Electrochemical performance of diamond thin-film electrodes from different commercial sources

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.     

Optically transparent diamond electrode for use in ir transmission spectroelectrochemical measurements

A new analytical spectroelectrochemical methodology is reported on that utilizes an optically transparent boron-doped diamond thin film. The film was deposited on undoped Si by microwave-assisted chemical vapor deposition using a 4-h growth with a 0.5% CH4/H2 source gas mixture and 2 ppm B2H6 added for boron doping. The thin-film electrode possessed a transparency of 40-60% in the mid- and far-IR regions of the electromagnetic spectrum. The physical, electrical, optical, and electrochemical properties of the electrode were characterized by scanning electron microscopy, Raman spectroscopy, X-ray diffraction, four-point probe electrical resistance measurements, IR spectroscopy, and cyclic voltammetry. The film's electrochemical behavior was evaluated using both aqueous (Fe(CN)(6)3-/4-, methyl viologen, Ru(NH3)(6)3+/2+, and IrCl(6)2-/3-) and nonaqueous (ferrocene) redox systems. The film exhibited a low and stable background current and a nearly reversible voltammetric response for all these redox systems. The diamond/Si optically transparent electrode (OTE) and a thin-layer transmission cell were used to record the spectroelectrochemical response for 10 mM Fe(CN)(6)3-/4- in 1 M KCl. Difference IR spectra (oxidized minus reduced), recorded at various applied potentials, showed that the CN vibrational mode at 2039 cm-1 for Fe(CN)(6)4- reversibly shifted to 2116 cm-1 upon oxidation to Fe(CN)(6)3-, as expected. Difference IR spectra (oxidized minus reduced) were also recorded for 20 mM ferrocene in 0.1 M TBABF4/CH3CN. A shift of the C-H bending mode of the cyclopentadienyl ring from 823 to 857 cm-1 occurred upon oxidation of ferrocene to ferricenium. The key finding from the work is that the diamond OTE provides sensitive, reproducible, and stable spectroelectrochemical responses for aqueous and nonaqueous redox systems in the mid- and far-IR.     

Structure and electrochemical properties of carbon films prepared by a electron cyclotron resonance sputtering method

This paper describes the characterization, electrochemical properties, and applications of carbon films prepared by the electron cyclotron resonance (ECR) sputtering method. The ECR-sputtered carbon film was deposited within several minutes at room temperature. The optimized sputtering conditions significantly change the film structure, which includes many more sp3 bonds (sp3/sp2 = 0.702) than previously reported film (sp3/sp2 = 0.274)1 with an extremely flat surface (0.7 A). The ECR-sputtered carbon films exhibit excellent electrochemical properties. For example, they have nearly the same potential window in the positive direction as that of high-quality, boron-doped diamond (moderately doped, 10(19)-10(20) boron atoms/cm3)2 and an even wider potential window in the negative direction with a low background current, high stability, and suppression of fouling by electroactive species without pretreatment. The electron-transfer rates at ECR-sputtered carbon films are similar to those of glassy carbon (GC) for Ru(NH3)(6)(2+)/(3+) and Fe(CN)(6)(3-)/(4-), whereas they are much slower than those of GC for Fe2+/3+, dopamine oxidation, and O2 reduction due to weak interactions between electroactive species and the ECR-sputtered carbon film surface. Such a response can be attributed to the ultraflat surface and low surface O/C ratios of ECR-sputtered carbon films. ECR-sputtered carbon film is advantageous for measuring biochemicals with high oxidation potentials because of its wide potential window and high stability. Highly reproducible and well-defined cyclic voltammograms were obtained for histamine and azide ions with a peak potential at 1.25 and 1.12 V vs Ag/AgCl, respectively. The film is very stable for continuous voltammetry measurements in 10 microM bisphenol A, which usually fouls the electrode surface with oxidation products.     

