Electro-catalyzed oxidation of reduced glutathione and 2-mercaptoethanol by cobalt phthalocyanine-containing screen printed graphite electrodes

Electro-catalytic behavior of screen printed graphite electrodes modified with cobalt phthalocyanine (CoPc) towards the oxidation of reduced glutathione (GSH) and 2-mercaptoethanol (2-ME) is reported. We find, by using cyclic voltammetry, that the oxidation of 2-ME occurs at 0.2 V vs Ag/AgCl and − 0.3 vs Ag/AgCl V at pH = 7 and pH = 13, respectively and that of GSH occurs at 0.4 V vs Ag/AgCl and 0.0 V vs Ag/AgCl at pH = 7 and 13, respectively. The electro-catalytic activity depends on the method of electrode modification and the amount of catalyst incorporated in the ink used to fabricate the SPCEs. The highest activity was obtained with electrodes prepared with 2.5% (w:w) of CoPc.     

Preconcentration of f-elements from aqueous solution utilizing a modified carbon paste electrode

An evaluation using paraffin oil based, Acheson 38 carbon paste electrodes modified with α-hydroxyisobutyric acid (HIBA) to preconcentrate f-elements cathodically is described. The modified paste was made by directly mixing solid HIBA into the carbon paste. A chemically reversible cyclic voltammogram for HIBA was observed on this modified carbon paste, which was found to be a non-Nerstian, single electron transfer process. Lanthanides (less promethium) were found to accumulate onto the electrode surface during a 30 s electrodeposition step at -0.4 V vs Ag/AgCl from 0.1 M LiCl. The elements were then stripped off into a 2% HNO(3) solution by an oxidative step at +0.8 V vs Ag/AgCl; quantitative removal from the electrode was confirmed by ICPMS. Ultratrace solutions with initial concentrations down to 5 parts per quadrillion (ppq) were preconcentrated in 5 min above our instrumental limit of detection (LOD) of around 1 ppt for lanthanides.     

Thin films of polyelectrolyte-encapsulated catalase microcrystals for biosensing

Polyelectrolyte (PE)-encapsulated catalase microcrystals were assembled onto gold electrodes by their sequential deposition with oppositely charged PEs, utilizing electrostatic interactions to form enzyme thin films for biosensing. The PE coating around the microcrystals provided a regular surface charge, thus facilitating the stepwise film growth, and it effectively prevented catalase leakage from the assembled films. The encapsulated catalase was shown to retain both its biological and its electrochemical activity. Direct electron transfer between catalase molecules and the gold electrode was achieved without the aid of any electron mediator. In pH 5.0 phosphate buffer solution, the apparent formal potential (E(o)') of catalase was -0.131 V (vs Ag/AgCl). As a H2O2 biosensor, films consisting of one layer of the encapsulated catalase displayed considerably higher (approximately 5-fold) and more stable electrocatalytic responses to the reduction of H2O2 than did corresponding films made of one layer of nonencapsulated catalase or solubilized catalase. An increase in either the number of "precursor" PE layers between the gold electrodes and the catalase microcrystal layers in the film or the number of PE layers encapsulating the catalase microcrystals was found to decrease the electrocatalytic activity of the electrode. At low precursor PE layer numbers (approximately 2) and PE encapsulating layers (approximately 4), the current response was proportional to the H2O2 concentration in the range 3.0 x 10(-6) to 1.0 x 10(-2) M. The overall electroactivity of the multilayer film increased for the first two layers of encapsulated catalase, after which a plateau was observed. This was attributed to the increasing difficulty of electron transfer and substrate diffusion limitations. The current approach of using immobilized PE-encapsulated enzyme microcrystals for biosensing provides a versatile method to prepare high enzyme content films with high and tailored enzyme activities.     

Nanotextured metal copper substrates as powerful and long-lasting fuel cell anodes

Fuel cells (FCs) are promising electrochemical devices that convert chemical energy of fuels directly into electrical energy. We present a new anode material based on nanotextured metal copper for fuel cell applications. We have demonstrated that low-cost copper catalyst anodes act as highly efficient and ultra-long-lasting materials for the direct electro-oxidation of ammonia-borane and additional amine derivatives. High power densities of ca. 1W·cm(-2) (ca. -1 V vs Ag/AgCl at 1 A) are readily achieved at room temperature. We fabricate fuel cell devices based on our nanotextured Cu anodes in combination with commercial air cathodes.     

