Observation of gold nanoparticle Brownian motion via localized surface plasmon resonance enhanced highly inclined illumination microscopy

Recently, single-particle tracking has emerged as a key tool for revealing transport and interactions at the nanoscale. Traditional fluorescence methods are limited by photobleaching, while interferometric scattering requires stringent optical stability. This study demonstrates highly inclined and laminated optical sheet (HILO) microscopy enhanced by localized surface plasmon resonance (LSPR) of gold nanoparticles as a simple alternative on a commercial inverted microscope. The oblique illumination suppresses an out-of-focus background while LSPR selectively amplifies elastic scattering, delivering order-of-magnitude gains in single-particle signal-to-noise without fluorescent labels. Rapid image sequences are processed with TrackMate-MATLAB to generate thousands of Brownian trajectories in each acquisition. Ensemble mean-square-displacement analysis yields the measured diffusion coefficients that match Stokes-Einstein predictions to within 2% and exhibit coefficients of variation below 3.5% across independent replicates. The optical adjustment requires only rotation of the excitation beam, making the platform low-cost, high-throughput, and readily scalable for interrogating nanoparticle dynamics in physiological environments. It also lays the foundation for integrating chemical fingerprinting modalities such as surface-enhanced Raman scattering (SERS) at the single-particle level in the future.