Standard electrochemical behavior of high-quality, boron-doped polycrystalline diamond thin-film electrodes

Standard electrochemical data for high-quality, boron-doped diamond thin-film electrodes are presented. Films from two different sources were compared (NRL and USU) and both were highly conductive, hydrogen-terminated, and polycrystalline. The films are acid washed and hydrogen plasma treated prior to use to remove nondiamond carbon impurity phases and to hydrogen terminate the surface. The boron-doping level of the NRL film was estimated to be in the mid 1019 B/cm3 range, and the boron-doping level of the USU films was approximately 5 x 10(20) B/cm(-3) based on boron nuclear reaction analysis. The electrochemical response was evaluated using Fe-(CN)6(3-/4-), Ru(NH3)6(3+/2+), IrCl6(2-/3-), methyl viologen, dopamine, ascorbic acid, Fe(3+/2+), and chlorpromazine. Comparisons are made between the apparent heterogeneous electron-transfer rate constants, k0(app), observed at these high-quality diamond films and the rate constants reported in the literature for freshly activated glassy carbon. Ru(NH3)6(3+/2+), IrCl6(2-/3-), methyl viologen, and chlorpromazine all involve electron transfer that is insensitive to the diamond surface microstructure and chemistry with k0(app) in the 10(-2)-10(-1) cm/s range. The rate constants are mainly influenced by the electronic properites of the films. Fe(CN)6(3-/4-) undergoes electron transfer that is extremely sensitive to the surface chemistry with k0(app) in the range of 10(-2)-10(-1) cm/s at the hydrogen-terminated surface. An oxygen surface termination severely inhibits the rate of electron transfer. Fe(3+/2+) undergoes slow electron transfer at the hydrogen-terminated surface with k0(app) near 10(-5) cm/s. The rate of electron transfer at sp2 carbon electrodes is known to be mediated by surface carbonyl functionalities; however, this inner-sphere, catalytic pathway is absent on diamond due to the hydrogen termination. Dopamine, like other catechol and catecholamines, undergoes sluggish electron transfer with k0(app) between 10(-4) and 10(-5) cm/s. Converting the surface to an oxygen termination has little effect on k0(app). The slow kinetics may be related to weak adsorption of these analytes on the diamond surface. Ascorbic acid oxidation is very sensitive to the surface termination with the most negative Ep(ox) observed at the hydrogen-terminated surface. An oxygen surface termination shifts Ep(ox) positive by some 250 mV or more. An interfacial energy diagram is proposed to explain the electron transfer whereby the midgap density of states results primarily from the boron doping level and the lattice hydrogen. The films were additionally characterized by scanning electron microscopy and micro-Raman imaging spectroscopy. The cyclic voltammetric and kinetic data presented can serve as a benchmark for research groups evaluating the electrochemical properties of semimetallic (i.e., conductive), hydrogen-terminated, polycrystalline diamond.     

Strong interaction between imidazolium-based polycationic polymer and ferricyanide: toward redox potential regulation for selective in vivo electrochemical measurements

This study effectively demonstrates a strategy to enable the ferricyanide-based second-generation biosensors for selective in vivo measurements of neurochemicals, with glucose as an example. The strategy is based on regulation of redox potential of ferricyanide mediator by carefully controlling the different adsorption ability of ferricyanide (Fe(CN)(6)(3-)) and ferrocyanide (Fe(CN)(6)(4-)) onto electrode surface. To realize the negative shift of the redox potential of Fe(CN)(6)(3-/4-), imidazolium-based polymer (Pim) is synthesized and used as a matrix for surface adsorption of Fe(CN)(6)(3-/4-) due to its stronger interaction with Fe(CN)(6)(3-) than with Fe(CN)(6)(4-). The different adsorption ability of Fe(CN)(6)(3-) and Fe(CN)(6)(4-) onto electrodes modified with a composite of Pim and multiwalled carbon nanotubes (MWNTs) eventually enables the stable surface adsorption of both species to generate integrated biosensors and, more importantly, leads to a negative shift of the redox potential of the surface-confined redox mediator. Using glucose oxidase (GOD) as the model biorecognition units, we demonstrate the validity of the ferricyanide-based second-generation biosensors for selective in vivo neurochemical measurements. We find that the biosensors developed with the strategy demonstrated in this study can be used well as the selective detector for continuous online detection of striatum glucose of guinea pigs, by integration with in vivo microdialysis. This study essentially paves a new avenue to developing electrochemical biosensors effectively for in vivo neurochemical measurements, which is envisaged to be of great importance in understanding the molecular basis of physiological and pathological events.     

