Self-Assembled Plasmonic Interfaces for Wearable SERS: From Substrate Fabrication Toward Online Biofluid Sensing

Wearable health monitoring is expanding from physical signals, such as heart rate and body temperature, to molecular information at body interfaces and in biofluids. Raman spectroscopy provides chemically specific vibrational fingerprints and supports label-free molecular analysis. Surface-enhanced Raman scattering (SERS) further improves sensitivity through plasmonic enhancement, enabling analysis of low-abundance biomarkers. For wearable sensing, SERS-active structures must provide reliable hotspots, integrate with flexible or three-dimensional supports, and remain compatible with biofluid sampling. Self-assembly offers a bottom-up route to generate nanoscale interparticle gaps and integrate plasmonic structures with diverse supports. Existing reviews have mainly focused on biomedical SERS, flexible SERS substrates, or nanoparticle assembly, while links between self-assembly routes, biofluid sampling, and wearable integration have received less systematic attention. This review summarizes recent progress in self-assembled plasmonic structures for wearable SERS. Reported strategies are classified as fluid-interface, evaporation-induced, and template-assisted self-assembly according to the dominant assembly process and spatial constraints. Representative studies involving sweat, interstitial fluid, tears, and wound exudate are compared across biofluid-sensing scenarios. Finally, we discuss the stability of the effective sensing area under on-body conditions and quantitative validation in real biofluids.