AFM Study of pH‐Dependent Adhesion of Single Protein to TiO2 Surface

Abstract The effect of pH‐induced electrostatic conditions on the molecular interaction force of a single lysozyme molecule with TiO 2 is investigated using atomic force microscopy (AFM). The force between the charged or neutral lysozyme molecule and the TiO 2 surface is measured at different pH from 3.6 to 10.8. It is found to be directly proportional to the contact area, given by an effective diameter of the lysozyme molecule, and is further qualitatively verified by the AFM‐measured friction coefficients. The results of the Derjaguin–Landau–Verwey–Overbeek theory show that the pH can change the surface charge densities of both lysozyme and TiO 2 , but the molecular interaction force at different pH is only dependent on the pH‐induced effective diameter of lysozyme. The molecular interaction forces, quantified at the nanoscale, can be directly used to design high‐performance liquid chromatography measurements at macroscale by tuning the retention time of a protein under varied pH conditions. They can also be applied to develop a model for predicting and controlling the chromatographic separations of proteins.