Tuning the number density of nanoparticles by multivariant tailoring of attachment points on flat substrates

We report on the organization of nanoparticles on a flat surface when there is strong yet tunable interaction between the particles and the surface. Specifically, we tailor the number density of citrate-stabilized gold nanoparticles on flat substrates by varying the concentration of the grafted amino groups on the surfaces and their degree of ionization. While the former effect is accomplished by decorating silica-based substrates with a molecular gradient of (3-aminopropyl)triethoxysilane (APTES), the latter effect is achieved by varying the degree of ionization of the -NH₂ groups in APTES by varying the pH of the gold sol. We show that the measurement of particle number density on an APTES concentration gradient substrate at different pH values provides a simple, non-spectroscopic means to deduce the relative molecular concentration profile of APTES on the substrate.