Role of the diffusion boundary layer in the molecular imprinting of PFAS in poly(ortho-phenylenediamine) toward improving MIP-based sensors

Abstract

Molecularly imprinted polymers (MIPs) and electrochemical sensors offer a promising route for rapid and onsite detection of per- and polyfluoroalkyl substances (PFAS). The quantity and quality of the imprinted cavities in MIPs underpin the selective recognition and sensing performance of MIP-based sensors. Thus, understanding the role of various synthesis parameters during the electropolymerization of MIPs is crucial to control the imprinting process for various PFAS templates. Herein, we demonstrate that the synthesis scan rate used during electrosynthesis of MIPs can be leveraged to modulate the imprinting efficiency of PFAS with different tail lengths within a poly(ortho-phenylenediamine) (PoPD) film. Specifically, we test the hypothesis that increasing the scan rate, which reduces the thickness of the diffusion boundary layer during electropolymerization, significantly increases the density of imprinted PFAS in the resulting MIP-based sensors. We characterize the total amount and the spatial distribution of the imprinted cavities via cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) sputter depth profiling (SDP), respectively. We demonstrate that both properties depend on the nature of the diffusion boundary layer and the identity of the PFAS templating molecules (i.e., perfluorooctane sulfonic acid, PFOS; perfluorohexane sulfonic acid, PFHxS; perfluorobutane sulfonic acid, PFBS). We further show that the cyclic voltammogram during the electrosynthesis can be modeled using finite element analysis to describe the effect of different synthesis scan rates. We anticipate that our results will provide further insights into the development and optimization of PoPD MIP-based sensors for perfluoroalkyl sulfonic acids (PFSA) towards the applications of decentralized sensors.