Electrochemistry at chemically modified graphenes

Electrochemical applications of graphene are of great interest to many researchers as they can potentially lead to crucial technological advancements in fabrication of electrochemical devices for energy production and storage, and highly sensitive sensors. There are many routes towards fabrication of bulk quantities of chemically modified graphenes (CMG) for applications such as electrode materials. Each of them yields different graphene materials with different functionalities and structural defects. Here, we compare the electrochemical properties of five different chemically modified graphenes: graphite oxide, graphene oxide, thermally reduced graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide. We characterized these materials using transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, which allowed us to correlate the electrochemical properties with the structural and chemical features of the CMGs. We found that thermally reduced graphene oxide offers the most favorable electrochemical performance among the different materials studied. Our findings have a profound impact for the applications of chemically modified graphenes in electrochemical devices.     

High-performance liquid chromatography of sulfur-containing amino acids and related compounds with amperometric detection at a modified electrode

Organic disulfides generally are not oxidized at bare electrodes under conditions that are suited to routine amperometric detection, and thiols are typically oxidized in a manner that leads to partial blockage of the surface. Modification of a carbon electrode with a film of Ru(III,IV) oxide stabilized with cyanocross-links permits the amperometric detection of cystine, cysteine, glutathione, methionine, and glutathione disulfide under conditions compatible with their chromatographic separation on a strong cation-exchange column. Detection limits of 0.2-0.6 microM and linear dynamic ranges of at least 1-50 microM were obtained. The electrode was stable for at least 11 days with a pH 1 citrate, phosphate mobile phase.     

Liquid chromatography-electrochemical detection of ferro- and ferricytochrome c at a chemically modified gold electrode

Chemically modified electrodes, constructed by adsorption of 4,4'-dithiodipyridine onto a polyvinylferrocene-treated gold surface, were employed for the amperometric detection of cytochrome c following size-exclusion chromatography. The electrode response was nearly reversible, permitting quantitation both of the ferro-form of the protein at +0.15 V vs. Ag/AgCl and of the ferri-form at -0.15 V. The limit of detection for the reduced species was 3 pmol injected, and the response was linear over three orders of magnitude. Using the chemically modified electrode approach, cytochrome c monitoring was sufficiently selective that the compound could be determined in human plasma pretreated only by dilution and particulate filtration.     

Determination of 3, 4-dihydroxybenzoic acid by electrocatalytic oxidation at an ionic liquid modified electrode

An electrode modified with an ionic liquid was used for the electrochemical determination of 3,4-dihydroxybenzoic acid (DHBA). Cyclic voltammetry indicated a pair of well-defined quasi-reversible redox peaks with a formal peak potential located at 586 mV (vs. the SCE). The voltammetric response to DHBA is largely improved compared to a traditional carbon paste electrode. This is attributed to a larger interface (due to the presence of an ionic liquid) with higher conductivity and inherent catalytic capability. The charge transfer coefficient, the standard rate constant and the apparent diffusion coefficient were calculated. The oxidation peak current was linearly related to the concentration of DHBA in the range 0.8–1.5 mM, and the detection limit was 0.62 µM (at 3σ). The effect of potential interferents was investigated, and the method was successfully applied to the determination of DHBA in different samples.

     

化学修饰电极的研究: 1:12磷钼杂多阴离子薄膜修饰电极的电化学性质

用循环伏安法研究了1:12磷钼杂多阴离子(PMo12)薄膜修饰电极的制备及其电化学行为, 发现PMo12膜强烈地吸附在玻璃碳电极表面, 溶液氢离子在PMo12膜改性电极的电化学过程中起着重要的作用, 而其它阴离子不参与这一过程, 在循环伏安法扫描过程中PMo12膜改性电极的稳定性很好。     

Rotating disk electrode study of electrocatalytic oxidation of ascorbate at Prussian blue modified electrode

