Uniform Spray-Coated Flexible SERS Substrates for Enhanced Molecular Detection

Regulating uniformity and stability of substrates remains a key challenge in developments of flexible surface enhanced Raman spectroscopy (SERS) sensors. Herein, we fabricated a flexible SERS platform by integrating ultra-stable nitrogen-doped graphite-coated gold nanoparticles (Au@NG) with a polydimethylsiloxane (PDMS) film via optimized microarray spray-coating techniques, forming a composite substrate denoted as Au@NG@PDMS. The structure and chemical stability of the Au@NG nanoparticles were confirmed by TEM and Raman spectroscopy. The presence of a thin, nitrogen-doped graphite shell effectively protected the Au core against acidic, alkaline, and oxidative environments. Benefiting from the superior mechanical flexibility of PDMS, the Au@NG@PDMS substrate maintained excellent SERS signal reproducibility under repeated bending and stretching cycles. Furthermore, we demonstrated that adjusting the solvent evaporation rate by selecting solvents in spraying process significantly improved the uniformity, reproducibility, and overall SERS performance of the substrate. Using this platform, we achieved highly sensitive and quantitative detection of crystal violet across a concentration range from 10 nM to 10 µM and successfully identified trace levels (20 ng/mL) of thiram residues directly on the surface of apples. The resulting flexible SERS substrate exhibits outstanding structural stability, signal uniformity, and surface conformability making it highly promising for practical applications in on-site pesticide residue detection in agricultural monitoring.