopotentiometry (see above). The key drawback is currently the relatively long time required to acquire a full titration curve, since the sample is re-equilibrated with the contacting solution between each perturbation step. The same group extended this approach to calcium, using a calcium ion pump together with a calcium-selective electrodes placed opposite the thin layer sample.

116

In an alternate configuration, the readout

was performed at the very same membrane in a tubular design, using a coulometric protocol. In the absence of calcium in the sample, the device was shown to quantitatively recover incrementally delivered concentrations at the detection electrode. The detection of EDTA in the sample was subsequently demonstrated with good precision.

Thin Layer Sensing Films A number of researchers aimed for a dramatic reduction of the sensing membrane thickness, to a micrometer or less. While this will certainly be accompanied by some loss of robustness compared to macro-scale membranes, there are some very attractive advantages. Thin layer membranes will equilibrate quickly, which is essential for ion transfer stripping voltammetry. During a linear potential sweep in contact with a reasonably concentrated solution, the sample–membrane interface may be assumed to re-establish equilibrium at every applied potential. This can be used to introduce voltammetric methodologies that give information previously only accessible by zero current potentiometry.


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The group of Amemiya used thin plasticized PVC films spin coated onto electrodes overcoated with lipophilic PEDOT intermediate layers for the stripping ion transfer voltammetric detection of a range of ions. A recent review described progress by this group,

117

and includes the description of

work on nanopipette based ion transfer voltammetry. A more recent example describes the stripping voltammetric detection of calcium, see Fig. 9.

118

In

this careful work, numerical simulations are presented, and varying preconcentration times of up to 1 h for 10 nM calcium concentration suggest no saturation of the observed stripping charge. Concentrations as low as 0.1 nM were detected, but required background subtraction. The authors emphasized the important risk of contamination for this ubiquitous ion. This work was extended to the stripping voltammetric detection of perfluoroalkyl sulfonates by membranes of the same architecture, but without ionophore.

119

Owing to the high membrane selectivity that resulted from the lipophilicity of the target ion in addition to the accumulation step that precedes the stripping voltammetric protocol, a detection limit as low as 50 pM was achieved after background correction. Unfortunately, the authors did not report on the measurement of this contaminant in actual environmental samples. The groups of Harris and Bond reported on their efforts to detection blood electrolytes by linear sweep voltammetry on relatively thin PVC or ionic liquid based ion-selective membranes using crystalline TTF–TCNQ as ion to electron transducing layer.

120

When an ionophore such as valinomycin was

present in the ion-selective membrane, the extraction of potassium into and from the membrane resulted in a peak shaped voltammetric wave whose peak position was a direct function of ion activity. The results suggest that the membranes used in this work do not exhibit ideal thin layer behavior. The use of a redox couple as reference couple such as ruthenium hexamine to avoid a traditional reference electrode is an elegant idea, but may be problematic because trivalent species will show rather dramatic changes in activity coefficients as a function of solution ionic strength. This substantial paper demonstrates measurements in a variety of samples, including whole blood. Crespo et al. found that the general principle described above forms the basis for the true multianalyte detection of ion activity.

121

This was demonstrated

with 230 nm thin plasticized PVC films spin coated onto gold or glassy carbon


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electrodes coated with an electropolymerized POT transduction layer. Undesired anion transfer was completely eliminated by avoiding the lipophilic salt from the membrane phase. With a molar excess of cation-exchanger relative to lithium and calcium ionophore simultaneously present in the membrane, two voltammetric waves were observed that gave a selective response to lithium and calcium. This marks the first time that a single membrane can give a selective response to multiple ion activities. In this mode, linear sweep voltammetry essentially becomes a potentiometric technique. Amemiya's group has used similarly formulated films, but backside contacted with lipophilic PEDOT, to assess ion–ionophore complex formation constants.

122

They found that a pronounced tailing of the cation transfer

waves is observed for ionophores forming complexes of higher stoichiometry. The authors also managed to find experimental conditions where the successive decomplexation (1:2 to 1:1 stoichiometry) of sodium–ionophore could be observed. The use of thin layer membranes for these types of studies appear to be much more elegant than previous potentiometric or dynamic electrochemistry techniques reported before. The same group used such membranes for distinguishing adsorption and extraction behavior of the polycationic analyte protamine.

123

The two processes appear to occur at very

well separated potentials and can be visualized by voltammetric techniques. The Lindner group used thin polymeric films to preferentially extract electrically neutral drug molecules that subsequently are oxidized with a two electrode cell in direct contact with the membrane.

124

This approach

effectively introduces an orthogonal selectivity, with the membrane providing separation selectivity and a preconcentration while the electrochemical readout again provides some discrimination power. This particular work integrated the measurement principle into a flow injection apparatus for the measurement of propofol.

124

Subsequent work demonstrated the direct

detection of this important anesthetic drug in undiluted whole blood.

125

The

exciting work demonstrated that the polymer coating protects the underlying electrode from fouling, with attractive detection limits of less than 1 µM. The paper equally demonstrates propofol detection with a catheter prototype for the closed loop detection and administration of propofol.


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Zazoua and co-workers used spin coated sensing films of unspecified thickness containing a calix[8]arene derivative for the impedimetric detection of Cd(II).

126

A linear relationship between the charge transfer resistance and

the logarithmic ion concentration was found, with detection limits reaching 100 nM. Unfortunately, this work did not report on any selectivity data and a detailed mechanistic discussion is largely missing. The group of Dryfe used relatively thin membranes doped with lipophilic electrolyte and contacted on both sides with aqueous electrolyte to control transmembrane ion transport by electrochemistry.

