Zinc Oxide/Ag Water-Gated Field-Effect Transistor Coupled with Surface-Enhanced Raman Scattering for Indigo Dye Detection

Abstract Water-gated field-effect transistors (WGFETs) based on a solution-processed ZnO–Ag composite are developed as a complementary electrical–spectroscopic sensing platform, combining electrical detection of indigo in aqueous media with SERS-based molecular identification on the same ZnO–Ag active layer. The resulting WGFETs operate below 1.5 V and exhibit stable transfer characteristics with reduced gate leakage and reproducible hysteresis behavior. Threshold voltage shifts (ΔVth0) induced by indigo exposure were described using a Langmuir–Freundlich model, providing a phenomenological representation of the nonlinear concentration-dependent electrical response, rather than definitive proof of adsorption heterogeneity. An apparent electrical LoD in the nanomolar range was estimated using the 3σ criterion. Simultaneously, surface-enhanced Raman scattering (SERS) performed on the same active layer enabled molecular fingerprint identification with a signal-to-noise ratio >3 at 10–8 mol L–1. The integration of WGFET and SERS is presented as a complementary sensing strategy in which the WGFET provides a sensitive electrical response in water and SERS supplies molecular confirmation through the characteristic vibrational bands of indigo. These results demonstrate that hybrid ZnO–Ag interfaces provide a multifunctional platform for low-voltage electrolyte-gated electronic materials capable of sensitive dye pollutant monitoring, with molecular selectivity supported by spectroscopic validation.