Non-linear plasmon response to protein binding at a nanostructured gold particle plasmon resonance surface.

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.

Interfacial pH at an isolated silica-water surface.

Tethering a pH-sensitive dye to a silica surface enables the interfacial pH to be measured optically and is found to be 2 pH units lower than that of the bulk. The positive H+ ions are attracted by the surface potential and have an enhanced concentration consistent with the previously determined surface potential of order -120 mV, with respect to zero in the bulk. A stable near-surface charged layer once formed is not disrupted by the bulk pH.

pH dependence of the crystal violet adsorption isotherm at the silica-water interface.

The pH-dependent adsorption isotherms for the charged chromophore crystal violet, CV(+), have been measured with three different bases by a free-running cavity implementation of evanescent wave cavity ring-down spectroscopy. The ratio of the maximal absorbance measurements at pH 5.10 and 9.05 is consistent with a Q2:Q3 silanol site ratio of 72.8:27.2. The adsorption isotherms have been interpreted in terms a cooperative binding adsorption allowing more than one ionic species to bind to each silanol group. The surface concentration is consistent with a silanol charge density of 1.92 +/- 0.55 nm(-2) and a total neutralized interface layer structure extending 9 nm from the surface. Binding constants and stoichiometric coefficients are derived for CV(+) to both the Q2 and Q3 sites. A variation of the adsorption isotherm with base is observed so that the isotherm at pH 9.05 adjusted with ammonium hydroxide sets up a competitive acid-base equilibrium with the SiOH groups with only 49% of the surface silanol sites dissociated. The implications for functionalized surfaces in chromatography are discussed.