Processing-defined organization governs plasmonic response and stability in lipid-coated gold nanoparticles

Lipid-coated gold nanoparticles (LC-AuNPs) are widely explored as plasmonic nano-bio interfaces that combine membrane-mimetic surface organization with optical functionality. However, modest LSPR red-shifts and apparent salt stability are often treated as indicators of conformal bilayer formation, although population-averaged optical and colloidal measurements do not uniquely resolve nanoscale interfacial organization. Here, we compared sonication- and extrusion-based coating routes to determine how processing history governs LC-AuNP organization, plasmonic response, and colloidal stability. Hydrodynamic sizing, zeta potential analysis, spectral line-shape descriptors, PBS stability assays, electron microscopy, and semi-empirical LSPR response modeling were integrated into a unified characterization framework. Sonication-driven coating supported lipid association but produced heterogeneous populations with condition-dependent hydrodynamic sizes, reaching > 400 nm under prolonged conditions, and modest LSPR red-shifts (Δλ ≈ 2 nm) that did not uniquely resolve interparticle organization. In contrast, mechanically constrained extrusion (11 ×-13 × passes) reduced coupling-associated spectral heterogeneity, yielding optically coherent populations converging toward ∼200 nm and typically accompanied by small but reproducible blue-shifts (Δλ ≈ 0.4-1.6 nm). Incorporation of DSPE-PEG further reduced population dispersion and improved optical stability under physiological ionic strength, consistent with steric regulation of interparticle organization.Electron microscopy of DSPE-PEG-containing multicomponent formulations revealed lipid-associated layers with an apparent thickness of ∼4.6 nm, compatible with bilayer-scale organization but not diagnostic of conformal coverage across the entire population. LSPR response modeling supported a shift-direction-resolved interpretation of ensemble spectra, in which positive-shift responses were evaluated for effective dielectric/coupling contributions, whereas blue-shifted states were assessed using a physically constrained required-decoupling framework. Together, these results show that LC-AuNP plasmonic peak shifts are governed by fabrication-defined interparticle organization in addition to lipid association. The absence of a red-shift therefore does not imply the absence of lipid coating. Rather, LSPR peak position should be interpreted as an ensemble-level descriptor of processing-defined organization rather than as a direct reporter of conformal bilayer formation.