19

While cyclic voltammetry

on such double polarized interfaces has been described before, this group used it to control permeation of ionized species, using Rhodamine B as a model example since it could directly observed in the receiving phase by UVVis spectroscopy.

Nanoscale Ion Transfer Voltammetry Reducing the lateral scale of ion-selective membranes to the micrometer or submicrometer scale has always been extremely valuable for the localized detection of ionic species. In recent years, efforts have moved from potentiometric sensors to their dynamic electrochemistry counterparts. One approach is the use of nanopipettes filled with organic solvent or polymeric material and appropriate sensing ingredients. Bae Ho Park and co-workers modified the interrogation protocol of ion-selective nanopipette electrodes to use an AC current as readout.

127

While this methodology was unfortunately

not directly compared to potentiometry, low noise and good reproducibility were demonstrated for the detection of small ions such as potassium. The group of Mei Shen described the detection of the neurotransmitters acetylcholine, tryptamine, and serotonin at nanopipette based interfaces exhibiting diameters of just 7 to 25 nm.

128

While no biological applications

were reported and the detection limits are still rather modest, this direction may prove to be more broadly applicable relative to redox turnover based measurements. The group of Michael Mirkin put forward an interesting combination ion and electron transfer voltammetry with nanopipettes used in SECM, see Fig. 10.


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Page 34 of 76

Specifically, an electrically neutral substrate for the redox reaction is

delivered diffusionally from the nanopipette probe and undergoes a redox reaction at the imaged substrate material. The same nanopipette probe subsequently detects the ionic product of the reaction by ion transfer voltammetry. The concept was subsequently put in practice by the passive delivery of oxygen, which was transformed at the platinum substrate to the short lived superoxide radical that was directly detected at the nanopipette. 130 The resutling current–distance curves gave direct information on the lifetime of the generated superoxide. This paper marks the first time that this short lived intermediate was detected with ion transfer voltammetry. Of note is also the finding of Amemyia's group that pyrolithic carbon materials appear to be coated with a layer of contaminant material that acts in some analogy to ion transfer voltammetry when probed by conventional SECM.

131

This important work shows that one should be very vigilant in the characterization of seemingly pure materials. Arrigan and collaborators have reported on the microfabrication of microarrays for ion transfer voltammetry (for recent reviews, see132,133). In a recent report, they described microarrays where the radial diffusion layers were shown to no longer overlap, giving an array where each microhole behaved independently of the others.

134

The group has focused particularly

on the detection of large biomolecules by ion transfer voltammetry. While the exact mechanism of response is not always entirely clear, the protein of interest adsorbs onto the liquid-liquid interface where it becomes detectable without any additional label. In one example, insulin was directly detected at such microinterfaces by adsorptive stripping voltammetry.

135

Although the

calibration curves are not perfectly linear, the authors achieved detection limits in the low nanomolar range, which is rather spectacular. A certain degree of selectivity (relative to albumin) work was achieved by optimizing the adsorption potential. The same general strategy was also used for the direct detection

of

haemoglobin,

136

but

here

the

influence

of

other

biomacromolecules or other sample constituents were unfortunately not yet explored. Note that Bobacka also reported on the direct detection of proteins by adsorption, but using ion-selective electrodes measured at zero current. 137 These authors explained the mechanism of response by the change of the


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local ion concentration by the formation of a Donnan exclusion membrane arising from the electrically charged protein on the surface of the electrode. The group of Arrigan characterized such micro-hole arrays by a range of electrochemical techniques, especially to find information on capacitive charging currents.

138

Surprisingly, the microhole diameter had negligible

influence on the time response in chronoamperometry, and long times on the order of a few seconds were required to eliminate capacitive effects. Such arrays (11 µm pore diameter) were then also used for the detection of small molecules such as the ionized drug ractopamine.

139

The recognition of the

cationic drug was possible in artificial serum on the basis of its lipophilicity relative to the background electrolyte. A wide range of potential interfering ions were screened and only 5 mM potassium was found to give significant interfere. The same group also reported on the detection of the β-blocker propranolol with the same type of liquid–liquid electrochemistry architecture. 140

While the analytical application was important, this study included a range

of fundamental studies, especially numerical simulations, to correlate the observed faradaic and charging current responses with theory. The groups of Molina and Compton described a theoretical model to understand the reversible ion transfer voltammetry across an asymmetric microhole filled with organic liquid.

141

The asymmetric waves arising from the

unique geometry of microhole interfaces is well described, giving a peak shapes associated with microelectrode and macroelectrode behavior, depending on scan direction. The treatment allows one to determine the ion transfer potential from cyclic voltammetric and square wave voltammetric data. Hye Jin Lee and co-workers developed a portable stick-shaped microhole array doped with PVC–NPOE and bis(dibenzoylmethanato)Ni(II) for the recognition of perchlorate.

142

The device was interrogated by differential

pulse stripping voltammetry, giving detection limits in the low ppb range, and was successfully applied for the analysis of spiked river samples. Morita and co-workers gave a detailed mechanistic view of the microscopic barrier mechanism of ion transport across liquid-liquid interfaces by molecular dynamics simulation.

143

They modeled the formation and break up of water

fingers reaching into the organic phase that accompany the gradual


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Page 36 of 76

dehydration of the transferred ion. This process was argued to be the very origin of the energy barrier for charge transfer, and should correlate with experimentally observed charge transfer rates.

Sensor Materials Imprinted Polymer Materials Wei Qin and co-workers have focused their attention on the use of ionophoremodified nanomaterials for use in ion-selective electrode membrane (see

144

for a review). For example, molecularly imprinted nanoscaled polymeric beads were fabricated to be used as selective capturing agents suspended in ionselective membranes, using the drug triclosan as a model system.

