Plasmonic coupling driven spectral and sensitivity evolution in TIR-excited discrete AuNP arrays

Localized surface plasmon resonance (LSPR) in discrete nanoparticle monolayers is difficult to control because disorder and aggregation obscure the link between microscale coupling and macroscopic spectra. We demonstrate that surface coverage (SC) and incidence angle (θ) provide a coordinated control under total internal reflection. A compact finite element method and an effective-medium theory framework, supported by a dipole-oscillator picture, predict that tightening gaps strengthens dipole–dipole coupling, redshifts, deepens the resonance, and increases intensity sensitivity (SI) until multipolar modes emerge and reduce SI. Increasing θ reconditions the evanescent field and shifts the SC threshold for strong coupling to higher values. Using pH-responsive self-assembly to tune SC from 8.07% to 23.62% and a prism-based angle-resolved setup under unpolarized illumination, we experimentally verify a nonmonotonic SI and a θ-dependent right-shift of the optimum. These results provide actionable design rules and a scalable route to high-sensitivity, self-assembled LSPR sensors.