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Electrochemically Switchable Polymeric Membrane Ion-Selective Electrodes Elena Zdrachek, and Eric Bakker Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01282 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018
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Analytical Chemistry
Electrochemically Switchable Polymeric Membrane Ion-Selective Electrodes Elena Zdrachek, Eric Bakker* Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland. Corresponding Author:
[email protected]
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Abstract: We present here for the first time a solid contact ion-selective electrode suitable for the
simultaneous
sensing
of
cations
(tetrabutylammonium)
and
anions
(hexafluorophosphate), achieved by electrochemical switching. The membrane is based on a thin plasticized polyurethane membrane deposited on poly(3octylthiophene) (POT) and contains a cation-exchanger and lipophilic electrolyte (ETH 500). The cation-exchanger is initially in excess, the ion-selective electrode exhibits an initial potentiometric response to cations. During an oxidative current pulse, POT is converted into POT+, which results in the expulsion of cations from the membrane followed by the extraction of anions from the sample solution to fulfil the electroneutrality condition. This creates a defined excess of lipophilic cation in the membrane, resulting in a potentiometric anion response. A reductive current pulse restores the original cation response by triggering the conversion of POT+ back into POT, which is accompanied by the expulsion of anions from the membrane and the extraction of cations from the sample solution. Various current pulse magnitudes and durations are explored, and the best results in terms of response slope values and signal stability were observed with oxidation current pulse of 140 µA cm-2 applied for 8 s and reduction current pulse of –71 µA cm-2 applied for 8 s.
Keywords: ion-selective electrodes, switchable cation/anion response, thin layer membrane, poly(3-octylthiophene).
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Analytical Chemistry
Introduction Ion-selective electrodes (ISEs) are widely used analytical sensors. Their fields of application has been diversified owing to the discovery of selective receptors (ionophores) that can be incorporated into the polymeric membrane1 as well as owing to the development of all-solid-state ISEs by eliminating the inner filling solution and replacing it with ion-to-electron transducing material. Conducting polymers were one of the first classes of these materials. Nowadays polypyrrole2, poly(3-octylthiophene) (POT)3 and poly(3,4-ethylenedioxythiophene) (PEDOT)4 are the most commonly used ones among the many conducting polymer solid contacts that have been investigated over the last decades5. It is well-known that the unintentional accumulation of a thin layer of water at the ISE membrane/solid contact interface affects the electrode potential and results in potential drifts, sensitivity to osmolality changes, and ultimately membrane detaches from the underlying metal substrate.5 Consequently, the development of new highly hydrophobic ion-to-electron transducing materials is considered to be the key for solving this problem. In particular, Lindner and co-workers recently showed that PEDOT-C14, a highly hydrophobic derivative of PEDOT, can be used as an intermediate layer for allsolid-state ISEs (H+, K+, and Na+) providing excellent performance characteristics (theoretical response slope, short equilibration time, excellent potential stability etc.).6 Moreover, it was proved that the superhydrophobic properties of this new conducting polymer prevent the accumulation of an aqueous film at ISE membrane/solid contact interface.6 Besides conducting polymers other electroactive species, such as lipophilic silver complexes,7 redoxactive self-assembled monolayers based on fullerene and tetrathiafulvalene (TTF),8,9 ferrocene,10 Prussian blue,11 lipophilic Co(II)/Co(III) salts,12-15 7,7,8,8-tetracyanoquinodimethane (TCNQ),16,17 and TTF with its radical 3 ACS Paragon Plus Environment
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salts18 can provide the redox properties needed for effective ion-to-electron transduction. In recent years different nano- or microstructured materials such as three-dimensionally
ordered
macroporous
(3DOM)
carbon,19,20
carbon