145

The

nanoscaled beads appear to improve the resulting selectivity dramatically relative to microbeads. The doping of only a surface layer of the membrane with ion-exchanger allowed the authors to achieve a very low detection limit in the nanomolar range. In another example, nanobeads imprinted for the environmental pollutant bisphenol A were doped together with anionexchanger sites in polymeric membranes after confirmation that the nonimprinted beads did not exhibit bisphenol A affinity. The modified membranes gave unusual transient responses that needed to be carefully analyzed, but an excellent detection limit down to 20 nM was achieved. An imprinting approach was also used by Alizadeh for the fabrication of a membrane electrode for As3+.

146

Arsenic(methacrylate)3 was used as the

templating molecyle, and subsequently dispersed in a plasticized PVC membrane. Non-imprinted polymer did not give an arsenic response. While the selectivity coefficients were obtained with the Matched Potential Method, and therefore are difficult to relate to thermodynamic values, and the technical quality of the data is not very high, the detection limits were below micromolar and therefore already rather impressive.


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Chemically Modified Polymers Bakker’s group used modified PVC where a fraction of the chlorine groups were substituted by azide functionalities to achieve a facile surface modification by Click chemistry after membrane casting (for a review, see 147). In one work, solvent cast ion-selective membranes were exposed to aqueous solutions of alkyne terminated PEG with an appropriate catalyst and shown to exhibit improved biocompatibility.

148

In contrast, the attachment of cysteine to

enhance biocompatibility by NO release did not give the desired results as evidenced by SEM imaging after blood exposure. This strategy of modifying the surface of ion-selective membrane was subsequently used to design an apparently label free immunosensor for the detection of viruses.

149

The

capturing agent was covalently attached to the membrane surface by the Click chemistry attachment of a biotinylated group, which allowed the surface binding of the antibody through a streptavidin linker. The presence of virus could be detected by the modulation of the ion flux of a quaternary ammonium potential determining marker ion from the membrane into the aqueous sample. This materials strategy was subsequently used to bind D-mannose to the surface of an ion-selective membrane. 150 As this carbohydrate binds to its analyte, Concanavalin A, the build up of marker ion (potassium or tetrabutylammonium) released by a constant current was altered, and a potential change was observed.

Ionic Liquid Based Membranes Ion-selective electrodes that obey the Hofmeister selectivity sequence have been established for many years, and conventional wisdom asks for a high ion-exchanger loading to kinetically discriminate dilute interfering ions (known as the Hulanicki effect). Cecylia Wardak used the ion-exchanging ionic liquid trihexyltetradecylphosphonium chloride together with PVC and plasticizer for the fabrication of nitrate-selective electrodes, where the ion-exchanger also stabilized the potential with the internal Ag/AgCl element in this solid state configuration.

151

Membranes up to 15% of ionic liquid were evaluated, but in

the latter case a super-Nernstian jump at lower concentrations was still observed, even after extensive conditioning in the nitrate solutions, and a 5%


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Page 38 of 76

loading was preferred. While the results are promising, one may wonder about the long term potential stability of the inner interface, since nitrate is eventually expected to replace the chloride ion needed to define the inner potential. A similar approach was used by the group of Bin Su with a silver tetraphenylborate salt in potassium-selective membranes, using a carbine inner transducing layer.

152

Again, good results were obtained, including a

suppression of any water layer at the inner interface, but silver ions needed to stabilize the inner interface potential are expected to eventually leach out of the membrane by ion-exchange with potassium ions. Yu Qin’s group showed that the triblock copolymer polystyrene-blockpolybutadiene-block-polystyrene with and without added ionic liquid are attractive for the fabrication of solid contact ion-selective electrodes, with Na(I) and Pb(II) as model analytes.

153

The advantage of the formulation is the

improved hydrophobicity, giving no evidence for a water layer formation between membrane and substrate, and a long lifetime compared to ionic liquid based membranes.

Other Polymers and Additives for Ion Sensing Launay’s group developed a sodium-selective field effect transistor using a fluoropolysiloxane polymer doped with a sodium ionophore as coating.

154

This polymer reportedly adheres well on silicon-based substrates, gives low resistivities and can be inkjet printed. Compared to the previous literature on Na+-ISFETs, the selectivity over potassium was most remarkable (logKpotNa,K = –3). The group of Bachas reported on a polymeric plasticizer to realize PVC based ion-selective membranes with improved lifetimes, with K(I) as a model ion.

155

In analogy to sebacate plasticizers, the authors explore polyester sebacate with good results. A higher plasticizer to PVC ratio than traditionally used for necessary to achieve Nernstian response slopes and adequate detection limits. Extended sonication for up to 170 h was used to accelerate the aging of ion-selective membranes, showing clearly that the resulting lifetime is at least an order of magnitude higher than with conventionally plasticized PVC. Interestingly, the work suggests that a neutral colorimetric ionophore exhibits a higher lipophilicity in the polymeric plasticizer based membrane as well.


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O’Neil et al. showed that graphene oxide dispersed in ion-selective membranes acts as cation-exchanger and may replace tetraphenylborates in some applications.

156

The selectivity coefficients of a membrane containing

no ionophore was found to be compressed. While the authors did not argue in this way, the results point to a significant coulombic interaction of the anionic graphene oxide with ionic species, suggesting that tetraphenylborates will likely not be replaced soon.

Ion-Exchange Nanopore Membranes Ion-exchange Donnan exclusion membranes (such as Nafion) have been known for many years, but curiously only have been explored as membrane materials in ion-selective electrodes quite recently.

157

In this work, a range of

cation and anion permselective membranes were explored, giving Nernstian response slopes to monovalent ions with a compressed selectivity sequence in comparison with hydrophobic membrane materials. The same group subsequently used the same type of membranes in chronopotentiometry (pulstrodes approach), giving nearly identical calibration curves for a range of ions.

