Use of atomic force microscopy to probe surface charge densities in electrolyte solutions on a nanometer scale

The use of Atomic Force Microscopy (AFM) to investigate the surface of hybrid bio-electronic devices is proposed. This work is focused on the determination of charge densities on insulating layers immersed in electrolyte solutions. Computer simulations which modes the tip-to-sample interaction forces are discussed. The expression for electrostatic interaction takes into account the H⁺ density distribution, according to the site binding theory. Experimental data, obtained by operating two commercial microscopes in the “force versus distance mode”, are therefore compared to the simulated ones. As a result of the comparison, the amount of charge density on SiO ₂ surfaces is estimated as a function of pH. Possible extensions of the model to include dipole layers are also briefly discussed