Ultrasensitive Food and Water Sensing by Recyclable, Femtosecond Laser-Structured Surfaces

We report on ultrasensitive detection of dangerous trace elements in food and water by elaborating reusable, crystalline silicon surface-enhanced Raman scattering (SERS) substrates via femtosecond laser processing. Limits of detection (LoDs) of 10^-7 μg/L for microcystin-LR (a waterborne algal toxin) and 10^-12 M for malachite green (a banned aquaculture additive) are achieved by the fabricated substrates with experimental results closely matching the predictions of finite-element simulations. Interestingly, substrates fabricated in ambient air exhibit an order-of-magnitude higher SERS signal intensity than those produced under vacuum, evidencing an important influence of the processing conditions. The signal enhancement arises from synergistic electromagnetic effects: (1) subwavelength inter-ripple cavities induce intense field confinement, (2) laser-induced nanoscale roughness from nanoparticle redeposition creates additional field localization sites, and (3) Au nanoparticle decoration establishes hybrid plasmonic coupling via particle-substrate and interparticle interactions, generating gradient-distributed 3D hotspots with simulated enhancement factors (EFs) > 10¹⁰. SERS mapping demonstrates high signal uniformity across the platform, while also revealing distinct intensity variations between regions with differing surface roughness modulated by processing conditions. Our experimental findings validate a simple and effective strategy for food safety and environmental monitoring with laser-fabricated SERS substrates, also highlighting a critical influence of the processing environment on their final sensitivity.