New Signal Readout Principle for Solid-Contact Ion-Selective

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A new signal readout principle for solidcontact ion-selective electrodes (SC-ISEs) Ulriika Vanamo, Elisa Hupa, Ville Yrjänä, and Johan Bobacka Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04800 • Publication Date (Web): 27 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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Analytical Chemistry

A new signal readout principle for solid-contact ion-selective electrodes (SC-ISEs) Ulriika Vanamo (a,b), Elisa Hupa(a, c),Ville Yrjänä(a) and Johan Bobacka(a)* a

Laboratory of Analytical Chemistry, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo, Finland

b

Laboratory of Materials Chemistry and Chemical Analysis, University of Turku, Vatselankatu 2, 20500 Turku, Finland

c

Graduate School in Chemical Engineering (GSCE), Finland

*corresponding author, email: [email protected]

Abstract A novel approach to signal transduction concerning solid-contact ion-selective electrodes (SC-ISE) with a conducting polymer (CP) as the solid contact is investigated. The method presented here is based on constant potential coulometry, where the potential of the SC-ISE vs the reference electrode is kept constant using a potentiostat. The change in the potential at the interface between the ion-selective membrane (ISM) and the sample solution, due to the change in the activity of the primary ion, is compensated with a corresponding, but opposite change in the potential of the CP solid contact. This enforced change in the potential of the solid contact results in a transient reducing/oxidizing current flow through the SC-ISE. By measuring and integrating the current needed to transfer the CP to a new state of equilibrium, the total cumulated charge that is linearly proportional to the change of the logarithm of the primary ion activity, is obtained. In this work, different thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contact. Also, coated wire electrodes (CWEs) were included in the study to show the general validity of the new approach. The ISM employed was selective for K+ ions, and the selectivity of the membrane under implementation of the presented transduction mechanism was confirmed by measurements performed with a constant background concentration of Na+ ions. A unique feature of this signal readout principle is that it allows amplification of the analytical signal by increasing the capacitance (film thickness) of the solid contact of the SC-ISE.

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Introduction One by one, the problems related to the implementation of solid-contact ion-selective electrodes (SCISEs) in modern applications, including portable health diagnostics and remote environmental monitoring, are scrutinized by the research community. The inherent properties of SC-ISEs, i.e. robustness, technical simplicity and low power consumption, are good starting points to reach the goal that is reliable, maintenance-free sensors for long-term measurements, and low-cost, easy-to-use disposable sensing chips for various on-site analyses. For example the lack of a good solid-state reference electrode (RE) that has been an issue for very long, seems to be solved [1, 2]. Fabrication of SC-ISEs in configurations suitable for mass production on various interesting substrates, such as paper, is emerging

[3-8]

. One of the remaining

problems is the inadequate stability and reproducibility of the standard potential E°

[9]

that is a necessary

requirement for performing calibration-free measurements in conventional, open circuit potential (OCP) mode [10, 11]. It is hampering both the applications that require long-term stability [12], and the ones based on single-use sensors, where piece-to-piece reproducibility is important the irreproducibility of the E° is under intensive research

[13]

. Solving the problem related to

[14-18]

. In many approaches the ion-selective

electrodes and membranes are used with non-zero current pretreatment or measurement mode [19-24]. Introduction of a conducting polymer film as ion-to-electron transducer between the electrical conductor and the ion-selective membrane (ISM) was proven to increase the stability of the signal redox capacitance of the conducting polymer (CP)

[25]

due to the

[26]

. The properties of the CPs also offer the possibility

to externally adjust and control the redox state and thus the potential of the conducting polymer

[16, 17]

determined by its ionic content and redox equilibrium. These characteristics can be used in exploitation of CP-based ISEs in a novel way that would ensure calibration-free utility after optimization. In a recent proof-of-concept short communication we presented a novel signal transduction method for SC-ISEs based on constant potential coulometry

[27]

