Surface chemical reactivity of diamond-like carbon films

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Owing to their outstanding properties, diamond-like carbon (DLC) films have found applications in fields as diverse as mechanical engineering on the one hand and biomaterials on the other. Worldwide research activities on DLC films have resulted in profound knowledge and understanding of its bulk microstructure and properties, as well as physical surface characteristics such as surface free energy, wetting and adhesion properties. Knowledge about the chemical nature of DLC film surfaces and especially about their chemical reactivity, on the other hand, is very poorly developed. In a few studies reported in literature, XPS analyses were used in conjunction with standard chemical derivatization in order to gain information about the temporal development of surface chemistry during aging under ambient conditions, generally resulting in oxygen uptake (autoxidation) and formation of oxygen-containing electrophilic functional groups. From these reports, however, no conclusions can be drawn about the ability of DLC surfaces to react under smooth conditions with organic molecules, especially biomolecules carrying nucleophilic functional groups. For this reason the main objective of the proposal is to advance substantially the knowledge and understanding of surface chemistry in general and reactivity specifically on aging hydrogen-free and hydrogenated DLC films, a-C and a-C:H, respectively. A powerful combination of immersion-IRRAS (ImIRRAS) and XPS, together with non-standard chemical derivatization techniques, will provide so far unavailable information regarding the autoxidation mechanism itself as well as the emerging reactive groups. These functional groups are of importance in biomedical applications where DLC surfaces interact with biomolecules, primarily proteins. Due to their size of typically 3 to 10 nm, very small densities of reactive functional groups, down to 0.01 nm2, may suffice to bind a monolayer. Preliminary studies have shown that ImIRRAS can quantify such low densities. This holds also true for hydroperoxy groups, presumed central intermediates in the oxidation process. Results of the project will be of fundamental interest, filling a gap in the knowledge about DLC films. Understanding the aging of DLC films is important for a more general comprehension of their physico-chemical surface properties. Presumably, however, surface functional groups of DLC also play an important role for its outstanding properties as a hemocompatible coating. Therefore the relation between surface chemical reactivity and several properties allowing a first rating of blood compatibility will be explored in this project: Using FTIR-ATR spectroscopy in situ, the presence of reactive groups on a-C:H or a-C will be correlated with the ability of the surfaces to adsorb or immobilize proteins playing a major role for the hemocompatibility of a surface. These studies will be complemented by investigations of platelet deposition and by blood clotting tests.