Spectroscopic ellipsometry investigation of amorphous carbon films with different fraction of sp3 bonds: relation with protein adsorption

Diamond like carbon (DLC) is known to posses high mechanical hardness and chemical inertness along with good bloodcompatibility. The latter is determined by protein - DLC surface interaction that is not properly addressed in the literature. These properties of DLC are determined to a large extent by sp3 bonds fraction and hydrogen content. Therefore, DLC films with different sp3 content have been produced by Ion Beam Assisted Deposition (IBAD) (low sp3) and Filtered Cathodic Vacuum Arc (FCVA) (high sp3) by changing deposition parameters.
The films were characterized by spectrocopic ellipsometry (SE) within the wavelength range l=300-1700 nm after deposition. The spectral dependences of ellipsometric parameters, D and Y, were acquired in autoretarder mode of VASE spectroscopic ellipsometer (J.A. Woolam Co., Inc., USA). To improve the quality of SE data reduction, multiple sample analysis was carried out for set of different thickness DLC samples produced by IBAD at the same deposition parameters on different substrates. The optical constants of DLC both with low and high content of sp3 bonds were parameterized in semiconductor model approach [2]. The SE data reduction yielded refraction and absorption index values, film thickness, band gap values of the DLC and surface roughness. Relation of SE results for samples produced by FCVA with Raman scattering data, which demonstrate D and G bands displacements, allowed proper characterization of the bonding structure of DLCs and estimation of sp3/sp2 fraction ratio. By using film thickness from SE results DLC film densities were estimated due to elastic recoil detection analysis (ERDA). ERDA also demonstrated only low amount of hydrogen mainly near film-substrate interface in DCLs produced by FCVA technique. Thus, it demonstrates that DLC properties modification is due to sp3/sp2 ratio variation and are not caused by variation of hydrogen content.
Deposition of human serum albumin labeled by fluorescein on DLC layers with different sp3 fraction was investigated. Selective binding of polyclonal antibody to the adsorbed albumin was quantified. The ratio of antibody binding to total adsorbed protein as measured by florescence gives the information about availability of epitopes and protein deformation.