The electrochemistry of proteins and related substances: I. Cystine and cysteine at the mercury electrode

Studies of the electroreduction of cystine (RSSR) and oxidation of cysteine (RSH) at several pH's at a hanging Hg drop electrode by cyclic voltammetry and at a Hg pool by coulometry are described. The proposed mechanism for RSSR involves reduction of an adsorbed monolayer (maximum coverage of 41 μC cm^?2) to form solution phase RSH at the adsorption prewave. A diffusion controlled reduction of RSSR to RSH occurs at more negative potentials. Oxidation of RSH involves formation of an adsorbed organomercury species, e.g., Hg(RS)₂ (maximum coverage of 80 μC cm^?2), which is reducible back to RSH. At higher RSH concentrations, anodic and cathodic current spikes appear on the cyclic voltammograms which are ascribed to formation of a tight or compact film when monolayer coverage of Hg(RS)₂ is attained, corresponding to strong interactions between adsorbed species.     

Elektrochemische untersuchung von bis(diphenyldithiophosphin) disulfid

The electrochemical behaviour of bis(diphenyldithiophosphine)disulphide (RSSR) and mercuric diphenyldithiophosphinate [(RS)₂Hg] in ethanol-lithium perchlorate and ethanol-sulphuric acid media was studied by the methods of classical polarography, and electrolysis at controlled-potential and at a rotating disc platinum electrode. The data obtained show that RSSR is not reduced directly on the dropping mercury electrode but is adsorbed. It then undergoes a rapid chemical reaction causing the formation of (RS)₂Hg, which is electroactive. The electrolysis at controlled potential proves that (RS)₂Hg undergoes a two-electron reduction, giving diphenyldithiophosphinic acid (RSH) as a main product, whereas the oxidation of RSH leads to the production of (RS)₂Hg. Regardless of the fact that the chemical and adsorption equilibria during reduction of RSSR and (RS)₂Hg are complex, the coulometric generation of RSH is not difficult to achieve, and permits the use of RSSR as a coulometric reagent.     

Scanning electrochemical microscopy imaging of rhodochrosite dissolution using gold amalgam microelectrodes

Gold/mercury amalgam (Au/Hg) microelectrodes with a diameter of 25 microm were developed for the detection of environmentally relevant analytes such as manganese and iron by scanning electrochemical microscopy (SECM), and applied to investigate the controlled dissolution of manganese carbonate (MnCO(3); rhodochrosite) in acidic conditions. Characterization of the amalgam electrode geometry via approach curves recorded during SECM experiments revealed Au/Hg microelectrodes with sphere cap geometry. Quantitative determination of Mn(2+) has been achieved by calibration of the Au/Hg microelectrode in bulk solution experiments. Subsequent SECM imaging experiments confirm the applicability of amalgam microelectrodes for imaging of Mn(2+) production during dissolution of MnCO(3) at pH 3.9. This study confirms feasibility and provides the fundamental basis of SECM imaging with amalgam microelectrodes to address biogeochemically relevant questions.     

Capillary electrophoresis with constant potential amperometric detection using a nickel microelectrode for detection of carbohydrates

Carbohydrates were separated by capillary electrophoresis (CE) and detected electrochemically using a nickel microelectrode which was operated at a constant applied potential (∼0.6 V vs. Ag/AgCI). A simple capillary electrode holder design facilitated alignment between the separation capillary and the working microelectrode without the use of micro-positioning equipment. The separations were performed under alkaline conditions (pH > 11), matching the high pH requirements for amperometric detection at the nickel electrode. The analytical procedure developed showed detection limits for the carbohydrates studied in the micromolar range, showing a linear response in the range tested (micromolar to millimolar). The procedure was used to identify sugars in two real samples (i.e., urine and in a common beverage). The potential use of the system for the determination of amino acids was also demonstrated.     

Electroanalytical Determination of Some Phenolic Acids by High‐Performance Liquid Chromatography at Gold Electrodes

The electrochemical and amperometric behavior of a gold electrode was investigated towards the oxidation of several common phenolic acids in neutral phosphate solutions. Au electrodes show an appreciable stability and reproducibility of the amperometric signals by using a constant applied potential of 1.0 V vs. Ag/AgCl. Separations of selected phenolic acids using a reverse phase C₁₈ analytical column with a mobile phase containing 10 mM NaH₂PO₄ plus 10 mM Na₂HPO₄ (pH 7) and methanol as organic modifier, are achieved isocratically in less than 30 min. The detection limits at the level of nmol/L and linear ranges of four‐five orders of magnitude are generally achieved. The proposed chromatographic strategy coupled with the electrochemical detection at the Au electrode was successful tested for the quantitative determination of phenolic acids in beer, red wine and brandy with good sensitivity and recovery.     