Glucose oxidase electrodes of polyaniline,poly(<Emphasis Type="Italic">o</Emphasis>-anisidine) and their co-polymer as a biosensor: a comparative study

Polyaniline (PA), poly(o-anisidine) (POA) and their co-polymer poly(aniline-co-o-anisidine) (PA-co-POA) thin films were electropolymerized in solution containing 0.1 M monomer(s) and 1 M H₂SO₄ as a electrolyte by applying sequential linear potential scan rate 50 mV/s between −0.2 and 1.0 V vs. Ag/AgCl electrode on platinum electrode. A simple technique is described for constructing a glucose sensor by the entrapment of glucose oxidase (GOD) in PA, POA and their co-polymer PA-co-POA thin films, which were electrochemically deposited on a platinum plate in phosphate and acetate buffer. The maximum current response was observed for PA, POA, and PA-co-POA GOD electrodes at pH 5.5 and potential 0.60 V (vs. Ag/AgCl). The phosphate buffer gives fast response as compared to acetate buffer in amperometric measurements. PA GOD electrode shows fast response (means time taken for sense the glucose is lees) followed by PA-co-POA and POA GOD electrodes.     

Electrochemical behavior of ascorbate oxidase immobilized on graphite electrode modified with Au-nanoparticles

Direct electrochemistry of ascorbate oxidase was observed when immobilized on graphite modified with nano-sized gold structures. Au-structures were electrodeposited onto the graphite surface by means of cyclic voltammetry, then the enzyme was chemisorbed onto their surface. The electron transfer between the enzyme active center and the modified electrode surface was probed by square wave voltammetry (SWV) and cyclic voltammetry (CV). The dependence of the current maxima on the scan rate was found linear, suggesting that the redox process is controlled by surface chemistry. Bioelectrocatalytic oxidation of the enzyme substrate l-ascorbic acid was explored by constant potential amperometry over the potential range from 200 to 350 mV (vs. Ag/AgCl, 3 M KCl) at the рНs 5.6 and 7.0. At a potential as low as 200 mV, pH 7.0 and temperature 25 °C following operational parameters were determined for the enzyme electrode: a sensitivity: 1.54 μA mM⁻¹ mm⁻² (r² = 0.99₅), linear dynamic range up to 3.3 mМ, detection limit of 1.5 μМ, response time up to 20 s.     

Electrochemical detection of carbamate pesticides at conductive diamond electrodes

Conductive boron-doped diamond thin-film electrodes were used for the electrochemical detection of selected N-methylcarbamate pesticides (carbaryl, carbofuran, methyl 2-benzimidazolecarbamate, bendiocarb) after liquid chromatographic separation. Two kinds of detection methods were adopted in this study. In the first method, a direct detection of underivatized pesticides was carried out at an operating potential of 1.45 V versus Ag/AgCl, which resulted in the detection limits of 5-20 ng/mL (or 5-20 ppb) with S/N = 2 due to the low background current and wide potential window of the diamond electrode. In the second method, the detection limits were improved by subjecting the pesticide samples to alkaline hydrolysis in a separate step prior to injection. The phenolic derivatives obtained by alkaline hydrolysis oxidize at a relatively lower potential (0.9 V vs Ag/AgCl), which increases the sensitivity drastically. The advantage of the diamond electrode for the detection of phenolic derivatives is that it offers excellent stability in comparison to other electrodes. This method gives the detection limits of 0.6-1 ng/mL (or 0.6-1 ppb), which are well below the maximum residue levels allowed for carbaryl, carbofuran, and bendiocarb. While the lowest detection limits (LOD) obtained by the direct detection of pesticides are comparable to the those reported by the well-established HPLC-fluorescence, the LODs of the alkaline hydrolysis method are found to be even lower than the reported limits. On-line reactivation of the diamond electrode surface was shown to be possible by an anodic treatment of the electrode at approximately 3 V for 30 min in case of electrode fouling, which may occur after a prolonged use. Such a treatment damages the glassy carbon (GC) and metal electrodes, while the diamond electrode remains stable. These results suggest that the diamond electrode is superior to the other previously used electrodes such as GC and Kelgraf type for highly sensitive and stable detection of carbamate pesticides.     

High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na_0.5MnO₂ nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na_0.5MnO₂ nanowall arrays can be extended to 0–1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g^?1. The extended potential window for the Na_0.5MnO₂ electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon‐coated Fe₃O₄ nanorod arrays are synthesized as the anode and can stably work in a negative potential window of ?1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na_0.5MnO₂ nanowall arrays as the cathode and carbon‐coated Fe₃O₄ nanorod arrays as the anode. In particular, the 2.6 V Na_0.5MnO₂//Fe₃O₄@C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg^?1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO₂‐based supercapacitors. This work provides new opportunities for developing high‐voltage aqueous asymmetric supercapacitors with further increased energy density.     

