Heterogeneous catalysis of a copper-coated atomic force microscopy tip for direct-write click chemistry

We report a constructive scanning probe lithography method that uses heterogeneous copper-coated atomic force microscopy tips to catalyze azide-alkyne cycloadditions (CuAAC) between solvated terminal alkyne molecules and azide-terminated self-assembled monolayers on silicon surfaces. Spatially controlled surface functionalization was carried out successfully with 50 mM ethanolic solutions of small molecules bearing terminal alkyne groups--propargylamine, 4-pentynoic acid, and an alkynyl-oligoethyleneoxide. We observed that reaction occurs only where the copper tip is in contact with an azide-terminated surface resulting in features with linewidths on the order of 50 nm. The extent of surface functionalization, as measured by changes in surface topography and lateral force microscopy, depends on the scanning force (31-350 nN) and scanning speed, with significant surface patterning observed even at speeds as high as 64 microm/s. In contrast with related SPL techniques, this approach affords a direct-write lithographic approach to constructively modifying and patterning surfaces at the nanoscale without the need for auxiliary reagents. All that is required is (1) an azide surface, (2) a solution of a terminal alkyne, and (3) a copper-coated AFM tip. These advantages allow the direct attachment of a potentially limitless library of molecules that bear terminal alkyne functionalities, including biomolecules, under relatively mild conditions, with sub-100 nm spatial resolution.