Aggregation Kinetics of Gold Nanoparticles at the Silica−Water Interface

The aggregation kinetics have been observed for citrate-reduced 15 nm gold nanoparticles at the native silica and modified silica−water interfaces. At the native, negatively charged silica−water interface, two-phase adsorption is observed: a pseudo-Langmuirian adsorption phase and, after an acid wash to remove the citrate ligand from the adsorbed particles, another pseudo-Langmuirian adsorption phase. A kinetic analysis of these phases shows an average adsorption rate constant of (1.5 ± 0.4) × 106 M−1 s−1 with no measurable desorption. Another acid wash induces a nearly linear aggregation phase with a 4-fold faster nanoparticles deposition rate. SEM imaging of the aggregation-phase surface shows the formation of small nanoparticle clusters of 10. The refractive index sensitivity of the produced surfaces was measured by varying the analyte refractive index at each phase: the aggregated clusters on the positively charged surface are up to 4 times more sensitive.