Nathan S. Lawrence

Analytical determination of homocysteine: a review

The determination of homocysteine has gained high interest within the biomedical community over recent years as the molecule is an important biomarker for a wide range of diseases. The current report gives a brief overview into the biological significance of homocysteine and a rigorous account of the existing detection protocols for homocysteine in biological fluids, as well as trying to anticipate possible future trends in the development of rapid, low-cost and sensitive assays for its detection.

Analytical strategies for the detection of sulfide: a review

The detection of sulfide, particularly hydrogen sulfide, has long held the attention of the analytical community and, as such, a large number of protocols have arisen. Built around a core of reactions, some of which are highly specific to sulfide, there is considerable scope for exploitation across a number of instrumental platforms. Such flexibility may provide options for alleviating potentially intractable matrix effects through switching to an alternative detection methodology. This review details the need for sulfide measurements and describes the basis of many of the current approaches to sulfide detection. The collation of information from a broad range of sources, covering the major approaches, has been achieved and the salient points of each technique presented and critically appraised. The various peculiarities of the systems are compared and contrasted and a summary of the analytical characteristics (matrix, methodology and detection range) from each has been constructed.

Electrochemical determination of thiols: A perspective

Monitoring thiol concentrations within physiological fluids has risen to considerable importance within the biomedical communities as our appreciation of the various roles that these compounds can perform has increased. Their utilization as biomarkers for numerous clinical conditions has prompted considerable interest in the development of analytical protocols that enable speedy measurements to be made. As such, a large number of procedures have arisen. This perspective highlights the contexts where physiological thiol measurements are required and describes the basis of many of the current electrochemical approaches to their determination. The collation of information from a broad range of sources covering the major electrochemical approaches (direct, indirect, and post column/capillary detection) has been achieved and the salient points of each technique critically appraised. The various characteristics and limitations of the systems are compared and contrasted with the analytical parameters (matrix, methodology, detection limit etc) from each major grouping tabulated to aid direct comparison.

The electrochemical analog of the methylene blue reaction: A novel amperometric approach to the detection of hydrogen sulfide

The electrochemical oxidation of N,N-dimethylphenylenediamine at glassy carbon electrodes in aqueous solution in the pH range 3-6 is shown to lead to currents which are considerably amplified in the presence of dissolved hydrogen sulfide. The mechanism of the electrode process is investigated and the reaction is shown to provide an attractive basis for the amperometric sensing of sulfide with an accessible range of ca. 3 to 150 muM (0.1 to 4.8 ppm).

Electrochemistry of immobilised redox droplets: Concepts and applications

The use of microdroplet modified electrodes provides a simple methodology with which to study the biphasic electrochemistry of a plethora of species, encouraging the use of such techniques to mimic emulsion media. Furthermore, since the droplets may be miniaturised, this approach may assist in the field of biomimetic electrochemistry. For these reasons, this paper reviews the voltammetry of electrodes modified with electrochemically active droplets. The primary focus of the review is of unsupported droplets, where electron transfer processes occur at the three phase boundary, the base circumference of the individual droplets (of a volume range spanning nine orders of magnitude, going from microlitre to femtolitre volumes). The voltammetry of such systems is categorised via a semi-quantitative appreciation of the voltammetric characteristics. Finally, several topical examples illustrating the potential of application of this technology are described.

Electrochemical determination of sulphide at nickel electrodes in alkaline media: a new electrochemical sensor

The electrochemistry of a nickel hydroxide electrode has been studied both in the presence and absence of sulphide at microdisc and macroelectrodes. With sulphide present a new oxidative wave is observed. At a macroelectrode, this response produced a linear range from 20 to 200 μM with a corresponding limit of detection of 19 μM. Under the microelectrode regime, the response was found to be linear from 20 to 200 μM with a detection limit of 10 μM. The protocol has been developed into the design of a simple and cheap electrochemical sensing cell for the detection of sulphide in aqueous media.

Electrochemical detection of thiols in biological media.

The utilisation of catechol as an electrochemical indicator for the presence of sulphydryl thiols (RSH) has been investigated. The electrochemical oxidation of the catechol within tissue culture media was examined with the influence exerted on the redox chemistry by cysteine evaluated in terms of the development of an analytical protocol. The electro-generation of o-quinone was found to be followed by a 1,4-addition reaction with available cysteine such that an increase in the current, attributed to the re-oxidation of the thiol-catechol adduct, could be exploited as means of quantifying the concentration of the thiol. The selectivity of the reaction has been assessed with no interference from lysine, tyrosine, methionine or cystine. Other amino acids possessing sulphydryl thiol functionalities (homocysteine and glutathione) were, however, found to react through a similar route to that observed with cysteine.

Homogeneous chemical derivatisation of carbon particles: a novel method for funtionalising carbon surfaces

The homogeneous reduction of diazonium salts by hypophosphorous acid, provides an easy methodology for the derivatisation of carbon powder. The resulting derivatised carbon was characterised electrochemically showing the behaviour expected of a surface bound species. This procedure provides an easy, inexpensive methodology for functionalising carbon which might find applications in ion-exchange resins or combinatorial chemistry.

Electrochemically initiated 1,4 additions: a versatile route to the determination of thiols

The electrochemical generation of quinoid intermediates and their subsequent reaction with sulphydryl thiols (RSH) have been investigated as a method by which the latter can be quantified. Nucleophilic addition of the thiol species to the electrogenerated quinoid structure leads to the production of a chemically reduced adduct whose subsequent re-oxidation at the electrode leads to an amplification of the oxidative current. This can be correlated to the concentration of thiol present within the medium. The influence of indicator composition on the nature of the analytical response has been assessed through examining the electrochemical properties of 15 derivatives and the validity of the proposed reaction pathway corroborated by comparison of the voltammetric response with computer simulations. Interference by other electroactive species (ascorbate, urate, tyrosine) and other physiological constituents (cystine, lysine) has also been assessed.

Anthraquinone-derivatised carbon powder: reagentless voltammetric pH electrodes

A simple pH probe is developed based upon the covalent chemical derivatisation of carbon particles with anthraquinone. The amperometric response of electrodes constructed from this material is examined and shown to produce a Nernstian linear response to pH from 1 to 9, over a range of temperatures from 20 to 70 degrees C, consistent with a two-electron, two-proton electrochemical process.