Chemical pulsed-force microscopy of single polyethyleneimine molecules in aqueous solution

Using scanning probes modified with amino- and methyl-terminated groups in situ offers the possibility of scanning force microscopy (SFM) imaging with enhanced chemical sensitivity. The sample, a positively charged high molecular weight polyethyleneimine (branched), is adsorbed on the negatively charged mica surface to achieve an arrangement of single polyethyleneimine (PEI) molecules. Chemical imaging of individual PEI molecules is revealed by differences in adhesion and contact stiffness obtained from the pulsed-force mode measurements by SFM. We conclude that electrostatic and hydrophobic forces are responsible for the adhesion contrast in the system. The measurements also show a pH dependence based on the type of interaction between the modified tip and the substrate. We demonstrate that the chemical pulsed force microscopy is a suitable tool for the investigation of strongly charged polymers on an oppositely charged surface.