Calibration-free Solid-State Ion-Selective Electrode Based on a Polarized PEDOT/PEDOT-S-Doped Copolymer as Back Contact

Solid-contact ion-selective electrodes (ISEs) have the inherent advantage of being miniaturized in addition to maintaining high selectivity and sensitivity of the ionophore-based ISE. The major disadvantage of ISEs is the necessity of performing a calibration curve (varying the intercept in the linear calibration curve equation) each time before running experiments, which limits their application as one-time disposable sensors or for use in remote water sample analysis. To overcome these challenges, we designed a unique back contact made of 3,4-ethylenedioxythiophene (EDOT) and 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butanesulfonic acid, sodium salt (EDOT-S). The calibration-free ISEs showed near Nernstian responses of 57.2 +/- 0.2 mV/log [K+] and 28.5 +/- 0.3 mV/log [Ca2+], while maintaining their respective selectivity against major interfering ions. The detection limits for Ca2+ and K+ ISEs were 0.45 +/- 0.01 and 1.68 +/- 0.18 mu M, respectively. The charging cycles of the PEDOT: PEDOT-S back contact allowed us to fix the background potential at a desired fixed intercept value across different ionophores (K+, Ca2+). This protocol was used to determine the K+ and Ca2+ contents in creek water samples. The activity and concentration of [Ca2+] and [K+] in a local creek was found to be 257 +/- 7.3 and 28.1 +/- 1.1 mu M, respectively.

Automated summary

Solid-contact ion-selective electrodes have the advantages of miniaturization, high selectivity and sensitivity. However, the need for calibration curves limits their applicability as disposable sensors or for remote water sample analysis. To overcome this, a unique back contact was designed to fix the background potential and determine the contents of potassium and calcium in creek water samples.