, and here the said method is demonstrated more

thoroughly. The principle of the method is shown in Figure 1. The potential of the SC-ISE is maintained constant vs the RE through the means of a potentiostat. The potential of the conducting polymer (CP) solid-contact contributes to the overall potential of the SC-ISE, and the potential of the CP-film itself is defined by its redox-state and ionic composition, which are related to each other [28, 29]. When the activity of the primary ion in the sample solution is altered, there is a change in the boundary potential at the interface between the ISM and the sample solution. Since the potential of the whole electrode is kept constant using the potentiostat, this potential change at the ISM | sample interface is counterbalanced by an equally large, but opposite change in the potential of the CP solid contact, resulting in a reducing/oxidizing current that can be measured. By integration of the current at the CP solid contact, 2 ACS Paragon Plus Environment

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resulting from the potential change induced by the change of the primary ion activity in the sample, a total (cumulated) charge (Q) is obtained. The logarithm of the activity (change) shows a linear relationship to the total cumulated charge, and the slope of the curve depends on the redox capacitance of the solid contact. The K+-selective SC-ISEs employed in this study were prepared by introducing a CP solid contact between the ISM and the electrical conductor. Various thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contacts, and even a coated wirearrangement (CWE) without any PEDOT(PSS) was included in the study. The currents were measured while the primary ion concentration was decreased by stepwise dilutions of the electrolyte. By using a constant ionic background of the interfering ion, it was proven that the selectivity of the ISE was maintained with this method.

Experimental Materials Valinomycin, potassium tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate (KTFPB), tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH-500), poly(vinyl chloride) of high molecular weight (PVC), bis(2ethylhexyl)sebacate (DOS) and tetrahydrofuran (THF) were from Fluka and of Selectophore® grade. Poly(sodium 4-styrenesulfonate) (NaPSS, Mw ~70 000) and 3,4-ethylenedioxythiophene (EDOT, 97%) were from Sigma-Aldrich. Deionized water (ELGA Purelab Ultra) was used throughout the measurements. Preparation of electrodes The SC-ISEs used in the study were prepared by inserting a glassy carbon rod (GC, Sigradur G, HTW Hochtemperatur – Werkstoffe GmbH, D-86672, Thierhaupten, Germany) into a PVC housing and polishing the disk-shaped end with sandpapers, 1 µm diamond paste, and finally with 0.3 µm Al2O3 paste. Prior to electropolymerization, the electrodes were cleaned chemically by immersing them in 1 M HNO3 followed by ultra-sonication in ethanol and water baths, respectively.

The electropolymerization was performed galvanotstatically in a conventional three-electrode cell using an Autolab General Purpose Electrochemical System (AUT30.FRA2-AUTOLAB, Eco Chemie, B.V., The 3 ACS Paragon Plus Environment


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Netherlands). A GC-rod was used as a counter electrode (CE) and the reference was a Metrohm Ag/AgCl/3 M KCl/0.1 M NaPSS double-junction electrode, while the polished GC-disk electrode functioned as the working electrode.

The polymerization solution (0.01 M EDOT and 0.1 M NaPSS) was left to stir overnight to ensure proper dissolution of the monomer. Prior to polymerization, the solution was de-aerated by allowing nitrogen to bubble through it for 20 min, after which nitrogen was positioned to flow above the solution to prevent interference with oxygen. The diameter of the GC-rod was 3 mm and the area of the GC disk was thus 0.07 cm2. Films of different thicknesses were prepared by applying a 0.2 mA/cm2 current for 71, 143, and 286 seconds to obtain films with 1 mC, 2 mC, and 4 mC total charge, respectively. Deposition of the conducting polymer galvanostatically enables precise control of the polymerization charge and thus the redox capacitance of the PEDOT(PSS) layer [26].

After the polymerization, the GC/PEDOT(PSS) electrodes were soaked in 0.1 M KCl solution overnight, rinsed with deionized water, and allowed to dry in air for one day. Then, 50 µl of K+-selective membrane cocktail was drop-cast on each electrode. The composition of the membrane was 1% (wt) valinomycin, 0.5% (wt) KTFPB, 1% (wt) ETH-500, 65.3 % (wt) DOS, and 32.2 % (wt) PVC dissolved in THF (dry content 15%). Also coated-wire type electrodes (CWE), without any PEDOT(PSS) between the ISM and GC were prepared. After evaporation of THF, the K+-selective SC-ISEs were conditioned in 0.1 M KCl for approximately 5 days.