Anthraquinonedisulfonate electrochemistry: a comparison of glassy carbon, hydrogenated glassy carbon, highly oriented pyrolytic graphite, and diamond electrodes

The electrochemistry of anthraquinone-2,6-disulfonate (2,6-AQDS) at glassy carbon (GC), hydrogenated glassy carbon (HGC), the basal plane of highly oriented pyrolytic graphite (HOPG), and boron-doped diamond was investigated by cyclic voltammetry and chronocoulometry. Quantitative determination of the surface coverage and qualitative assessment of the physisorption strength of 2,6-AQDS adsorption on each of these electrodes were done. The diamond and HGC surfaces are nonpolar and relatively oxygen-free, with the surface carbon atoms terminated by hydrogen. The polar 2,6-AQDS does not adsorb on these surfaces, and the electrolysis proceeds by a diffusion-controlled reaction. Conversely, the GC and HOPG surfaces are polar, with the exposed defect sites terminated by carbon-oxygen functionalities. 2,6-AQDS strongly physisorbs on both of these surfaces at near monolayer or greater coverages, such that the electrolysis proceeds through a surface-confined state. Less than 40% of the initial surface coverage can be removed by rinsing and solution replacement, reflective of strong physisorption. The results show the important role of the surface carbon-oxygen functionalities in promoting strong dipole-dipole and ion-dipole interactions with polar and ionic molecules such as 2,6-AQDS. The results also support the theory that diamond electrodes may be less subject to fouling by polar adsorbates, as compared to GC, leading to improved response stability in electroanalytical measurements. The relationship between the 2,6-AQDS surface coverage, the double-layer capacitance, and the heterogeneous electron-transfer rate constant for Fe(CN)(6)(3)(-)(/4)(-) for these four carbon electrodes is presented.     

Diffusion boundary layer of a rotating disk electrode as a thin-layer spectroelectrochemical cell

A UV-visible rapid scan spectrophotometer (RSS) was coupled to a Au rotating disk electrode (RDE) for monitoring at near-normal incidence the reflection-absorption spectrum of the diffusion boundary layer in [Fe(CN)(6)](4)(-) aqueous solutions over a potential region in which [Fe(CN)(6)](4-) oxidizes, generating highly absorbing [Fe(CN)(6)](3-) (lambda(max) = 420 nm). Measurements were performed under steady-state conditions at rotation rates, omega, in the range 300 相似文献   

Determination of heterogeneous electron transfer kinetics in the presence of ultrasound at microelectrodes employing sampled voltammetry

The technique of sampled voltammetry at microelectrodes irradiated with ultrasound is demonstrated for the first time. This technique is used to determine the heterogeneous electron transfer rate constants for the redox couples ferrocene/ferrocenium, Ru(NH(3))(6)(3+/2+), and IrCl(6)(3)(-)/IrCl(6)(2)(-). Determination of the heterogeneous rate constants is also achieved for comparison purposes by analysis of fast sweep rate voltammetry of the redox systems studied at microelectrodes and comparison of the results obtained to the theory developed by Nicholson and Shain. The heterogeneous rate constants determined using sampled voltammetry were 1.0, 0.6, 1.23, and 0.18 cm s(-)(1) for the ferrocene/ferrocenium (0.1 mol dm(-)(3) TEATFB, CH(3)CN), Ru(NH(3))(6)(3+/2+) (0.1 mol dm(-)(3) KCl), IrCl(6)(3)(-) (1 mol dm(-)(3) KCl), and IrCl(6)(3)(-)/IrCl(6)(2)(-) (1 mol dm(-)(3) NaCl), respectively, in agreement with those obtained in the absence of ultrasound.     

