Flexible SERS sensor with piezoelectric enrichment based on PVDF/AgNWs for cortisol detection in sweat

Wearable sensing platforms for non-invasive monitoring of stress biomarkers in sweat are highly desirable for personalized health management. This work presents a flexible surface-enhanced Raman scattering (SERS) sensor with piezoelectric enrichment based on a PVDF/AgNWs composite membrane for sensitive detection of cortisol. The PVDF fibrous membrane is prepared by electrospinning, which endows it with a high β-phase content and good piezoelectric response. Under mechanical bending, the piezoelectric PVDF membrane generates a non-uniform electric field that polarizes the neutral cortisol molecules, driving them toward the AgNW-decorated SERS hotspots via dielectrophoretic forces, thereby achieving active analyte enrichment without an external power source. In artificial sweat, it shows a wide linear range of 1.0 × 10^-9 to 1.0 × 10^-5 M and a low detection limit of 1.3 × 10^-10 M, along with acceptable uniformity and mechanical robustness. The sensor was further assembled into a wearable patch and preliminarily evaluated using human sweat samples collected during sleep, normal activity, and exercise. Differences in SERS intensity were observed among these states. This flexible SERS sensor shows promise for non-invasive detection of cortisol in sweat and offers a potential strategy for wearable hormone monitoring.