Article
Reference
Potentiometric Sensor Array with Multi-Nernstian Slope
ZDRACHEK, Elena, BAKKER, Eric
Abstract
The sensitivity of potentiometric sensors functioning in equilibrium mode is limited by the value predicted according to the Nernst equation and inversely proportional to the charge of the analyte ion. Therefore, an increased ion charge results in a dramatic decrease of sensor sensitivity. We propose an approach to allow one to increase the sensitivity of potentiometric measurements by using a combined electrochemical cell composed of several identical ion-selective electrodes immersed into separate sample solutions of equal composition. The combination of n electrodes demonstrating individually a Nernstian slope in one electrochemical cell allows to amplify the signal and associated response slope by n times.
The proposed approach is shown to provide a double and triple Nernstian slope for potassium-, calcium-, nitrate and carbonate-selective electrodes by combining two or three identical electrodes, correspondingly. Each ion-selective electrode functions in an equilibrium mode, hence ensuring response stability and reproducibility.
ZDRACHEK, Elena, BAKKER, Eric. Potentiometric Sensor Array with Multi-Nernstian Slope.
Analytical Chemistry , 2020, vol. 92, no. 4, p. 2926-2930
DOI : 10.1021/acs.analchem.9b05187
Available at:
http://archive-ouverte.unige.ch/unige:136525
Disclaimer: layout of this document may differ from the published version.
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Potentiometric Sensor Array with Multi-Nernstian Slope
Elena Zdrachek and Eric Bakker*
Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland.
ABSTRACT: The sensitivity of potentiometric sensors functioning in equilibrium mode is limited by the value predicted according to the Nernst equation and inversely proportional to the charge of the analyte ion. Therefore, an increased ion charge results in a dramatic decrease of sensor sensitivity. We propose an approach to allow one to increase the sensitivity of potentiometric meas- urements by using a combined electrochemical cell composed of several identical ion-selective electrodes immersed into separate sample solutions of equal composition. The combination of n electrodes demonstrating individually a Nernstian slope in one elec- trochemical cell allows to amplify the signal and associated response slope by n times. The proposed approach is shown to provide a double and triple Nernstian slope for potassium-, calcium-, nitrate and carbonate-selective electrodes by combining two or three identical electrodes, correspondingly. Each ion-selective electrode functions in an equilibrium mode, hence ensuring response sta- bility and reproducibility.
It is well-known that the sensitivity of a potentiometric sensor functioning in equilibrium mode is limited by the slope pre- dicted from the Nernst equation as 59.2/z mV at the room temperature of 298 K, where z is the charge of analyte ion. An increase of sensor sensitivity would allow one to lower exper- imental errors when assessing analytes that exhibit a narrow concentration range, for example, ionized calcium in blood plasma and sweat or potassium in blood serum. It would also facilitate the potentiometric detection of ions with very large valences as an increased analyte ion charge results in a signifi- cant decrease of the corresponding sensor sensitivity.
An apparent non-equilibrium super-Nernstian response for ion-selective electrodes (ISEs), known as the Hulanicki ef- fect,1 was previously demonstrated as a result of the induced concentration gradients and counterdiffusion of primary and interfering ions within the membrane phase due to ion- exchange and co-extraction equilibria at the membrane inter- face. While the control of analyte ion fluxes from the mem- brane to the sample phase was shown to lower the detection limit of ISEs by several orders of magnitude,2-5 an excessive inward concentration gradients within the membrane may cause analyte depletion at the membrane surface and generate a super-Nernstian response.2,3
Subsequently, similar principle of counterdiffusion kinetics of primary ions with interfering ones was used by the group of Meyerhoff to develop a potentiometric sensor with analytical- ly meaningful super-Nernstian slope for the anticoagulant heparin with an average charge of one molecule about -70.6-8 This approach resulted in further potentiometric sensors for polyionic species, including protamine,9 the food additive car- rageenan,10 DNA,11 and polyquaternary species.12
Despite these efforts, non-equilibrium potentiometry remains less robust than the response of well conditioning classical ion-selective electrode membranes. For example, once a membrane is in contact with an analyte sample for a prolonged time period, the equilibrium state is eventually reached and the response slope reverts back to its equilibrium value. Therefore,
a non-equilibrium response often depends on time and hydro- dynamic conditions making it incompatible with a continuous monitoring or wearable sensor applications.
