SERS-Active Flexible Substrates Engineered with Self-Assembled Gold Nanostar Arrays for Ultra-Sensitive Detection of Pesticide Residues on Fruit Surfaces

In this work, we report the development of a highly sensitive, flexible, and conformable surface-enhanced Raman scattering (SERS) substrate engineered by self-assembling gold nanostar (AuNS) arrays onto polydimethylsiloxane (PDMS) films for the non-destructive, in situ detection of pesticide residues on fruit surfaces. Gold nanostars, characterized by their multiple sharp tips, act as efficient plasmonic "hotspots" that generate exceptionally high local electromagnetic field enhancements. By utilizing an interfacial self-assembly method mediated by block copolymer templates, we achieved a dense, uniform monolayer of AuNSs on the elastomeric PDMS substrate. The resulting AuNS/PDMS substrate exhibits excellent mechanical flexibility, optical transparency, and outstanding SERS reproducibility (relative standard deviation < 7.5%). Using rhodamine 6G (R6G) as a probe molecule, the substrate demonstrated an enhancement factor of 1.4 × 10⁷. For real-world applications, the flexible substrate was directly conformed to the curved surfaces of apples and tomatoes to extract and detect thiram and chlorpyrifos residues. Under optimized conditions, the limit of detection (LOD) reached as low as 1.2 ng/cm² for thiram and 8.5 ng/cm² for chlorpyrifos, which are well below the maximum residue limits established by international regulatory agencies. This work provides a robust, cost-effective, and practical platform for point-of-care food safety screening and environmental monitoring.