Malcolm R. Smyth

Capillary electrophoresis with electrochemical detection employing an on-column Nafion joint

Abstract The construction and evaluation of an on-column joint utilizing Nafion tubing for the isolation of the electrical circuit from the detection end of a capillary zone electrophoresis system is described. The joint enables electrochemical detection to be performed without adverse effects from the applied high voltage. The joint is both simple to construct and durable. The electrochemical detector employing a carbon fiber working electrode exhibited high coulometric efficiencies and a detection limit of 6 · 10−9M or 34.8 amol for hydroquinone. A high efficiency, of the order of 185 000 theoretical plates, was achieved for this compound. This system was evaluated for the detection of phenolic acids in apple juice and for the determination of naphthalene-2,3-dicarboxaldehyde derivatized amino acids in a brain homogenate. The use of voltammetry as a method of compound verification was also demonstrated.

Conducting electroactive polymer-based biosensors

Abstract Conductive electroactive polymers are materials discovered just on two decades ago. Originally heralded for their high conductivity/weight ratio, it is the unique chemical properties they possess that now arouse much attention. The ability to synthesise these materials under mild conditions enables a range of biological moieties (enzymes, antibodies and even whole living cells) to be incorporated into the polymer structure. The unique electronic properties then allow direct and interactive communication with the biochemistries incorporated to produce a range of analytical signals. This work reviews the options available for immobilisation of biocomponents and signal generation using conducting polymer-based biosensors.

An introduction to bismuth film electrode for use in cathodic electrochemical detection

Abstract A new electrode surface design, the bismuth film electrode (BiFE), is presented as a promising alternative to mercury and other solid electrodes for direct cathodic electrochemical detection of organic compounds. The preparation of the BiFE, involving an ex situ electroplating of metallic bismuth onto a glassy carbon (GC) substrate electrode, was optimised. The useful negative potential windows of the BiFE in the pH range 1 (−0.2 to −0.8 V vs Ag/AgCl) to 10 (−0.2 to −1.5 V) were determined. The reproducibility of measuring 2-nitrophenol as a model compound (relative standard deviation, r.s.d., n =10) was found to be 0.5% at the same BiFE, and 1.0% at successive newly prepared BiFEs. No polishing or any other pre-treatment of the substrate GC surface was required prior to re-plating of a new Bi film. The BiFE showed similar or even favourable voltammetric behaviour when compared to mercury and bare GC electrodes, and was successfully tested for amperometric detection under hydrodynamic conditions. The results revealed that BiFE is an attractive new non-mercury metallic electrode particularly suitable for cathodic electrochemical detection in flow analytical systems.

The application of conducting polymer nanoparticle electrodes to the sensing of ascorbic acid.

An ascorbic acid sensor was fabricated via the drop-casting of dodecylbenzene sulphonic acid (DBSA)-doped polyaniline nanoparticles onto a screen-printed carbon-paste electrode. The modified electrode was characterised with respect to the numbers of drop cast layers, optimum potential and operating pH. The sensor was found to be optimal at neutral pH and at 0V vs. Ag/AgCl. Under these conditions, the sensor showed good selectivity and sensitivity in that it did not respond to a range of common interferents such as dopamine, acetaminophen, uric acid and citric acid, but was capable of the detection of ascorbic acid at a sensitivity of 0.76 microA mM(-1) or 10.75 microA mM(-1) cm(-2) across a range from 0.5 to 8mM (r2=0.996, n=6), and a limit of detection of 8.3 microM (S/N=3). The sensor was compared to a range of other conducting polymer-based ascorbate sensors and found to be comparable or superior in terms of analytical performance.

Sodium-selective polymeric membrane electrodes based on calix[4]arene ionophores

Sodium-selective electrodes were prepared with polymeric membranes containing ester and ketone derivatives of p-tert-butyl calix[4]arene. The electrodes were prepared with 2-nitrophenyl octyl ether as the solvent mediator and potassium tetrakis(p-chlorophenyl)borate as the ion exchanger in a poly(vinyl chloride) matrix. Several of the ligands examined produced electrodes with near-Nernstian slopes, detection limits of ca. 10–5M for sodium and excellent selectivity against a range of common cations.