Electrooxidation of ascorbate has been studied with the use of a rotating disk electrode. The results obtained show an efficient electrocatalytic oxidation of ascorbate at the Prussian blue (PB) modified electrode to proceed in solutions of pH 5.5 and 7.3. Depending on solution pH, the onset potential for ascorbate electrooxidation at PB modified electrode appears shifted by 0.1–0.2 V to lower values, as compared to an unmodified glassy carbon electrode. Within the electrode potential window of 0.3 to 0.5 V vs. Ag/AgCl, and electrode rotation velocity of 50–2000 rpm, the catalytic current obeys Koutecky-Levich equation at a submillimolar ascorbate concentration. Kinetic current densities, obtained from the data treatment, are higher for a pH 5.5 solution, and also at higher electrode potential.      

亚硝酸根在纳米金修饰玻碳电极上的电催化氧化行为及其测定

采用直接电化学沉积法制备出纳米金修饰玻碳电极,研究了其对亚硝酸根的电催化氧化作用。结果表明,亚硝酸根在该修饰电极上于0.8 V处出现了一个良好的氧化峰。在最优实验条件下,亚硝酸根的峰电流与其浓度在2×10-6～2×10-3mol/L范围内呈一定的线性关系,检出限为6.0×10-7(S/N=3),提出了用循环伏安法测定亚硝酸根的方法。纳米金修饰电极用于东莞自来水水样中亚硝酸根的测定,回收率在98.1%～101.4%之间。对比本方法,用分光光度法对东莞自来水样中亚硝酸根进行了测定,结果满意。     

Investigation of the electrocatalytic behavior of halogenated acids at a C60-p-tert-butylcalix[8]arene chemically modified electrode

The C₆₀-p-tert-butylcalix[8]arene (represented as C₆₀-L) chemically modified electrode has two redox wave couples in mixed solvents of MeCN and water (3:2, v/v), corresponding to C₆₀-L in the solution originating from partial film dissolution and from film reaction, respectively. Both the C₆₀-L in the solution state and in the film state exhibit catalytic activity towards the reduction of halogenated acids. Electronic Publication     

肾上腺素在咖啡酸修饰电极上的电化学研究

研究了咖啡酸(CFA)修饰电极的性质,测定了电极反应的动力学常数.结果表明在pH 7.0的的磷酸盐缓冲溶液中,肾上腺素(Ep)在该修饰电极上产生一灵敏的氧化峰,峰电流与Ep浓度在5.0×10-6～1.0×10-4 mol/L范围内线性关系良好,检出限为7.0×10-7 mol/L.修饰电极制备简单,稳定性好.     

Amperometric detection of catecholamines with liquid chromatography at a novelly constructed prussian blue chemically modified electrode

A novel Prussian blue chemically modified electrode (CME) was constructed and characterized for liquid chromatography electrochemical detection (LCEC) of catecholamines. Both anodic and cathodic peaks could be obtained by monitoring at constant applied potential at anodic and slightly cathodic potential ranges (0.3-0.7 and -0.2-0.1 V vs. SCE), respectively. When arranged in a series configuration, using the modified electrodes as generating and collecting detectors, extremely high effective collection efficiencies of 0.91 (for norepinephrine) and 0.58 (for dihydroxyphenylacetic acid) were achieved in dual-electrode LCEC for catecholamines; and a linear response range over 3 orders of magnitude and a detection limit of 10 pg were obtained with a downstream CME as the indicating detector.     

Direct electrocatalytic reduction of p-nitrophenol at room temperature ionic liquid modified electrode

Direct electrochemical reduction ofp-nitrophenol （PNP） was investigated on a room temperature ionic liquid N-butylpyridinium hexafluorophosphate （BPPF6） modified carbon paste electrode （CILE）. The cathodic peak potential was positively shifted and the peak currents were increased compared to that obtained on traditional carbon paste electrode （CPE）. The results indicated that the presence of ionic liquid BPPF6 on the electrode surface showed excellent catalytic ability to the electrochemical reduction of PNP. The electrochemical behaviors of PNP on the CILE were investigated by cyclic voltammetry and the conditions such as the scan rate, the buffer pH, the substrate concentration were optimized. The electrochemical parameters were further calculated with the results of the electron transfer number （n）, the charge-transfer coefficient （α） and the surface concentration （Гr） as 1.76, 0.37 and 2.47 × 10^-9 mol/cm^2, respectively, for the selected reductive peak. The results indicated that PNP showed an irreversible adsorption-controlled electrode process on the CILE.     

