Flexible SERS substrate based on active nanoparticle enrichment for ultra-trace in situ detection

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful technique for applications in food safety testing, environmental monitoring, disease diagnosis, and molecular characterization. However, conventional SERS substrates frequently suffer from limited sensitivity, poor signal uniformity, and dependence on skilled operators. In this study, a flexible and high-performance SERS sensing strategy is proposed, which is based on surface acoustic wave (SAW)-induced clustering of 50 nm gold nanoparticles (AuNPs). When a $5\mu$L AuNP colloidal droplet is dispensed onto the SAW propagation region, acoustic streaming drives the aggregation of nanoparticles. Upon complete evaporation of the droplet, densely packed SERS-active hotspots are formed, enabling strong Raman signal enhancement. By optimizing the thickness of the PDMS flexible film and the input power, the resulting Raman substrate exhibits minimum detectable concentration reaching 10⁻¹³ M for rhodamine 6G (R6G), 10⁻¹⁰ M for 4-aminothiophenol (4-ATP), and 10⁻¹¹ M for crystal violet (CV)—as well as excellent reproducibility. The relative standard deviation (RSD) for CV detection decreased from 45.32% (SAW-free) to 3.59% under SAW-assisted enrichment, confirming the significant improvement in signal uniformity. Finally, the flexible SAW-SERS sensor demonstrates a sensitivity of up to 10⁻⁸ M for detecting Thiram residues on apple surfaces. This flexible SAW-assisted SERS substrate offers a promising strategy for portable, efficient, and reliable environmental and food safety monitoring.