Heidi B. Martin

Hydrogen and Oxygen Evolution on Boron‐Doped Diamond Electrodes

The evolution of hydrogen and oxygen was studied on diamond electrodes containing approximately 1021 boron atom/cm3. Voltammetry showed a wide potential window [−1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp2 content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp2 content. Anodic polarization made the diamond surface hydrophilic; x‐ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon‐oxygen bonds. The estimated capacitance of the interface ranged from 0.05 μF/cm2 for high quality diamond to 5 μF/cm2 for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.

Voltammetry Studies of Single‐Crystal and Polycrystalline Diamond Electrodes

Boron-doped polycrystalline and near-single-crystal quality diamond electrodes were studied by voltammetry. A redox couple with E 1/2 = + 1.83 V vs. standard hydrogen electrode was detected on the polycrystalline electrodes but was absent on the single-crystal electrodes. The results strongly suggest that the couple is associated with reactivity at the grain boundaries. Plasma fluorination of polycrystalline diamond electrodes using CF 4 in a radio frequency plasma eliminated the redox couple at + 1.83 V but did not alter the potential range of water stability. Cathodic polarization of as-grown, polycrystalline diamond electrodes caused an irreversible addition of oxygen to the surface. Subsequent anodic polarization added additional oxygen and made the surface hydrophilic. Single-crystal electrodes also displayed an increase in oxygen coverage upon both cathodic and anodic polarization. Voltammetry studies of electrodes covered with a thin sp 2 carbon surface layer indicate that the redox couple at + 1.83 V corresponds to multiple processes including the etching of sp carbon in the grain boundaries.

Platinum for neural stimulation: voltammetry considerations

The underlying cause of electrical stimulation-induced tissue trauma is debated. Our focus has been to study effects of generating electrochemical by-products at the electrode-electrolyte interface, using the pulse-clamp technique coupled with voltammetry to analyze charge transfer. The platinum-H(2)SO(4) system has been a standard for analyzing electrochemistry on platinum-stimulating electrodes, even though the chemical differences between H(2)SO(4) and the living body are obvious. Experiments were designed to determine whether phosphate-buffered saline (PBS) could serve as a more accurate emulation of living tissue. It had been rumored that platinums performance in PBS deviates from that in H(2)SO(4) at larger potentials. Voltammetry in PBS was performed in two potential ranges. In a conventional potential range (-0.6 V to +0.9 V versus Ag/AgCl), characteristic peaks appear very similar to published voltammograms of platinum in H(2)SO(4). However, in an extended range (-1.0 V to +1.7 V versus Ag/AgCl), platinum exhibited additional electrochemical activity: one oxidation peak and two reduction peaks. Therefore, voltammetry was performed in NaCl and a sodium phosphate mixture (i.e. PBS components) to separate their activity. The altered electrochemical performance of platinum in PBS suggests that certain reactions on platinum at potentials outside the water window will not reflect what happens in vivo.

Diamond-on-Polymer Microelectrode Arrays Fabricated Using a Chemical Release Transfer Process

This paper reports the design, fabrication, and characterization of diamond-on-polymer microelectrode arrays. A “diamond-first” chemical release transfer process was implemented to integrate diamond electrodes, grown at temperatures >; 700 °C, on a temperature-sensitive polynorbornene-based (PNB) substrate, allowing for the advantageous neural interfacing properties of diamond to be utilized in a flexible device. Intracortical probes with two electrodes and peripheral nerve electrode arrays with ten electrodes ranging in area from 800 to 41 000 μm2 were fabricated. Mechanical testing showed that the structures were flexible, with the composite structure having mechanical characteristics similar to bare PNB. Electrical testing confirmed that ohmic contacts were formed without a postanneal step and determined a diamond-on-polymer electrode impedance of ~ 1.5 MΩ at 1 kHz.