158

This is promising as a universal ion detector in chromatography, but

the lack of background electrolyte means that electrical migration effects change the sensitivities relative to that predicted by the Sand equation in a predictable manner. The authors subsequently used such membranes as separators for counter electrodes in dynamic electrochemistry experiments. 159

This is especially useful to protect the counterelectrode from fouling, for

example with measurement in biological matrices, and to achieve a defined chemical environment at that location relative to the sample. Kang Wang and co-workers was able to quantify the Donnan potential at DNA polyelectrolyte layers by cyclic voltammetry, using the peak potential change of ferrocene/ ferrocenium as marker ion.

160

Changes in perchlorate concentration gave

Nernstian responses in the range of 50 mM to 1 M. This principle may form the basis for new sensing approaches after appropriate functionalization to render these layers chemically selective. Nanopores decorated with ionophoric functionalities are a potentially attractive approach to the fabrication of ion-selective electrodes since they do not contain leachable components and they are less sensitive to surface defects


39


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Page 40 of 76

than planar electrodes decorated with a monolayer. Their underlying principle is similar to the Donnan exclusion membranes described above. Makra and Gyurcsanyi wrote a mini-review on nanopore based electrochemical approaches that include resistive pulse sensing at a single nanopore that function in analogy to coulter counters, as well as Donnan exclusion membranes that can be endowed with ion selectivity.

161

It is unfortunate that

not much new progress has been achieved in this exciting field in the past two years.

Modified Electrodes A somewhat related approach is the voltammetric modulation of a redox marker by the surface binding event with the ion of interest. These systems are often coined ion channel mimetic sensors. The groups of Hong-Seok Kim and Jun Ho Shim reported on such voltammetric ion sensors for ammonium using a gold surface decorated with thiazole benzo-crown ether ethylamine thioctic acid.

162

The voltammetric signature to the marker ion, [Ru(NH3)6]3+/2+,

was shown to change upon binding of ammonium. This detailed work includes an excellent study of selectivity and reversibility, which compares well to that of ion-selective membranes containing the same ionophore. Cyclic voltammetry was also used by the groups of Thompson and MacFarlane to detect pH with a surface confined conducting polymer that was modified with the pH responsive biomolecule riboflavin.

163

The voltammetric

peak position was shown to depend directly on solution pH, in analogy to earlier work by Compton. While the system was demonstrated to be useful even in non-aqueous solutions, the reported uncertainties unfortunately appear to be dramatically larger than with established potentiometric sensing systems.


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Ionophores Anion-selective Ionophores The group of Malinowska studied Al(III)-tetraazaporphyrin as ionophore for the detection of fluoride ions.

164

A range of membrane compositions were

tested with the aim of competing with the established single crystal LaF3 electrode in terms of detection limit and selectivity. The detection limit was highly dependent on pH, and a pulsed chronopotentiometric technique failed to reduce the interference of hydroxide. Because of this, an acidification of the sample to pH 2.2 was necessary for optimum behavior, but otherwise the selectivity coefficients over all tested anions were excellent (logKpotF,Cl = –3.9). Abbas et al. described a sulfide electrode based on ceric acrylohydrazine as ionophore that was covalently attached to a polyacrylamide backbone, which was then mixed with a methacrylic copolymer to form the membrane.

165

While the resulting detection limits were excellent (nanomolar), and sulfide could be reliably detected in a variety of environmental samples, including the River Nile, the reported selectivity coefficients are surprisingly unattractive. Perhaps this points to an experimental bias in their determination. Decheng Jiang and co-workers reported on three differently substituted sulfate-selective ionophores based on asquaramide-based tripodal molecules for the detection of sulfate.

166

The best compositions showed a remarkable

logarithmic selectivity coefficient of sulfate over chloride of –3.4, which compares to +3.8 for an anion-exchanger based membrane. The detection limit was about 1 micromolar. A collaborative effort between the Ortuño and Curiel groups resulted in a carbazolo[1,2-a]carbazole ionophore for the detection of dicarboxylic species.

167

Relative to an anion-exchanger based membrane, the selectivity

enhancement for a range of dicarboxylic acids relative to common monovalent anions was remarkable, about 4 orders of magnitude. Still, most common anions are more lipophilic than the di-anions of interest and cannot be tolerated at high concentration. A membrane electrode for monohydrogen phosphate was described by Tonelli et al., using a dispersion of synthetic hydrotalcite in a poly vinyl butyral


41


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membrane.

168

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A reasonable selectivity was obtained, but unfortunately the

values appear to be significantly inferior to membranes based on established uranyl salophene based ionophores, especially over the chloride anion. The group of Yoon-Bo Shim also described membranes for this anion, using an anthracene thiurea derivative as ionophore.

169

The selectivity over the very

lipophilic perchlorate anion is excellent (logKpotHPO4,ClO4 = –2.7), the selectivity over chloride is again not better than previous approaches (logKpotHPO4,Cl = – 1.8). That established uranyl salophene ionophore used by the group of Dechen Jiang in an all solid state configuration using multiwall carbon nanotubes as capacitive inner transducing layer.

170

The resulting membranes

are responsive to dihydrogen phosphate and require a control of sample pH. The authors reported similar selectivities to their inner electrolyte solution counterparts but a very attractive long term stability of many days of continuous use. The Meyerhoff group reported on a Cobalt(III) 5,10,15-tris(4-tert-butylphenyl) corrole for the measurement of nitrite by potentiometry.