nanotubes,21-25 fullerene,26,27 graphene,28-31 colloid-imprinted mesoporous (CIM) carbon,32 conducting polymers,33-35 or modified gold nanoparticles36,37 have been also tested as transducers. Potentiometric sensing probes are operated under zero current conditions, but a number of attractive analytical characteristics have recently been reported when such membranes are interrogated by either controlled current or potential techniques. Recent examples include constant current chronopotentiometry38-40, thin layer coulometry41-48 and ion transfer stripping voltammetry49-60. All-solid-state ISEs with thin polymer membrane have been widely exploited by ion transfer stripping voltammetry over the last decade. It has been found that the oxidation / reduction properties of some conducting polymers that are widely used as an intermediate ion-to-electron transducer layer between the ion-selective membrane and the underlying solid electron conductor in potentiometry, can initiate ion-transfer processes across the overlaid membrane/sample interface. This concept was introduced by Amemiya and co-workers with thin polyvinylchloride (PVC) membranes deposited onto POT or PEDOT films.49,50 A subnanomolar detection limit of perchlorate was achieved using a 0.72 µm thin membrane doped with a lipophilic electrolyte and deposited on a thin POT film.51 The thin polymeric film was electrochemically enriched with perchlorate during a preconcentration step, followed by stripping of the accumulated anion during a linear potential scan. Potassium detection at the nanomolar level was also demonstrated by using a thin layer membrane based on potassium ionophore valinomycin deposited on a thin PEDOT film.52
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Analytical Chemistry
Si et al.53 extended this approach, establishing the reversible ion-transfer process at even thinner membranes (10 times thinner than for the previous systems49-51) deposited on POT film. It was shown that the ion-transfer peak potentials follow a linear relationship with the logarithm of the ion activity in the sample phase, in complete analogy to a potentiometric readout.53 More recently, our group achieved the simultaneous and selective determination of multiple cations (namely, Li+ and Ca2+ or Li+, Na+ and K+) by ion transfer across thin
multi-ionophore-based
membranes
back-contacted
with
POT.54-56
Voltammograms with multiple peaks were observed where each peak corresponds to the transfer of an individual cation since the difference in cation-ionophore formation constants results in the expulsion of cations from the membrane at distinct applied potentials. A detailed model considering the diffusional mass transport at the sample solution interface was presented.57,58 It allows one to predict the voltammetric current and peak positions for a wide concentration range of analyte and to estimate the limit of detection.57,58 Thus, over the last decade a selective detection of a range of compounds (heparin,49 tetrapropylamonnium,50 perchlorate,49,51 ammonium,52 potassium,52 calcium,59 hydrogen chromate ions60) and multi-cation mixtures54-56 was demonstrated, as a result of coupling the oxidation / reduction of the conducting polymers to the ion-transfer across membrane and the theory of ion-transfer voltammetry with polymer films was thoroughly developed. But despite the great success accompanying the introduction of thin membranes by ion-transfer voltammetry technique into the field of electrochemistry, to the best of our knowledge, the ISEs with thin membranes have not yet been explored in potentiometry. At the same time, ISE with thin membrane may exhibit some unique properties, namely the simultaneous potentiometric detection of cations and anions. 5 ACS Paragon Plus Environment
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It is well-known that the concentration of cation (or anion) of interest must remain constant in the membrane phase to observe a Nernstian slope towards this ion. This is ordinarily achieved by incorporating a lipophilic cation- or anion-exchanger61. As any given membrane exhibits either cation or anion permselectivity, not both, the simultaneous or sequential potentiometric detection of cations and anions using a single polymer membrane is thought not to be possible. We report here for the first time on the potentiometric detection of cations (tetrabutylammonium) and anions (hexafluorophosphate) using a ion-selective membrane. It consists of a thin (of hundreds of nanometer thickness) plasticized polyurethane membrane containing an ion-exchanger and backside contacted with a POT layer as ion to electron transducer as initial example. Electrochemical switching was achieved by an applied current pulse, followed by zero current potentiometric measurements.
Experimental Section Reagents,
Materials,
and
Equipment.