Amperometric biosensor for glutamate using prussian blue-based "artificial peroxidase" as a transducer for hydrogen peroxide

The specially deposited Prussian Blue denoted as "artificial peroxidase" was used as a transducer for hydrogen peroxide. The electrocatalyst was stable, highly active, and selective to hydrogen peroxide reduction in the presence of oxygen, which allowed sensing of H2O2 around 0.0 V (Ag/AgCl). Glutamate oxidase was immobilized on the surface of the Prussian Blue-modified electrode in a Nafion layer using a nonaqueous enzymology approach. The calibration range for glutamate in flow injection system was 1 x 10(-7)-1 x 10(-4) M. The lowest concentration of glutamate detected (1 x 10(-7) M) and the highest sensitivity in the linear range of 0.21 A M-1 cm-2 were achieved. The influence of reductants was practically avoided using the low potential of an indicator electrode (0.0 V Ag/AgCl). The attractive performance characteristics of the glutamate biosensor illustrate the advantages of Prussian Blue-based "artificial peroxidase" as transducer for hydrogen peroxide detection.     

Synthesis and enhanced electrochemical properties of pod-shaped gold/silica nanoparticles

Pod-shaped gold/silica nanoparticles (PGSNPs) were prepared using perfluorooctanoic acid (PFOA) and cetyltrimethylammonium bromide (CTAB) as cotemplates. The PGSNPs were utilized to explore a novel biosensor through coupling myoglobin (Mb) with chitosan (Chi). Compared with Mb-Chi-PSNPs (pod-shaped silica nanoparticles)/GC modified electrode, Mb-Chi-PGSNPs/GC electrode exhibited a pair of much stronger redox peaks at − 0.28 V (vs. Ag/AgCl). Moreover, facilitated direct electron transfer of the metalloenzymes with smaller peak-to-peak separation (ΔEp) of about 46 mV was acquired on the PGSNPs-based enzyme electrode. The PGSNPs-based biosensor demonstrated significant electrocatalytic activity for the reduction of hydrogen peroxide with a wide linear range (1-540 µM) and high sensitivity (661 mA cm^− 2 M^− 1). Together, the Mb-Chi-PGSNPs film is one of ideal candidate materials for direct electrochemistry of redox proteins, and may find potential applications in biomedical, food, and environmental analysis and detection.     

Ascorbic acid sensor based on molecularly imprinted polymer-modified hanging mercury drop electrode

Ascorbic acid sensor based on molecularly imprinted polymer (MIP) is reported for sensitive and selective analysis, without any cross-reactivity or matrix effect, in aqueous, blood serum and pharmaceutical samples. The sensor was developed by the direct coating of ascorbic acid-imprinted polymer, prepared from melamine and chloranil, on the surface of a hanging mercury drop electrode (HMDE) at + 0.4 V (vs. Ag/AgCl). The molecular recognition of ascorbic acid was highly specific using non-covalent (hydrophobically driven hydrogen-bonding and electrostatic) interactions. The analyte was preconcentrated and oxidised instantaneously in the imprinted polymer layer giving voltammetric signal on cathodic stripping at optimised operational conditions: accumulation potential + 0.4 V (vs. Ag/AgCl), polymer deposition time 120 s, template accumulation time 120 s, pH 7.0, scan rate 10 mV s^? 1, pulse amplitude 25 mV. The proposed MIP sensor is able to enhance sensitivity substantially so as to detect serum ascorbic acid level as low as 0.26 ng mL^? 1 (R.S.D. 0.5%, S/N 3) for the diagnosis of hypovitaminosis C (Vitamin C deficiency).     

Flavin adenine dinucleotide as precursor for NADH electrocatalyst

The generation of a new electrocatalytic system for NADH after oxidizing flavin adenine dinucleotide (FAD) is shown. The oxidation is performed in alkaline medium until +1.4 V (Ag/AgCl) at graphite electrodes. The catalytic activity is ascribed to the electrooxidized moiety of FAD and not to quinone surface groups. A comparison between this catalyst and that attributed to poly(FAD) (Karyakin, A. A.; Ivanova Y. N.; Revunova, K. V.; Karyakina, E. E. Anal. Chem. 2004, 76, 2004-2009.) is presented. It is concluded that the surface quinone groups generated during the strong anodization of the electrode in acidic medium at 2-2.5 V and not the poly(FAD) are responsible for the catalytic activity described in the above mentioned work.     