171

The elegance of

the approach is the use of metal binding ligand structure of a net charge of -3, which means that the metal complex must function as an electrically neutral ionophore for the binding of nitrite. The logarithmic binding constant with nitrite was determined by the sandwich membrane technique as 5.6, which is weaker than with cationic Co(III) complexes. Nonetheless, the resulting selectivities were quite attractive. The Bakker group also reported on a nitriteselective ionophore based on cobalt(II) tert butyl salophen, which was synthesized to achieve a higher lipophilicity than previous work.

172

Lipophilic

anions such as perchlorate were discriminated against more significantly than with the corrole derivative described above, and micromolar detection limits were observed for nitrite. A cyanide-selective ionophore based on an Fe(III) containing porphyrin, Tetrakis-4-chlorophenylporpherinatoiron(III) Shirmardi-Dezaki et al.

173

chloride,

was

described

by

While the electrode slope was unfortunately very

sensitive to the membrane composition, the optimized formulation exhibited a remarkable selectivity for cyanide ion. A thiocyanate-selective electrode was described by Seungwon Jeon and coworkers,

using

1-benzyl-3-(4-nitrophenyl)

thiourea


as

ionophore.

174

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Perchlorate and thiocyanate gave approximately the same responses, while the selectivity over chloride was moderately attractive (logKpotSCN,Cl = –2.7). The group of Tuntulani found that di-tripodal amine calix[4]arene derivatives showed unusual Donnan failure when exposed to Cu(II) salts. 175 Interestingly, the resulting anionic responses after conditioning in CuCl2 showed a remarkable non-Hofmeister selectivity to thiocyanate. The membrane was demonstrated to detect Fe(III) indirectly via their affinity to this anion. Kiliç and co-workers reported on a pyrene decorated calix[4]arene ionophore for the detection of salicylate.

176

The key advantage of the proposed

ionophore is that it does not rely on organometrallic complexes. It seems odd that the optimum membrane composition did not contain added ion-exchanger sites, but the selectivity of the membrane was interesting, with even the lipophilic perchlorate discriminated against by one order of magnitude. Soleymanpour et al. used a zirconium salan complex in an ion-selective membrane to detect oxalate.

177

Unfortunately, addition of ion-exchangers

was not explored. While the selectivity coefficients are attractive and measurements in human serum and urine were demonstrated, the mechanistic role of the ionophore was not clearly described. Arsenite was targeted by the group of Singh with two different phenylhydrazone based ionophores, who explored these compounds in solution phase as colorimetric reagents as well as in traditional and in coated graphite polymeric ion-selective membranes.

178

The best composition

achieved a detection limit of about 100 nM, and selectivity coefficients were very attractive (logKpotAsO2,Cl = –3.9). Even though the structures appear not to be very lipophilic, the lifetime in continuous contact with solution was reported to be one to two months.

Cation-Selective Ionophores Ionophores for Hydrogen Ions, Alkali Metals and Ammonium Ertürün et al. reported on an ionophore-based pH electrode based on a calix[4]arene derivative, 5,17-bis(4-benzylpiperidine-1-yl)methyl-25,26,27,28tetrahydroxy calix[4]arene. PVC membranes plasticized with NPOE exhibited


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Page 44 of 76

a wide measuring range of about 1.9–12.7. The authors did not distinguish effects arising from the membrane solvent and sample electrolyte composition, which are known to strongly influence the pH measuring range. Oddly, the best measuring range was observed in the absence of any added cation-exchanger in the membrane. Xie et al. introduced a new class of pH responsive ionophores based on oxaxinoindolines that also exhibit unique spectral changes and hence are equally useful in optical sensors.

179

Three

derivatives were explored in ion-selective membranes with potentiometry and chronopotentiometry, and the measuring ranges were Nernstian up to pH 5 and 8, respectively. Wagner–Wysiecka et al. described the synthesis and characterization of a family of functionalized azo-benzocrown ionophores.

180

Binding studies in

solution were accompanied by incorporation in ion-selective membranes. A remarkable selectivity for Na(I) was observed over a range of other potential interferences, including potassium (logKpotNa,K = -2.5), similar to calix[4]arene derivatives. Potassium selectivity was also observed with some derivatives, which was explained by the formation of a sandwich complex geometry. The group of Blondeau reported on a K(I)-selective membranes on the basis of an ionophore, pyrene based benzo-18- crown-6 ether, adsorbed onto multiwalled carbon nanotubes.

181

These carbon nanotubes play the role of

the ion to electron transducer as well as a retention agent for the ionophore. While the authors emphasize that this approach allows for a higher loading than with covalent attachment to the polymer matrix, the advantages over simply dissolved and sufficiently lipophilic ionophore are less evident. Note that this is a model system and selectivities are not very attractive. Pazik and Skierawaska reported on eleven di- and triamide functionalized tetrazole derivatives as ionophores in ion-selective membranes.

182

One

compound showed considerable selectivity for ammonium over most other tested ions, including potassium (logKpotNH4,K = –2.0). Other compounds were shown to exhibit some preference for either Cd(II) or for Pb(II), but with more modest selectivities compared to the current state of the art. Four new calixarene based Tl(I) selective ionophores were reported by the group of De Marco, which were incorporated into plasticized PVC and methacrylic solid contact membranes with drop cast POT as ion to electron


44

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transduction layer.

183

Logarithmic complex formation constants in the

membranes were reported as 6.4, and detection limits as low as 10 nM were achieved. Ionophores for Transition Metals The groups of Bobacka and Bochenska reported on calix[4]arene based ionophores containing thioamide functionalities that exhibited attractive selectivities for Pb(II) over most common cations and demosntrated nanomolar detection limits for the detection of Pb(II) by potentiometry.

184

The

same group subsequently described an even more promising calixarene based ionophore, 25,26,27,28-tetrakis(piperidinylthiocarbonylmethylene)-ptert-butylcalix[4]arene, for the detection of Pb(II).