Sodium
tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (NaTFPB), tetradodecylammonium tetrakis(4chlorophenyl)borate (ETH 500), polyurethane (PU), bis(2-ethylhexyl) sebacate (DOS) and tetrahydrofuran (THF) were of Selectophore grade (Sigma-Aldrich, Switzerland). Tetrabutylammonium chloride (97%), sodium hexafluorophosphate (98%), lithium acetate, lithium chloride (99%), sodium chloride (99.5%), potassium chloride (99.5%), calcium chloride tetrahydrate (99.995%) were purchased from Sigma-Aldrich. Regioregular poly(3-octylthiophene-2.5-diyl) (POT) with a weight-average molecular weight of ca. 34,000 g/mol was purchased from Sigma-Aldrich. Chloroform with analytical reagent grade was obtained from Fisher Scientific. Aqueous solutions were prepared by dissolving the appropriate salts in Milli-Q purified distilled water.
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Analytical Chemistry
Electrochemical equipment and electrode preparation. Glassy carbon (GC) electrode tip with an electrode diameter of 3.00 ± 0.05 mm were sourced from Metrohm
(Switzerland).
Cyclic
voltammetry
and
chronopotentiometric
measurements were performed with a PGSTAT 302N (Metrohm Autolab B.V., Utrecht, The Netherlands) controlled by Nova 1.10 software running on a PC. A double-junction Ag/ AgCl/ 3M KCl/ 1M LiOAc reference electrode (Metrohm, Switzerland) and a platinum electrode (Metrohm, Switzerland) were used in the three-electrode cell. A Faraday cage was employed to protect the system from undesired noise. Potentiometric measurements were carried out with a high impedance input 16channels EMF monitor (Lawson Laboratories, Inc., Malvern, PA) using doublejunction Ag/ AgCl/ 3M KCl/ 1M LiOAc reference electrode (Metrohm, Switzerland). A rotating disk electrode (Autolab RDE, Metrohm Autolab B.V., Utrecht, The Netherlands) was used to spin coat the membranes on the electrodes. Preparation of the Electrodes. a) Fabrication of solid contact ion-selective electrodes (SC-ISEs). 0.125 mM POT solution in chloroform was applied by dropcasting 10µl on GC surfaces. The film was left to dry for 30 min and a volume of 25 µl of membrane cocktail was then spin coated at 1500 rpm on top of the POT layer. The membrane cocktail was prepared by dissolving 0.51 mg (22.1 mmol·kg1
) NaTFPB, 0.16 mg (5.5 mmol·kg-1) ETH500, 12.488 mg PU and 12.892 mg DOS
in 1 mL of THF. According to previous results obtained in our group this protocol should allow one to produce reproducible and sufficiently thin films of about 200 nm54. b) Fabrication of electrodes with solvent cast polymer membrane. 2.95 mg (19.3 mmol·kg-1) NaTFPB, 0.95 mg (4.8 mmol·kg-1) ETH500, 84.410 mg PU and 84.410 mg DOS were dissolved in 2 mL of THF and poured into a glass ring (22 7 ACS Paragon Plus Environment
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mm ID) affixed onto a glass slide. The solution was allowed to evaporate overnight, forming a parent membrane. The parent membrane was cut with a metallic hole puncher into disks of 8 mm diameter and mounted into Ostec electrode bodies (Oesch Sensor Technology, Sargans, Switzerland). Selectivity
coefficient
determination. The
selectivity
coefficients
were
determined by modified separate solution method (MSSM).62 The conventional electrodes with thick solvent cast membrane were previously conditioned in 10-3 M CaCl2 solution for 3 h and the same solution was used as the inner reference solution. SC-ISEs with thin layer membrane were conditioned in the same solution of 10-3 M CaCl2 for only 10-15 min. Then, EMF values in different ions solutions in the order of increasing preference at different concentration levels (10-4 – 10-1 M) were measured. The experimental slopes were close to Nernstian in all cases. The selectivity coefficients were calculated according to Eqn. 1 using the EMF values for the highest measured ion activities corresponding to the Nernstian range of an electrode response: log K ijPot =
( E j − Ei ) zi F 2.303RT
+ log
ai aj
(1)
zi / z j
Activity coefficients were calculated according to ref.63
Results and Discussion Even though the thin layer membranes deposited onto conducting polymers have been widely used in ion transfer voltammetry,49-60 their properties and applicability have not yet been explored in potentiometry. The goal of this work is to demonstrate that thin polymeric membranes can be also successfully used in potentiometry and moreover, their unique properties could open up new opportunities for developing a single sensor suitable for cation and anion detection.