Measurements The prepared K+-selective SC-ISEs were calibrated with a multichannel potentiometer (EMF16 Interface, Lawson Labs, Inc) over KCl concentrations ranging from 10-1 to 10-9 M. A total of 16 dilutions (½ decade per dilution) were performed with 5 minute pauses between each dilution. The data from this measurement was then used to plot the open circuit potential (OCP) versus the logarithm of activity for each electrode to check that the SC-ISEs functioned as expected. Two Metrohm 700 Dosino units connected to and controlled by a Metrohm 711 Liquino were used to dilute the electrolyte in potentiometric calibrations as well as in chronoamperometric measurements. The pumps were programmed to remove 34.2 ml of the sample solution and replaced the removed volume with an equivalent volume of either deionized water (when no constant ionic background was used in the measurement) or 0.1 M NaCl solution (when a constant ionic background was used in the measurement).


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The magnitude of the alteration of activity per dilution step was controlled by adjusting the starting volume of the electrolyte to be 50 ml (0.50 decades / step), 100ml (0.18 decades / step) or 200 ml (0.08 decades / step). The reference electrode used in all the measurements after the polymerization was a Metrohm Ag/AgCl/3 M KCl/1 M LiAc double-junction electrode. Chronoamperometric measurements were performed with an Iviumstat (Ivium Technologies, the Netherlands) in a three-electrode cell with a GC rod as CE. The potential of the K+-selective SC-ISE was set to a constant value, 0.0 mV vs the RE in this case, and the current resulting from oxidation/reduction of the CP solid contact was measured. The potential was kept at the selected 0.0 mV while the electrolyte was stepwise diluted analogously to the potentiometric calibration described above, only applying 15 min pauses between each dilution to allow time for the equilibration. The measured current-time (I-t) curves were integrated to obtain the total (cumulated) charge (Q) passed through the SC-ISE after each dilution. The concentration for the initial KCl solution was 0.1 M when 0.50 decade dilutions were made and 0.01 M when 0.18 or 0.08 decade dilutions were made. The measurements were also repeated with a constant 0.1 M NaCl as ionic background, in which case the withdrawn electrolyte volume was replaced with 0.1 M NaCl solution. Measurements were performed with and without stirring, and - when stirring was present - the stirring rate was constant.

Results and Discussion In the chronoamperometric measurements, the electrodes were allowed to equilibrate in the measurement electrolyte under the applied 0.0 mV potential before starting the dilution procedure, so that the measured current was stable and close to zero. The threshold current for 1 mC films was ≤ ±1 nA and slightly higher for the thicker films. This was done in order to control that the experiment was commenced from the equilibrium state of the set conditions to reduce background drift.

The choice of the applied potential is fundamentally related to the potential of the reference electrode, the activity of primary ion in solution and the potential stability window of PEDOT. Fortunately, the conducting polymer PEDOT has a wide potential stability window, i.e. approximately -500 mV to +500 mV (vs Ag/AgCl/KCl(aq)) in aqueous solution, which allows some flexibility in choosing the applied potential. Applying a potential that is close to the open circuit potential of the SC-ISE at a given ion concentration is expected to give the shortest initial equilibration time. 5 ACS Paragon Plus Environment


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The thickness (or absence, in case of CWEs) of the PEDOT(PSS) solid-contact influenced the time required for equilibration so that the 4 mC films needed the longest time in this study. Also the distance of the open circuit potential of the SC-ISE from the applied potential is of significance. In some cases the current did not reach zero during the equilibration step, resulting in a small additional background drift in the current measured during the dilution steps. From our previous studies it was clear

[16, 17, 27, 30]

that the

resistance of the ISM also influences the equilibration time required so that a large membrane resistance slows down the equilibration process of the CP under the membrane. KTFPB, a typical salt additive for plasticized polymeric membranes, decreases the membrane resistance but cannot be added in molar excess vs. the ionophore (in order to preserve the selectivity). Therefore, the membrane resistance was decreased further by addition of ETH-500 where both the anion and the cation are lipophilic and both stay in the membrane phase. Additionally, the volume of membrane cocktail (drop-cast) was only 50 µl for each electrode (instead of 100 µl used earlier) to lower the membrane resistance. Between the measurements, the SC-ISEs were conditioned in 0.1 M KCl solution.

