The atomic force microscope (AFM) is a potentially powerful probe of the electrostatic properties of biological interfaces. In addition to the nanometer-scale resolution achieved by the AFM, the high force sensitivity allows one to observe screened electrostatic interactions beyond the Debye length for minimum sample perturbation. An analytical expression for the tip-sample force due to electric double layer interactions has been derived based on the Gouy-Chapman theory. Although the functional dependence of this expression has been confirmed, its quantitative validity has not been tested largely due to the unknown electrostatic properties of the AFM probe tip. We have developed methods to fully characterize the tip properties and other AFM parameters to allow a quantitative electrostatic analysis by AFM. Measurements over supported lipid membranes with varying mole fractions of negatively charged DOPS in zwitterionic DOPC provide a variable surface for comparison to other measurements such as zeta potential and fluorescent probes. We find that the analytical expression results in an underestimate of the surface potential, and have carried out numerical simulations of the full Poisson-Boltzmann equation to achieve quantitative results from the AFM force curves. In addition, I will discuss our observation and analysis of the surface potential of pure DOPC membranes, and a method to map the local charge density at high resolution.
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