Fluid Electric Force Microscopy for Charge Density Mapping in Biological Systems

Electric force microscopy has been adapted for mapping surface charge density in aqueous solutions. The electric double layer interaction between the tip and the sample provides a contrast mechanism sensitive only to the surface charge density as other parameters are held constant. The resulting charge maps are acquired at typical atomic force microscopy scanning rates and approximately 25-nm resolution. The contrast is well-described by a simple expression for the tip−sample double layer interaction in electrolyte solutions. Fluid electric force microscopy is highly sensitive and nondestructive, as demonstrated with charge density maps of fluid-phase-supported bilayer membranes and single DNA molecules.