Extension of microscale surface ion conduction (μSIC) to the sensing of charged small molecules: application to per- and poly-fluoroalkyl substances (PFASs)

Surface ion conduction-based sensing, which detects changes in ionic conductivity upon target binding, has been widely used for molecular detection. However, many sensors in this class, such as chemically modified solid-state nanopores, face challenges related to complex fabrication and inconsistent immobilization of target-specific probes at the nanoscale. In contrast, we previously introduced the microscale surface ion conduction (μSIC) sensor as a solution for detecting biological analytes. In this work, we demonstrate the adaptability of the μSIC sensor for organic pollutant detection, specifically targeting per- and polyfluoroalkyl substances (PFASs) in drinking and brackish waters. By integrating C18 reversed-phase silica gel chromatographic beads as a solid substrate, we enable label-free, non-optical detection of PFASs. This approach offers several advantages, including being labor-efficient, portable, and cost-effective, compared to conventional detection methods. By integrating an additional preconcentration step using faradaic ion concentration polarization (fICP) with a lateral flow assay (LFA) (fICP-LFA), the sensor's sensitivity was increased almost 77 times while the limit of detection (LOD) of perfluorooctane sulfonic acid (PFOS) was improved 1137-fold. It was also observed that the shift in current (signal) produced by the sensor does not change significantly over a range of background electrolyte (BGE) concentrations spanning three orders of magnitude. This work is significant as it implements a chromatographic solid phase as a substrate for label-free sensing, creating an avenue to develop a new class of sensors or to monitor chromatographic interactions. Further, we demonstrate that the μSIC sensor can be adopted for non-biological molecules, and to the best of our knowledge, this work is the first surface charge-based detection of PFASs.