Imaging and determining friction forces of specific interactions between human IgG and rat anti-human IgG

Covalently immobilized rat anti-human immunoglobulin (IgG) monolayers on thiol-modified gold substrates and human IgG linked with the tips were fabricated using the self-assembled monolayer method, and interactions between these systems were studied by friction force microscopy (FFM). In addition to observation of distinct nanostructures of protein monolayers due to recognition events, FFM also quantified the friction force due to protein–protein-specific interactions. The average friction force due to interactions between the antigen functionalized tip and the antibody monolayer was determined as 200–250 pN, significantly greater than that between either the bare tip and the antibody monolayer (0–50 pN), or the blocked antigen tip and the antibody monolayer (50–100 pN), indicative of antigen/antibody-specific interactions. These results, taken together, suggest that FFM is not only capable of tracking recognition events, but also quantifying the friction force due to specific interactions between biological molecules, such as antigen and antibody.