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Analytical Chemistry
It was shown earlier in our group54-56 that polyurethane (PU) provides enhanced stability and robustness as a matrix in thin layer membranes in comparison with conventional PVC. In particular, the use of PU prevents leaching of lipophilic additives after rinsing with water and resists detachment from the underlying electrode surface.54-56 As this makes this polymer matrix more promising for real sample analysis, we focused our attention on this material for this study. A decrease in membrane thickness of an ISE may be beneficial for potentiometric measurements. In particular, it should allow one to completely eliminate the preconditioning step during electrode fabrication and reduce potential drifts that originate from time dependent mass transport processes in the membrane. Indeed, according to the ref64 the conditioning of the membrane was deemed unnecessary as the estimated equilibration time should be about 20 ms for a film with thickness of 200 nm and diffusion coefficient of 10−8 cm2 s−1. To evaluate the influence of the membrane thickness on the response characteristics (slope, selectivity) of the ISE, the selectivity coefficients for a conventional ISE with inner reference solution and the SC-ISE with thin membrane were determined according to the MSSM62 procedure. Figure 1 shows the responses of the two types of electrodes to calcium, potassium, sodium, lithium and tetrabutylammonium cations, in this order. In both cases near-Nernstian response slopes were exhibited for all cations. One observes a comparable EMF shift in the series of investigated cations for both thin and conventional thick membranes which suggests that they exhibit the same selectivity coefficients values. Indeed, as shown in Table 1, the observed selectivity coefficients values correspond well to each other, even though there is some positive shift for the values the thin membrane SC-ISEs compared to the conventional ISEs. The observed selectivity sequence for both types of electrodes was: TBA+ >Li+ > Na+ > K+ > Ca2+, which is different from the Hofmeister sequence expected for non9 ACS Paragon Plus Environment
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specific ion-exchanger based cation-selective membranes: TBA+ > K+> Na+ > Li+> Ca2+. This is attributed to the structure of polyurethane, which contains some cation binding functionalities.65,66 The EMF values are found to vary more significantly for the thin membrane SC-ISEs (see Figure 1b). This is explained with the solid contact fabrication procedure that involves manual drop casting on different electrode surfaces. Table 1 Observed Logarithmic Selectivity Coefficients for Traditional and Thin Membrane Ion-Selective Electrodes
(
Pot log K TBA + ,j
Interfering ion
)
MSSM
(95%, n=3) ISE with inner filling solution
SC-ISE with thin layer membrane
Ca2+
-5.20 ± 0.05
-4.90 ± 0.24
K+
-5.15 ± 0.02
-4.72 ± 0.19
Na+
-4.98 ± 0.06
-4.61 ± 0.21
-4.25 ± 0.06
-3.87 ± 0.24
0.00
0.00
Li+ +
TBA
The sensing mechanism of ion-transfer voltammetry for polymeric thin films containing
a
cation-exchanger
(R–)
and
backside
contacted
with
an
electropolymerized POT layer has been described before,53,54 and involves four main steps: a) POT is partially oxidized to POT+, thus incorporating R– into the conducting polymer layer and expulsing all counterions M+ from the membrane into the aqueous phase; b) further POT oxidation may force the extraction of sample anions A– into the membrane; c) reversing the applied potential scan will result in the expulsion of A– from the membrane, and d) subsequently uptake of M+ to fully reduce the underlying POT layer.
