DC Versus AC Electrokinetic-Driven Strategy Coupled with a Miniaturized Electrochemical-SERS Hydrogel Chip toward Dynamic Monitoring of Microplastics

Electrokinetic-driven surface-enhanced Raman scattering (ED-SERS) integrates the ultrasensitive fingerprint spectral identification capability of SERS with flexible electric field-driven molecular interfacial control, enabling it an emerging advanced analytical technique. However, previous studies of ED-SERS primarily relied on direct current (DC) operation, limiting the mechanistic understanding of the strategy. In this study, we introduce a miniaturized electrochemical-SERS sensing chip functionalized with poly(vinyl alcohol oxide)-gold nanoparticle hydrogel (PVA-AuNPs/SPE) for dynamic SERS monitoring of typical charged microplastics under both DC and alternating current (AC) modulation. We confirm that PVA-AuNPs/SPE achieves charge-selective molecular attraction/exclusion and nanoconfined capture under electric field control. Density functional theory (DFT) simulations are performed to investigate the potential molecular mechanism underlying ED-SERS. Furthermore, in situ dynamic ED-SERS monitoring of microplastics is realized through DC/AC voltage application in both simulated and river water samples. Our results show that the DC mode is particularly suitable for trace-level contaminant detection, while AC modulation can provide in-depth insights into the dynamics of molecular migration and the adsorption process. This research seeks to establish a novel AC-modulated ED-SERS strategy as a versatile platform for both high-performance monitoring of emerging contaminants and new perspectives for decoding electrokinetically driven molecular interactions with interfacial environments.