Morphology-driven hotspot engineering of femtosecond laser-assisted gold nanoparticles for surface-enhanced Raman detection of paraquat

Gold nanoparticles with tunable size and morphology were synthesized via a green femtosecond laser-assisted seed-growth method and evaluated as surface-enhanced Raman spectroscopy (SERS) substrates for paraquat detection. Particle size increased from 7.3 to 27.2 nm with a morphology evolution from quasi-spherical to anisotropic triangular and nanoplate structures. After deposition on silicon wafers, the average interparticle gap progressively decreased to an optimum hotspot distance of ∼4.9 nm before partial coalescence occurred at later growth stages. SERS measurements at 532 nm were strongly influenced by fluorescence background, whereas 785 nm excitation produced clearer and more reproducible Raman signals. The highest enhancement was obtained for the G3 substrate, corresponding to the most favorable nanogap configuration, with an enhancement factor on the order of 10⁶ and a detection limit of 0.01 ppm for paraquat. T-matrix electromagnetic simulations supported the experimental trend, showing maximum local field enhancement for the G3 geometry and a plasmon resonance centered near the 785 nm excitation wavelength. The combined experimental and computational results indicate that controlled nanoparticle growth offers a chemical-reductant-free route to engineer plasmonic hotspots and improve gold-based SERS substrates for pesticide monitoring.