Figure 1. A schematic presentation of the novel signal transduction principle. In the beginning, the system is at equilibrium at an applied, constant potential. When the activity of the primary ion in the electrolyte solution is altered (1), the potential difference at the sample | ISM interface is changed (2). Since the potential of the whole SC-ISE is enforced to remain constant, the potential change at the sample | ISM interface gives rise to an oxidation/reduction current (3) until the potential change at the sample | ISM is compensated by an equal but opposite change in the potential of the solid contact (4). By integrating the current peak caused initially by the alteration of the primary ion activity, the total (cumulated) charge (Q) resulting from the redox reaction of the solid contact is obtained (5).


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Figure 2a shows chronoamperograms recorded for the K+-selective SC-ISEs when the starting solution of 10-2 M KCl was gradually diluted to 10-4.9 M KCl with 0.18 decade concentration steps. The potential was retained at 0.0 mV vs the RE and the currents resulting from the oxidation / reduction of the PEDOT(PSS) solid contact, shown in eq.1, were measured. When the K+ activity in the electrolyte is decreased, the phase boundary potential (EPB) at the ISM | solution interface decreases with -∆E. In order to maintain the constant potential of the SC-ISE (enforced by the potentiostat), the PEDOT(PSS) film is oxidized so that its potential increases with +∆E and thus compensates for the negative potential change at the ISM | sample interface. If the membrane was selective for any anion, the dilution of the solution containing that anion would lead to an increase in the EPB which would, in turn, be compensated by reduction of the CP solid contact. And further, an increase in the activity of the cation K+ would lead to positive values of ∆EPB for the K-ISE, resulting in reduction and negative ∆E of the CP solid contact. ↔

(eq.1)

Figure 2. a) Chronopotentiograms recorded for K+-selective SC-ISEs with (1) 1 mC, (2) 2 mC, and (3) 4 mC PEDOT(PSS) solid contacts. The starting solution of 10-2 M KCl was gradually diluted to 10-4.9 M KCl with 0.18 decade steps, while the potential was retained constant at 0.0 mV vs the RE. No stirring or ionic background was used in this measurement. b) Enlargement of the 2nd and the 3rd dilution steps shown in Figure 2 a. Figure 2b presents the enlargement of the second and the third dilution steps. It can clearly be seen that the current peaks, resulting from the change of the activity of the primary ion in the sample, become broader with increasing thickness (redox capacitance) of the solid contact. The potential of the conducting polymer solid contact (in this case PEDOT(PSS)) is dictated by its redox state and ionic content, i.e. the ratio of oxidized and neutral states (PEDOT+/PEDOT0). This ratio should be the same for all thicknesses at a given potential. When the potential changes at the membrane | ISM interface, it must be compensated by a change in the ratio PEDOT+/PEDOT0, when the total potential of the SC-ISE is forced to remain 7 ACS Paragon Plus Environment


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constant. Thus, for thicker PEDOT films, a larger amount of material is oxidized/reduced in order to achieve a given change in the redox state of PEDOT, which leads to broader current peaks and higher Q. This phenomenon can be employed to amplify the measurement signal. During transition from one equilibrium state to another, contribution from the diffusion potential inside the PEDOT(PSS) layer and the ion-selective membrane may also be significant, and therefore it is important to allow adequate equilibration time after each change in the primary ion concentration. After reaching the new equilibrium state we assume here that the main contribution to the potential compensation is due to the change in the oxidation state of the conducting polymer solid contact with possible contributions also from the interfacial potentials the GC | PEDOT(PSS) and the PEDOT(PSS) | ISM interfaces.