We propose here a new way to enhance the sensitivity of po- tentiometric sensors and achieve double or triple Nernstian slope. The principle is based on the use of several identical ISEs to form a joined electrochemical cell with an increased number of membrane / sample phase boundaries placed in series. An equilibrium is established at each phase boundary and provides a corresponding increase of the potential meas- ured for the joined cell. The sensors are functioning in an equi- librium mode, which makes the response highly reproducible and independent of time and hydrodynamic conditions.
EXPERIMENTAL SECTION
Reagents. Poly(vinyl chloride) (PVC), bis(2-ethylhexyl) seba- cate (DOS), 2-nitrophenyl octyl ether (o-NPOE), bis(2- ethylhexyl) phthalate (DEHP), bis(2-ethylhexyl) adipate (DOA), sodium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (NaTFPB), potassium tetrakis [3,5bis(trifluoromethyl) phe- nyl]borate (KTFPB), tridodecylmethylammonium nitrate (TDMAN), tridodecylmethylammonium chloride (TDMACl), valinomycin (potassium ionophore I), N,N-Dicyclohexyl- N′,N′-dioctadecyl-3-oxapentanediamide (calcium ionophore IV), N,N-Dioctyl-3α,12α-bis(4-trifluoroacetylbenzoyloxy)-5β- cholan-24-amide (carbonate ionophore VII), tetradodec- ylammonium tetrakis(4-chlorophenyl)borate (ETH 500) and tetrahydrofuran (THF) were of Selectophore grade (Sigma- Aldrich, Switzerland). Lithium acetate dihydrate, potassium chloride (>99.5%), calcium chloride (>97%), sodium nitrate (>99%), sodium bicarbonate (>99.7%), tris(hydroxymethyl) aminomethane (Tris, >99.9%) and sulfuric acid (95-97 %) were purchased from Sigma-Aldrich. Aqueous solutions were prepared by dissolving the appropriate salts in Milli-Q water (18.2 MΩ·cm). Multi-walled carbon nanotubes (MWCNTs) with >95% wt. purity (0.5-200 µm length and 30-50 nm out- side diameter, M4905) were purchased from HeJi, Inc. (Shen-
zhen, China). MWCNTs were modified in-house to make them soluble in THF by following the protocol of Yuan et.
al.13 Briefly, MWCNTs were oxidized to produce carboxylic acidic groups followed by amide formation with octadecyla- mine to yield functionalized MWCNTs (f-MWCNTs).
Preparation of Ion-Selective Electrodes. The casting solu- tion for potassium selective membranes as prepared by dis- solving 2.2 mg of potassium ionophore I, 0.9 mg of NaTFPB, 63.2 mg of DOS and 33.3 mg of PVC in 1 mL of THF; for calcium selective membranes, 1.2 mg of calcium ionophore IV, 0.5 mg of KTFPB, 33.2 mg of PVC and 67.5 mg of o- NPOE dissolved in 1 mL of THF; for nitrate selective mem- branes, 0.6 mg TDMAN, 1.7 mg inert lipophilic salt ETH 500, 33.5 mg of PVC and 63.6 mg of plasticizer DEHP in 1 mL of THF; for carbonate selective membranes, 4.1 mg of carbonate ionophore VII, 1 mg of TDMACl, 50 µL of DOA and 30.2 mg of PVC dissolved in 1 mL of THF.
Commercial glassy carbon electrodes (Glassy carbon electrode tip 6.1204.300, Metrohm) were modified with a film of f- MWCNTs deposited on top of each electrode by drop casting of 10 µL of a f-MWCNT solution in THF (1 mg·ml-1) 10 times, allowing each layer to dry for 10 min before the next layer deposition. Then, the corresponding membrane cocktail was drop cast on the top of the f-MWCNT film using 3 times an aliquot of 50 µL, and each layer was allowed to dry for 20 min. Finally, the electrodes were conditioned overnight (∼12 h) in the solution of the corresponding salts, namely 10-4 M KCl, 10-4 M NaNO3, 10-3 M CaCl2, or 10-3 M NaHCO3 (corre- sponding to ~0.5·10-6 M of CO32- at pH=8.6).