Liquid Chromatography with Pulsed Amperometric Detection for Speciation of Mercury

Liquid chromatography with pulsed amperometric detection (PAD) at an Au electrode was successfully applied for speciation of mercury: inorganic mercury, methylmercury and ethylmercury. The optimized conditions for triple-step potential waveforms utilized in PAD were: E₁ 1800 mV, t₁ 50 ms; E₂ 300 mV, t₂ 3850 ms; E₃ 750 mV, t₃ 100 ms; mercury oxidation was monitored at E₃. With the eluent of HClO₄ (0.10 M) + KCl (1.0 mM) + CH₃CN (1.0%) at a flow rate 0.80 mL/min, three mercury species were fully separated in 6 min on a glass column (3 mm × 10 cm) packed with Chemcosorb C₁₈ (5 μm). However, the first peak (inorganic mercury) was commonly overlapped by the solvent front. The relative standard deviations (n = 6) for methylmercury (15 ppb) and ethylmercury (30 ppb) were 4.2% and 2.8%, respectively. The linear range tested was 2 – 500 ppb (r = 0.9998) for methylmercury, and was 4 – 1000 ppb (r = 1.000) for ethylmercury. The detection limits (S/N = 3) were 1.2 ppb and 1.8 ppb for methylmercury and ethylmercury, respectively. The results of determination of the mercury-containing species thimerosat in three commercial contact-lens solutions agreed satisfactorily with the expected values.     

An Investigation into the Suitability of Bismuth as an Alternative to Gold‐Amalgam as a Working Electrode for the In Situ Determination of Chemical Redox Species in the Natural Environment

The suitability of the bismuth film working electrode was investigated as an alternative to the gold‐amalgam electrode used in solid state microelectrodes for in situ voltammetric analysis of redox chemistry in the natural environment (e.g., lakes, oceans, sedimentary pore waters). Chemical redox species measurable with the Au‐amalgam include O₂, H₂S, S , S₂O , Fe²⁺, Fe³⁺, Mn²⁺, I^?. Bismuth was electrochemically deposited to form a solid film analogous to the Au‐amalgam on a polished gold disk electrode. The useable potential window of the Bi‐film was found to be narrower than that of the Au‐amalgam, precluding the detection of dissolved O₂, I^? and S₂O , whose redox potentials fall outside the Bi‐film's range. The Bi‐film was able to detect free H₂S/HS^? and total sulfide (AVS), but not Fe²⁺ or Mn²⁺. The Bi‐film was less sensitive to low levels of total sulfide (<10 μM) than the Au‐amalgam; however, the Bi‐film was able to accurately quantify very high concentrations of sulfide (at least 15 mM), with a linear response up to an order of magnitude higher than that of the Au‐amalgam. Thus the Bi‐film appears to have limited application as an alternative to the Au‐amalgam microelectrode for in situ analysis of redox species in natural waters.     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part II. A chronocoulometric investigation

Both the oxidation of Cu⁰ at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of Cu^II at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump E_i→E_f upon the initial and final potentials E_i and E_f, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a Cu^I compound on a mercury electrode immersed in a Cu^II solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a Cu^I compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I.     

Sensitive and Rapid Flow Injection Amperometric Hydrazine Sensor using an Electrodeposited Gold Nanoparticle Graphite Pencil Electrode