The electrocatalytic examination of cephalosporins at carbon paste electrode modified with CoSalophen

The electrocatalytic oxidation of cephalexin and cefazolin has been studied at a carbon paste electrode modified with cobalt salophen (CoSal) by cyclic voltammetry. The selectivity of the carbon paste modified with CoSal in detecting cephalexin and cefazolin was examined. To suggest the electrocatalytic mechanism for electro-oxidation of cefazolin, the electrochemical behavior of ceftriaxone was investigated which has a thiol group out of the beta lactam ring. The electrocatalytic oxidation of these antibiotics is shown to be irreversible at the CoSal modified electrode. Scan rate dependence of cefazolin, which is a sulfur-containing compound, has been examined. The results indicated that the electrocatalytic oxidation of the compounds is diffusion controlled. The responses of the modified electrode were compared with those of unmodified electrode and it has shown that the modified electrode has better sensitivity than unmodified electrode to the detection of cefazolin. The overall number of electrons contributed to the oxidation of cefazolin is obtained 1 by chronoamperometry; the number of electron involved in the rate-determining step was 1. The results of differential pulse voltammetry (DPV) using the modified electrode with high sensitivity were applied for the determination of cefazolin in human synthetic serum samples. The linear range was obtained from 1 × 10⁻⁵ to 1 × 10⁻³ M for DPV determination of cefazolin in buffered solutions (pH 3.0).     

Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode

In this work, we have developed a convenient and efficient method for the functionalization of ordered mesoporous carbon (OMC) using polyoxometalate H₆P₂Mo₁₈O₆₂·xH₂O (P₂Mo₁₈). By the method, glassy carbon (GC) electrode modified with P₂Mo₁₈ which was immobilized on the channel surface of OMC was prepared and characterized for the first time. The large specific surface area and porous structure of the modified OMC particles result in high heteropolyacid loading, and the P₂Mo₁₈ entrapped in this order matrix is stable. Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption-desorption isotherm and X-ray diffraction (XRD) were employed to give insight into the intermolecular interaction between OMC and P₂Mo₁₈. The electrochemical behavior of the modified electrode was studied in detail, including pH-dependence, stability and so on. The cyclic voltammetry (CV) and amperometry studies demonstrated that P₂Mo₁₈/OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. The mechanism of catalysis on P₂Mo₁₈/OMC/GC electrode was discussed. Moreover, the development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors.     

Electrochemistry of monoazahemin reconstituted myoglobin at an indium oxide electrode

Monoazahemin reconstituted myoglobin was prepared and its electrochemical behavior was studied in comparison with native myoglobin. For both myoglobins well-defined voltammograms were clearly obtained at highly hydrophilic surfaces of indium oxide electrodes. Although monoazahemin showed a more positive redox potential than hemin (measured in methanol), monoazahemin reconstituted myoglobin showed a more negative redox potential than native myoglobin in a 50 mM bis-Tris buffer solution (pH 6.5), suggesting that for both native and reconstituted myoglobins the heme environment including proximal histidine as an axial ligand of the redox center plays an important role in determining the redox potential. Also, a unique electrochemical response of cyano-monoazahemin reconstituted myoglobin was demonstrated.     