Electroanalytical performance of nitrogen-containing tetrahedral amorphous carbon thin-film electrodes

Tetrahedral amorphous carbon (ta-C) consists of a mixture of sp(3)- and sp(2)-bonded carbon ranging from 60 to 40% (sp(3)/sp(3)+sp(2)) depending on the deposition conditions. The physical, chemical, and electrochemical properties depend on the sp(2)/sp(3) bonding ratio as well as the presence of incorporated impurities, such as hydrogen or nitrogen. The ability to grow ta-C at lower temperatures (25-100 °C) on a wider variety of substrates as compared to CVD diamond is an advantage of this material. Herein, we report on the structural and electrochemical properties of nitrogen-incorporated ta-C thin films (ta-C:N). The incorporation of nitrogen into the films decreases the electrical resistivity from 613 ± 60 (0 sccm N(2)) to 1.10 ± 0.07 Ω-cm (50 sccm N(2)), presumably by increasing the sp(2)-bonded carbon content and the connectedness of these domains. Similar to boron-doped diamond, these materials are characterized by a low background voltammetric current, a wide working potential window (~ 3 V), and relatively rapid electron-transfer kinetics for aqueous redox systems, including Fe(CN)(6)(-3/-4) and Ru(NH(3))(6)(+3/+2), without conventional pretreatment. Additionally, there is weak molecular adsorption of polar molecules (methylene blue) on the ta-C surface. Overall, the properties of the ta-C and ta-C:N electrodes are such that they could be excellent new choices for electroanalytical measurements.     

Surface nanostructuring of boron-doped diamond films and their electrochemical performance

Uniform and vertically aligned nanocone and nanopillar arrays were successfully constructed on heavily boron-doped nanocrysatlline diamond films by carrying out bias-assisted reactive ion etching in hydrogen/argon plasmas. The electrochemical properties of the nanostructured boron-doped diamond films were investigated by cyclic voltammetry using 1 mM [Fe(CN)6](3-/4-) as redox couple. Compared to the planar boron-doped nanocrystalline diamond film electrode, the surface nanostructuring of boron-doped diamond film electrodes demonstrate enhanced sensitivity due to their enlarged electro-active surface areas. The results indicated that boron-doped diamond nanocones and nanopillars are promising electrode materials which benefit to improve the efficiency, sensitivity and reproducibility of biomedical and chemical sensors.     

Oxidation-state speciation of

The analytical utility of chemically modified microelectrodes for oxidation-state speciation of redox couples by cyclic voltammetry has been explored. [Re(I)(DMPE)3]+/[Re(II)(DMPE)3]2+, where DMPE = 1,2-bis(dimethylphosphino)ethane, was studied at carbon-fiber microelectrodes of approximately 5 microm in radius coated with Nafion-entrapped solgel-derived silica (Nafion-silica) composite. The results are compared with cyclic voltammetry of [Fe(CN)6]3-/[Fe(CN)6]4- at bare carbon-fiber microelectrodes. At both microelectrodes, the cathodic and anodic limiting currents are linearly proportional to the concentrations of the reducible and oxidizable species of a redox couple, respectively. The shape of the cyclic voltammogram and the magnitude of the steady-state limiting current are not affected by the potential at which the scan starts. Speciation of both forms of a redox couple could be achieved voltammetrically at the microelectrodes. However, a considerably slower scan rate was required to achieve steady state at the modified electrode because of the smaller diffusion coefficients of [Re(I)(DMPE)3]+ and [Re(II)(DMPE)3]2+ in the Nafion-silica composite. The detection limit at the modified electrode was considerably lower (5 x 10(-9) M for [Re(I)(DMPE)3]+) than at the bare electrode (6 x 10(-5) M for [Fe(CN)6]3- and [Fe(CN)6]4-) because of the substantial preconcentration of [Re(I)(DMPE)3]+ by the Nafion-silica composite.     

