The deposition, characterization, and electrochemical responsiveness of boron-doped nanocrystalline diamond thin-film electrodes is reported. The films consist of clusters of diamond grains, ∼50-100 nm in diameter, and possess an rms surface roughness of 34 nm over a 5 × 5 µm 2 area. The individual and randomly ordered diamond grains are approximately 10-15 nm in diameter, as evidenced by TEM. The ∼4-µm-thick films were deposited by microwave-assisted chemical vapor deposition (CVD) using a CH 4 /H 2 /Ar source gas mixture (1%/5%/95%). Under these conditions, C 2 , rather than CH 3 • , appears to be the dominant nucleation and growth precursor. The nanocrystallinity is a result of a growth and nucleation mechanism discovered by Gruen, which involves the insertion of C 2 carbon dimer into C-H bonds on the growth surface (MRS Bull. 1998, 23, 32). The nanocrystalline morphology results from a high renucleation rate. However, unlike previously reported nanocrystalline diamond thin films that have electrical properties dominated by the high fraction of π-bonded carbon atoms in the grain boundaries, the present films are doped with boron, either using B 2 H 6 or a solid-state boron diffusion source, and the electrical properties appear to be dominated by the charge carriers in the diamond. The films were characterized by scanning-electron microscopy, atomic-force microscopy, transmission-electron microscopy, visible-Raman spectroscopy, X-ray diffraction, boron-nuclear-reaction analysis, and cyclic voltammetry, using Fe(CN) 6 3-/4-, Ru(NH 3 ) 6 3+/2+ , IrCl 6 2-/3-, methyl viologen, Fe 3+/2+ , and 4-tertbutylcatechol. Analytical application of this advanced carbon electrode material for the detection of trace metal ions is discussed.

No abstract

The pattern of conductivity and electrochemical activity at the surfaces of hydrogen-terminated boron-doped diamond electrodes, with different boron doping levels, were measured using conductive probe atomic force microscopy (CP-AFM) and scanning electrochemical microscopy (SECM). CP-AFM showed that the surface was predominantly insulating, with discrete conducting areas of less than 2 μm in diameter, randomly and nonuniformly distributed on the surface. SECM imaging correlated these conductive areas with electrochemical activity and showed that the electrode surface was only partly electrochemically active and that the active area of the electrodes increased with boron doping level. Cyclic voltammograms and SECM approach curves obtained using Ru(NH3)6 3+/2+ as the redox mediator were characteristic of those obtained at partially blocked electrodes of nonuniform activity, or a microelectrode array. By use of this model, the SECM approach curves could be fit to obtain values of the fraction of the surface that was electrochemically active. The active area of the electrode was related to the boron doping level.

The electrochemical properties of two commercial (Condias, Sumitomo) boron-doped diamond thin-film electrodes were compared with those of two types of boron-doped diamond thin film deposited in our laboratory (microcrystalline, nanocrystalline). Scanning electron microscopy and Raman spectroscopy were used to characterize the electrode morphology and microstructure, respectively. Cyclic voltammetry was used to study the electrochemical response, with five different redox systems serving as probes (Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/)(2+), IrCl(6)(2)(-)(/3)(-), 4-methylcatechol, Fe(3+/2+)). The response for the different systems was quite reproducibile from electrode type to type and from film to film for electrodes of the same type. For all five redox systems, the forward reaction peak current varied linearly with the scan rate(1/2) (nu), indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, k degrees (app), for all five redox systems were determined from deltaE(p)-nu experimental data, according to the method described by Nicholson (Nicholson, R. S. Anal. Chem. 1965, 37, 1351.). The rate constants were also verified through digital simulation (DigiSim 3.03) of the voltammetric i-E curves at different scan rates. Good fits between the experimental and simulated voltammograms were found for scan rates up to 50 V/s. k degrees (app) values of 0.05-0.5 cm/s were observed for Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/2+), and IrCl(6)(2)(-)(/3)(-) without any extensive electrode pretreatment (e.g., polishing). Lower k degrees (app) values of 10(-)(4)-10(-)(6) cm/s were found for 4-methylcatechol and Fe(3+/2+). The voltammetric responses for Fe(CN)(6)(3)(-)(/4)(-) and Ru(NH(3))(6)(3+/2+) were also examined at all four electrode types at two different solution pH (1.90, 7.35). Since the hydrogen-terminated diamond surfaces contain few, if any, ionizable carbon-oxygen functionalities (e.g., carboxylic acid, pK(a) approximately 4.5), the deltaE(p), i(p)(ox), and i(p)(red) values for the two systems were, for the most part, unaffected by the solution pH. This is in contrast to the typical behavior of oxygenated, sp(2) carbon electrodes, such as glassy carbon.

The fabrication and characterization of boron-doped diamond microelectrodes for use in electrochemical detection coupled with capillary electrophoresis (CE-EC) is discussed. The microelectrodes were prepared by coating thin films of polycrystalline diamond on electrochemically sharpened platinum wires (76-, 25-, and 10-microm diameter), using microwave-assisted chemical vapor deposition (CVD). The diamond-coated wires were attached to copper wires (current collectors), and several methods were explored to insulate the cylindrical portion of the electrode: nail polish, epoxy, polyimide, and polypropylene coatings. The microelectrodes were characterized by scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry. They exhibited low and stable background currents and sigmoidally shaped voltammetric curves for Ru(NH3)6(3+/2+) and Fe(CN)6(3-/4-) at low scan rates. The microelectrodes formed with the large diameter Pt and sealed in polypropylene pipet tips were employed for end-column detection in CE. Evaluation of the CE-EC system and the electrode performance were accomplished using a 10 mM phosphate buffer, pH 6.0, run buffer, and a 30-cm-long fused-silica capillary (75-microm i.d.) with dopamine, catechol, and ascorbic acid serving as test analytes. The background current (approximately 100 pA) and noise (approximately 3 pA) were measured at different detection potentials and found to be very stable with time. Reproducible separation (elution time) and detection (peak current or area) of dopamine, catechol, and ascorbic acid were observed with response precisions of 4.1% or less. Calibration curves constructed from the peak area were linear over 4 orders of magnitude, up to a concentration between 0.1 and 1 mM. Mass limits of detection for dopamine and catechol were 1.7 and 2.6 fmol, respectively (S/N = 3). The separation efficiency was approximately 33,000, 56,000, and 98,000 plates/m for dopamine, catechol, and ascorbic acid, respectively. In addition, the separation and detection of 1- and 2-naphthol in 160 mM borate buffer, pH 9.2, was investigated. Separation of these two analytes was achieved with efficiencies of 118,000 and 126,000 plates/m, respectively.