A gold (Au) nanoparticle-modified graphite pencil electrode was prepared by an electrodeposition procedure for the sensitive and rapid flow injection amperometric determination of hydrazine (N₂H₄). The electrodeposited Au nanoparticles on the pretreated graphite pencil electrode surface were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction spectroscopy, and electrochemical impedance spectroscopy. Cyclic voltammograms showed that the Au nanoparticle-modified pretreated graphite pencil electrode exhibits excellent electrocatalytic activity toward oxidation of hydrazine because the highly irreversibly and broadly observed oxidation peak at +600?mV at the pretreated graphite pencil electrode shifted to ?167?mV at the Au nanoparticle pretreated graphite pencil electrode; in addition, a significant enhancement in the oxidation peak current was obtained. Thus, the flow-injection (FI) amperometric hydrazine sensor was constructed based on its electrocatalytic oxidation at the Au nanoparticle-modified pretreated graphite pencil electrode. The Au nanoparticle-modified pretreated graphite pencil electrode exhibits a linear calibration curve between the flow injection amperometric current and hydrazine concentration within the concentration range from 0.01 to 100?µM with a detection limit of 0.002?µM. The flow injection amperometric sensor has been successfully used for the determination of N₂H₄ in water samples with good accuracy and precision.     

Simultaneous Determination of Caffeine,Ibuprofen, and Paracetamol by Flow‐injection Analysis with Multiple‐pulse Amperometric Detection on Boron‐doped Diamond Electrode

This work presents a simple, fast and low‐cost method for the simultaneous determination of three drugs by flow‐injection analysis with multiple‐pulse amperometric (MPA) detection using a wall‐jet flow cell with a boron‐doped diamond electrode. The amperometric determination of caffeine (CF), ibuprofen (IB) and paracetamol (PC) was performed by the application of a four‐potential waveform using the MPA technique. PC is oxidized at E₁ (1.20 V/70 ms) and thus selectively detected; PC and CF are oxidized at E₂ (1.49 V/40 ms); PC, CF and IB are oxidized at E₃ (1.70 V/70 ms); and E₄ (1.80 V/100 ms) is applied for electrode cleaning. The subtraction of currents obtained at the different potentials did not provide accurate determinations of CF and IB, thus it was required to investigate correction factors to determine CF and IB without the interference from PC and CF using the respective amperometric signals obtained at E₂ and E₃. The proposed method was successfully applied for the determination of three drugs in pharmaceutical samples with low generation of residues and a high analytical frequency (150 h^?1) in comparison with HPLC‐DAD method.     

纳米材料修饰电极上吸附态葡萄糖氧化酶的酶活性和电活性的比较

采用石英晶体微天平(QCM)技术, 监测了裸金(Au)电极、电沉积纳米金的金电极(Au_ed/Au)、多壁碳纳米管(MWCNTs)修饰的金电极(MWCNTs/Au)以及MWCNTs 修饰后再电沉积纳米金的金电极(Au_ed/MWCNTs/Au)上葡萄糖氧化酶(GOx)的吸附过程, 测算了吸附固定的GOx质量. 通过阳极恒电位检测吸附酶与葡萄糖发生酶反应所产生的过氧化氢, 考察了这些酶电极的安培响应, 并测算了各吸附态GOx的质量比生物活性(MSBAi).也通过循环伏安法研究酶的直接电化学, 测算了各吸附态GOx的电活性百分数(EAP_i). 实验结果表明, 酶吸附量和酶电极的安培响应满足MWCNTs/Au > Au_ed/MWCNTs/Au > Au_ed/Au > Au 的顺序; MSBA_i满足Au > Au_ed/MWCNTs/Au > Au_ed/Au > MWCNTs/Au的顺序; 而EAP_i则满足MWCNTs/Au > Au_ed/MWCNTs/Au > Au_ed /Au > Au的顺序. 根据酶和纳米材料的亲疏水作用以及酶的吸附量对实验结果进行了合理解释, 也定量验证了电极上吸附酶分子的总生物活性与酶电极的安培响应呈正相关关系, 所得数据和结论有助于纳米材料固定酶及其安培酶电极的研究.     

Polarographic behaviour of alizarincomplexan,anodic wave and its analytical applications

Summary Polarographic Behaviour of Alizarincomplexan, Anodic Wave and Its Analytical Applications The polarographic DPP and DC techniques have been utilized to carry out the study of the anodic wave of alizarincomplexan (AC) in aqueous medium. At pH>5.7 the anodic current is produced by the complex formed between the AC and Hg(II) proceeding from the dropping electrode. At pH=7.3 theE _1/2 of this anodic wave is 0.000 V vs. Ag/AgCl. The main characteristics of the wave have been studied, and the corresponding electrode mechanism process is proposed. Since Hg(II) forms a complex with AC, a method for the amperometric determination of Hg(II) is proposed. We have also studied the analytical possibilities for doubly charged ions which form complexes with the reagent, and an amperometric method for Co(II) determination is also proposed.     