185

This ionophore provides

outstanding selectivity over common cations, with a discrimination of Cd(II) by nine orders and Zn(II) by nearly 14 orders of magnitude. Despite these exciting characteristics, measurements at ultra-trace levels were not yet reported. Mei-Rong Huang et al. described the use of polymeric microparticles containing a variety of functional binding groups. suspended

into

ion-selective

membranes

186

for

The particles were characterization

by

potentiometry but the reported coefficients, while interesting, unfortunately appear to be biased. Sanjay Singh et al. evaluated two different diethylsulfide containing chelating ionophores for the detection of Pb(II).

187

A range of membrane compositions

were explored, with the best composition giving a nanomolar detection limit. The selectivities were outstanding, with Cd(II) discriminated by 4 orders of magnitude. Although the structure appear not to be sufficiently lipophilic, the lifetime was reported to be several weeks. Kulesza et al. reported on four different lower rim functionalized calix[4]arene ionophores for the recognition of Pb(II) in ion-selective membranes and by ion extraction.

188

Selectivity coefficients were dramatically influenced by the

plasticizer, with NPOE giving particularly excellent discrimination of sodium ions and Cu(II). One derivative was also found to result in a discriminaton of Cd(II) by about five orders of magnitude. Detection limits were not optimized in this work. The detection of Pb(II) was also the focus of Bushra et al., using a Ti(IV) Arsenophosphate Composite Cation Exchanger as selective reagent


45


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and polyaniline as ion to electron transducer.

Page 46 of 76

189

Despite an attractive

discrimination of Cd(II), the technical quality of the paper is not very high and the resulting detection limits only in the µM range and therefore rather poor. A Cu(II) selective ionophore based on a salophen templated schiff base, 2,3bis(salicylaldimino)pyridine, was characterized by the group of Demir in solid state ion-selective electrodes.

190

The reported selectivity coefficients were

rather modest and the small electrode slopes to interfering ions hints at significant experimental bias in the data. A family of four Ag(I) selective ionophore based on calix[4]arenes were synthesized by the group of Tuntulani and characterized in ion-selective membranes.

191

The resulting complexes were found to be relatively weak,

with logarithmic binding constants of at most 5.3 as determined by the segmented sandwich membrane technique. The selectivities were attractive, but alkali and alkaline earth metals were less discriminated against than previously reported ionophores. Nonetheless, this is a valuable class of ionophores since the structures contain no sulfur groups, giving less interference from Hg(II). The electrodes were applied to the detection of DNA hybridization by dissolution of silver nanoparticle labels. Wardak used 2-(2-Hydroxy-1-naphthylazo)-1,3,4 –thiadiazole as ionophore for the determination of Zn(II), with a 70 nM detection limit.

192

The solid state

configuration involved an Ag/AgCl element, and a chloride containing ionic liquid was doped into the membrane to help stabilize the inner electrode potential. Suprisingly, this ionic liquid did not give rise to a nitrate response in the absence of ionophore, which would be expected, neither to Zn(II). The selectivity relative to most common ions was excellent (logKpotZn,K = –5.4), with Ag(I) and Hg(II) identified as key interferences. Ramirez-Silva and co-workers described ion-selective membranes for Hg(II) using a dithiophosphate based ionophore.

193

While the ionophore does not

appear to be very extremely lipophilic, the membranes selectivities were attractive and the electrode behaved similarly to a mercury drop electrode in the titration of Hg(II) with EDTA. The detection limit was about one micromolar and therefore not very low. The group of Sardohan-Koseoglu reported on oxime based ionophores for the detection of Hg(II), without sulfur containing functionalities. 194 Unfortunately, the resulting membrane selectivity was rather


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modest, the detection limits again on the order of 1 micromolar, and the observed endpoints in the complexometric titration of mercury by EDTA were not very sharp. Another mercury ion-selective electrodes based on a triazine compound dissolved in a carbon paste electrode was reported by Mashhadizadeh et al.

195

The modest observed selectivity coefficients were

experimentally biased, but nanomolar detection limits were achieved. Logarithmic complex formation constants with mercury in methanol were on the order of 9, which is very attractive. It would have been interesting to see complex formation constants directly in the membrane phase. Inamuddin and co-workers also put forward ion-selective electrodes for Hg(II), using a the polymeric cation-exchanger poly-o-toluidine Zr(IV) tungstate.

196

While the

technical quality of the paper is not very high and the detection limits were likely biased with the 0.1 M Hg(II) solution used as inner electrolyte, the lifetime and selectivity of the membrane appear to be promising. On the other hand, Wei Qin and coworkers showed that membranes containing the commercially available Lead ionophore IV are responsive to the HgCl3– species, allowing its detection in chloride rich media.

197

While this anion is

already rather lipophilic, the selectivities were significantly improved relative to those using membranes containing anion-exchanger only, with chloride discriminated against by nearly 9 orders of magnitude. Recognition of Lipophilic Ions Zorin et al. reported on the use of polyelectrolytes embedded in plasticized PVC membranes for the detection of lipophilic cations, with the main target cetyltrimethyl ammonium.

198

The authors found a significantly improved

stability and excellent selectivity over other cations, but comparisons with membranes doped with more traditional cation-exchangers were unfortunately not presented. El-Sayed showed that functionalized cyclodextrin or calix[8[arene remarkably improved the selectivity of a cation-exchanger based membrane for the detection of the cationic drug rivastigmine.

199

The selectivity gain over the

small cations sodium and potassium was about two orders of magnitude, showing that these basket-shaped molecules are effective ionophores.


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Page 48 of 76

Sami Elhag et al. also used cyclodextrin doped membranes, on a Co3O4 nanowire electrode substrate, to detect dopamine.

