Abstract
The irreproducibility of the standard potential (E°) is probably the last major challenge for the commercialization of solid-contact ion-selective electrodes (SCISEs) as single-use or wearable sensors. To overcome this issue, we are introducing for the first time a perfluorinated alkanoate side chain functionalized poly(3,4-ethylenedioxythiophene) (PEDOTF) as a hydrophobic SC in potassium-selective electrodes (K-SCISEs) based on plasticized poly(vinyl chloride). The SC incorporates the tetrakis(pentafluorophenyl)borate (TFAB–) anion, which is also present as a lipophilic additive in the ion-selective membrane (ISM), thus ensuring thermodynamic reversibility at the SC/ISM interface and improving the potential reproducibility of the electrodes. We show here that the PEDOTF-TFAB solid contact, which was prepolarized prior to the ISM deposition to either its half or fully conducting form (i.e. different oxidation states) in acetonitrile containing 0.01 M KTFAB, had a very stable open-circuit potential and an outstanding potential reproducibility of only ±0.5 mV (n = 6) for 1 h in the same solution after the prepolarization. This shows that the oxidation state of the highly hydrophobic PEDOTF-TFAB film (water contact angle 133°) is stable over time and can be precisely controlled with prepolarization. The SC was also not light sensitive, which is normally a disadvantage of conducting polymer SCs. After the ISM deposition, the standard deviation of the E° of the K-SCISEs prepared on glassy carbon was ±3.0 mV (n = 5), which is the same as that for conventional liquid contact K-ISEs. This indicates that the ISM deposition is the main source for the potential irreproducibility of the K-SCISEs, which has been overlooked previously.
Highlights
Solid-contact ion-selective electrodes (SCISEs) have been the subject of intensive research
The results presented in this work show that the deposition of the PVC-ion-selective membrane (ISM) on the solid contacts (SC) is the main source causing the potential irreproducibility of the K-solid-contact ion-selective electrodes (SCISEs), which has been overlooked previously
We found that the PEDOTF film growth was not fully reproducible with cyclic voltammetry (CV) and the PEDOT-SC film thickness could not be controlled precisely
Summary
We report here for the first time the electropolymerization of EDOTF with the TFAB− anion as the charge-compensating counterion on GC and gold (Au) substrates. It shows that the prepolarization of the SC is essential to obtain electrodes with high E° reproducibility We demonstrate this in Figure S-9 for KSCISEs having nonpolarized PEDOTF-SCs. The potential response of one of the SCISEs deviates considerably (>300 mV) from the other five electrodes, which show a relatively good E° reproducibility of ±5.2 mV in the beginning of the initial potential stability test (Table 1), but ±10.7 mV already after 24 h in 0.01 M KCl. We observed that the K-SCISEs that were prepolarized to the half conducting form (0.22 V) had a lower initial potential drift in comparison to the electrodes prepolarized to the fully conducting form (0.40 V). This redox capacitance in combination with high hydrophobicity of the SC and the thermodynamically reversible charge (ion) transfer at the SC/ISM interface is sufficiently high to provide the K-SCISEs with a reproducible and stable potential
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