A general and efficient cantilever functionalization technique for AFM molecular recognition studies

Atomic force microscopy (AFM) is a versatile technique for the investigation of noncovalent molecular associations between ligand-substrate pairs. Surface modification of silicon nitride AFM cantilevers is most commonly achieved using organic trialkoxysilanes. However, susceptibility of the Si−O bond to hydrolysis and formation of polymeric aggregates diminishes attractiveness of this method for AFM studies. Attachment techniques that facilitate immobilization of a wide variety of organic and biological molecules via the stable Si−C bond on silicon nitride cantilevers would be of great value to the field of molecular recognition force spectroscopy. Here, we report (1) the formation of stable, highly oriented monolayers on the tip of silicon nitride cantilevers and (2) demonstrate their utility in the investigation of noncovalent protein-ligand interactions using molecular recognition force spectroscopy. The monolayers are formed through hydrosilylation of hydrogen-terminated silicon nitride AFM probes using a protected α-amino-ω-alkene. This approach facilitates the subsequent conjugation of biomolecules. The resulting biomolecules are bound to the tip by a strong Si−C bond, completely uniform with regard to both epitope density and substrate orientation, and highly suitable for force microscopy studies. We show that this attachment technique can be used to measure the unbinding profiles of tip-immobilized lactose and surface-immobilized galectin-3. Overall, the proposed technique is general, operationally simple, and can be expanded to anchor a wide variety of epitopes to a silicon nitride cantilever using a stable Si−C bond.