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) x 10(6) 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 particles. Adsorption on a positively charged, aminated silica surface shows a kinetic trace consistent with aggregations at low surface coverages and has a similar adsorption rate to the aggregation phases on the negatively charged surfaces. SEM imaging reveals clusters of many particles 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.