Microstructure And Properties Of Phosphorus Incorporated Tetrahedral Amorphous Carbon And Its Application As Bioelectrode

Great attention has been given to the development of new bioelectrode materials in the field of material and electrochemistry sciences. Tetrahedral amorphous carbon (ta-C) is suggested to be a preponderant material due to its excellent mechanical properties, good biocompatibility, high wear resistant, long-term stability, chemical inertness against aggressive media and ambient temperature growth on virtually any substrate. However, the high resistance and intrinsic compressive stress of ta-C film limit its practical application as a semiconductor electrode. In this paper, phosphorus impurity was incorporated into the carbon network during ta-C film preparation. The effect of phosphorus content in the film on the microstructure, mechanical properties, compressive stress, photoelectrical and electrochemical behaviors, haemocompatibility and corrosion resistance was investigated, and the catalytic and analytical abilities of the resulted film towards biochemical matters were estimated.Phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films were deposited on conductive silicon wafers by filtered cathodic vacuum arc system with PH3 gas as the dopant source at varying flow rate of PH3 and substrate bias. The chemical compositions and microstructures of ta-C:P films were examined by the analyses of X-ray photoemission spectroscopy (XPS), Raman spectroscopy, Fourier Transform infrared (FTIR) spectrometer and atomic force spectroscope. The XPS results show that phosphorus is bonded with carbon as C-P and C=P forms and the contents of these bonds increase with phosphorus level in ta-C:P films. Phosphorus incorporation results in an increased content of sp2-hybridized carbon atoms in the film and the clustering of sp2 groups, especially, an evolution of sp2 configurations from olefinic groups to rings. The downshift of G peak, the fall in the full width at half maximum of G peak and the increase of the intensity ratio of D and G peaks, ID/IG, are related to the increase of phosphorus fraction in ta-C:P films. The IR absorption band in the range of 1000-1800 cm-1 contains both CC skeleton modes and CP stretching species, and the spectrum at 2800-3100 cm-1 is attributed to C-H stretching modes. Hydrogen content increases with PH3 flow rate varying from 3 to 30 sccm, which modifies the carbon network by saturating isolated sp3 dangling bonds and =C bonds as =CHx groups. This contributes to the increase in photoluminescence efficiency of ta-C:P film. Some isolated and irregular nanoclusters induced by phosphorus impurities are dispersed on the film surfaces. High bombardment energy (-2000 V bias) increases the surface roughness of ta-C:P films, which makes the films more loose and porous.The mechanical properties and intrinsic stress of ta-C:P films were investigated by nanoindentation and substrate curvature measurements. Phosphorus incorporation results in a decreased coordination number of carbon and a released local distortion, which reduces the Young's modulus and compressive stress of the films. The increase of sp2 content and the decrease of sp3 content lead to the loss of hardness of ta-C:P films. When phosphorus content of ta-C:P films increases from 0 to 6.8 at.%, the hardness of the films varies from 50 GPa to 22 GPa, and Young's modulus from 400 GPa to 230 GPa.By measuring the photoelectricial performances of ta-C:P films, the results show that low-phosphorus ta-C:P films have smaller E04 and represent an electronic structure with a large number of'n-type'band tail states. The film with 6.8 at.% phosphorus shows the best conductive ability. Progressive saturation of sp2 sites by excessive H induced by high flow rate of PH3 is responsible for the slight narrowing of E04 and the loss of conductivity for the high-phosphorus ta-C:P films. The carriers represent hopping conduction and thermally activated conduction mechanisms in the temperature range from 293 to 573 K.The surface and interface behaviors, and the haemocompatibility of ta-C:P films in the biological environment were evaluated using contact angle and blood compatibility measurements. ta-C:P films represent more hydrophilic surfaces, and the interfacial tensions between the films and water are more closer to the cell-medium interface tension (1-3 mJ/m2). Phosphorus incorporation also increases the surface energy and the ratio of the polar component to dispersive component. There is little destruction action of ta-C:P films to red blood cells and the haemolytic effect is lower than 1 %. The adsorption of albumin and fibrinogen on ta-C:P surfaces is weak and the adsorption ability of albumin is stronger than that of fibrinogen. The numbers of adhered platelets on ta-C:P films are significantly lower than those attached to Ti alloy and ta-C surfaces. Platelet adhesion tests show that ta-C:P films with less spreading area and circularity index represent lower adhesion and activation of platelets, and therefore lower thrombosis risk. The stability and corrosion resistance of ta-C:P films were studied via potentiodynamic polarization experiments. ta-C:P films in 0.89 wt. % NaCl solution have high corrosion potential and low corrosion current density. Phosphorus incorporation increases the polarization resistance, decreases the porosity of ta-C film and provides efficient protection to Ti alloy.The electrochemical activity of ta-C:P films was investigated by an electrochemical workstation. ta-C:P electrodes show a wide potential window around 2.0 V and low background current density of 1μAcm-2 in H2SO4 solution. Acid pre-treatments with different concentrations develop active CP sites and remove inactive PO sites, accelerate the electron exchange between ta-C:P electrode and aqueous solution, widen the potential window of ta-C:P electrodes, and improve the reversibility of the electrodes. The modification of gold nanoparticles to ta-C:P surfaces further favors the electrochemical reaction on ta-C:P electrodes. The progressive nucleation and diffusion-controlled growth of Au on ta-C:P surface are confirmed. The existed Au sites decrease the energy barrier and favor the growth of Au nuclei. The size and coverage of Au nanoparticles can be adjusted by controlling the deposition time, which dominates the electrochemical properties of ta-C:P/Au electrodes.The detection capability of ta-C:P films as bioelectrodes was evaluated by cyclic voltammetry and differential pulse voltammetry. ta-C:P electrodes can determine Cu2+, Pb2+ and Cd2+ simultaneously. The stripping peak positions of these three ions are about -0.9, -0.6 and -0.2 V, respectively, with no interference. ta-C:P/Au electrode reveals high electrocatalytic ability towards the electrooxidation of hydrogen peroxide because the three-dimensional Au nanoparticles accelerate the electron exchange between ta-C:P electrode and H2O2 in aqueous solution. The current of H2O2 at ta-C:P/Au electrode is linear within a wider concentration range from 0.2μM to 1 mM with a sensitivity of 20.0±0.2 nA/μM (n=6). The detection limit at signal-to-noise ratio of 3 to 1 is calculated to be 0.08±0.01μM. The value is one order lower than that obtained at ta-C:P electrode under the same conditions. The neurotransmitter of dopamine (DA) and ascorbic acid (AA) can be oxidized on ta-C:P/Au electrodes and the oxidation peaks of those are separated by 0.2 V. Therefore, ta-C:P/Au electrodes can provide effective detection to DA in the presence of large amount of AA. Phosphorus and Au nanoparticles increase the conductivity of ta-C, which attracts positive DA and repulses negative AA due to the electrostatic repulsion.