Theoretical considerations on the performance of electrochemical flow-through detectors

A survey of the equations on the limiting current in electrochemical flow-through cells is presented. For voltammetric detectors, a generalized equation for the limiting current is given. The conditions to be considered in designing an electrochemical flow-through detector for optimal signal-to-noise ratios are outlined for tubular, thin-layer, wall-jet and disk electrodes.     

Tissue- and microbe-based electrochemical detectors for liquid chromatography

A novel electrochemical detection approach for liquid chromatography is described, utilizing rapid-responding tissue- and microbe-based carbon paste electrodes. This approach adds a new dimension of selectivity to liquid chromatography/electrochemistry, based on bioactivity (substrate specificity). Complex chromatograms (e.g., of urine samples) are greatly simplified as only substrates of the enzymes present in the cellular material are detected. Selectivity can be greatly improved by obtaining two chromatograms, one profiling only the biologically active solutes. Because of their higher enzymatic activity and stability, cellular materials are more suitable for liquid chromatographic detection than isolated enzymes. This concept is tested on several classes of analytes, using banana-, mushroom- and yeast-modified electrodes. Results are also given for a series configuration with a tissue-generator/electrode collector. These developments should lead to increased use of natural materials for monitoring chromatographic effluents.     

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.     

Large-volume wall-jet cells as electrochemical detectors for high-performance liquid chromatography

The wall-jet electrode is an attractive configuration for electrochemical detectors for high-performance liquid chromatography (h.p.l.c.) on account of its high convective mass-transfer characteristics. Another important, though less recognized, feature is its small effective cell volume, which is shown to be almost independent of the geometric cell volume. The effective volume is less than the volume of the hydrodynamic boundary layer, or only a few microlitres. The practical use of the wall-jet detector in both normal-phase and reverse-phase h.p.l.c. is discussed with particular reference to the distance between the jet and the electrode. A new cell design is proposed.     

Optimization of electrolysis in the cylindrical electrochemical cell rotating in the magnetic field

The relationships between the rotational rate of electrochemical cell, its ohmic resistance, a ratio between the radii of inner and outer cylindrical electrodes, a sum of cathodic and anodic potentials, and the mechanical energy, which is consumed for passing a prescribed current and overcoming friction forces, are analyzed. It is shown that the relationships can be used to optimize the operation of rotating electrochemical cell.     

Corrosion and electrochemical behavior of metal-oxide anodes during electrolysis with an ion-exchange membrane

The reasons for a specific behavior of anodes in chlorine electrolysis with an ion-exchange membrane are considered. The corrosion rate in modeling conditions and the predicted lifetime of anodes of different compositions are studied. The anodes’ coatings contain mixed oxides of Ir, Ru, Sn, or Ti. The effect of the anode’s position relative to the membrane and a relative stability of the anode coatings are examined in conditions simulating a heavy alkalization of the anolyte caused by a membrane rupture. A considerable advantage of using the anode with a coating containing 15, 15, and 70 mol % of RuO₂, IrO₂, and TiO₂ is demonstrated     

Theoretical aspects of 2D electrochemical phase formation

Journal of Solid State Electrochemistry - Two-dimensional electrochemical phase formation as a result of nucleation, growth, and overlap of disk-shaped nuclei of a new phase on the electrode...     

Glassy-carbon amperometric transducers as electrochemical detectors in liquid chromatography The influence of oxygen

The electrochemical behaviour of oxygen on glassy-carbon electrodes and the suitability of this electrode material for electroreduction of organic compounds have been investigated. The observed oxygen overpotential on a glassy-carbon electrode was more negative than that on an amalgamated gold electrode, thus allowing the determination of easily reducible compounds such as polynitro-aromatics and quinones without the need for exhaustive removal of dissolved oxygen. The detection limits (3sigma) were about 0.2, 0.8, and 2.5 pmole for polynitro-aromatics, mononitro-aromatics and quinones, respectively. Though the glassy-carbon material has a negative-potential limit about 250 mV more positive than that for the amalgamated gold electrode, and requires a longer equilibration time before use, it is more convenient for routine use.     

The electrochemical reduction of succinimide : Reactivity of quaternary ammonium ions under electrolysis conditions

A solution of succinimide in ether acetonitrile or N, N-dimethylformamide containing tetra-n-butylammonium fluoborate shows a single, irreversible reduction wave at a platimum cathode by cyclic voltammetry. Coulometry demonstrates that a single electron is transferred. The reaction is accompanied by the evolution of hydrogen at a rate such that the passage of one Faraday of charge results in the generation of almost exactly 0.5 mole of hydrogen. The product of these reactions is the succinimide anion which is stable in the electrolysis solution, but reacts with tetra-n-butylammonium ion during vpc analysis or in refluxing N, N-dimethylformamide to form N-n-butylsuccinimide.     

An electrochemical reactivation method for solid electrodes used in electrochemical detectors for high-performance liquid chromatography and flow injection analysis

An electrochemical reactivation method for solid electrodes used in electrochemical detectors is optimized for glassy carbon electrodes. Application of a voltage pulse train for 5 min is effective in restoring the response of the electrode after deactivation by organic compounds. Potassium hexacyanoferrate(II) and dl-synephrine were used to test the reactivation of electrodes; 2,6-dihydroxybenzoic acid and urine served to deactivate them. The effects of the amplitude, d.c. level, and frequency of the applied voltage pulse train, as well as the mode of termination, are discussed.     