Electrochemical equipment and protocols. Potentiometric measurements were carried out with a high impedance input 16-channel EMF monitor (Lawson Laboratories, Inc., Mal- vern, PA) using a double-junction Ag/ AgCl/ 3M KCl/ 1M LiOAc reference electrode (Metrohm Autolab Utrecht, The Netherlands). Calibration curves for potassium-, nitrate- and calcium-selective electrodes were obtained by adding succes- sive aliquots of the concentrated solutions of the correspond- ing salts to Milli-Q water. The calibration curve for carbonate- selective electrode was carried out by adding successive ali- quots of 0.1 M NaHCO3 to a buffered solution at pH = 8.6 (0.1 M Tris-H2SO4). The carbonate concentration was calculated at this pH using the mass balance and the acidity constants (pKa1
= 6.3514 and pKa2 = 10.3314). Activity coefficients were calcu- lated according to ref.15 Electrochemical impedance spectra for the solid contact ISEs were measured at open circuit potential by an Autolab potentiostat/galvanostat PGSTAT204 (Metrohm Autolab Utrecht, The Netherlands). Measurements were performed in solutions of the corresponding salts, name- ly 10-2 M KCl, 10-2 M NaNO3, 10-2 M CaCl2, or 4.9·10-3 M NaHCO3 (corresponding to ~10-4 M of CO32-
at pH=8.6 buff- ered with 0.1 M Tris-H2SO4). The frequency range was from 100 kHz to 10 mHz, and the excitation amplitude was 10 mV.
THEORY
In potentiometric measurements the measured cell potential is equal to the sum of potential changes at all phase boundaries present in the cell. Normally, all potentials in the cell are con- stant and independent of the sample composition with the ex- ception of the potential value at the sample–membrane phase boundary. For the single cell configuration in Figure 1a one may ideally write:
𝐸𝑀𝐹!"#$%& !"##=𝐸!"#$%+𝑠 𝑙𝑜𝑔 𝑎!" (1)
where Econst is an experimentally determined constant potential value and s is the electrode slope, ideally 59.2 mV/z, where z is the charge of the analyte ion.
Figure 1. Schematic representation of a) single, b) double and c) triple cell configuration in potentiometric measurements.
Let us now consider two pairs of reference and indicator elec- trodes immersed in two separate beakers containing the same concentration of analyte ion. If one connects the reference electrode from the first pair to the indicating electrode from the second pair, while connecting the indicator electrode from the first pair and reference electrode from the second pair to the voltmeter, a joined double cell is constructed (see Figure 1b).
The observed potential of this double cell is described as above but with two potential values that are dependent on the sample composition, which are the phase boundaries between sample and membrane phase of the first and second electrodes correspondingly, giving the following:
𝐸𝑀𝐹!"#$%& !"##=2𝐸!"#$%+2𝑠 𝑙𝑜𝑔 𝑎!" (2)
According to eq 2 the double cell configuration allows to dou- ble the sensitivity of the potentiometric measurement because the slope of the observed response function is now given by the sum of two Nernstian slopes.
One may continue by merging pairs of reference and indicat- ing electrodes into a cell containing any number n of elec- trodes pairs (see Figure 1c for n=3), which should increase the sensitivity of the potentiometric response by n times as fol- lows,
𝐸𝑀𝐹! !"##$=𝑛𝐸!"#$%+𝑛𝑠 𝑙𝑜𝑔 𝑎!" (3) predicting an n times sensitivity increase compared to a con- ventional single electrode configuration.
The advantage of this configuration is its simplicity and possi- bility to be implemented using any of commonly available high-impedance voltmeter instruments. To gain a similar ef- fect with an integrated amplifier circuit multiple channels would have to be applied to the instrument circuit.
RESULTS AND DISCUSSION
To experimentally demonstrate the sensitivity enhancement of ISEs described in the theory section above, potentiometric sensors for four different analyte ions were prepared. We chose sensors sensitive for single and doubly charged cations (potassium and calcium) and anions (nitrate and carbonate) to establish the general applicability of the approach. All potenti- ometric sensors exhibited a solid state configuration with a deposited layer of f-MWCNT as a capacitive transducer layer.
The membrane compositions were chiefly adopted from pre- vious reports describing field applications of the correspond- ing sensors16-18 with the possibility of a future implementation of this approach in real sample analysis. For each analyte three potentiometric sensors were prepared, conditioned and tested individually in a single cell configuration. Subsequently, two electrodes were combined with two reference electrodes to construct a double cell. Finally, all three electrodes were com- bined with three reference electrodes and used to build a triple cell. The response curves obtained for potassium-selective electrodes in a single, double and triple cell configurations are shown in Figure 2.
