Diamond for bio-sensor applications

A summary of photo- and electrochemical surface modifications applied on single- crystalline chemical vapour deposition ( CVD) diamond films is given. The covalently bonded formation of amine- and phenyl- linker molecule layers is characterized using x- ray photoelectron spectroscopy, atomic force microscopy ( AFM), cyclic voltammetry and field- effect transistor characterization experiments. Amine- and phenyl- layers are very different with respect to formation, growth, thickness and molecule arrangement. We detect a single- molecular layer of amine- linker molecules on diamond with a density of about 10(14) cm(-2) ( 10% of carbon bonds). Amine molecules are bonded only on initially H- terminated surface areas to carbon. In the case of electrochemical deposition of phenyl- layers, multi- layer formation is detected due to three- dimensional ( 3D) growths. This gives rise to the formation of typically 25 angstrom thick layers. The electrochemical grafting of boron- doped diamond works on H- terminated and oxidized surfaces. After reacting such films with hetero- bifunctional crosslinker molecules, thiol- modified ss- DNA markers are bonded to the organic system. Application of fluorescence and AFM on hybridized DNA films shows dense arrangements with densities of up to 10(13) cm(-2). The DNA is tilted by an angle of about 35. with respect to the diamond surface. Shortening the bonding time of thiol- modified ss- DNA to 10 min causes a decrease of DNA density to about 10(12) cm(-2). Application of AFM scratching experiments shows threshold removal forces of around 75 nN for DNA bonded on phenyl- linker molecules and of about 45 nN for DNA bonded to amine- linker molecules. DNA sensor applications using Fe( CN6)(3-/4-) mediator redox molecules, impedance spectroscopy and DNA- field effect transistor devices performances are introduced and discussed.