" High-resolution Probing of Compliant Surfaces using Tapping-Mode AFM"

Ijeoma Nnebe (Advisor: James Schneider)

Intermolecular and inter-surface interactions are the foundation for the function of many technologies in a broad variety of industries. Direct force measurement techniques are now established and well-valued tools in the measurement of these interactions. In traditional atomic force microscopy (AFM) force measurement (dc), 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 nanosharp tip is used as the probe. However with these tips, high contact loads 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 probe-sample contact time with the purpose of providing a gentler and less-destructive measurement, is tapping-mode (TM) AFM. Here, the cantilever is forced to oscillate at ~kHz frequencies and large amplitudes causing intermittent contact between the tip and the sample. This minimizes the tip-sample contact time and provides early detection of forces before significant compression takes place. TM-AFM in liquid is now commonly utilized for high-resolution imaging of soft surfaces in their appropriate environment. However, its force measurement capabilities have not been widely studied.

TM force measurements of normalized amplitude versus normalized mean tip-sample separation - illustrating the sensitivity of the oscillation amplitude to repulsive polymer steric forces.

In our research, we therefore investigate the utility of TM-AFM as a force measurement tool for the gentle and less-destructive measurement of intermolecular forces on/between a variety of compliant surfaces in their relevant aqueous environments. Here, we monitor changes in the dynamic properties of the cantilever’s oscillation, such as its amplitude of oscillation, its mean deflection, and its phase lag from the driving signal as a function of the mean tip-sample separation. Though tip-sample interactions cannot be directly calculated from the measured dynamic quantities, we can successfully and quantitatively extract tip-sample interactions through numerical modeling of the cantilever dynamics. Specifically, we use a forced damped harmonic oscillator model with distance-dependent dissipation. We conduct TM force measurements in liquid on a variety of surfaces and compare the results to those obtained from the standard dc mode of AFM force measurement. Specifically, we investigate the capabilities of TM-AFM through the measurement of three different intermolecular interactions: (i) steric forces from covalently grafted poly(ethylene) glycol chains; (ii) bridging of physically adsorbed cationic poly(ethylenimine) segments to the negatively-charged silicon nitride tip; (iii) and the adhesion between the receptor-ligand pair, streptavidin-biotin, in homogeneous and heterogeneous environments. These molecules are relevant to applications that utilize colloidal steric stabilization and flocculation, and to the fouling prevention of biomaterials. Through our work we illustrate the feasibility and great potential of TM in liquid as a force measurement technique, with application towards the less-destructive and more sensitive measurement of interactions on biomaterials and polymer-coated surfaces.