Relative surface charge density mapping with the atomic force microscope

An experimental approach for producing relative charge density maps of biol ogical surfaces using the atomic force microscope is presented. This approa ch, called D minus D (D-D) mapping, uses isoforce surfaces collected at dif ferent salt concentrations to remove topography and isolate electrostatic c ontributions to the tip-sample interaction force. This approach is quantita tive for surface potentials below 25 mV, and does not require prior knowled ge of the cantilever spring constant, tip radius, or tip charge. In additio n, D-D mapping does not require tip-sample contact. The performance of D-D mapping is demonstrated on surfaces of constant charge and varying topograp hy (mechanically roughened mica and stacked bilayers of dipalmitolphosphati dylserine), a surface of varying charge and varying topography (patches of dipalmitolphosphatidylcholine on mica), and bacteriorhopsin membranes adsor bed to mica.