Electrochemical detection of neurotransmitters at structurally small electrodes

Electroanalytical chemistry has been widely developed and applied to the study of neurochemical systems. This then leads to a better understanding of many aspects of neurotransmission, for example, neural circuitry and neural substrates of compulsive drug use. This feasibility partly stems from the ease of oxidative detection of many neurotransmitters including dopamine, acetylcholine, norepinephrine, serotonin, glutamic acid and γ–aminobutyric acid. At the same time, this has also stimulated the development of structurally small electrodes for applications to the detection of neurotransmitters in biological microenvironments. In this respect, the small dimension of such electrodes permits minimal tissue damage upon implantation and, of equal importance, permits very careful selection of the region of tissue where measurements can be performed. In addition, the inherent fast response time of structurally small electrodes makes it feasible to follow biochemical events frequently taking place on a millisecond time scale (e.g. neuronal firing). Various electrode materials used to construct structurally small electrodes of different geometries and sizes have hitherto been reported. Common electrode materials both modified and otherwise, include metals such as tungsten and aluminium, gold nanoparticledeposited aluminium, various forms of carbon e.g. doped diamond, nanocrystalline diamond, pyrolysed carbon, carbon fibres, and gold nanoparticles deposited onto glassy carbon. A common problem encountered while performing in vivo electrochemical analyses of neurotransmitters is the adsorption of lipids, peptides and high molecular weight proteins present in biological matrices on the electrode surface. Formation of these layers leads to electrode fouling which distorts the voltammetric signal and suppresses the sensitivity of the electrode. Considerable research effort has been devoted to addressing electrode fouling problems. Approaches ranging from fast scan voltammetry, immobilising a protective organic film on the electrode surface, completely altering the surface termination, fabrication of nanocrystalline diamond coated electrodes, or of doped diamond electrodes, to gold electrodes modified with gold nanorod and gold nanoparticles have been developed. Apart from overcoming fouling, the latter methods have also demonstrated other advantages such as wider potential windows, greater durability, increased robustness and enhanced sensitivity. In this paper, we aim to thoroughly review the techniques used in developing structurally small electrodes of different geometries, which were then applied to the detection of neurotransmitters. We will also pay special emphasis on the strategies used to minimize electrode fouling during electrochemical detection of neurotransmitters at these electrodes. A comparison of these methods and possible future directions in the development of structurally small electrodes for detection of neurotransmitters will conclude the review

Minimizing fouling at hydrogenated conical - tip carbon electrodes during Dopamine detection in vivo

In this paper, physically small conical-tip carbon electrodes (∼2−5 μm diameter and ∼4 μm axial length) were hydrogenated to develop a probe capable of withstanding fouling during dopamine detection in vivo. Upon hydrogenation, the resultant hydrophobic sp3 carbon surface deters adsorption of amphiphilic lipids, proteins, and peptides present in extracellular fluid and hence minimizes electrode fouling. These hydrogenated carbon electrodes showed a 35% decrease in sensitivity but little change in the limit of detection for dopamine over a 7-day incubation in a synthetic laboratory solution containing 1.0% (v/v) caproic acid (a lipid), 0.1% (w/v) bovine serum albumin and 0.01% (w/v) cytochrome C (both are proteins), and 0.002% (w/ v) human fibrinopeptide B (a peptide). Subsequently, during dopamine detection in vivo, over 70% of the dopamine oxidation current remained after the first 30 min of a 60-min experiment, and at least 50% remained over the next half-period at the hydrogenated carbon electrodes. On the basis of these results, an initial average electrode surface fouling rate of 1.2% min−1 was estimated, which gradually declined to 0.7% min−1. These results support minimal fouling at hydrogenated carbon electrodes applied to dopamine detection in vivo

Evaluation of physically small p-phenylacetate-modified carbon electrodes against fouling during dopamine detection in vivo

In this paper, the effectiveness of anionic p-phenylacetate film was evaluated in protecting physically small conical-tip carbon electrodes (∼2�m radius and ∼4�m axial length)from fouling during dopamine detection in vivo. After characterising p-phenylacetate film-modified carbon electrodes in several redox systems in vitro, they were found to exhibit an 11% loss in dopamine oxidation signal over a 40-day storage period in ambient laboratory conditions, compared to over a 90% loss at bare carbon electrodes. In addition, by incubating in a synthetic laboratory solution containing the fouling reagents, 1.0% (v/v)caproic acid (a lipid), 0.1%(w/v) bovine serum albumin and 0.01%(w/v) cytochrome C (both are protein)and 0.002% (w/v) human fibrinopeptide B (a peptide), film-modified carbon electrodes showed a 29% reduction in the limit of detection and a 25%decrease in sensitivity for dopamine over 7 days, compared to undeterminable results arising from a severely degraded surface at bare carbon electrodes. During dopamine detection invivo,70- 95% of the dopamine oxidation current remained after the first 40min of the experiment, and at least 50% over the next 20min. In contrast, constant degradation in the dopamine oxidation signal was observed at bare carbon electrodes throughout the experiment. An average electrode surface fouling rate of 0.54%min−1 was estimated at the p-phenylacetate film-modified carbon electrodes during the first 40min of the experiments

Diffusion - limited chronoamperometry at conical - tip microelectrodes

In this paper, we present simulated diffusion-limited time-variant currents at conical-tip microelectrodes fabricated by depositing a carbon film in and on pulled quartz capillaries. These mechanically strong microelectrodes are suitable probes for detecting neurotransmitters in vivo. The simulated results show that the currents obtained at conical-tip microelectrodes are larger than those at finite conical microelectrodes (e.g. etched carbon fibres protruding from an insulating plane) of comparable dimensions. The currents at conical-tip microelectrodes and finite conical microelectrodes both converge to that of a microdisk electrode at small cone heights and large cone angles, and to that of a cylindrical electrode portion of equal length and half the radius at large cone heights and small cone angles. At short times (scaled by the electrode dimensions), Cottrellian current is achieved at conical-tip microelectrodes and the current densities collapse to the expected chronoamperometric response at a microdisk electrode, subject to some simulation errors. Comparison between a simulated chronoamperogram and an experimental chronoamperogram then allows an estimate of parameters (such as electrode surface area and dimensions) that define the electrode geometry. Steady-state currents based on empirical functions have also been computed for conical-tip microelectrodes and finite conical microelectrodes

Real time analysis of laterodorsal tegmentum modulation of nucleus accumbens dopamine release

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