Analysis of Triorganotin Compounds by High-Performance Liquid Chromatography with Reverse-Pulse Amperometric Detection

High-performance liquid chromatography (HPLC) coupled with the reverse-pulse amperometric (RPA) detection method has been developed for the analysis of triorganotin compounds in aqueous solutions. The major advantage of RPA vs. conventional amperometric detection is its ‘in situ’ elimination of interference from dissolved oxygen in the chromatographic eluent; therefore, no extra chemicals or apparatus are required for oxygen removal. With a Partisil-10 SCX column and an eluent of methanol/0.01 M sodium acetate buffer (70:30, pH 5.5), the four triorganotins, viz., trimethyl-, triethyl-, tripropyl-, and tributyltin, can be totally separated. Detection by RPA was performed with a static dropping mercury electrode with an initial potential of ?1.15 V and a final potential of +0.15 V. The absolute detection limit (S/N = 3) ranged from 12 ng of tributyltin (as tin) to 0.3 μg of trimethyltin (as tin). Applications of the method to the analysis of trace tributyltin in marine antifoulant leachate and sea water are described.     

CE coupled with amperometric detection using a boron-doped diamond microelectrode: validation of a method for endogenous norepinephrine analysis in tissue

The level of endogenous norepinephrine (NE) in several tissue types was determined by CE with amperometric detection. We report herein on the method validation by HPLC using both amperometric and coulometric detection (CD). Keys to the method were the use of a diamond microelectrode for detection and off-line SPE for sample preparation. The run buffer was a 250 mM borate solution adjusted to pH 8.8 with potassium hydroxide. The diamond microelectrode exhibited a low and stable background current, and a low peak-to-peak noise < or =0.65 pA at the detection potential of +0.86 V versus Ag/AgCl. For standard solutions, the detector signal (i.e., oxidation current) changed linearly with the NE concentration (r(2) = 0.999) between 60 and 1000 nmol/L with an estimated LOD of 51 nmol/L (S/N = 3) and a response variability of 4.5% (RSD, n = 5). An Oasis MCX sorbent was used for SPE and the procedure produced an NE recovery of 95.1 +/- 5.6% (n = 6) from tissue homogenates. NE levels in the spleen, small intestine, and heart of a normotensive rat were found to be in the range of 0.77-0.97, 0.22-0.32, and 0.29-0.45 microg/g tissue (n = 3), respectively.     

Amperometric detection of cytochrome c by capillary electrophoresis at a sol–gel carbon composite electrode

Capillary electrophoresis (CE) was employed for the determination of cytochrome c using a wall-jet amperometric detector consisting a copper(I) oxide-modified sol–gel carbon composite electrode (CCE), which exhibits a sensitive electrocatalytic response for the oxidation of cytochrome c. The optimum conditions of separation and detection are 0.08 M NaOH for the separation solution, 12 kV for separation voltage and +0.60 V versus saturated calomel electrode (SCE) for the detection potential. Calibration was linear over the concentration range 1–600 μM with the limit of detection of 3.4 μM, based on a signal-to-noise ratio (S/N) of 3.     

Study of the Interaction of Some Potential Anticancer Gold(III) Complexes with Biologically Important Thiols Using NMR,UV–Vis,and Electrochemistry

The interaction of gold(III) complexes [Au(en)Cl₂]Cl, [Au(en)₂]Cl₃, [Au(cis‐DACH)Cl₂]Cl, and [Au(cis‐DACH)₂]Cl₃ (en = ethylenediamine, DACH = cis‐1,2‐diaminocyclohexane) with biologically important thiols, such as glutathione (GSH), dl ‐penicillamine (PSH), mercaptoacetic acid (MAA), and N‐(2‐mercaptopropionyl)glycine (MPG), has been studied using ¹H, ¹³C NMR, UV–vis spectroscopy and electrochemistry in aqueous solution. Kinetic data revealed that the reactivity of their substitution reaction followed the order: [Au(en)Cl₂]⁺ > [Au(en)₂]³⁺ > [Au(cis‐DACH)Cl₂]⁺ > [Au(cis‐DACH)₂]³⁺. The thiol reactivity increased with decreasing its size, viz. MAA ≫ MPG > PSH > GSH. Square wave stripping voltammetry displayed peaks for Au(III) and Au(I) at +0.875 V and +1.4 V respectively. The interaction of the complexes with thiols resulted in reduction of gold(III) to gold(I) and thiol ligands (RSH) were oxidized to disulfide (RSSR).     

