Glycine Adsorption onto DLC and N-DLC Thin Films Studied by XPS and AFM

Abstract

An understanding of protein adsorption to surfaces of materials is required for the control of biocompatibility and bioactivity. Amorphous carbon, commonly known as diamond-like carbon (DLC) is reported to have excellent biocompatibility. Hydrogenated amorphous-carbon thin films (DLC) and nitrogen doped a-C:H thin films (N-DLC) were prepared by plasma-enhanced chemical vapour deposition (PECVD). Glycine adsorption onto the surface of the films was investigated in order to aid in the elucidation of the mechanisms involved in protein adhesion. The physicochemical nature of the surfaces, before and after adsorption of glycine, was analysed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The XPS spectra highlighted a slight increase the ratio of sp3/sp2 at low levels of N (5.4 atom %) whilst increasing the nitrogen dopant level (> 5.4 atom %) resulted in a decrease of the sp3/sp2 ratio. Following exposure to solutions containing glycine, the presence of peaks at 285.0 eV, 399 eV and 532 eV indicated the adsorption of glycine to the surfaces with a quantitative change in the amount of C, N and O on the surfaces. Glycine was bound to the surface of the DLC films via both de-protonated carboxyl and protonated amino groups while, in the case of N-DLC gylcine was bound to the surface via anionic carboxyl groups and the amino group did not interact strongly with the surface. AFM analysis showed a change in surface roughness of the films with the ratio of rms values increasing following exposure to glycine. These results show that low levels of nitrogen doping in DLC enhances the adsorption of the amino acid, while, increased doping levels (> 5.4 atom %) led to a reduced adsorption, as compared to undoped DLC. Doping of DLC may allow control of protein adsorption to the surface. [DOI: 10.1380/ejssnt.2009.217]