Low pH Exposure Alters Mediator-Dependent Response in Poly( o -phenylenediamine)-Based Molecularly Imprinted Polymer Electrochemical Sensors (MIP-ES)

Abstract

Molecularly imprinted polymer electrochemical sensors (MIP-ES) are a promising approach for detecting less electrochemically active contaminants, such as per- and polyfluoroalkyl substances (PFAS). However, exposure of poly(o-phenylenediamine) [poly(o-PD)]-based MIP films to strongly acidic conditions can alter the polymer environment and complicate the interpretation of electron mediator based electrochemical readout. In this study, we examined how low pH exposure influences the response of the anionic perfluorooctanoic acid (PFOA) templated poly(o-PD) MIP-ES by comparing four commonly used electron mediators with different charge states: ferrocenemethanol, ferrocyanide, ferricyanide, and hexaammineruthenium(III). Apparent association constants (KA) and maximum PFOA-dependent current differences (Δimax) derived from a fractional response two-parameter Langmuir binding isotherm model, together with ratiometric comparisons of normalized current differences, showed that mediator-dependent trends changed after acidic pH exposure of the polymer. The anionic mediators, ferrocyanide and ferricyanide, exhibited the largest increases in apparent KA and Δimax, whereas the neutral mediator, ferrocenemethanol, retained a similar apparent KA but exhibited a lower fitted response amplitude. Hexaammineruthenium(III) produced substantial variability that limited Langmuir isotherm interpretation. These results are consistent with the interpretation that changes in the protonation state of the poly(o-PD) film influence mediator access and interactions within the sensing environment, thereby biasing the mediator-based current response. Overall, this work highlights that mediator choice can strongly affect the apparent performance of poly(o-PD)-based MIP-ES after exposure to low pH and should be considered carefully when interpreting sensor response under challenging conditions.