Anchoring of Aminophosphonates on Titanium Oxide for Biomolecular Coupling

Aminophosphonates were chosen for a first step functionalization of TiO2 grown on titanium, as they possess a phosphonate group on one end, that can be exploited for coupling with the oxide surface, and an amino group on the other end to enable further functionalization of the surface. The deposition of aminophosphonates with different chain lengths (6 and 12 methylenes) was investigated. Oxygen plasma treatment proved useful in increasing the number of −OH groups at the TiO2 surface, thus helping to anchor the aminophosphonates. By combining different surface-sensitive experimental techniques, we found the existence of a discontinuous monolayer where the molecules are covalently coupled to the TiO2 surface. For the molecules with longer chains, we find evidence of their covalent coupling to the surface through Ti–O–P bond formation, of the exposure of the amino groups at the outer surface, and of an increase in the order of the layer upon thermal annealing.