Use of atomic force microscope force measurement to probe solid/liquid interfacial processes

I describe here how the diffuse double-layers at various solid/liquid interfaces can be probed at nanometer resolution with a modified tip on the cantilever of an atomic force microscope (AFM). The double-layer structure and thickness is examined by measuring the force (with nanonewton resolution) between a silica probe and a substrate as the probe penetrates the double-layer. The substrate surface electrostatic potential and charge can then be calculated by theoretical fits of the force data to solutions of the complete nonlinear Poisson-Boltzmann equation with the knowledge of silica probe surface potentials.; By applying this newly developed AFM force measuring technique, several interesting solid/liquid interfacial processes were addressed. First, the diffuse double layer forces at an n-type {dollar}rm TiOsb2{dollar} semiconductor/electrolyte interface were measured under various conditions. From the force measurements, the isoelectric point and flat-band potential of the semiconductor were determined. In another project, I was particularly interested in determining the surface {dollar}rm pKsb{lcub}a{rcub}{dollar} of surface-confined carboxylic acids. The surface force titration curve was obtained by correlating the surface potentials to the different electrolyte pH values. A theoretical fit to the titration curve provided surface {dollar}rm pKsb{lcub}a{rcub}{dollar} and an explanation for the broadening of the titration curve. In a separate project, the adsorption of sodium dodecyl sulfate (SDS) on charge-regulated substrates was investigated by AFM force measurement. The surface charge reversal through surfactant adsorption was directly observed for the first time. The experimental results also led to the conclusion that the formation of a compact and uniform SDS hemimicelle or bilayer on a positively charged surface did not occur as proposed earlier. Finally, the growth of aluminum (III) alkanebisphosphonate multilayer thin films on gold surfaces and the immobilization of both single-stranded (ss) and double-stranded (ds) DNA on positively charged surfaces in aqueous solutions were monitored in situ by probing the surface charge. The AFM force measurements revealed that the formation of the multilayer films followed a regular layer-by-layer growth mechanism and that both ss-DNA and ds-DNA could be immobilized on positively charged surfaces.