Carbon nanotube and diamond as electrochemical detectors in microchip and conventional capillary electrophoresis

As two important polymorphs of carbon, carbon nanotube (CNT) and diamond have been widely employed as electrode materials for electrochemical sensing. This review focuses on recent advances and the key strategies in the fabrication and application of electrochemical detectors in microchip and conventional capillary electrophoresis (CE) using CNT and boron-doped diamond. The subjects covered include CNT-based electrochemical detectors in microchip CE, CNT-based electrochemical detectors in conventional CE, boron-doped diamond electrochemical detectors in microchip CE, and boron-doped diamond electrochemical detectors in conventional CE. The attractive properties of CNT and boron-doped diamond make them very promising materials for the electrochemical detection in microchip and conventional CE systems and other microfluidic analysis systems.     

Complex rate constant for an electrochemical system involving an adsorbed intermediate

Two different methods of obtaining the impedance of a system involving an electrochemical reaction with an adsorbed intermediate are compared. The earlier method combines in parallel an impedance derived separately for the overall reaction with that for the rest of the system. It is appropriate for a fully supported electrolyte system but seems much less applicable to the unsupported situation. In contrast, the method derived and discussed herein involves complex reaction rate constants which may be directly incorporated in expressions for the total system impedance. This method may, therefore, be applied to either supported or unsupported situations. Several specific cases of the general complex rate constant results are discussed, particularly, various frequency dependencies possible. The overall small-signal impedance may be inductive in some parts of the frequency range and the low frequency limiting differential resistance negative, zero, or positive under various conditions.     

Theoretical analysis of the telegraph-like fluctuations in bridged electrochemical contacts

Steady-state current-voltage characteristic of an electrochemical bridged contact and the telegraph noise power are calculated in terms of the model of two redox states. It is shown that the overvoltage dependence of the tunnel current at a constant bias voltage is S-shaped, which was observed in works on the electron tunneling in similar systems. The overvoltage dependence of the contact conductance has a maximum near the equilibrium potential of the bridge-molecule redox group. The overvoltage dependence of the noise power at zero frequency has a maximum whose position is determined by the bias voltage.     

Theoretical analysis of proton relays in electrochemical proton-coupled electron transfer

The coupling of long-range electron transfer to proton transport over multiple sites plays a vital role in many biological and chemical processes. Recently the concerted proton-coupled electron transfer (PCET) reaction in a molecule with a hydrogen-bond relay inserted between the proton donor and acceptor sites was studied electrochemically. The standard rate constants and kinetic isotope effects (KIEs) were measured experimentally for this double proton transfer system and a related single proton transfer system. In the present paper, these systems are studied theoretically using vibronically nonadiabatic rate constant expressions for electrochemical PCET. Application of this approach to proton relays requires the calculation of multidimensional proton vibrational wave functions and the incorporation of multiple proton donor-acceptor motions. The decrease in proton donor-acceptor distances due to thermal fluctuations and the contributions from excited electron-proton vibronic states play important roles in these systems. The calculated KIEs and the ratio of the standard rate constants for the single and double proton transfer systems are in agreement with the experimental data. The calculations indicate that the standard PCET rate constant is lower for the double proton transfer system because of the smaller overlap integral between the ground state reduced and oxidized proton vibrational wave functions, resulting in greater contributions from excited electron-proton vibronic states with higher free energy barriers. The theory predicts that this rate constant may be increased by modifying the molecule in a manner that decreases the equilibrium proton donor-acceptor distances or alters the molecular thermal motions to facilitate the concurrent decrease of these distances. These insights may guide the design of more efficient catalysts for energy conversion devices.     

纳米NiO-Y复合阴极材料的制备及微生物电解池催化产氢性能

在Y分子筛的溶胶反应体系中加入碳球,经老化、水热晶化反应得到纳米Y分子筛,通过等体积浸渍（incipient-wetness impregnation,IWI）方式负载镍盐前驱体,经焙烧制备纳米NiO-Y复合材料,采用XRD、SEM、TEM、XPS、TG-DTG和N₂吸附-脱附等手段对其物理化学性质进行表征。结果表明,合成的NiO-Y复合材料样品的晶粒粒径为500 nm,具有微-介孔多级孔道结构。总比表面积达到774.3 m²/g,孔容为0.495 cm³/g,有利于暴露更多的活性位。通过线性扫描和塔菲尔曲线电化学测试评价发现,当镍盐负载量为30%（质量分数）时,纳米NiO-Y复合材料作为微生物电解池阴极具有较高的电催化活性。在运行周期内,样品的最大析氢电流密度达到22.87 A/m²,产气总量中H₂含量占73.71%,产氢效率为0.393 m³/（m³·d）,与Pt/C阴极产氢效率相近。     

Comparison of high-performance liquid chromatography electrochemical detectors for the determination of aporphines,catecholamines and melatonin

A Bioanalytical Systems detector showed a greater sensitivity for low-level determinations of apomorphine, norepinephrine, epinephrine, dopamine, and melatonin compared to a Brinkmann Instruments system. Modifications of the Bioanalytical Systems flow cell, which have been developed in our laboratories, increase the sensitivity toward certain catecholic compounds, generally decrease baseline noise, and allow for serial coupling of detectors.²