Figure 2 shows clearly that the response time (equilibration time) increases with increasing thickness of the PEDOT(PSS) layer, i.e. with increasing redox capacitance of the solid contact and with increasing signal amplification. This implies that SC-ISEs with a high redox capacitance of the ion-to-electron transducer are most suitable for such applications where the detection of very small activity changes is of primary importance. When smaller dilution steps are taken, analogously smaller current peaks and charges (Q) are observed. Earlier it was shown using cation-sensitive SC-ISEs with 10 mC PEDOT(PSS) as solid contact that an alteration of concentration from 0.02 M to 0.01 M KCl and back to 0.02 M could easily be detected with this method. Even a concentration change from 0.02 M to 0.0185 M (0,034 decades) was clearly detectable [27]. In this work, the change of the concentration with 0.08 decade steps was well detected for a concentration range from 0.01 M to 10-3.3 M KCl even with the thinnest PEDOT(PSS) film (1 mC) used. Naturally, larger changes in the activity of the primary ion in the sample lead to larger charges (Q) measured. Integration of the current peaks recorded during dilution allows for inspection of cumulated charge (Q) transferred in the process. The charge passed through the K+-selective SC-ISE with 1 mC PEDOT(PSS) film during stepwise dilution (0.18 decade / step) from 10-2 M to 10-4.9 M KCl vs time is presented in Figure 3. The same measurement using the same electrode individual was performed both with and without 0.1 M NaCl background electrolyte and/or stirring. The use of the background electrolyte proves that selectivity, which is one of the most important characteristics of the ISEs, is maintained also with this novel signal transduction technique. The charges resulting from the 10 first dilution steps (in Figure 3) are given in Table 1.


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Table 1 Charge per activity change (∆Q/∆log a) resulting from the 10 first dilution steps for SC-ISE with 1 mC PEDOT(PSS) film. Numbers indicate measurements done (1) without background electrolyte or stirring (2) without background electrolyte with stirring (3) with 0.1 M NaCl as background electrolyte and (4) with 0.1 M NaCl as background electrolyte and with stirring. Measurements were done with the same electrode individual.

log c -2.00 -2.18 -2.36 -2.55 -2.73 -2.91 -3.09 -3.27 -3:45 -3.64 -3.82

1 x -2.74 -3.07 -3.21 -3.35 -3.41 -3.48 -3.55 -3.59 -3.66 -3.74

Average ∆Q/log a SD

-3.38 0,29

(∆Q / ∆log a) / µC 2 3 x x -2.61 -2.53 -2.94 -2.85 -3.11 -3.04 -3.22 -3.14 -3.31 -3.23 -3.37 -3.29 -3.42 -3.33 -3.47 -3.37 -3.52 -3.41 -3.57 -3.43

-3.25 0.28

-3.16 0.27

4 x -2.01 -2.51 -2.80 -2.96 -3.09 -3.18 -3.24 -3.30 -3.33 -3.35

-2.98 0.41

Figure 4 shows that there is a linear dependency between the logarithm of the activity and the charge passed through the electrode. Only a slight decrease in the slope of the calibration curve (∆Q vs ∆log a) is observed, when the constant 0.1 M NaCl background is used. Curves in Figures 3 and 4 were recorded in in following order (slopes given in parenthesis): 1 (3,60 µC/∆log a) → 3 (3.30 µC/∆log a) → 2 (3.46 µC/∆log a) → 4 (3.17 µC/∆log a). This indicates that differences in the curves are not solely related to a gradual change in the electrode properties. The slope for the linear part (log a from -1.1 to -5.5) of curve 2 in Figure 5 (measured with the same 1 mC electrode that was used in measurements for Figures 3 and 4) is 3.63 µC / ∆log a, and it was recorded 3 weeks before curve 1 in Figures 3 and 4, thus proposing excellent reproducibility for this method. It should though be taken into account that dilutions in experiment for Figure 5 were made in 0.50 decades steps, whereas 0.18 decade dilution steps were used in measurements for Figures 3 and 4. Also in measurements using SC-ISEs with 2 mC and 4 mC PEDOT(PSS) films, the slopes were lower when ionic background was used, and very good



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reproducibility for calibrations without background electrolyte was observed. Largest deviations in slopes derive from the initial background drift, i.e. a small residual current at the beginning of the dilution sequence, when a measurement has been started before the electrode has reached equilibrium. It is thus very important to wait for equilibrium before commencing a measurement in order to achieve high reproduciblity.