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Analytical Chemistry
To assess if the electrodes with solvent cast POT film can be used in ion-transfer voltammetry, five consecutive cyclic voltammograms of freshly prepared SC-ISE in 1mM TBACl solution in the potential range of –0.8 to 1.0 V vs Ag/AgCl were performed. Figure S1a shows that in these conditions one anodic and one cathodic peak is observed that can be interpreted as TBA+ transfer peaks according to the mechanism described above. The signal is stable and does not appreciably change with time. The extraction /expulsion of Cl– ions predicted according to the mechanism described above was not observed, likely because of the limited applied potential window. In a separate experiment, it was shown that the TBA+ peak position shifts towards more positive potential values with increasing TBA+ concentrations in the sample. In Figure S2 one can see that the difference between TBA+ peak positions for 100 and 10 mM TBACl solutions is 53.5 mV, which is close to the expected Nernstian shift. Further studies showed that the anodic and cathodic TBA+ transfer peak potentials show a negligible dependence on scan rate in the range from 10 to 100 mV·s-1 (see Figure 2a), along with a linear relationship between peak current and scan rate, confirming the thin layer behavior for the studied electrode (see Figure 2b). Figure S1b demonstrates five consecutive cyclic voltammograms in 1mM NaPF6 solution in the potential range from –0.8 to 1.0 V vs Ag/AgCl for SC-ISE that was earlier exposed to TBACl solution. During the forward scan three distinct peaks are observed at ∼230 mV, ∼420 mV and ∼700 mV. First two peaks can be ascribed to the cation expulsion from the membrane phase and they represent the competition between TBA+ ions that were originally present in the membrane and Na+ ions from the sample solution. The peak at 420 mV that is ascribed to TBA+ transfer gradually disappears, making way for a new peak corresponding to Na+ transfer at less positive potential of 230 mV. 11 ACS Paragon Plus Environment
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The peak at 700 mV is stable and does not change with time. It is ascribed to anion extraction, specifically the extraction of PF6–. In contrast to the previous experiments in TBACl solution, the anion wave appears here owing to the difference in lipophilicity between Cl– and PF6–. As PF6– is a more lipophilic ion than Cl–, it is more easily transferred to the membrane phase and the potential for the anion peak shifts to the left towards less positive potentials. It is well-known that the ion transfer voltammetric behavior of the electrode depends on the ion selectivity of the overlaid polymer membrane.53,55,56 It was found that the response of SC-ISEs towards different cations does not follow the Hofmeister sequence (see Figure 2c: TBA+ >Li+ > Na+ > K+ > Ca2+), but it correlates well with potentiometrically measured selectivity coefficients (see Figure 1). In particular, the cation transfer peak for 1mM NaCl was observed to shift relative to that for TBA+ by –282 mV, corresponding to a logarithmic Pot selectivity coefficient of log KTBA + , Na +
= –4.76 which is comparable to
potentiometric results (see Table 1). The fact that the studied SC-ISEs could be exploited as potentiometric and voltammetric sensors inspired us to propose a new protocol allowing to use a single electrode for potentiometric cation and anion detection. An idealized mechanism of switching between cationic and anionic potentiometric responses is illustrated in Figure 3 where sufficiently hydrophobic cation and anion TBA+ and PF6– were chosen as a model system. A freshly prepared electrode with a membrane containing cation-exchanger (R-) and lipophilic electrolyte (R1+R2-) backside contacted with POT exhibits a cationic TBA+ potentiometric response. As POT is oxidized to POT+, it starts to form ion pairs with the lipophilic anions (R− and R2−) present in the membrane, resulting in the expulsion of TBA+ from the membrane. This process eventually results in the 12 ACS Paragon Plus Environment