Design and development of a miniaturised total chemical analysis system for on-line lactate and glucose monitoring in biological samples

A miniaturised Total chemical Analysis System (μTAS) for glucose and lactate measurement in biological samples constructed based on an integrated microdialysis sampling and detection system. The complete system incorporates a microdialysis probe for intravascular monitoring in an ex vivo mini-shunt arrangement, and a silicon micromachined stack with incorporated miniaturised flow cell/sensor array. The prototype device has been developed based on state-of-the-art membrane and printed circuit board technology. The flow-through detection system is based on a three-dimensional flow circuit incorporating silicon chips with stacked micromachined channels. An integrated biosensor array (comprising enzyme sensors specific for glucose and lactate) is placed at the base of the stack allowing the detector to be incorporated within the μTAS assembly. These glucose and lactate biosensors are prepared using photolithographic techniques, with measurement based on the detection of hydrogen peroxide at glucose oxidase and lactate oxidase modified platinum electrodes. The resulting amperometric current (at 500 mV vs, Ag/AgCl) is proportional to the concentration of analyte in the sample. All instrumentation is under computer control and the complete unit allows continuous on-line monitoring of glucose and lactate, with fast stable signals over the relevant physiological range for both analytes. The microdialysis system provides 100% sampling efficiency. Sensor performance studies undertaken include optimisation of sensitivity, linearity, operational stability, background current, storage stability and hydration time. The total system (sampling and detection) response time is of the order of 4 min, with sensor sensitivity 1-5 nA mM-1 for glucose and lactate over the range 0.1-33 and 0.05-15 mM, respectively.

Monitoring excitatory amino acid release in vivo by microdialysis with capillary electrophoresis- electrochemistry

Capillary electrophoresis (CE) with electrochemical detection (ED) was used to determine extracellular levels of aspartate, glutamate and alanine in samples from the frontoparietal cortex of the rat which were obtained by microdialysis. The method was used to monitor the effect on the overflow of the excitatory amino acids aspartate and glutamate of an influx of high concentrations of potassium ion. Samples were derivatized with naphthalenedialdehyde-cyanide prior to analysis. Detection limits for aspartate and glutamate were 80 and 100 nM, respectively. CE-ED is extremely useful for the analysis of microdialysis samples because of the very small sample volumes required by this analytical technique. The use of ED provides the requisite sensitivity and allows verification of peak purity by voltammetry.

Bismuth film electrode for simultaneous adsorptive stripping analysis of trace cobalt and nickel using constant current chronopotentiometric and voltammetric protocol

Abstract Bismuth film electrode (BiFE) is presented as a promising alternative to mercury electrodes for the simultaneous determination of trace cobalt and nickel in non-deoxygenated solutions. The preplated BiFE was employed under adsorptive stripping constant current chronopotentiometric and adsorptive stripping voltammetric conditions in the presence of dimethylglyoxime complexing agent. BiFE exhibited well-defined and undistorted signals with favorable overall resolution for cobalt and nickel cations, with the signals for both metal cations being practically independent of each other. The stripping performance of BiFE is characterized by good reproducibility (RSD 1.4% for Co(II), and 4.3% for Ni(II)), low detection limits of 0.08 μg l−1 for Co(II) and 0.26 μg l−1 for Ni(II) employing a deposition time of 60 s, in addition to good linearity. The non-toxic character of bismuth imparts the possibility of tailoring disposable and one-shot electrochemical sensors for decentralized environmental, clinical and industrial monitoring of trace cobalt and nickel.

The preparation of a molecular imprinted polymer to 7-hydroxycoumarin and its use as a solid-phase extraction material

A molecular imprinted polymer (MIP) was prepared to 7-hydroxycoumarin (7-OHC). A number of preparation parameters were examined by ultraviolet (UV) spectroscopy, including the amount of solvent used for reaction, equilibration time, selectivity and capacity of material. The polymer which showed the most selectivity for 7-OHC was then packed into cartridges and used as a solid-phase extraction sorbent. An extraction procedure was then developed from first principles. The cartridges were examined for selectivity of 7-OHC over some other members of the coumarin family. 7-OHC was then extracted from urine using this solid-phase extraction (SPE) method, and its concentration determined using capillary zone electrophoresis (CZE). The method was found to be linear over the range 10-50 micrograms ml-1. Inter- and Intra-assay precision studies were performed to validate the method.

Glucose oxidation at ruthenium dioxide based electrodes

The oxidation of glucose at RuO2–carbon paste composite electrodes in alkaline solution was examined. It is proposed that the catalytically active species are surface-bound oxyruthenium groups, and that the latter mediate substrate oxidation via a process of cyclic heterogeneous redox catalysis. The latter process can be described in terms of a Michaelis–Menten mechanism involving the formation of a substrate–catalyst complex which subsequently decomposes to form product and pre-catalyst; the latter can subsequently be regenerated electrochemically. Hence the oxyruthenium surface groups operate as inorganic enzyme analogues.