Plasmonic nanostructure-based optical nano-sensing for ultrasensitive detection of environmental pollutants

• Ag-Au/porous sapphire heterostructure enables stable SERS sensing. • Gaps smaller than 10 nanometers form high-density hotspots, achieving a detection limit of 10 −14 M. • It exhibits excellent uniformity (relative standard deviation of approximately 2%) and reproducibility. • It enables ultrasensitive detection of organic dyes and various microplastics (PE, PET, PP, PS). • Functionalization of the substrate with 4-ATP allows for specific capture and detection of trace formaldehyde. Surface-enhanced Raman spectroscopy (SERS), owing to its ultrahigh sensitivity, rapid response, and molecular fingerprint recognition capability, has emerged as a powerful analytical technique for trace environmental pollutant detection. However, the performance of SERS is largely determined by the plasmonic properties of the substrate, and thus the development of nanostructured substrates with simultaneously high enhancement factors, excellent signal uniformity, and reliable batch-to-batch reproducibility remains a critical challenge. In this work, we constructed a composite SERS sensing platform based on noble-metal heterostructures. A 100 nm-thick gold film was first deposited onto a periodically ordered nanoporous sapphire array via electron-beam evaporation (Au/porous sapphire). Subsequently, chemically reduced silver nanoparticles (Ag NPs) were mixed with the analyte solution and self-assembled onto the Au/porous sapphire surface. Finite element simulations revealed that the Ag NP/Au film heterostructure could generate a larger number of electromagnetic “hotspots” compared with single-metal counterparts. Using rhodamine 6G (R6G), malachite green (MG), melamine, and methylene blue (MB) as probe molecules, the platform exhibited outstanding detection sensitivity, excellent spectral reproducibility, and uniform signal distribution. Furthermore, the substrate enabled ultrasensitive detection of common microplastics (PE, PET, PP, and PS) as well as trace formaldehyde, with the latter facilitated by a 4-aminothiophenol (4-ATP)-mediated capture strategy. Collectively, this work establishes a stable and reproducible SERS sensing platform, offering a promising pathway for trace pollutant monitoring and environmental safety assessment.