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Analytical Chemistry
extraction of PF6– to balance free cationic sites (R1+) that are being liberated (Figure 3a). Consequently, as the PF6– concentration in the organic phase is constant and determined by the concentration of cationic sites (R1+), the phase boundary potential of the electrodes will now depend on the PF6– activity change in the aqueous phase. Therefore, one should expect to observe an anionic PF6– potentiometric response instead of the original cationic one. On the other hand, during the reduction of POT+ to electrically neutral POT, one should expect the opposite processes (see Figure 3b), namely, the expulsion of PF6– from the membrane and the extraction of TBA+ from the solution to counterbalance the liberated anionic sites (R− and R2−). As a result of this electrochemical modulation one may restore the cationic potentiometric response. To verify the validity of the mechanism discussed above four SC-ISEs were prepared following the described earlier procedure. The original potentiometric TBA+ response was confirmed. In Figure 4 one can see that all electrodes yield near-Nernstian slopes (56.5 ± 1.0, 56.4 ± 1.4, 56.8 ± 1.3 and 55.1 ± 1.3 mV·dec-1 for electrodes 1, 2, 3 and 4 correspondingly) in the range of TBA+ concentration from 10-6 to 10-3 M. The oxidation/reduction of the underlying POT film was achieved by applying a constant current pulse. The oxidation/reduction current pulse was followed by monitoring the open circuit potential (OCP) for 240 s before starting the potentiometric calibration of the electrodes. To obtain a desirable switchable cation/anion potentiometric response, the magnitude and the duration of applied current pulse was optimized. To oxidize POT a current of 10 µA (140 µA cm-2) was applied to electrodes 1, 2, 3 and 4 for 3, 6, 8 and 10 s, respectively, in 1 mM NaPF6 solution resulting in the corresponding charge density values of 420, 840, 1120 and 1400 µC cm-2. The current pulse values were chosen to ensure that only a small portion of available 13 ACS Paragon Plus Environment
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POT will be oxidized. In particular, for the most extreme case of the applied current pulse of 10 µA for 10 s one should expect the removal of 1.0 nmol of electrons from POT film according to Faraday's law. Taking into account the average molecular weight of the purchased polymer (34,000 g/mol) and the amount of POT casted on the electrode surface (1.25 nmol) one obtains 0.23 µmol octylthiophene units that can be oxidized. This estimation implies that POT would be oxidized to a degree of less than 1 %, which should not cause a degradation of the polymer and should be fully reversible. The constant-current chronopotentiogramms and OCP as functions of time for all electrodes are presented in Figures S3–S6. One can see that all constant-current chronopotentiogramms exhibit the same shape but the observed potential values vary at a given time. As the applied current is constant one would ideally expect the same recorded potential values. The variation may originate from differences in the characteristics of the obtained POT films for four separately prepared SC-ISEs. Figure 4 shows that independently of the applied charge value the original cationic potentiometric response was successfully replaced by an anionic one. All electrodes exhibit near-Nernstian slopes (–64.1 ± 2.3, –65.0 ± 0.7, –65.9 ± 1.2 and –67.5 ± 1.9 mV·dec-1 for electrodes 1, 2, 3 and 4 correspondingly) in the range of PF6– concentration from 5·10-6 to 10-3 M. None of the electrodes show a cationic response after being exposed to solutions of different concentrations of TBACl (see Figures S7–S10). The subsequent reduction of the underlying POT film was performed by applying a constant current of –5 µA (–71 µA cm-2) for the same time intervals of 3, 6, 8 and 10s in 1 mM TBACl solution, resulting in the corresponding charge density values of–213,–426,–568 and –710 µC cm-2. The constant-current chronopotentiogramms and OCP as a function of time for all electrodes are presented in Figures S3–S6.
