Plasmonic–Photonic Hybrid Hydrogels for Slow-Light-Amplified Surface-Enhanced Raman Scattering Detection of Small Molecules

Surface-enhanced Raman scattering (SERS) enables nondestructive, label-free detection by amplifying weak Raman signals through localized electromagnetic fields. Coupling SERS with photonic crystals enhances light-matter interaction via the slow-light effect, further boosting signal intensity. However, conventional plasmonic-photonic hybrids often require complex fabrication and are vulnerable to structural defects and contamination from adhesive molecules. Here, we present a scalable strategy to fabricate plasmonic-photonic hybrid hydrogel (P2H2) films for sensitive, contamination-free SERS detection in complex media. P2H2 films are created through sequential shear-induced particle alignment, photopolymerization, particle etching, and in situ photoreduction, with conditions optimized for maximal SERS enhancement. Upon drying, the films lose their photonic structure, but fully recover it upon rehydration while rapidly accepting analyte along with water. This reversible structural restoration allows efficient analyte uptake and retention of optical functionality. Critically, tuning the long-wavelength photonic band edge to the laser excitation wavelength enables slow-light-assisted SERS amplification. Moreover, the hydrogel matrix permits size-selective permeation, enabling the detection of small target molecules even in the presence of large interfering species. As a demonstration, we achieve direct, label-free detection of thiram in milk and pyocyanin in saliva without sample pretreatment.