Ultrasensitive SERS detection via hydrodynamic focusing coupled with electric field induced migration.

Surface-enhanced Raman spectroscopy (SERS) offers a powerful platform for ultrasensitive molecular detection, yet its performance is often limited by inefficient analyte transport to plasmonic hotspots. In this work, we present a hybrid microfluidic-SERS detection platform that integrates hydrodynamic focusing and electric field-induced migration to overcome diffusion limitations and enhance molecular delivery to the sensing interface. The system employs a sheath-flow microfluidic device with a thermally evaporated silver nanostructured substrate, serving both as a SERS-active surface and as an electrochemical working electrode. Rhodamine 6G, a model organic dye, was used to evaluate performance under four flow and biasing conditions. The strongest SERS signals were achieved under combined sheath flow and -1 V electric field, demonstrating a synergistic enhancement from both physical and electrokinetic focusing mechanisms. Quantitative analysis yielded Langmuir adsorption behavior over the tested range of concentration (0.1 nM-10 μM) and a limit of detection of 0.076 nM. Mechanistic confirmation was obtained through Stark-shift measurements of 4-mercaptobenzonitrile, which exhibited a measurable blue shift in the CN vibrational mode under +5 V bias, verifying the presence of localized electric fields at the metal-solution interface. This dual-mode transport-enhancement strategy provides a robust and generalizable method for improving sensitivity and control in microfluidic SERS detection platforms, with broad potential applications in biomedical diagnostics, environmental monitoring, and trace contaminant analysis.