Ag-iPyramid SERS Enables Single-Particle Detection of Sub-200 nm Nanoplastics in Real Samples

The detection of nanoplastics smaller than 200 nm is critical because of their potential to cross biological barriers, yet it remains a significant analytical challenge in complex environmental matrices due to weak signals and matrix interference. Here, we developed an ordered silver-coated inverted pyramid array, termed the Ag-iPyramid substrate, as a reproducible surface-enhanced Raman scattering (SERS) platform for high-fidelity single-particle analysis of sub-200 nm nanoplastics. Systematic comparisons showed that Ag-iPyramid produced substantially higher signal-to-noise ratios, apparent single-particle enhancement ratios, and spectral matching scores than Au- and Cu-coated counterparts. Finite-difference time-domain simulations further showed that Ag generated the strongest local electric-field confinement within the inverted-pyramid cavity, consistent with the experimental performance ranking of Ag > Au > Cu. Beyond SERS enhancement, the Ag-iPyramid platform enabled high-quality detection of individual 100 nm polystyrene (PS) nanoplastics. When applied to real samples, the platform identified PS nanoplastics released from disposable food containers with a minimum Feret diameter of ∼140 nm and topographical heights of ∼22-45 nm. In authentic rainwater, the platform further resolved a polyethylene (PE) particle, a nylon-like nanoplastic (∼188 nm), and polyethylene-like (PE-like) particles in the sub-150 nm size range. These results demonstrate that Ag-iPyramid provides a robust strategy for the single-particle characterization of sub-200 nm plastics, bridging a crucial gap in monitoring the smallest and potentially most hazardous fraction of plastic pollution.