Figure 2. Potentiometric response of potassium-selective electrode in single, double and triple cell configuration. Error bars are standard deviations (n = 3). The single cell configuration plot corresponds to the response of one out of three tested electrodes.
In a conventional two-electrode setup the tested electrodes demonstrated a close-to-Nernstian slope of 58.8±0.4 mV in the concentration range from 5.0·10-6 to 1.0·10-2 M, which is simi- lar to that frequently reported in the literature for this mem- brane composition and electrode configuration. When two pairs of potassium-selective and reference electrodes were combined to form a double cell, the response slope changed to 120±0.4 mV, which is effectively a doubling of the originally observed slope for the single electrodes. The combination of three electrodes pairs gave a response slope of 178.7±0.5 mV which is close to a tripling of the originally observed slope.
One may also see in Figure 2 that a corresponding increase in the intercept value of the calibration curves was observed, which is caused by the addition of an extra sample–membrane phase boundary potential to the cell as predicted by eq 3. This observation also supports the theoretical basis of the proposed approach. Importantly, the linear response range and the lower detection limit remained the same for a single, double and triple cell configuration.
It should be noted that the response was highly reproducible, giving similar standard deviation values for all cell configura- tions (see error bars in Figure 2 and data in Table 1). It is ap- parent that in a traditional single cell configuration at room temperature of 298 K 1 mV uncertainty in the potential meas- urements translates into an error in analyte activity of 4 and 8% for a single and double charged ion, correspondingly.
Thus, for the same level of the potential uncertainty a triple increase of the slope should decrease the error in analyte activ- ity to 1 and 3% for a single and double charged ion. Indeed, according to the observed standard deviations of the slope the predicted relative standard deviation of potassium activity decreased from 1.4% to 0.8% and 0.6% when moving from single to double and triple cell configurations (see Table 1).
Similar behavior was observed for potentiometric sensors se- lective to calcium, nitrate and carbonate operated in single, double and triple cell configurations (see Figure S1, Support- ing Information). The calibration parameters for all tested ISEs are summarized in Table 1. The observed response slopes were double- and triple-Nernstian for double and triple cell configurations, correspondingly, while the linear response range and a lower detection limit did not change in compari- son with the values observed for a single cell. Thus, the pro- posed approach allows one to significantly increase the sensi- tivity of potentiometric sensor without compromising its un- derlying response characteristics.
The calibration parameters for the single cell configuration summarized in Table 1 correspond to one out of three prepared and tested ISEs. The calibration parameters for the other two ISEs measured against the same reference electrode are given in Table S1 (Supporting Information). According to Tables 1 and 1S, there is a satisfactory agreement between the sum of individual Econst values for two or three ISEs and the experi- mental nEconst observed for double and triple cell configura- tion. The deviation between expected and experimental values is in the range from 9 to 36 mV.
It is apparent that combination of two and three pairs of ion- selective and reference electrodes in one measurement cell should result in the corresponding increase of the cell’s re- sistance, which should make it more prone to electrical noise.
Therefore, the signal to noise ratio for conventional potenti- ometric cell and double/triple cell configuration were com- pared. The resistance of the membranes for the tested elec- trodes was estimated by electrochemical impedance spectros- copy (see Figure S2, Supporting Information), giving values ranging from 150 kΩ for the calcium-selective electrode to 8.9 MΩ for the nitrate-selective electrode. The obtained mem- brane resistance values were in a good agreement with the ones usually observed for ISEs.
The signal-to-noise ratio was estimated for single and triple cell configurations by introducing equal concentration changes and estimating the signal change and the standard deviation of the potential values measured every 0.4 s within 5 min. In this simple setup used here, the triple cell measurements required three stirring bars to mix the sample solutions in three separate beakers after introducing the concentration change, which can be a source of additional electrical noise. Consequently, tests were performed in a Faraday cage and the potential values were recorded after turning off all stirring bars for both single and triple cell configurations.
For a potassium-selective electrode operated in a single cell configuration, a 10-fold increase of potassium concentration
resulted in a 61 mV potential change, which was accompanied by an uncertainty of the measured potential value of 0.1 mV
Table 1. Calibration parameters, slope (s), an intercept value (nEconst) and linear response range obtained for potassium-, nitrate-, calcium- and carbonate-selective electrodes operated in single, double and triple cell configurations.