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Analytical Chemistry
Figure 4 demonstrates that this electrochemical modulation of the POT layer allows us to restore the original cationic potentiometric response of the membranes, providing near-Nernstian slopes to TBA+ (56.7 ± 3.7, 56.2 ± 3.2, 58.8 ± 2.3 and 58.8 ± 6.1 mV·dec-1 for electrodes 1, 2, 3 and 4 correspondingly) in the concentration range from 10-6 to 10-3 M. To check the presence of an anionic response the electrode potential was measured in the solutions of NaPF6 in the concentration range from 10-6 to 10-3 M. An anionic response was not observed. Instead, cationic response with sub-Nernstian slope towards Na+ was demonstrated (see Figures S7–S10). To evaluate the stability of the established anionic and restored cationic responses the calibration curves initially observed after applying the corresponding current pulse were compared with the ones obtained after 24 h of keeping the electrodes in 1mM solution of NaPF6 or TBACl. Figure 5c,d demonstrates that the restored TBA+ potentiometric response is stable for all electrodes which exhibit nearNernstian slopes even for 24 hours after applying the reduction current pulse without respect to the corresponding charge values. On the other hand, one can see the important influence of the applied charge values on the stability of the PF6– response (see Figure 5a,b). For 420 µC cm-2 and 840 µC cm-2 one can see a negative shift in measured EMF values and a significant loss in the exhibited slope values. In fact, the increase in standard deviation values for the PF6– calibration curve obtained for electrode 1 in comparison with other electrodes is also the result of observed gradual shift in measured EMF values. One possible explanation for such behavior is likely the instability of the oxidized POT+ form that reduces spontaneously back to POT as suggested earlier by Amemiya and co-workers.49 The observation that a lower reduction current pulse is required to restore the original TBA+ potentiometric response can be considered as indirect
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evidence for this spontaneous POT reduction with time, although the large associated anodic potentials also suggest possible side reactions. Nevertheless, it was discovered that the stability of PF6– response can be enhanced by increasing the applied charge value. For 1120 µC cm-2 near-Nernstian slopes are observed both for the initial calibration curve and the one obtained after 24 h. A subsequent increase of the applied charge value up to 1400 µC cm-2 provides similar results (see Figure 5a,b). Therefore, the protocol involving the oxidation of the underlying POT film by applying a constant current pulse of 140 µA cm-2 for 8 s (1120 µC cm-2) and its subsequent reduction by a constant current pulse of – 71 µA cm-2 µA for 8 s (–568 µC cm-2) may be considered optimal for switching between potentiometric cationic and anionic responses. It was evaluated whether any components are being lost from the membrane phase as a result of leaching process and / or possible oxidation process coupled with spontaneous POT+ reduction. This was done by comparing the cyclic voltammetric responses in 1mM TBACl solution before and after switching between cationic and anionic response. As shown in Figure S11 the peak current values obtained for freshly prepared electrodes in 1mM TBACl and after restoring TBA+ potentiometric response within 1 h (applying oxidation and subsequent reduction current pulse) correspond well to each other, while after 48 h of electrochemical manipulations with the same electrodes (applying oxidation current pulse, waiting for 24 hours, applying reduction current pulse, waiting for 24 h) one observes some loss in the measured signals. The influence of the cast POT layer thickness on the switching properties was also studied. Two types of electrodes were prepared by drop-casting of 30 µL and 60 µL instead of the originally used 10 µL of 0.125 mM POT solution in chloroform on GC surfaces. Cyclic voltammetry experiments in aqueous 10-3 M TBACl 16 ACS Paragon Plus Environment
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(Figure S12) shows that the ion transfer waves were significantly suppressed with increasing POT layer thickness. Similar results had been reported earlier with ion transfer voltammetry experiments on electropolymerized POT layers covered with thin polymer membrane.67 The effect is explained with an increasingly dense layer that suppresses the counter ion doping of POT upon electrochemical oxidation.67 In contrast, an increased thickness of the POT film does not affect the original potentiometric TBA+ response as both types of electrodes yield near-Nernstian slopes (60.8 ± 0.5 and 57.6 ± 0.2, mV·dec-1 for 30 µL and 60 µL POT solution, respectively) in the TBA+ concentration range from 10-6 to 10-3 M (see Figure S13). In order to switch the cationic and anionic potentiometric responses the oxidation / reduction of the POT film was performed according to the previously established protocol that provided the best results (140 µA cm-2 / –71 µA cm-2 µA for 8 s). The obtained constant-current chronopotentiogramms for electrodes with a thicker underlying POT layer were similar to the ones observed with a thin POT layer, but the potential values during the reduction process were even more extreme (reaching -10 V in some cases). Figure S13 demonstrates that as a result of the oxidation / reduction of POT film the original cationic TBA+ potentiometric response could be switched to anionic and subsequently to the restored TBA+ response. However, the established anionic responses deteriorated considerably with increasing POT film thickness, see standard deviations for the PF6– calibration curves in Figures S14 and S15 that has its origin in a gradual shift in measured EMF values. Thicker POT films resulted in a stability loss of the imposed anionic response while it did not influence the cationic response. The electrode with 60 µl of solvent-casted POT solution demonstrated a pronounced cationic response after being exposed to solutions of different concentrations of TBACl for 24 h (see Figure S15). To conclude, the proposed approach for the reversible switching 17 ACS Paragon Plus Environment
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between potentiometric cation and anion responses can only be implemented for the electrodes with thin POT layers.
