Photopicking: In Situ Approach for Site-Specific Attachment of Single Multiprotein Nanoparticles to Atomic Force Microscopy Tips

Ligand-receptor interactions are fundamental in life sciences and include hormone-receptor, protein-protein, pathogen-host, and cell-cell interactions, among others. Atomic force microscopy (AFM) proved to be invaluable for scrutinizing ligand-receptor interactions at the single molecular level. Basically, a ligand is attached to the AFM tip while its cognate receptor is immobilized on a surface or vice versa, and interactions are studied following triggered ligand-receptor binding. However, with rising biological complexity it becomes increasingly challenging to attach a single intact biomolecule to the tip and ensure interaction-specific orientation. This study presents a novel strategy of inducible in situ tip functionalization with complex multiprotein nanoparticles exemplified by viral capsids, termed photopicking. It ensures a firm attachment of single 125 nm large capsids to the tip. Specific orientation is attained by weak immunosorption of capsids to the substrate and strong photoinducible covalent cross-linking to the tip. Validation of the tip functionalization success is immediate in situ. The versatility of the strategy is further demonstrated on 20-60 nm large amino-modified nanoparticles. In conclusion, considering the size range of the tested biomolecules, the presented strategy is applicable to viruses, viral particles, cellular organelles, multiprotein ligands/receptors, and therapeutic nanoparticles, among others. It therefore opens up exciting new avenues in broad biomedical research fields.