200

While a very attractive

nanomolar detection limit was achieved, the response selectivity was partly explained by the morphology of the nanowire substrate material, which seems difficult to understand given that a polymeric membrane coating was used.

About the Author Born in Switzerland from Dutch parents, Eric Bakker studied chemistry at ETH Zurich where he received his diploma of chemistry and his doctoral degree in 1989 and 1993, respectively. He then moved to the United States for a postdoctoral stay at the University of Michigan (1993-1995) before starting his independent academic career as Assistant Professor at Auburn University where he became Associate Professor 1998 and Full Professor in 2003. In 2005, he was called to Purdue University in West Lafayette as Professor and in 2007 he accepted a position as Professor and Director of the Nanochemistry Research Institute at Curtin University in Perth, Western Australia. In 2010 he moved to the University of Geneva in Switzerland as Chair of Analytical Chemistry. Eric Bakker’s research interests include electroanalytical fluorescent

chemistry,

nanosensors,

chemical light

sensors,

triggered

membrane

devices,

electrodes,

environmental

and

bioanalytical applications. He received the 2014 Robert Boyle prize from the Royal Society of Chemistry. Eric Bakker is currently serving as Editorial Advisory Board Member of Analytical Chemistry and Associate Editor for the newly launched journal ACS Sensors.


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Acknowledgments The author wishes to thank the Swiss National Science Foundation and the EU Seventh Framework Program (FP7-OCEAN 2013.2 SCHeMA project – Grant Agreement 614002) for supporting research in this field.

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Di- and Triamide Derivatives in Ion-Selective Membrane Electrodes. Sens. Actuators, B 2014, 196, 370–380. Chester, R.; Sohail, M.; Ogden, M. I.; Mocerino, M.; Pretsch, E.; Marco, R. D. A Calixarene-Based Ion-Selective Electrode for Thallium(I) Detection. Anal. Chim. Acta 2014, 851, 78–86. Guziński, M.; Lisak, G.; Sokalski, T.; Bobacka, J.; Ivaska, A.; Bocheńska, M.; Lewenstam, A. Solid-Contact Ion-Selective Electrodes with Highly Selective Thioamide Derivatives of P-TertButylcalix[4]Arene for the Determination of Lead(II) in Environmental Samples. Anal. Chem. 2013, 85, 1555–1561. Jasiński, A.; Guziński, M.; Lisak, G.; Bobacka, J.; Bocheńska, M. Solid-Contact Lead(II) Ion-Selective Electrodes for Potentiometric Determination of Lead(II) in Presence of High Concentrations of Na(I), Cu(II), Cd(II), Zn(II), Ca(II) and Mg(II). Sens. Actuators, B 2015, 218, 25–30. Huang, M.-R.; Ding, Y.-B.; Li, X.-G.; Liu, Y.; Xi, K.; Gao, C.-L.; Kumar, R. V. Synthesis of Semiconducting Polymer Microparticles as Solid Ionophore with Abundant Complexing Sites for Long-Life Pb(II) Sensors. ACS Appl. Mater. Interfaces 2014, 6, 22096–22107. Singh, S.; Rani, G.; Singh, G.; Agarwal, H. Comparative Study of Lead(II) Selective Poly(Vinyl Chloride) Membrane Electrodes Based on Podand Derivatives as Ionophores. Electroanalysis 2013, 25, 475–485. Kulesza, J.; Guziński, M.; Bocheńska, M.; Hubscher-Bruder, V.; Arnaud-Neu, F. Lower Rim Substituted P-Tert-Butyl-Calix[4]Arene. Part 17. Synthesis, Extractive and Ionophoric Properties of P-TertButylcalix[4]Arene Appended with Hydroxamic Acid Moieties. Polyhedron 2014, 77, 89–95. Bushra, R.; Shahadat, M.; Khan, M. A.; Inamuddin; Adnan, R.; Rafatullah, M. Optimization of Polyaniline Supported Ti(IV) Arsenophosphate Composite Cation Exchanger Based Ion-Selective Membrane Electrode for the Determination of Lead. Ind. Eng. Chem. Res. 2014, 53, 19387–19391. Demir, S.; Yılmaz, H.; Dilimulati, M.; Andac, M. Spectral and Thermal Characterization of Salophen Type Schiff Base and Its Implementation as Solid Contact Electrode for Quantitative Monitoring of Copper(II) Ion. Spectrochim Acta A Mol Biomol Spectrosc 2015, 150, 523–532. Janrungroatsakul, W.; Lertvachirapaiboon, C.; Ngeontae, W.; Aeungmaitrepirom, W.; Chailapakul, O.; Ekgasit, S.; Tuntulani, T. Development of Coated-Wire Silver Ion Selective Electrodes on Paper Using Conductive Films of Silver Nanoparticles. Analyst 2013, 138, 6786–6792. Wardak, C. Solid Contact Zn2+ -Selective Electrode with Low Detection Limit and Stable and Reversible Potential. Cent. Eur. J. Chem. 2013, 12, 354–364. Juarez-Gomez, J.; Perez-Garcia, F.; Ramirez-Silva, M. T.; RojasHernandez, A.; Galan-Vidal, C. A.; Paez-Hernandez, M. E. SolidContact Hg(II)-Selective Electrode Based on a Carbon-Epoxy Composite Containing a New Dithiophosphate-Based Ionophore.


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Page 64 of 76

Figure Captions Fig. 1

Two types of innovative reference electrode concepts where the phase boundary potential is dictated by ions originating from the membrane. A) A current pulse delivers an ion from the membrane into the sample.8 B) An electrolyte of adequate lipophilicity partially partitions into the aqueous phase.9 The latter principle appears to be most convincing since the equilibrium partitioning results in a thermodynamically defined equilibrium potential.