Conclusions A novel approach for potentiometric cation and anion detection using a single thin ion-selective polymeric membrane was demonstrated. This approach is based on exploiting the redox properties of underlying POT film for ion expulsion and extraction from and to the membrane phase. Freshly prepared membrane exhibits a cationic Nernstian response by virtue of the presence of a cation-exchanger in its composition. The oxidation of POT film to POT+ stimulate the expulsion of cations from the membrane phase following the extraction of anions from the sample solution to counterbalance released cationic sites from ETH 500. This switches the original cationic response of the ISE to an anionic one. In contrast, the reduction of POT+ to POT triggers the expulsion of anions from the membrane with subsequent extraction of cations from the sample solution, allowing one to restore the original cationic response. The reversible switching between cation and anion potentiometric detection was confirmed for tetrabutylammonium and hexafluorophosphate ions yielding nearNernstian responses which were stable for at least 24 h. The oxidation / reduction of POT film was performed by applying a constant current pulse. It was found that the protocol involving the oxidation of the underlying POT film by applying constant current pulse of 140 µA cm-2 for 8 s (1120 µC cm-2) and its subsequent reduction by applying constant current pulse of –71 µA cm-2 µA for 8 s (–568 µC cm-2) is an optimal one in terms of achieved potentiometric response slope values and signal stability.
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While the work has focused on POT as a well-established model system, the principle is not restricted to this conducting polymer. Lipophilic redox probes that exhibit mild reduction potentials are particularly promising for this application.
Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Constant-current chronopotentiogramms and open circuit potentials as functions of time. Potential stability after applying oxidation / reduction current pulses. Cyclic voltammograms before and after switching between cationic and anionic response. Comparison of electrodes with different POT layer thickness. (PDF)
Acknowledgements The authors thank the Swiss National Science Foundation for financial support of this research. The authors thank Luc Monnier and Dante Savoia for the assistance with this project during their traineeship.
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Figure Captions
Figure 1. Selectivity coefficient determination by the modified separate solution method62 for (a) a conventional ISE with inner reference solution and (b) an all solid contact ISE with a thin layer membrane. Error bars are standard deviations (n = 3). Figure 2. (a) Observed cyclic voltammograms in 1mM TBACl at different scan rates. (b) Observed relationship between ipeak and scan rate. (c) Response towards different cations. Scan rate: 100 mV s-1. Figure 3. Proposed mechanism of switching between cationic and anionic potentiometric responses for thin layer membrane containing cation-exchanger (R-) and lipophilic electrolyte (R1+R2-) backside contacted with POT as a result of applying (a) oxidation and (b) reduction current pulses. Figure 4. Potentiometric responses before and after applying oxidation/reduction current pulses of 140 µA cm-2/–71 µA cm-2 for (a) 3s, (b) 6s, (c) 8s and (d) 10s in NaPF6 and TBACl solutions. Error bars are standard deviations (n = 3). Figure 5. The stability of potentiometric response to PF6- (a, b) observed after applying oxidation current of 140 µA cm-2 for 3–10s and restored potentiometric response to TBA+ (c, d) after applying reduction current of –71 µA cm-2 for 3–10s. Error bars are standard deviations (n = 3).
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