ISE Calibration parameter Single cella, b, c Double cellb,c Triple cellb,c
K+-ISE
s, mV/dec 58.8 ± 0.4 (1.4%) 120.2 ± 0.4 (0.8%) 178.7 ± 0.5 (0.6%)
nEconst, mV 307.3 ± 4.1 693.6 ± 5.0 1072.6 ± 6.1
Linear response range, M 5.0·10-6 - 1.0·10-2 5.0·10-6 - 1.0·10-2 5.0·10-6 - 1.0·10-2
NO3--ISE
s, mV/dec -54.0 ± 0.1 (0.6%) -106.6 ± 0.2 (0.4%) -159.9 ± 0.5 (0.7%)
nEconst, mV 82.2 ± 2.1 162.3 ± 6.1 241.5 ± 6.1
Linear response range, M 5.0·10-6 - 1.0·10-2 5.0·10-6 - 1.0·10-2 5.0·10-6 - 1.0·10-2
Ca2+-ISE
s, mV/dec 29.6 ± 0.2 (1.9%) 59.2 ± 0.6 (2.4%) 91.2 ± 0.3 (0.7%)
nEconst, mV 262.8 ± 0.5 491.2 ± 7.6 783.5 ± 6.7
Linear response range, M 1.0·10-6- 1.0·10-2 1.0·10-6- 1.0·10-2 1.0·10-6- 1.0·10-2
CO32--ISE
s, mV/dec -26.5 ± 0.8 (6.5%) -57.3 ± 1.0 (4.0%) -83.5 ± 1.1 (3.1%)
nEconst, mV -115.0 ± 3.7 -243.4 ± 3.1 -356.2 ± 7.2
Linear response range, M 5.0·10-6 - 1.0·10-4 5.0·10-6 - 1.0·10-4 5.0·10-6 - 1.0·10-4
a The single cell configuration calibration parameters correspond to the response of one out of three tested electrodes; bStandard devia- tion values were calculated from the measurements repeated three times (n=3); cValues in parentheses indicate the relative standard devia- tion of an analyte activity calculated from the observed standard deviations of the slope with a constant intercept value.
Figure 3. Signal and noise comparison for single and triple cell configurations for potassium-selective electrodes upon 10-fold concentration increases. For easy comparison the two traces were shifted to give a potential near zero for the first studied concentra- tion. SD stands for standard deviation of the potential values ob- served during the last 5 min of each concentration measurement.
for each concentration (see Figure 3). Therefore, the signal to noise ratio can be estimated as 610. On the other hand, the triple cell configuration gave a 180 mV potential increase for the same concentration change (see Figure 3). With the ob- served uncertainty of the measured potential value of 0.2 mV and 0.3 mV, the averaged signal to noise ratio was found as 750. This simple test suggests that both single and triple cell configurations exhibit comparable signal to noise ratios. Thus,
we can conclude that triple cell configuration offers triple sen- sitivity increase without significant rise of electrical noise.
Analogous signal to noise tests were performed for potentiom- etric calcium, nitrate and carbonate sensors and gave similar results as for the potassium-selective electrode (see Figure S3).
CONCLUSIONS
We propose here a new methodology that allows one to en- hance the sensitivity of conventional potentiometric sensors by increasing the number of membrane / sample phase boundaries in a joined electrochemical cell. Each of the additionally intro- duced phase boundary potentials can be described by the Nernst equation. The resulting slope is calculated as a sum of the slopes for the individual electrodes if the analyte concen- tration at each membrane / sample phase boundary is the same. The applicability of the approach is demonstrated with ionophore- and ion-exchanger-based electrodes selective to single and double charge cations (potassium and calcium) and anions (nitrate and carbonate). A satisfactory agreement was demonstrated between experimentally observed and theoreti- cally predicted values of a slope and an intercept of the poten- tiometric responses in double and triple cell configurations.
All tested sensors demonstrated a reproducible increase of Nernstian slope by two and three times when operated in dou- ble and triple cell modes, without deterioration of the lower detection limit.
