Spectroelectrochemistry with Ultrathin Ion-Selective Membranes: Three Distinct Ranges for Analytical Sensing

We present spectroelectrochemical sensing of the potassium ion (K+) at three very distinct analytical ranges—nanomolar, micromolar, and millimolar—when using the same ion-selective electrode (ISE) but interrogated under various regimes. The ISE is conceived in the all-solid-state format: an ITO glass modified with the conducting polymer poly(3-octylethiophene) (POT) and an ultrathin potassium-selective membrane. The experimental setup is designed to apply a potential in a three-electrode electrochemical cell with the ISE as the working electrode, while dynamic spectral changes in the POT film are simultaneously registered. The POT film is gradually oxidized to POT+, and this process is ultimately linked to K+ transfer at the membrane-sample interface, attending to electroneutrality requirements. The spectroelectrochemistry experiment provides two signals: a voltammetric peak and a transient absorbance response, with the latter of special interest because of its correspondence with the generated charge in the POT and thus with the ionic charge expelled from the membrane. By modifying how the ion analyte (K+ but also others) is initially accumulated into the membrane, we found three ranges of response for the absorbance: 10–950 nM for an accumulation-stripping protocol, 0.5–10 μM in diffusion-controlled cyclic voltammetry, and 0.5–32 mM with thin-layer cyclic voltammetry. This wide response range is a unique feature, one that is rare to find for a sensor and indeed for any analytical technique. Accordingly, the developed sensor is highly appealing for many analytical applications, especially considering the versatility of samples and ion analytes that may be spotted.


Materials, instruments and preparations S3
Experimental Accumulation/Stripping protocol S3

Table S1
Peaks information and absorbance transition information in Figure 2 (main manuscript)

Table S2
Fitting Parameters calculated from Figure 2 (main manuscript) S5

Table S3
Peaks information and absorbance transition information in Figure 3 (main manuscript)

Table S4
Fitting Parameters calculated from Figure 3 (main manuscript) S7

Table S5
Peaks information and absorbance transition information in Figure 4 (main manuscript)

Table S6
Fitting Parameters calculated from Figure 4 (main manuscript) S9

Figure S1
Picture of the spectroelectrochemical setup S10

Figure S2
Scheme of the mechanism S11

Figure S3
Real dynamic absorbance corresponding to Figure 2b (main manuscript) S12

Figure S4
Reproducibility of dynamic absorbance for 1 mM KCl in 10 mM NaCl background S13

Figure S5
Calculated voltammograms and dynamic absorbance for K + detection at millimolar level S14

Figure S7
Reproducibility of dynamic absorbance for 5 μM KCl in 10 mM NaCl background S16

Figure S8
Calculated voltammograms and dynamic absorbance for K + detection at micromolar level S17

Figure S9
Voltammograms and dynamic absorbance for 10 nM KCl using different accumulation time S18

Figure S10
Voltammograms and dynamic absorbance for 10 nM KCl using different stirring speed S19

Figure S11
Voltammograms and dynamic absorbance for 450 nM KCl using different stripping scan rate S20

Figure S13
Calculated voltammograms and dynamic absorbance for K + detection at nanomolar level S22

Figure S14
Stripping voltammograms and dynamic absorbance for artificial samples Preparation of ITO-POT-membrane electrodes. First, the ITO electrode was cleaned with ethanol through ultra-sonification and rinsed with water. For the synthesis of POT, a solution comprising 0.1 M of both 3-octylthiophene and LiClO4 in ACN was used. After degassing by purging nitrogen for 15 min, a POT film was electrochemically polymerized on the ITO surface by performing cyclic voltammetry (0-1.5 V, 100 mV/s, 2 scans) and then discharging at 0 V for 120 s. A Platinum electrode and a home-made Ag/AgCl wire were used as counter electrode and reference electrode respectively. Thereafter, the synthesized POT film was immersed in ACN and after in THF for 30 min and 10 s respectively and finally dried with air, to clean it from any residuals. A stock membrane cocktail was prepared by dissolving 15 mg of PVC, 30 mg of DOS, 1.8 mg of NaTFPB and 4.5 mg of potassium ionophore I in 500 μL of THF. The cocktail was diluted by mixing 50 μL of the stock solution with 150 μL of THF. A volume of 30 μL of the diluted membrane cocktail was deposited on the POT-ITO electrode by spin coating (1500 rpm, 60 s) using a 6808P spin coater provided by PI-KEM.
Potassium samples. A volume of 0.5 mL of ultra-pure NaCl solution (1 M, prepared by dissolving 99.999% purity NaCl powder in Milli-Q water) was mixed with 50 mL of the commercial 0.001 M KCl solution (Sigma) to provide a 1 mM standard KCl sample in NaCl background (Sample 1). Standard samples containing 5 μM and 300 nM KCl concentrations (Sample 2 and Sample 3 respectively) were prepared by mixing 15 μL or 250 μL of the commercial 0.001 M KCl solution with 50 mL of 10 mM NaCl ultra-pure solution. A distilled water sample was collected from the lab tap supply. Ultra-pure NaCl powder was added into the distilled water sample to form a solution with 10 mM NaCl (Sample 4). Two powder samples (A: NaCl from Sigma and B: VWR, Sample 5 and Sample 6, respectively) were dissolved in milli-Q water to prepare two solutions of 10 mM NaCl concentration.