Wireless Amperometric Neurochemical Monitoring Using an Integrated Telemetry Circuit

An integrated circuit for wireless real-time monitoring of neurochemical activity in the nervous system is described. The chip is capable of conducting high-resolution amperometric measurements in four settings of the input current. The chip architecture includes a first-order DeltaSigma modulator (DeltaSigmaM) and a frequency-shift-keyed (FSK) voltage-controlled oscillator (VCO) operating near 433 MHz. It is fabricated using the AMI 0.5 mum double-poly triple-metal n -well CMOS process, and requires only one off-chip component for operation. Measured dc current resolutions of ~ 250 fA, ~ 1.5 pA, ~ 4.5 pA, and ~ 17 pA were achieved for input currents in the range of plusmn5, plusmn37, plusmn150, and plusmn600 nA, respectively. The chip has been interfaced with a diamond-coated, quartz-insulated, microneedle, tungsten electrode, and successfully recorded dopamine concentration levels as low as 0.5 muM wirelessly over a transmission distance of ~ 0.5 m in flow injection analysis experiments.

Anodically Generated Short-Lived Species on Boron-Doped Diamond Film Electrodes

Electrodes composed of boron-doped diamond (BDD) films on metal and semiconductor substrates have a wide range of applications in electrochemistry. This research investigated short-lived species (SLS) produced by anodic polarization of BDD electrodes in 1.0 M HClO 4 solutions. Normal pulse voltammetry experiments were performed to identify anodically produced SLS with lifetimes of less than 50 ms under open-circuit conditions. Potential step chronoamperometry experiments were performed to investigate the steady-state concentrations of SLS at the electrode-solution interface as a function of potential. Anodic potentials greater than 1.5 V with respect to the standard hydrogen electrode (SHE) were required to generate the SLS. Increasing anodic potentials between 1.5 and 3.0 V/SHE resulted in increasing concentrations of the SLS, until a saturation point was reached. Past work by other investigators suggests that the SLS likely consist primarily of HO . radicals produced from water oxidation.

Diamond Microelectrodes and CMOS Microelectronics for Wireless Transmission of Fast-Scan Cyclic Voltammetry

This paper reports on technology development at the sensor and circuit levels for wireless transmission of fast- scan cyclic voltammetry (FSCV) in neurochemical detection. Heavily conductive, boron-doped diamond is selectively deposited onto the polished tip of a tungsten microelectrode to fabricate versatile, implantable, micro-needle microprobes capable of neurochemical sensing in the brain. In addition, an integrated circuit is fabricated in a 0.5-mum CMOS technology for processing and wireless transmission of the electrochemical signals corresponding to extracellular concentration changes of various neurotransmitters. The chip consists of a current-based, second-order, front-end SigmaDelta ADC and an on-chip, RF-FSK transmitter at the back-end. The ADC core and the transmitter consume 22 muA and 400 muA, respectively, from a 2.6-V power supply. Major electroactive neurotransmitters such as serotonin and dopamine in micromolar concentration have been wirelessly recorded at 433 MHz using 300-V/s FSCV in flow injection analysis experiments.

Kinetic and Adsorption Studies of Biogenic Amine Neurotransmitters at Polycrystalline Diamond Microelectrodes

High sensitivity, 1-10 nM detection, of biogenic amine neurotransmitters was achieved using fast-scan cyclic voltammetry at hydrogen-terminated diamond microelectrodes. This reduced to 1-10 μM after 5-10 runs. To attempt to understand the sensitivity decrease, electron transfer kinetics, baseline current, and rate-limiting step for high (μM) and low (nM) concentrations were compared. Dopamine oxidation kinetics slowed after repeated scans, ΔEp increased from 0.142 to 0.805 V. Baseline current decreased by 50%. The rate-limiting step, measured only at the conditioned surface, was diffusion limiting. The high quality of our electrodes was verified by the voltage window in 0.5 M H2SO4 and by Raman spectroscopy. Dopamine and serotonin oxidation kinetics were characterized at more sensitive, sulfophenyl-terminated diamond microelectrodes; the responses were diffusion limited. These data were qualitatively consistent with surface chemistry changes after scanning, but do not adequately explain the sensitivity decrease. Additional testing of hydrogen- terminated surfaces is needed to provide insight.

Boron-Doped Diamond Films for Electrochemical Applications

Diamond electrodes possess unique chemical stability, a very wide potential window of water stability, and low background currents. These properties give rise to numerous possible applications, for example, electrosynthesis and electrodestruction reactions at extreme potentials and environmental conditions and as a sensor electrode in aggressive environments. Furthermore, the study of semiconducting diamond electrodes promises to lead to greater understanding of the surface chemistry of diamond and of electronic levels and surface states in doped diamond. In this paper the reactivity of diamond electrodes and their use in a molten salt environment, as a sensor element, and for characterizing diamond are discussed.