Figure 3. Cumulated charge passed through a K+-selective SC-ISE with 1 mC PEDOT(PSS) film during stepwise dilution (0.18 dec / step) of 10-2 M to 10-4.9 M KCl vs time. Numbers indicate measurements done (1) without background electrolyte or stirring (2) without background electrolyte with stirring (3) with 0.1 M NaCl as background electrolyte and (4) with 0.1 M NaCl as background electrolyte and with stirring. Measurements were done with the same electrode individual.


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Figure 4. Cumulated charge Q vs logarithm of the K+ activity for a K+-selective SC-ISE with 1 mC PEDOT(PSS) film during stepwise dilution (0.18 dec) of 10-2 M to 10-4.9 KCl. Numbers indicate measurements done (1) without background electrolyte or stirring (2) without background electrolyte with stirring (3) with 0.1 M NaCl as background electrolyte and (4) with 0.1 M NaCl as background electrolyte and with stirring. Measurements were done with the same electrode individual. The obtained analytical signal (Q) can be amplified by increasing the thickness, i.e. the redox capacitance, of the conducting polymer solid contact. This is illustrated in Figure 5 for K+-selective SC-ISEs without PEDOT(PSS) (CWE) and with three different thicknesses (1 mC, 2 mC, and 4 mC) of PEDOT(PSS). An enlargement of the response of the CWE is shown in the insert of Figure 5.

Figure 5. Charge vs logarithm of activity for K+-selective SC-ISEs with 0 mC (CWE), 1 mC, 2 mC and 4 mC PEDOT(PSS) film as solid contact during stepwise dilution (0.5 dec) of 10-1 M KCl to10-8.5 M KCl without stirring or background electrolyte. Enlargement of the response of the CWE is shown in the insert.



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The calibration slopes in Figure 5 are approximately proportional to the PEDOT(PSS) film thickness, as expected. By multiplying the cumulated charge (Figure 5) for the 1 mC film by 4 and for the 2mC film by 2, the results are slightly larger than for the 4 mC film. This is expected to be due to inadequate equilibration time for the thicker films, as can be seen also in Figure 2, when 15 min integration time between each dilution was applied in all cases. The very low charges recorded for the coated wire electrodes (CWE) are due to their much lower double layer capacitance compared to the redox capacitance of even the thinnest, 1mC PEDOT film [26]. However, the calibration plot for the CWE (Figure 5, insert) shows that the signal transduction mechanism works also for the CWE based on purely capacitive transduction at the GC | ISM interface, which is an additional support for the suggested transduction mechanism. Therefore, the method will likely work also for other solid contacts, including porous carbon [31, 32]

and other nanostructured materials having a higher capacitance than the polished GC disk used here