It is evident that the proposed methodology is not limited to the cases of double or triple cell configuration but may be scaled as needed. There are certainly a few limitations that can be foreseen. Firstly, in the current set up the increase of the number of the sample beakers inevitably will result in an in- crease of the required sample volume. Secondly, the increase of the number of reference electrodes with liquid junction will
lead to the corresponding increase of the impact of the liquid junction potential on the final measurements. Nevertheless, the liquid junction potential contribution can be readily taken into account with the Henderson equation since it has been previ- ously shown to give a satisfactory agreement with Planck's theory.19,20 The average error in the predicted potential was about 3% and exceeded 10% only in case of HCl samples.19,20 The two above-mentioned limitations may be overcome in the future by switching to a flow-cell configuration with all-solid state indicating and reference electrodes. This appears to be feasible because the principle has been already confirmed here with all-solid-state ISEs, and significant recent advances have been reported in the field of all-solid-state reference elec- trodes.21-24
Another limitation that one may think of is the overall increase of the resistance of the system. On one side, the instability of the measured signal can become an additional source of error in the analysis. Therefore, a special attention should be payed to system shielding and grounding to minimize noise artefacts.
On the other side, the value of the resistance of the individual membranes will define the maximum number of the cells that can be combined together, which may be particularly chal- lenging with micropipette-based membrane electrodes that are known to exhibit resistances on the order of Gigaohms.
ASSOCIATED CONTENT Supporting Information
The Supporting Information is available free of charge on the ACS Publications website: Potentiometric response, calibration parameters, signal noise and electrochemical impedance spectra for calcium-, nitrate- and carbonate-selective electrodes (PDF) AUTHOR INFORMATION
Corresponding Author
*E-mail: eric.bakker@unige.ch Author Contributions
The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manu- script.
ACKNOWLEDGMENT
The authors thank the Swiss National Science Foundation (SNSF) and the University of Geneva for financial support of this study.
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6
Table of Contents artwork
S1
Supporting information for:
Potentiometric Sensor Array with Multi-Nernstian Slope
Elena Zdrachek, Eric Bakker*
Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland.
Corresponding Author: eric.bakker@unige.ch
S2
Figure S1. Potentiometric response of a) nitrate-, b) carbonate- and c) calcium-selective
electrodes in single, double and triple cell configuration. Error bars are standard deviations
(n = 3). The single cell configuration plot corresponds to the response of one out of three
tested electrodes.
S3
Table S1. Calibration parameters, slope (s), an intercept value (E
const) and linear response range obtained for three individual potassium-, nitrate-, calcium- and carbonate-selective electrodes operated in single cell configurations.
ISE Calibration parameter Electrode 1 Electrode 2 Electrode 3 K
+-ISE
s, mV/dec 58.8 ± 0.4 58.8± 0.4 58.7± 0.3
E
const, mV 307.3 ± 4.1 364.5 ± 2.5 373.7 ± 2.2
Linear response range, M 5.0·10
-6- 1.0·10
-25.0·10
-6- 1.0·10
-25.0·10
-6- 1.0·10
-2NO
3--ISE
s, mV/dec -54.0 ± 0.1 -53.7± 0.4 -54.1± 0.6
E
const, mV 82.2 ± 2.1 88.9± 4.1 103.4± 4.2
Linear response range, M 5.0·10
-6- 1.0·10
-25.0·10
-6- 1.0·10
-25.0·10
-6- 1.0·10
-2Ca
2+-ISE
s, mV/dec 29.6 ± 0.2 29.6± 0.2 29.8± 0.2
E
const, mV 262.8 ± 0.5 264.8± 0.7 265.8± 0.8
Linear response range, M 1.0·10
-6- 1.0·10
-21.0·10
-6- 1.0·10
-21.0·10
-6- 1.0·10
-2CO
32--ISE
s, mV/dec -26.5 ± 0.8 -26.4± 0.7 -26.8± 0.7
E
const, mV -115.0 ± 3.7 -113.6± 3.0 -107.5± 3.4
Linear response range, M 5.0·10
-6- 1.0·10
-45.0·10
-6- 1.0·10
-45.0·10
-6- 1.0·10
-4S4
Figure S2. Electrochemical impedance spectra recorded for a) potassium- and calcium- selective electrodes and b) nitrate- and carbonate-selective electrodes. Measurements were performed at open circuit potential in solutions of the corresponding salts, namely 10
-2M KCl, 10
-2M NaNO
3, 10
-2M CaCl
2, or 4.9·10
-3M NaHCO
3(corresponding to 10
-4M of CO
32-at pH=8.6 buffered with 0.1 M Tris-H
2SO
4). The frequency range was 100 kHz to 10 mHz,
and the excitation amplitude was 10 mV.
S5
