AN ATOMIC FORCE MICROSCOPY BASED INVESTIGATION OF INTERFACIAL PROPERTIES OF BIOCOMPATIBLE CLUSTER ASSEMBLED THIN FILMS

Cluster-assembled nanostructured Titanium Oxide (ns-TiOx) deposited by Supersonic Cluster Beam Deposition (SCBD) recently proved to be a very promising biomaterial. The intrinsic nanostructure of this material, with fine granularity, high porosity and specific area, coupled to the chemical reactivity of the surface is likely to be a key element in determining the biological affinity of the material with nanometer-sized biomolecules, such as proteins. However, little is known of the specific role played by each of these surface properties in the interaction of proteins with nanostructured biocompatible materials. For understanding the role of different surface properties we used atomic force microscopy (AFM) to study morpho-chemical nature of ns-TiOx biocompatible surfaces. AFM Force-Spectroscopy measurements have been used to characterize local adhesive properties of ns-TiOx surfaces. In order to achieve this goal we have developed a patterning strategy based on the combined use of SCBD and Nanosphere Lithography (NSL), for the production of sub-micrometer patterns of ns-TiOx on glass and other substrates. With this methodology one can have both target and reference material in the same investigation area. Results indicated that atoms on the surface of ns-TiOx can form coordinate bond with protein molecules thereby aiding in irreversible protein adsorption at the same time retaining complete biological activity. To further understand how protein adsorption is affected by the buffer medium and by the surface properties of the substrate, we have measured the point of zero charge (PZC) of nanostructured cluster-assembled TiOx. As each kind of protein has different isoelectric point (IEP), hence their adsorption is greatly affected by pH of the buffering medium and concentrations of ions in the solutions. To this purpose, colloidal probes were developed to measure attractive and repulsive forces of a silica micro-sphere against metal oxide surface as a function of pH. Estimated PZC values for TiOx (rutile) and ns-TiOx is 4.9 ± 0.5 & 3.0 ± 0.5, the latter being significantly smaller than PZC typically measured on crystalline surfaces. These results can open up new avenues towards understanding adsorption characteristics of various proteins on metal oxide surfaces.