Acid-base properties and the chemical imaging of surface-bound functional groups studied with scanning force microscopy

In this paper we present a scanning force microscopy (SFM) study on electrostatic and hydrogen-bonding interactions between chemically modified SFM probes and surface functional groups. pH-dependent adhesion force measurements in aqueous media between various ionizable functional groups showed a dependency of the adhesion force on the state of ionization (acid-base properties) of these surface-bound groups. The local degree of ionization and surface pK values can be determined from the obtained ''adhesion force titration curves'', which resemble conventional solution pH titration curves. In the presence of a 1:1 electrolyte, the electric double layer was probed and the measured Debye lengths were found to agree with theoretical predictions. pH-dependent lateral force imaging was found to directly map the ionization state of surface-bound functional groups.