Advanced SERS sensor with horizontally aligned sub-5 nm silicon nanowires and high-density silver nanoparticles for ultra-sensitive molecular analysis.

Surface-enhanced Raman scattering (SERS) is indispensable for its unparalleled ability to amplify Raman signals and detect low-concentration analytes. However, its full potential remains unrealized due to challenges in fabricating complex and tightly controlled nanostructured substrates with an optimized balance of synergistic chemical and electromagnetic enhancement. Here, we present an ultra-sensitive SERS sensor utilizing high-density small silver nanoparticles (AgNPs, 1-10 nm) coated onto highly dense, horizontally aligned sub-5 nm silicon nanowires (SiNWs) designed to achieve single-molecule detection. By a well-controlled dip-coating technique without reducing agents, AgNPs were nucleated and tightly adhered to sub-5 nm SiNWs synthesized using a catalyst-free chemical vapor etching (CVE) method. Such innovative structures significantly increase SERS sensitivity by minimizing interparticle gaps and ensuring more stable and consistent signal amplification across the substrate. Using Rhodamine 6G (R6G) and crystal violet (CV) as probe molecules, the sensor demonstrates exceptional dual-sensing capabilities, achieving precise detection of R6G at trace concentrations as low as 10 M, indicative of single-molecule sensitivity. The sensor's performance was validated using Raman mapping, revealing stable and reproducible single-molecule detection with high-resolution spectra. Our SERS sensor system, based on aligned sub-5 nm SiNW arrays decorated with high-density AgNPs, enables ultra-sensitive molecular detection, providing significant advancements in environmental pollution monitoring and biomedical analysis applications.