Amperometric determination of hydrazine at manganese hexacyanoferrate modified graphite-wax composite electrode

Fabrication, characterization and application of a manganese hexacyanoferrate (MnHCF) modified graphite-wax composite electrode are described. The MnHCF mixed with graphite powder was dispersed into molten paraffin wax to yield a conductive composite, which was used as electrode material to construct a renewable three-dimensional MnHCF modified electrode. The characterization of the modified electrode has been studied by electrochemical techniques. The cyclic voltammogram of the MnHCF modified graphite-wax composite electrode prepared under optimum composition, showed a well-defined redox couple due to Fe(CN)(6)(4-)/Fe(CN)(6)(3-) system. The electrocatalytic oxidation of hydrazine by MnHCF modified graphite-wax composite electrode has been investigated in an attempt to develop a new sensor for its determination. It was found that the mediator catalyzed the oxidation of hydrazine. The electrocatalytic oxidation of hydrazine was also studied under hydrodynamic and chronoamperometric conditions. The anodic current increases linearly with increase in the concentration of hydrazine in the range of 3.33x10(-5)M to 8.18x10(-3)M. The detection limit was found to be 6.65x10(-6)M (S/N=3). The modified electrode can also be used for on-line detection of hydrazine. The proposed method has also been applied for the determination of hydrazine in photographic developer solution.     

Amplified electrochemical detection of a cancer biomarker by enhanced precipitation using horseradish peroxidase attached on carbon nanotubes

An electrochemical nanoimmunosensor based on multiwall carbon nanotubes (MWCNTs)/gold nanoparticles (AuNPs) was developed for the amplified detection of prostate specific antigen (PSA). The amplified detection was achieved by the enhanced precipitation of 4-chloro-1-naphthol (CN) using a higher number of horseradish peroxidase (HRP) molecules attached on MWCNTs. The PSA nanoimmunosensor was fabricated by immobilizing a monoclonal anti-PSA antibody (anti-PSA) on the AuNP-attached thiolated MWCNT on a gold electrode. The sensor surface was characterized using scanning electron microscope, transmission electron microscope, quartz crystal microbalance, and electrochemical techniques. Cyclic and square wave voltammetric techniques were used to monitor the enhanced precipitation of CN that accumulated on the electrode surface and subsequent decrement in the electrode surface area by monitoring the reduction process of the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) redox couple. Under the optimized experimental condition, the linear range and the detection limit of PSA immunosensor were determined to be 1.0 pg/mL to 10.0 ng/mL and 0.40 ± 0.03 pg/mL, respectively. The validity of the proposed method was compared with an enzyme-linked immunosorbent assay method in various PSA spiked human serum samples.     

Electrochemical sensing of DNA hybridization based on duplex-specific charge compensation

A nonlabeling voltammetric detection method for DNA hybridization has been developed, in which [Fe(CN)(6)](3-) in solution can readily approach an electrode surface covered with a charge-compensated DNA duplex layer and thus provides a strong redox-sensing current. Charge compensation for negative charges on the DNA backbone has been specifically accomplished on DNA duplexes by discouraging nonspecific binding of positively charged intercalating molecules with single strands. A pretreatment of DNA-modified electrodes with sodium dodecyl sulfate before the intercalator binding process is essential in preventing the nonspecific binding. Since ferricyanide, the only electrochemically active species, is present in the voltammetric solution, the detection signal can be amplified by increasing its concentration. Combination of the duplex-specific charge compensation with the signal amplification has achieved a remarkable signal difference: in 30 mM [Fe(CN)(6)](3-), the area ratio between cyclic voltammograms of the hybridized and unhybridized electrodes is approximately 200 when 3,6-diaminoacridine is used as the intercalator. High sensitivity of the method has been demonstrated by detecting 10 fM (100 zmol in amount) of a target probe DNA.     

