Gold Nanoparticle Adsorption and Aggregation Kinetics at the Silica−Water Interface

Gold nanoparticles have been synthesized as colloids using a citrate and borohydride reduction of auric chloride producing 15 and 45 nm particles, respectively. The nanoparticle adsorption kinetics have been observed at the silica-water interface by evanescent wave cavity ring-down spectroscopy. AFM images have been used to determine the surface coverage from which the extinction coefficients of the particles in water have been determined at two interrogation wavelengths: 15 nm particles ε(635 nm) = 6.4 ± 1.6 × 10 7 M -1 cm -1, ε(830 nm) = 9.8 ± 0.2 × 10 6 M -1 cm -1; and 45 nm particles ε(635 nm) = 3.1 ± 1.8 × 10 9 M -1 cm -1, ε(830 nm) = 9.2 ± 1.5 × 10 8 M -1 cm -1 . These values are larger than the Mie scattering calculations would predict. Mono- and multilayer adsorption kinetics have been observed with monolayer binding constants K D = 2.75 ± 0.55 nM for the 15 nm particles and 0.74 ± 0.47 nM for the 45 nm particles. An initial slope analysis of the binding kinetics shows the 15 nm particles undergo aggregation at the surface whereas the 45 nm particles do not. A multilayer co-operative sequential adsorption aggregation model is developed indicating that gold−gold particle aggregation affinity is not as strong as the affinity of the gold to the surface. The refractive index sensitivity of the resulting particle plasmon surfaces has been measured, and the surfaces are sensitive to changes of typically 7 × 10 -4 but optimally 2.5 × 10 -6 .