A Highly Stable and Flexible Ca²⁺ Ion-Selective Sensor Based on Treated PEDOT:PSS Transducing Layer

In this work, a highly stable Ca ²⁺ ion-selective sensor was successfully developed by depositing a poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonate) (PEDOT:PSS) layer on top of a carbon paste electrode as an ion-electron transducing layer. The PEDOT:PSS was treated by a co-solvent of ethylene glycol (EG) using the bath-sonication technique to enhance electrical and electrochemical characteristics, and finally coated with Ca ²⁺ ion-selective membrane cocktail. The developed Ca ²⁺ ion-selective electrode (Ca ²⁺ -ISE) showed excellent electrochemical properties by much-improved charge transfer kinetics with remarkable electric conductivity. The fabricated Ca ²⁺ -ISE exhibited an excellent sensitivity of 37.7 mV/decade (n = 4) in the range from 10 ⁻⁴ to 10 ⁻¹ M with a rapid response (< 20 seconds). Moreover, the EG-treated PEDOT:PSS (PEDOT:PSS-EG) based ISE showed negligible responses for primary interfering ions of human biofluid samples, proving significant potential selectivity. Furthermore, the negligible drift of open circuit potential ( $2.78~ { \mu }\text{V}$ /s) proved the stability of the PEDOT:PSS-EG compared to pristine PEDOT:PSS( $10.63~{ \mu }\text{V}$ /s) based ISE. Based on these analyses, it can be expected that the organic solvent treatment on PEDOT:PSS will pave the way for long-term monitoring of other biochemical compounds.