Optimization of the experimental conditions for the accumulation/stripping spectroelectrochemical protocol with enhanced accumulation (Operational mode 3).
To optimize the accumulation/stripping protocol, the influence of the accumulation time and stirring speed on the electrode response were explored using a solution containing 10 nM K + concentration. The effect of scan rate was studied with a solution of 450 nM K + concentration.
Accumulation times ranging from 0 to 1500 s were evaluated while keeping constant the rest of parameters (stirring speed of 300 rpm, Eapp=-0.2 V and scan rate of 50 mVs -1 ). The results are shown in Figure S7. It was found that the voltammogram corresponding to an accumulation time as shorter as 30 s displays a similar shape as that observed at 0 s (i.e., no enhanced accumulation). When further expanded the accumulation time up to 700 s, the K + peak current in the CV keeps growing while the Na + peak current decreases. This indicates that a higher amount of K + is accumulated into the membrane, as the mass transport from the bulk solution to the membrane is promoted with increasing time. In the cases of an accumulation time larger than 700 s, any increase in the K + was less drastic. Regarding the dynamic absorbance curves, it is observed that more absorbance change occurs in the potassium region (second sigmoidal portion) when the accumulation process is enlarged from 30 s to 700 s. Notably, this change is less evident when accumulating for more than 700 s. Therefore, 700 s was selected as the optimized accumulation time.
The effect of the stirring speed of the sample solution during the accumulation step on the electrode response was studied from 100 to 800 rpm ( Figure S8). A higher speed is not convenient in order to preserve the membrane integrity. A higher K + peak current was observed at increasing stirring speed, which is in principle ascribed to a diminishment in the thickness of the diffusion layer thickness, since K + mass transport becomes more efficient. Notably, there is a large difference in the peak currents display for 300 rpm and 400 rpm, after which the current profile did not show any significant change. Concerning the dynamic absorbance curves, the curve for 400 rpm demonstrates a larger absorbance change than the curves for 100, 200 and 300 rpm, while being rather similar as the curve for 500 rpm. As a result, a stirring speed of 400 rpm was selected as the optimal one. Different scan rates in the stripping step, ranging from 25 to 100 mV s -1 , were also tested ( Figure  S9). When the scan rate was increased, a higher peak current was obtained for both Na + and K + . However, the ratio between the two peak currents slightly differs, as K + peak is slightly favored over the Na + one. Inspecting the optical curves, two distinct transitions, corresponding to Na + transfer and K + transfer, can be clearly recognized in the curves for 10 mV/s, 25 mV/s and 50 mV/s. When using an even higher scan rate, the boundary between the two transitions is hard to be distinguished, because the time for collecting optical signal seems to be insufficient to provide an acceptable resolution. Accordingly, a scan rate of 50 mV s -1 was selected as the optimal one.  Table S2. Parameters for the calculated voltammograms and dynamic normalized absorbance at increasing concentration of KCl at millimolar level in 10 mM NaCl background. The average values are then used to simulate the absorbance curves and the voltammograms provided in Figure S5. SD=standard deviation.  Table S4. Parameters for the calculated voltammograms and dynamic normalized absorbance at increasing concentration of KCl at micromolar level in 10 mM NaCl background. The average values are then used to simulate the absorbance curves and the voltammograms provided in Figure S8. SD=standard deviation.   Table S6. Parameters for the calculated voltammograms and dynamic normalized absorbance at increasing concentration of KCl at nanomolar level in 10 mM NaCl background. The average values are then used to simulate the absorbance curves and the voltammograms provided in Figure S13. SD=standard deviation.  Figure S1. Picture of the spectroelectrochemical setup based on the cell presented in Figure 1 in the main manuscript. WE: working electrode, CE: counter electrode, RE: reference electrode.  Notably, the experimental results (c and d) are added for comparative purposes. S15 Figure S6. Plot of the peak charge corresponding to the Na + and K + voltammetric peaks in the experiment in Figure 3 (main manuscript). The sum of both charges versus the K + concentration is also presented. Linear fittings: 9: -( ) = −3.425 × ! ( ) + 59.892, ; = 0.9960 and ! -( ) = 3.415 × ! ( ) + 2.342, ; = 0.9956. S16 Figure S7. Normalized dynamic absorbance of three electrodes for 5 µM KCl in 10 mM NaCl background at five consecutive cyclic voltammetry scans. Scan rate: 50 mV s -1 .  Figure S11. Accumulation/stripping protocol optimization: Stripping voltammograms and the associated dynamic absorbance curves for 450 nM KCl in 10 mM NaCl solution at increasing scan rates of 10, 25, 50, 75, 100 mV s -1 . Electrochemical protocol: Eapp=-0.2 V during 700s, stirring speed of 400 rpm, linear sweep stripping from -0.2 to 1.4 V. Figure S12. Plot of the peak charge corresponding to the Na + and K + voltammetric peaks in the experiment in Figure 4 (main manuscript). The sum of both charges versus the K + concentration is also presented. Linear fittings: 9: -( ) = −4.45 × 10 <; ! ( ) + 54.282, ; = 0.9949 and ! -( ) = 4.43 × ! ( ) + 6.992, ; = 0.9937. are added for comparative purposes. Figure S14. Cyclic voltammograms and dynamic absorbance curves for increasing concentrations of KCl in sample 4 (distilled water), sample 5 (NaCl purchased from Sigma) and sample 6 (NaCl purchased from VWR). Electrochemical protocol: Eapp=-0.2 V during 700s, stirring speed of 400 rpm, linear sweep stripping from -0.2 to 1.4 V, scan rate of 50 mV s -1 .