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 (635 nm) ) 6.4 ( 1.6 × 10 7 M -1 cm -1 , (830 nm) ) 9.8 ( 0.2 × 10 6 M -1 cm -1 ; and 45 nm particles (635 nm) ) 3.1 ( 1.8 × 10 9 M -1 cm -1 , (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 goldgold 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 .

The adsorption kinetics of thioctic acid (ToA) self-assembled monolayer (SAM) formation has been observed by monitoring small changes in the extinction of a gold nanoparticle surface, interrogated by evanescent wave cavity ring-down spectroscopy. A direct measurement of the adsorption kinetics for charged and neutral SAM formation has been made and modeled and shows significant differences in the lateral interactions between surface moieties:  the interaction parameter for the charged species is 20 times larger than for the neutral species. Titration of ToA SAMs deposited from basic conditions demonstrate a pH-switchable surface with charge densities varying from 0.1 to 1.0 e nm-2. Similar measurements were performed from SAMs deposited under acidic conditions producing charge densities of 1.8 e nm-2. The surface potential varies between −44 and −198 mV for all SAMs, which produces an interfacial pH 0.8−3.4 units more acidic than the bulk and interfacial capacitances varying from 5.5 to 88 μF cm-2.

A self-assembled acid-functionalised monolayer on a gold surface has an interfacial pH 2.93 more acidic than the bulk and surface pK(a) very similar to that of the free acid.

Rapid adsorption kinetics have been observed for protein binding to a 800 nm aggregated nanoparticle, showing extreme sensitivity resulting from a non-linear particle plasmon response.

No abstract