for the CWE. The effect of stirring was studied by repeating the same measurement, dilution of 0.1 M KCl to 10-8.5 M KCl with 0.5 decade dilution steps with and without stirring, while keeping the potential constant and recording the current. In Figure 6, this test is presented for a K+-selective SC-ISE with 1 mC PEDOT(PSS) film. It seems that with stirring it is possible to increase the linear range and thus lower the detection limit. The influence of stirring was not visible at higher concentrations, as can be seen in Figures 4 and 6. At lower concentrations, the linear range for both CWE and SC-ISEs with 1 mC PEDOT(PSS) films was improved by stirring. For SC-ISEs with 2 mC and 4 mC PEDOT(PSS) films, stirring stabilized the response faster while an extension of the linear range was not observed in the performed measurements for these thicker films. For each dilution step, the potential at the ISM | solution interface decreases, which means that PEDOT(PSS) is oxidized (positive current transient), i.e. the reaction in (eq. 1) proceeds from right to left. This implies that K+ ions are released from PEDOT(PSS) via the ISM to the solution upon dilution. When the thickness of the PEDOT(PSS) layer is increased, the flux of K+ ions also increases for a given change of the K+ activity in the solution. The experimental results above therefore indicate that the stirring applied here did not cause a sufficient increase in K+ transport away from the ISM | solution interface for the SC-ISEs with 2 mC and 4 mC PEDOT(PSS) films (under the experimental conditions used in this work). Furthermore, in order to extend the linear range for SC-ISEs with thicker PEDOT(PSS) films, also the integration time may have to be increased. The influence of ion transport at low concentrations, when applying this new transduction principle, is an important topic that deserves a more detailed separate study.


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Figure 6. Charge vs logarithm of activity for K+-ISE with 1 mC PEDOT(PSS) film during dilution (0.5 dec / dilution step) of 10-1 M KCl to 10-8.5 M KCl with (2) and without (1) stirring. No background electrolyte was used. With traditional, direct open circuit potentiometric measurements, the slope of the calibration curve decreases with the increasing valency of the primary ion. With the presented method it is expected to be possible to amplify the sensitivity of SC-ISEs for divalent and trivalent ions by increasing the redoxcapacitance of the solid contact and detect even small changes in activity.

With this signal read-out method there is a trade-off between the sensitivity of the SC-ISE and the speed of the response, since both are affected by the thickness (redox capacitance) of the film. The said trade-off can be optimized for a specific application in order to utilize the possibility to amplify the signal, which is an advantage compared to classical potentiometry.

Concerning practical applications, the novel signal readout principle is expected to work well in standard addition mode, where either a known standard is added to an unknown sample (standard addition) or a known volume of unknown sample is added to a known standard and the charge is measured. We believe that the method can be useful also for continuous monitoring when the main goal is to detect small deviations from a constant (set value) of ion activity (concentration). It could be seen that as small as 0.08 decade changes were detectable in this work, and a change of 0.034 decades (0.02 M to 0.0185 M) in our earlier work

[27]

. The applicability of the proposed signal readout principle also to gradual changes in

concentration (continuous monitoring) is expected as long as the changes are slow enough in comparison to the response time of the electrode. Furthermore, very rapid concentration changes to both higher and 13 ACS Paragon Plus Environment


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lower concentrations can be detected as (negative and positive) current spikes, while obtaining an exact concentration value of such fast concentration changes will require electrodes with a sufficiently short response time. This is a topic of our on-going work.

Conclusions A novel signal readout principle for SC-ISEs was described. The method is based on constant potential coulometry. The measured charge Q was linearly proportional to alteration in the logarithm of the activity of the primary ion in the sample solution. This correlation between Q and activity is expected, when the ion-selective membrane gives a Nernstian response (E is linearly proportional to log a) and the redox capacitance of the solid contact is practically independent of the potential. The measured signal could be amplified by increasing the redox-capacitance (thickness) of the conducting polymer solid contact. The traditional selectivity of the ion-selective membrane was preserved also with measurements implemented using this novel method. The method is expected to be particularly useful in standard addition mode and for continuous measurements where it is important to detect very small changes in ion activity. Based on the results obtained with the CWE, the presented signal transduction method is expected to work for SCISEs employing also other solid-contact materials than conducting polymers. This work may thus open up a new research direction in the field of solid-contact ion sensors and increase the applicability of ion sensing in analytical chemistry.

Acknowledgements U. Vanamo would like to acknowledge financial support of the Magnus Ehrnrooth Foundation and E. Hupa that of the Graduate School in Chemical Engineering, Finland. This work is part of the activities of the Johan Gadolin Process Chemistry Centre at Åbo Akademi University.

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FOR TOC ONLY (graphical abstract)


New Signal Readout Principle for Solid-Contact Ion-Selective

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