化学修饰电极的研究及其分析应用

化学修饰电极的出现推动了电化学的发展,是现代电分析化学领域中一个重要的研究方向。本文较详细地评述了化学修饰电极的制备方法及其电分析应用,对其未来发展作了展望。     

Retention behavior of monosaccharides and disaccharides on titania

Summary The ability of titania to recognize the position of hydroxyl groups was previously found to result from the formation of a chelate ring between a titanium ion and the two oxygen atoms of the hydroxyl group and carboxylate anion of 2-hydroxycarboxylic acids. We investigated how titania could recognize the position of hydroxyl group of monosaccharides and disaccharides. The retention behavior of monosaccharides and disaccharides on titania was found to be characteristic. Thus, sucrose was eluted much faster than the other sugars, whereas D-ribose, D-fructose and L-sorbose were most strongly retained on titania of all of the sugars tested. It is apparent that the sugars strongly retained on titania have an axial hydroxyl group or neighboring hydroxyl groups of the same conformation.     

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate-surfactant film modified electrode

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) were investigated successfully in liquid crystal films of didodecyldimethylammonium bromide (DDAB) on pyrolytic graphite (PG) electrodes in aqueous solution. This new bi-CoPc-surfactant film modified electrode could reduce aggregation of bi-CoPc in aqueous solution, which was confirmed by the electronic absorption spectra in the visible region and images of transmission electron microscopy (TEM). The charge transport diffusion coefficient (D_ct) and apparent heterogeneous electrode reaction rate constant (k_s) for this modified electrode were estimated. The reduction of bi-CoPc ligand showed excellent electrocatalytic ability for the reduction of trichloroacetic acid (TCA) and catalytic current had a linear relationship with the concentration of TCA in the range of 3×10⁻⁵-7.5×10⁻³ M. The reduction of bi-Co(III)Pc/bi-Co(II)Pc couple exhibited new pattern of catalytic reactivity in the reduction of oxygen. Peak current for the cathodic reduction of oxygen is proportion to the square root of the scan rate in the range of 5-1000 mV s⁻¹ in oxygen-saturated solution.     

Electrochemistry and electrocatalytic activity of polypyrrole/ferrocyanide films on a glassy carbon electrode

Functionalized polypyrrole films were prepared by incorporation of Fe(CN)₆ ³⁻ as doping anion during the electropolymerization of pyrrole at a glassy carbon electrode from aqueous solution. The electrochemical behavior of the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry. An obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole film with ferrocyanide incorporated was demonstrated by oxidation of ascorbic acid at the optimized pH of 4 in a glycine buffer. The catalytic effect for mediated oxidation of ascorbic acid was 300 mV and the bimolecular rate constant determined for surface coverage of 4.5 × 10⁻⁸ M cm⁻² using rotating disk electrode voltammetry was 86 M⁻¹ s⁻¹. Furthermore, the catalytic oxidation current was linearly dependent on ascorbic acid concentration in the range 5 × 10⁻⁴–1.6 × 10⁻² M with a correlation coefficient of 0.996. The plot of i _p versus v ^1/2 confirms the diffusion nature of the peak current i _p. Received: 12 April 1999 / Accepted: 25 May 1999     

Understanding and improving direct UV detection of monosaccharides and disaccharides in free solution capillary electrophoresis

Direct UV detection of carbohydrates in free solution capillary electrophoresis at 270 nm is made possible by a photo-oxidation reaction. Glucose, rhamnose and xylose were shown to have unique UV absorption spectra hypothesizing different UV absorbing intermediates for their respective photo-oxidation. NMR spectroscopy of the photo-oxidation end products proved they consisted of carboxylates and not malondialdehyde as previously theorized and that oxygen thus plays a key role in the photo-oxidation pathway. Adding the photo-initiator Irgacure^® 2959 in the background electrolyte increased sensitivity by 40% at an optimum concentration of 1 × 10⁻⁴ mM and 1 × 10⁻⁸ mM for conventional 50 μm i.d. capillaries and for the corresponding extended light path capillaries, respectively.     

Electrochemical and photophysical properties of a chemically modified electrode

A surface extinction coefficient for an inorganic complex, bis(2,3′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylato) ruthenium(II), immobilised on a transparent electrode has been determined for the first time, and demonstrated to be significnatly larger than that of its analogue in homogeneous solution. The first observation of a formal redox potential, for an immobilised complex, which is sharply dependent upon the extent of surface coverage is also reported.