Strategy of fabricating enriched gold nanoparticles based on electrochemical methods

In this work, we report a new and clean electrochemical pathway to prepare enriched gold nanoparticles in aqueous solutions via the aid of chitosan without addition of any other stabilizer and reductant. First, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1 N NaCl and 1 g/L chitosan from − 0.28 to + 1.22 V vs Ag/AgCl at 500 mV/s with 500 scans. Then the Au working electrode was immediately replaced by a Pt electrode, and a cathodic overpotential of 0.6 V from the open circuit potential (OCP) of ca. 0.84 V vs Ag/AgCl was applied under sonification to synthesize Au nanoparticles. Experimental results indicate the concentration and the particle sizes of prepared Au nanoparticles are ca. 50 ppm and 10 nm in diameter, respectively. No aggregation of Au nanoparticles is observed in an ambient atmosphere for at least 3 months.     

Tight and uniform layer of covalently bound aminoethylophenyl groups perpendicular to gold surface for attachment of biomolecules

Strongly adhered layers of the compound with the primary amino group directed toward the solution were obtained at the gold surface by chronoamperometric electroreduction of 4-aminoethylobenzenodiazonium salt (AEBD) in acetonitrile solution at appropriately selected potential. The used techniques (EQCM, AFM, EIS, PM, IRRAS) showed that the nature and thickness of formed aminoethylophenyl layer strongly depend on the potential applied to the electrode. Electroreduction of AEBD salt at a potential more negative than -0.6 V (vs Ag/AgCl) leads to about monolayer on the gold surface. Additionally, such a layer was very tight and uniform. The electrochemical measurements indicate that the efficient and precise attachment of biomolecules to the aminoethylophenyl layer is only possible when this layer is formed at appropriate potential. This was shown for ss- and dsDNA.     

Chemical vapor deposition of amorphous molybdenum sulphide on black phosphorus for photoelectrochemical water splitting

Non-precious metal electrocatalyst molybdenum sulphide(MoS) and black phosphorus(BP) are highly promising catalysts for H₂ evolution reaction(HER).However,BP is environmentally unstable and the basal planes of crystal MoS₂ are inactive toward HER.Herein,amorphous molybdenum sulphide(MoSx)directly on BP/BiVO4 film dramatically improves the performance of photoelectrochemical water splitting compared with pure BiVO4.Additionally,we demonstrate that a BP layer,inserted between the MoSx and BiVO4,can enhance the photoelectrochemical performance and improve the stability of the electrodes.Finally,MoS_x/B P/BVO electrode shows the excellent current density of 2.1 mA/cm² at the potential of 1.2 V(vs Ag/AgCl),which is twice higher than that of pure BVO electrode.Our novel nanostructure materials will lead to a new class of non-precious metal photocatalysts for hydrogen production.     

Detection of glucose at 2 fM concentration

We report the amperometric detection of glucose at 2 fM concentration in a physiological buffer solution at 1 atm O2 pressure. The sensitive assay is based on the close to absolute electroreductive stripping of O2 from the solution near the glucose electrooxidizing anode. The glucose was detected by its electrooxidation on a stationary glassy carbon disk surrounded by an also stationary platinum ring. The disk was coated with a film of glucose oxidase (GOx), electrically "wired" with PVP-[Os(N,N'-dimethyl-2,2'-biimidazole)3]2+/3+ (polymer I), having a redox potential of -0.19 V versus Ag/AgCl. The ring was coated with bilirubin oxidase (BOD) "wired" with PAA-PVI-[Os(4,4'-dichloro-2,2'-bipyridine)2Cl]+/2+ (polymer II), having a redox potential of + 0.36 V versus Ag/AgCl. The ring-disk electrode was held facing up, and a 30-microL drop was placed on it for the assay, with the ring poised at -0.3 V/ AgAgCl and the disk poised at -0.1 V/ Ag/AgCl. Even though the atmosphere over the drop was O2 at 1 atm pressure, the wired BOD disk scavenged the O2 so effectively that the glucose-reduced FADH2 of GOx was not oxidized by O2, the natural cosubstrate of the enzyme.     