Fig. 2

Two principal approaches for the realization of solid-contact ionselective electrodes. A) The backside consists of a hydrophobic conductive material that exhibits a high capacitance, thereby resulting in small potential changes upon electrochemical perturbation. B) The membrane is back side contacted with a material consisting of a redox buffer, which can be a conducting polymer or a molecular pair in the oxidized and reduced form.

Fig. 3

Determination of unbiased selectivity coefficients according to Egorov’s group.49 Left: Prolonged contact with a solution containing interfering ion will result in the establishment of a membrane confined diffusion layer and a potential drift. Right: The experimental selectivity coefficients can be extrapolated to long times as shown, resulting in the desired unbiased values.

Fig. 4

Redox potentials are often difficult to assess because of hindered electron transfer kinetics.56 A nanostructured electrode coating (shown left) helps to increase the reversibility of electron transfer and results in better agreement with the Nernst equation, see right.

Fig. 5

Dissolved carbon dioxide can be directly measured potentiometrically (right) by measuring a pH electrode against a carbonate-selective membrane electrode (left).59 This alleviates the need for a gas permeable membrane as required with Severinghaus type gas sensing probes.

Fig. 6

Anion-exchanger based membranes are responsive to boronic acid and can be used to detect fructose.75 The response mechanism, shown on the left, is explained by an extraction of the boronic acid into the membrane along with water, which subsequently loses a hydrogen ion and the initial counter ion (shown as chloride) from the membrane. As shown on the left, this results in a potential decrease. The addition of fructose results in an effective decrease of the boronic acid activity in the sample, giving a positive potential change.


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Fig. 7

Controlling the ion flux at ion-selective membranes by a current pulse results in pulstrodes. Left: the readout is akin to a potentiometric sensor, either during the pulse or immediately afterwards during a zero current period. Right: Localized depletion of the ion of interest results in a potential change at a transition time, which is a direct function of ion concentration. This latter approach does not require a traditional reference electrode.

Fig. 8

Thin layer electrochemical proton pump in combination with a pH probe placed opposite a thin layer sample for chemically selective total alkalinity detection. 115 This strategy forms the basis for in situ pH titrations without pumps, traditional sampling or standardization and volumetric delivery of reagents. Counter and reference electrodes are placed outside the thin layer gap in the bulk solution.

Fig. 9

Thin Layer Ion-Selective Membranes for the Stripping Voltammetric Detection of Ca(II).118 While the high selectivity of the membrane for calcium is given by the ionophore, the stripping voltammetry protocol gives the opportunity for electrochemical accumulation and subsequent signal enhancement during the anodic stripping step. The extraction process is dictated by the oxidation state of the conducting polymer underlayer, as shown. Left: During the accumulation step, the conducting polymer is reduced, thereby freeing up the cation-exchanger R– to help extract calcium into the membrane, which forms a complex with the ionophore L. Right: during the stripping step, the polymer is oxidized, requiring the cation-exchanger as counter ion, thereby forcing the extraction of calcium back into the sample.

Fig. 10 Nanopipette based ion transfer detection of the reactive superoxide radical.130 Oxygen diffusionally delivered from the nanopipette is converted at the substrate to superoxide, which is detected at the same nanopipette by ion transfer voltammetry. The short diffusion distances make it possible to detect this short lived species at the appropriate time scale.


65


For TOC only

Sample

Membrane

R– K+

emf

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KL+

log a


Page 67 of 76

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Reference Electrodes A) Galvanostatic Pulse

B) Ionic Liquid

Sample

Sample

Membrane

Membrane

Flux R– K+

R–

KL+

R+R–


R+

Fig. 1


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Solid Contact Ion-Selective Electrodes A) High Capacitance

B) Redox Buffer

Sample Mem Contact

Sample Mem Contact

R– K+

KL+

+– +– +– +– +– +– +–

R– K+

KL+

R– Ox+ Red


Fig. 2

Page 69 of 76

Determining Unbiased Selectivity Coefficients Time Dependent Gradients

Extrapolated Selectivity

Sample Membrane t2

K

I+

R– IL+

t1 (exp)

t1 t2 pot I .J

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Unbiased Value t −1/4 ACS Paragon Plus Environment

Fig. 3


Nanostructured Electrodes for Redox Potentials Sample

Electrode With Catalyst

Red emf

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Page 70 of 76

Ox Without Catalyst [Ox] log [Red]


Fig. 4

Page 71 of 76

Direct Detection of PCO2

emf

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pH

CO32−

log PCO 2


Fig. 5


Boronic Acid Responsive Membranes Sample

Membrane Add RB(OH)2

Cl– RB(OH)2

OH–

Cl– R+

emf

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RB(OH)3– Add Fructose

Time


Fig. 6

Page 73 of 76

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Pulstrodes: Current Control of Membrane Electrodes A) Potential Readout

B) Transition Time Readout

Sample

Sample

Membrane

Flux

Flux R–

I+

Membrane

I+

IL+


R– IL+

Fig. 7


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Thin Layer Proton Pump for Direct Alkalinity Detection

Proton Pump Sample pH Electrode Charge

H+ R–

R–

HL+

HL+


Fig. 8

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Ion Transfer Stripping Voltammetry Accumulation

Stripping

Sample Mem PEDOT

Sample Mem PEDOT

Ca2+ L

R–+

e–

R– CaL2+ R–

R–+


e– +

Fig. 9


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Nanopipettes for Detecting Short Lived Ions

Pt Substrate

Nanopipette

O2

O2

O2•−

O2•−


Fig. 10

Electroanalysis with Membrane Electrodes and LiquidâLiquid Interfaces

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