Fabrication and Use of Dual‐function Iridium Oxide Coated Gold SECM Tips. An Application to pH Monitoring above a Copper Electrode Surface during Nitrate Reduction.

The fabrication of a gold microelectrode modified with iridium oxide film (IrO_x) and its use as tip with a dual function in SECM experiments is reported. The defective structure of the coating onto the microelectrode surface was used as strategy to combine the advantages of both amperometric (for current‐distance determination) and potentiometric (for pH sensing) SECM operation modes. Approach curves, using oxygen and hexaammineruthenium(III) as redox mediators, were obtained without significant loss of the performance and reproducibility of the potentiometric pH response. This allowed the precise positioning of the proposed tip above a substrate in SECM experiments and, subsequently, to monitor pH at the substrate surface. The IrO_x modified microelectrode was applied successfully in SECM experiments involving the local proton consumption during the nitrate reduction at a copper cathode surface.     

Pulsed coulometric detection of carbohydrates at a constant detection potential at gold electrodes in alkaline media

A significant increase in the signal-to-noise ratio for the pulsed amperometric detection (PAD) of carbohydrates at gold electrodes is obtained by increasing the length of the current integration period (t_i) from the traditional value of 16.7 ms (i.e., $\text{1}\text{60}$ Hz). For t_i > 16.7 ms, the integrated response (q, coulombs) is plotted as the signal. This pulsed coulometric detection (PCD) is applied in a flow-injection system. For t_i = 500 ms, the detection limit with the instrumentation used is 1 μM (S/N = 2) for glucose which is a significant improvement on the value 35 μM found with PAD. The absolute detection limits for glucose and sucrose are ca. 50 pmol and 125 pmol, respectively, in 50-μl samples. Calibration plots (q_p vs. C^b) for PCD are linear over significantly larger dynamic ranges than those observed for PAD because of the lower detection limits.     

Development of capillary electrophoresis methods for quantitative determination of taurine in vehicle system and biological media

CE methods have been developed for the determination of taurine in pharmaceutical formulation (microemulsion) and in biological media such as sweat. The CE system with end-column pulsed amperometric detection has been found to be an interesting method in comparison with UV and fluorescence detection for its simplicity and rapidity. A gold-disk electrode of 100 mm diameter was used as the working electrode. The effects of a field decoupler at the end of the capillary, separation voltage, injection and pressure times were investigated. A detection limit of 4 x 10(-5) mol/L was reached using integrated pulsed amperometric detection, a method successfully applied to taurine analysis of the biological samples such as sweat. For taurine analysis of oil-in-water microemulsion, fluorescence detector was the favored method, the detection limit of which was 4 x 10(-11) mol/L.     

Amperometric L‐cysteine Sensor Using a Gold Electrode Modified with Thiolated Catechol

A thiolated catechol (CA) consisting of 1,6‐Hexanedithiol (HDT) and CA was modified on a gold (Au) electrode to obtain an amperometric L‐cysteine sensor with detection limit of 60.6 nM. The preparation of thiolated CA was conducted via a thiol addition between HDT and electro‐oxidized CA (EOCA). Briefly, the thiol addition reaction was accomplished by potential cycling of HDT/Au electrode in 0.1 M phosphate buffer (PB, pH 7.2) containing CA, and an EOCA‐HDT/Au electrode was produced. The obtained EOCA‐HDT/Au electrode exhibits a pair of well‐defined redox peaks (at 0.22/0.10 V) of o‐quinone moiety, which effectively mediates the oxidation of L‐cysteine in a 0.1 M PB (pH 7.2), with an over‐potential decrease by ca. 0.12 V (versus bare Au electrode). Electrochemical quartz crystal microbalance, cyclic voltammetry and surface‐enhanced Raman spectra were used to study relevant processes and/or film properties. The amperometric L‐cysteine sensor has good anti‐interferent ability and reproducibility. It also has acceptable recovery for detection of L‐cysteine in urine samples.