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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/29 06:49:58」(JST)
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An ultramicroelectrode (UME) is a working electrode used in a voltammetry. The small size of UME give them relatively large diffusion layers and small overall currents. These features allow UME to achieve useful steady-state conditions and very high scan rates (V/s) with limited distortion. UME were developed independently by Wightman[1] and Fleischmann around 1980.[2] Small current at UME enables electrochemical measurements in low conductive media (organic solvents), where voltage drop associated with high solution resistance makes these experiments difficult for convention electrodes.[3] Furthermore, small voltage drop at UME leads to a very small voltage distortion at the electrode-solution interface which allows using two-electrode setup in voltammetric experiment instead of conventional three-electrode setup.
Contents
- 1 Design
- 2 Theory
- 2.1 Linear region
- 2.2 Steady-state region
- 3 Applications
- 3.1 Measurements in low conductive media
- 3.2 Measurements in biological systems
- 4 See also
- 5 References
Design
Ultramicroelectrodes are often defined as electrodes which are smaller than the diffusion layer achieved in a readily accessed experiment. A working definition is an electrode that has at least one dimension (the critical dimension) smaller than 25 μm. Platinum electrodes with a radius of 5 μm are commercially available and electrodes with critical dimension of 0.1 μm have been made. Electrodes with even smaller critical dimension have been reported in the literature, but exist mostly as proofs of concept. The most common UME is a disk shaped electrode created by embedding a thin wire in glass, resin, or plastic. The resin is cut and polished to expose a cross section of the wire. Other shapes, such as wires and rectangles, have also been reported. Carbon-fiber microelectrodes are fabricated with conductive carbon carbon fibers sealed in glass capillary with exposed tips. These electrodes are frequently used with in vivo voltammetry.
Theory
Linear region
Every electrode has a range of scan rates called the linear region. The response to a reversible redox couple in the linear region is a "diffusion controlled peak" which can be modeled with the Cottrell equation. The upper limit of the useful linear region is bound by an excess of changing current combined with distortions created from large peak currents and associated resistance. The charging current scales linearly with scan rate while the peak current, which contains the useful information, scales with the square root of scan rate. As scan rates increase, the relative peak response diminishes. Some of the charge current can be mitigated with RC compensation and/or mathematically removed after the experiment. However, the distortions resulting from increased current and the associated resistance cannot be subtracted. These distortions ultimately limit the scan rate for which an electrode is useful. For example, a working electrode with a radius of 1.0 mm is not useful for experiments much greater than 500 mV/s.
Moving to an UME drops the currents being passed and thus greatly increases the useful sweep rate up to 106 V/s. These faster scan rates allow the investigation of electrochemical reaction mechanisms with much higher rates than can be explored with regular working electrodes. By adjusting the size of the working electrode an enormous kinetic range can be studied. For UME only the very fast reactions can be studied through peak current since the linear region only exists for UME at very high scan rates.
Steady-state region
At scan rates slower than those of the linear region is a region which is mathematically complex to model and rarely investigated. At even slower scan rates there is the steady-state region. In the steady-state region linear sweeps traces display reversible redox couple as steps rather than peaks. These steps can readily be modeled for meaningful data.
To access the steady-state region the scan rate must be dropped. As scan rates are slowed, the relative currents also drop at a given point reducing the reliability of the measurement. The low ratio of diffusion layer volume to electrode surface area means regular stationary electrodes can not be dropped low enough before their current measurements become unreliable. In contrast, the ratio of diffusion layer volume to electrode surface area is much higher for UME. When the scan rate of UME is dropped it quickly enters the steady-state regime at useful scan rates. Even though UME supply small total currents their steady-state currents are high compared to regular electrodes.
Applications
Measurements in low conductive media
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This section requires expansion. (April 2014) |
Measurements in biological systems
|
This section requires expansion. (April 2014) |
See also
- Bioelectronics
- Multielectrode array
- Scanning electrochemical microscopy
- Fast-scan cyclic voltammetry
References
- ^ Wightman, R. Mark (August 1981). "Microvoltammetric electrodes". Analytical Chemistry 53 (9): 1125A–1134A. doi:10.1021/ac00232a004.
- ^ Heinze, Jurgen (September 1993). "Ultramicroelectrodes in Electrochemistry". Angewandte Chemie International Edition in English 32 (9): 1268–1288. doi:10.1002/anie.199312681.
- ^ Bond, A.M.; Fleischmann, M.; Robinson, J. (May 1984). "Electrochemistry in organic solvents without supporting electrolyte using platinum microelectrodes". Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 168 (1-2): 299–312. doi:10.1016/0368-1874(84)87106-3.
English Journal
- Electrochemical detection of a single cytomegalovirus at an ultramicroelectrode and its antibody anchoring.
- Dick JE1, Hilterbrand AT2, Boika A1, Upton JW2, Bard AJ3.
- Proceedings of the National Academy of Sciences of the United States of America.Proc Natl Acad Sci U S A.2015 Apr 28;112(17):5303-8. doi: 10.1073/pnas.1504294112. Epub 2015 Apr 13.
- We report observations of stochastic collisions of murine cytomegalovirus (MCMV) on ultramicroelectrodes (UMEs), extending the observation of discrete collision events on UMEs to biologically relevant analytes. Adsorption of an antibody specific for a virion surface glycoprotein allowed differentiat
- PMID 25870261
- Time of first arrival in electrochemical collision experiments as a measure of ultralow concentrations of analytes in solution.
- Boika A1, Bard AJ1.
- Analytical chemistry.Anal Chem.2015 Apr 21;87(8):4341-6. doi: 10.1021/acs.analchem.5b00037. Epub 2015 Apr 7.
- In electrochemical collision experiments, the frequency of collisions of nanoparticles (NPs) with an ultramicroelectrode (UME) is a measure of the solution concentration of NPs. The time of first arrival is evaluated as a measure of ultralow (sub-femtomolar) concentration of analytes in solution. Th
- PMID 25803279
- Measurement of temperature-dependent stability constants of Cu(I) and Cu(II) chloride complexes by voltammetry at a Pt ultramicroelectrode.
- Meng Y1, Bard AJ1.
- Analytical chemistry.Anal Chem.2015 Mar 17;87(6):3498-504. doi: 10.1021/acs.analchem.5b00052. Epub 2015 Mar 4.
- Cu(II)Cl complexes can undergo stepwise electrochemical reduction to Cu(I)Cl and then to Cu. In this paper, the Nernst equation is used to simultaneously calculate the temperature-dependent stability constants of both mononuclear Cu(II)Cl and Cu(I)Cl complexes for the first time. First, voltammogram
- PMID 25708623
Japanese Journal
- 形状と電気化学イメージの同時取得が可能な高解像度走査型電気化学顕微鏡の開発
- Characterization of electrochemiluminescence of tris(2,2′-bipyridine)ruthenium(II) with glyphosate as coreactant in aqueous solution
- Application of a Continuous Square-Wave Potential Program for Sub Nano Molar Determination of Ketotifen
Related Links
- Ultramicroelectrode, process for making same and its application Abstract The ultramicroelectrode comprises a wire or filament of noble metal and/or carbon on which an insulating layer is provided. The insulating layer is made ...
- An ultramicroelectrode assembly which generally comprises an electrically nonconducting host membrane having a plurality of micro-sized pores extending through the membrane, and a macro-sized substrat ... Other References
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