Bulk LSPR sensitivity and biomolecular response across immobilized nanoparticle-biointerface architectures

Bulk refractive-index sensitivity is widely used to characterize localized surface plasmon resonance (LSPR) sensors, but the response of a finite biomolecular interface also depends on how that optical response is realized after surface construction. We examined this relationship across independently generated, microplate-reader-compatible immobilized gold nanoparticle (AuNP) and gold nanourchin (AuNU) architectures spanning direct-IgG coupling, Protein A/anti-glial fibrillary acidic protein (GFAP) IgG, and anti-receptor-binding domain (RBD) nanobody (Nb) workflows. Selected nanobody branches were further evaluated using receptor-omitted RBD controls and leptin challenge, and the 100 nm AuNU/Nb branch was tested in 10% pooled human serum. A fixed, non-fitted sensing-envelope calculation based on assigned parameters provided a distance-based optical reference. Bulk sensitivities spanned 57-241 nm RIU^-1, with the largest value for the 100 nm AuNU ensemble. Downstream responses, however, depended on architecture and interface state. Linker loading did not consistently yield a resolvable receptor-associated step, and the 30 and 60 nm AuNP Protein A/anti-GFAP branches showed compressed GFAP-associated profiles. Anti-RBD nanobody branches produced concentration-dependent buffer responses; on the 100 nm AuNU/Nb branch, receptor-omitted RBD background remained low but non-zero and did not reproduce the larger middle-to-upper-range target response. Leptin also produced a measurable response, while the exploratory 10% serum experiment yielded small, partly non-monotonic serum-centered increments (0.12-0.28 nm). The data distinguish pre-biofunctionalization optical response from interface construction and analytical discrimination. The sensing-envelope calculations provide a conditional geometric reference for interpreting these observations.