Probing nanoscale interactions on biocompatible cluster-assembled titanium oxide surfaces by atomic force microscopy

We report on the investigation of the adhesive properties of cluster-assembled nanostructured TiOx (ns-TiOx) films against a Si3N4 AFM tip, in air and in water. The interacting AFM tip apex represents a model nanometer-sized probe, carrying both silanol (Si-OH) and silamine (Si2-NH) groups: it is therefore well suited to investigate biologically relevant molecular interactions with the biocompatible ns-TiOx surface. Coupling nanosphere lithography with supersonic cluster beam deposition we produced sub-micrometer patterns of ns-TiOx on a reference amorphous silica surface. These devices are ideal platforms for conducting comparative nanoscale investigations of molecular interactions between surfaces and specific groups. We have found that in the aqueous medium the adhesion is enhanced on ns-TiOx with respect to amorphous silica, opposed to the case of humid air. A comparative analysis of the different interactions channels (van der Waals, electrostatic, chemical bonding) led to the conclusion that the key for understanding this behavior can be the ability of incoming nucleophiles like nitrogen or oxygen on the Si3N4 tip to displace adsorbed molecules on ns-TiOx and link to Ti atoms via co-ordinate (dative covalent) bonding. This effect is likely enhanced on nanostructured TiOx with respect to crystalline or micro-porous TiO2, due to the greatly increased effective area and porosity. This study provides a clue for the understanding of interaction mechanisms of proteins with biocompatible ns-TiOx, and in general with metal-oxide surfaces.