Long-chain thiol-regulated hierarchical flower-like silver nanoparticles self-assembled on filter paper as highly sensitive flexible SERS substrates for pesticide residue detection

Liquid–liquid interface self-assembly enables efficient surface-enhanced Raman scattering (SERS) detection. Transferring self-assembled membranes to flexible/rigid substrates reduces liquid-phase instability and background interference. This study employed a straightforward chemical method to synthesize hierarchical flower-like silver nanoparticles (Ag NFs), and introduced long-chain thiols to construct a novel self-assembly platform. Incorporating thiols converted the repulsive forces between nanoparticles into van der Waals interactions, forming a compact and ordered self-assembled membrane. The self-assembled membrane rapidly transfers to the filter paper substrate by utilizing the capillary effect of the filter paper, thereby extending the applicability of the self-assembled SERS platform. These Ag NFs, characterized by hierarchical three-dimensional structures, can effectively generate numerous surface plasmon resonance ‘hotspots’ and exhibit remarkable SERS activity. The detection limit of the Ag NFs/filter paper substrate for rhodamine 6G (R6G) is reduced to 10⁻¹¹ M, and a significant linear correlation exists between the Raman intensity and the R6G concentration, with the R² value of the fitted curve being 0.96. The relative standard deviation (RSD) of the three typical characteristic peaks of R6G is 8.54%. Furthermore, the flexibility and porosity of the filter paper substrate enhance direct contact with analytes on curved surfaces, enabling the detection of methylene blue (MB), Sudan I, and carbendazim (CBZ) on apple surfaces through the 'stick-peel' technique. Due to their simple preparation and excellent sensitivity, an Ag NFs/filter paper substrate is suitable for practical applications in non-invasive sampling, while providing an effective strategy for the rapid construction of SERS substrate platforms.