This work presents molecular-level investigations of how well-characterized silica-supported phospholipid bilayers formed from either pure DOPC or a 9:1 mixture of DOPC:DOTAP interact with positively and negatively charged 4 nm gold metal nanoparticles at pH 7.4 and NaCl concentrations ranging from 0.001 to 0.1 M. Second harmonic generation (SHG) charge screening measurements indicate the supported bilayers carry a negative interfacial potential. Resonantly enhanced SHG measurements probing electronic transitions within the gold core of the nanoparticles show the particles interact irreversibly with the supported bilayers at a range of concentrations. At 0.1 M NaCl, surface coverages for the particles functionalized with the negatively charged ligand mercaptopropionic acid (MPA) or wrapped in the cationic polyelectrolyte poly(allylamine) hydrochloride (PAH) are estimated from a joint analysis of QCM-D, XPS, AFM, and ToF-SIMS to be roughly 1 × 107 and 1 × 1011 particles cm–2, respectively. Results from com...