Dynamic probing of compliant surfaces in liquid using intermittent-contact atomic force microscopy.

摘要

Direct force measurement techniques such as atomic force microscopy are widely used tools in the measurement of intermolecular and intersurface interactions on/between a variety of surfaces. In standard atomic force microscopy force measurement (DC-AFM), changes in the static deflection of a cantilever are used to measure forces between a specific probe and a sample of interest as they are approached toward and retracted away from each other. To achieve high lateral resolution, a sharp tip with apex radii of curvature between 5--25 nm is used as the probe. However with these nanosharp tips, high contact pressures can result between the tip and sample that can cause irreversible structural and functional damage to compliant and delicate molecules weakly adsorbed on either surface, such as diffuse polymers, proteins, and other biological molecules and organisms.; An alternate mode of AFM, purposely designed to minimize the tip-sample contact time with the purpose of providing more gentle and less-destructive measurement, is intermittent-contact (IC) AFM. Here, the cantilever is forced to oscillate at kHz frequencies with large amplitude resulting in intermittent contact between the tip and the sample. The large amplitudes also enable early detection of forces before significant compression takes place. IC-AFM in liquid is now commonly utilized for high-resolution imaging of soft surfaces in their appropriate environment. However, the force measurement capabilities of IC-AFM in liquid have not been widely studied.; In our research, we investigate the utility of a derivative of IC-AFM, namely TappingMode(TM) AFM (TM-AFM) for less-destructive force measurement of intermolecular forces on/between a variety of compliant surfaces in their relevant aqueous environments. We monitor changes in the cantilever's amplitude of oscillation as a function of the mean tip-sample separation. Long-ranged tip-sample forces are quantified through numerical modeling of the cantilever dynamics. Here, a forced damped harmonic oscillator model with distance-dependent dissipation is used to simulate cantilever motion. Short-ranged adhesive interactions are also quantified using Hookian physics, akin to the approach used in DC measurements.; We present novel measurements of: (i) steric forces from covalently grafted poly(ethylene glycol) chains; and (ii) the adhesion between the receptor-ligand pair, streptavidin-biotin, in homogeneous and heterogeneous environments, and compare the results to those obtained from the standard DC-AFM force measurement. (Abstract shortened by UMI.)