High-sensitivity, label-free DNA sensors using electrochemically active conducting polymers

Label-free oligonucleotide sensors that use a change in the electrode kinetics of the redox reaction of the negatively charged Fe(CN)(6)(3-/4-) redox couple to signal the formation of a DNA duplex with a surface-conjugated probe nucleotide are investigated. Electrochemically active conducting polymers (ECPs) can advantageously be used both as the active electrode and as the means of surface conjugation of the probe nucleotide. Here, we demonstrate that the sensitivity of the detection of the surface-complementary oligonucleotide can significantly be improved, into the low nanomolar range, by forming the ECP as a highly porous, very rough layer by growing it using electrochemical polymerization on a microelectrode. In comparison, smoother surfaces formed on macroelectrodes had detection sensitivity in the low micromolar range. We propose Donnan exclusion of the redox couple from small pores as the reason for the enhanced sensitivity. We discuss the effects using a simple patch model for the electrochemical kinetics and use the model to derive the equilibrium binding constant and binding kinetic rate constants for the surface hybridization reaction. We use the electrochemically active copolymer of pyrrole (Py) and 3-pyrrolylacrylic acid (PAA) [poly(Py-co-PAA)] as the sensing electrode and binding surface and measure the surface hybridization-induced changes in electrode kinetics of Fe(CN)(6)(3-/4-) by electrochemical impedance spectroscopy.     

Large-amplitude Fourier transformed high-harmonic alternating current cyclic voltammetry: kinetic discrimination of interfering Faradaic processes at glassy carbon and at boron-doped diamond electrodes

Significant advantages of Fourier transformed large-amplitude ac higher (second to eighth) harmonics relative to responses obtained with conventional small-amplitude ac or dc cyclic voltammetric methods have been demonstrated with respect to (i) the suppression of capacitive background currents, (ii) the separation of the reversible reduction of [Ru(NH(3))(6)](3+) from the overlapping irreversible oxygen reduction process under conditions where aerobic oxygen remains present in the electrochemical cell, and (iii) the kinetic resolution of the reversible [Ru(NH(3))(6)](3+/2+) process in mixtures of [Fe(CN)(6)](3-) and [Ru(NH(3))(6)](3+) at appropriately treated boron-doped diamond electrodes, even when highly unfavorable [Fe(CN)(6)](3-) to [Ru(NH(3))(6)](3+) concentration ratios are employed. Theoretical support for the basis of kinetic discrimination in large-amplitude higher harmonic ac cyclic voltammetry is provided.     

Equilibrium and kinetic behavior of Fe(CN)6(3-/4-) and cytochrome c in direct electrochemistry using a film electrode thin-layer transmission cell

We report on the design and performance of a thin-layer electrochemical cell optimized for use with optically transparent film electrodes in combination with UV/vis and IR transmission spectroscopic measurements. The cell allows for measurements under both aerobic and anaerobic conditions. The direct, unmediated electron transfer, as assessed by the current transient, and the corresponding optical response observed for the Fe(CN)(6)(3-/4-) couple were in good agreement with theoretical predictions for voltammetry and optical absorption by an analyte confined in a thin layer. Chronoamperometric and spectroscopic measurements of Fe(CN)(6)(3-/4-) on gold mesh electrode revealed fast kinetics strongly influenced by the electrolyte concentration. Maximal apparent rates exceeding 2 s(-1) in 1 M KCl were observed optically. The direct kinetic and thermodynamic behavior of cytochrome c was compared with several electrode materials using the cell. The results showed heme ligand-dependent changes in the protein-electrode interactions. Mid-UV/visible spectral changes upon redox transitions in native cytochrome c and its cyanide derivative, as well as dissociation of the ferrous cytochrome c-CN complex, are reported.     