Photoelectrochemical oxygen sensor using copper-plated screen-printed carbon electrodes

We report here an efficient photocatalytic amperometric sensor for the determination of dissolved oxygen (DO) in phosphate buffer solution using a disposable copper-plated screen-printed carbon electrode (CuSPE). The photoelectrochemical activity toward DO of the CuSPE was related to the formation of a p-type semiconductor Cu(I)2O. The solution pH and biased potential (E(bias)) were systematically optimized as pH 8 PBS and -0.7 V vs Ag/AgCl, respectively. Under optimized conditions, the calibration plot was linear in the range of 1-8 ppm with sensitivity and regression coefficient of 23.51 (microA cm2)(-1) ppm(-1) and 0.9982, respectively. The reproducibility of the system was good with seven successive measurements of DO yielding a RSD value of 1.87%. Real sample assays for groundwater and tap water were also consistent with those measured by a commercial DO meter. The principle used in DO measurement has an opportunity to extend into various research fields.     

Direct electrochemistry and electrocatalytic activity of cytochrome c covalently immobilized on a boron-doped nanocrystalline diamond electrode

Cytochrome c (Cyt c) was covalently immobilized on a boron-doped nanocrystalline diamond (BDND) electrode via surface functionalization with undecylenic acid methyl ester and subsequent removal of the protecting ester groups to produce a carboxyl-terminated surface. Cyt c-modified BDND electrode exhibited a pair of quasi-reversible and well-defined redox peaks with a formal potential (E(0)) of 0.061 V (vs Ag/AgCl) in 0.1 M phosphate buffer solution (pH 7.0) and a surface-controlled process with a high electron transfer constant (ks) of 5.2 +/- 0.6 s(-1). The electrochemical properties of as-deposited and Cyt c-modified boron-doped microcrystalline diamond (BDMD) electrodes were also studied for comparison. Investigation of the electrocatalytic activity of the Cyt c-modified BDND electrode toward hydrogen peroxide (H2O2) revealed a rapid amperometric response (5 s). The linear range of response to H2O2 concentration was from 1 to 450 microM, and the detection limit was 0.7 microM at a signal-to-noise ratio of 3. The stability of the Cyt c-modified BDND electrode, in comparison with that of the BDMD and glassy carbon counterpart electrodes, was also evaluated.     

Structure and electrochemical reactivity of 3-[tris(2-methoxyethoxy)silyl]-propanethiol adsorbed on silver surface

A novel stable self-assembled chemisorbed layers, providing protection of metal surface against electrooxidation capable of blocking propylene carbonate (PC) electroreduction and Li electrodeposition, were produced from 3-[tris-(2-methoxyethoxy)silyl]-propanethiol (SIS2) on silver. Reflection-absorption infrared spectroscopy (RAIRS) indicated cleavage of the S-H bond upon adsorption of SIS1 [3-(trimethoxysilyl)-propanethiol] and SIS2 species with the formation of S-Ag bonds on the metal surface. By cyclic voltammetry it was found that the primary adsorbate formed on a Ag electrode at E_ad (between − 0.2 and − 1.2 V vs. SCE) underwent reductive desorption at E < − 1.3 V vs. SCE. From the charge involved in this process, the saturation surface coverage was estimated as 4.2 10^− 10 mol cm^− 2. A compact SIS2 layer after long-time aging (hydrolysis and condensation) was electroinactive and thus non-desorbable from the electrode surface during the potential cycling. The structures of SIS2 and its complexes with Li⁺ cations on the Ag surface were calculated and visualized by the AM1d semi-empirical method.     

Characterization and electrochemistry of interfacial self-assembled multi-manganese(III)-porphyrin arrays

Electrochemical behaviors and catalytic oxidation of nitrite have been investigated by using the indium tin oxide (ITO) electrodes modified by the Langmuir-Blodgett (LB) films of manganese porphyrin and its palladium-mediated multiporphyrin arrays. The multiporphyrin arrays were prepared at the air-water interface. Surface pressure-area isotherms indicated that monolayers of the porphyrin of MnTPyP (TPyP: tetrapyridylporphyrin) could be stabilized on the sodium tetraphenylboron and K₂PdCl₄ subphase surfaces, but not on the pure water surface. These monolayers were transferred onto quartz and ITO substrate surfaces with the use of vertical dipping method, and characterized by using transmission electron microscope, UV-vis absorption and X-ray photoelectron spectra. For the electrodes modified by the porphyrin LB films, reversible Mn^(II)TPyP ↔ Mn^(III)TPyP and Mn^(III)TPyP ↔ Mn^(IV)TPyP redox couples were recorded, and centered at about − 0.17 and 0.52 V vs. Ag/AgCl, respectively. A high-valent Mn^(V)TPyP intermediate was detected when the cyclic voltammograms were measured in an electrolyte solution containing sodium nitrite. These multiporphyrin arrays modified electrode showed very strong stability and reproductivity, resulting in potential applications in the development for the nitrite sensors and molecular catalysts of organic compounds.