Achievement of near-reversible behavior for the

Voltammetric studies in the absence of added supporting electrolyte are presently dominated by the use of near-steady-state microelectrode techniques and millimolar or lower depolarizer concentrations. However, with this methodology, large departures from conventional migration-diffusion theory have been reported for the [Fe(CN)6](3-/4-) process at both carbon fiber and platinum microdisk electrodes. In contrast, data obtained in the present study reveal that use of the transient cyclic voltammetric technique at glassy carbon, gold, or platinum macrodisk electrodes and K4[Fe(CN)6] or K3[Fe(CN)6] concentrations of 50 mM or greater provides an approximately reversible response in the absence of added electrolyte. It is suggested that the use of very high [Fe(CN)6](3-) and [Fe(CN)6](4-) concentrations overcomes problems associated with a diffuse double layer and that large electrode surface areas and faster potential sweep rates minimize electrode blockage and passivating phenomena that can plague voltammetric studies at microelectrodes. The cyclic voltammetry of the [Fe(CN)6](3-/4-) couple at a range of concentrations at macroelectrodes in the absence of added inert electrolyte is compared with that obtained in the presence of 1 M KCl. The enhanced influences of uncompensated resistance, migration, and natural convection arising from density gradients under transient conditions at macrodisk electrodes also are considered.     

Electrochemical characterization of parylene-embedded carbon nanotube nanoelectrode arrays

A novel parylene-embedded carbon nanotube nanoelectrode array is presented for use as an electrochemical detector working electrode material. The fabrication process is compatible with standard microfluidic and other MEMS processing without requiring chemical mechanical polishing. Electrochemical studies of the nanoelectrodes showed that they perform comparably to platinum. Electrochemical pretreatment for short periods of time was found to further improve performance as measured by cathodic and anodic peak separation of K(3)Fe(CN)(6). A lower detection limit below 0.1?μM was measured and with further fabrication improvements detection limits between 100?pM and 10?nM are possible. This makes the nanoelectrode arrays particularly suitable for trace electrochemical analysis.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 2. Demonstration of selectivity in the presence of direct interferences

Three modes of selectivity based on charge-selective partitioning, electrolysis potential, and spectral absorption wavelength were demonstrated simultaneously in a new type of spectroelectrochemical sensor. Operation and performance of the three modes of selectivity for detection of analytes in the presence of direct interferences were investigated using binary mixture systems. These binary mixtures consisted of Fe(CN)(6)(3-) and Ru(bpy)(3)(2+) and of Fe(CN)(6)(4-) and Ru(CN)(6)(4)(-) in aqueous solutions. Results on the Fe(CN)(6)(3-)/Ru(bpy)(3)(2+) binary mixture showed that an anion-exchange coating consisting of PDMDAAC-SiO(2) [where PDMDAAC is poly(dimethyldiallylammonium chloride)] and a cation-exchange coating consisting of Nafion-SiO(2) can trap and preconcentrate analytes with charge selection. At the same time, such coatings exclude interferences carrying the same type of charge as that of the exchange sites in the sensor coating. Using the Fe(CN)(6)(4-)/Ru(CN)(6)(4-) binary mixture, the Fe(CN)(6)(4-) component can be selectively detected by restricting the modulation potential cycled to a range specific to the redox-active Fe(CN)(6)(4-) component and simultaneously monitoring the optical response at the overlapping wavelength of 420 nm. It was also shown that, when the wavelength for optical monitoring was chosen as 500 nm, which is specific to the Ru(CN)(6)(4-) component, interference from the Fe(CN)(6)(4-) component for spectroelectrochemical detection of Ru(CN)(6)(4-) was significantly suppressed, even though the cyclic modulation potential encompassed the redox range for the Fe(CN)(6)(4-) component.