Current Analytical Chemistry - Volume 4, Issue 3, 2008

Electrochemical Stripping Analysis has always been the electroanalytical technique of choice for trace and ultra-trace determination of metals and organics. It is based on a preconcentration step of the target analytes on a suitable working electrode followed by a conventional voltammetric or chronopotentiometric scan. Trace metals can be determined at μg L-1 or sub-μg L-1 levels after electrolytic preconcentration while those not amenable to electrolysis can be monitored after adsorptive accumula tion of their complexes with appropriate surface-active ligands. Similarly, numerous organic compounds can be determined at race levels after spontaneous adsorptive preconcentration. Application areas include environmental analysis, clinical and biochemical analysis, food control and industrial monitoring. This volume hosts a number of authoritative reviews on the main advances in Stripping Analysis over the last decade, authored by outstanding scientists in the field of electrochemistry. One major area that has attracted much research is the development of new electrode materials. Mercury electrodes, that have dominated Stripping Analysis since the invention of polarography, are no longer desirable due to the toxicity of mercury and, therefore, new types of alternative more environment-friendly electrodes are sought. The first review by Professor R.G. Compton's group focuses on the utility and the applications of boron-doped diamond (BDD) electrodes in Stripping Analysis. BDD consists of a diamond lattice p-type doped with boron to a level that ensures good conductivity and electrochemical properties. These electrodes offer a wide useful potential window, exhibit low background currents (and therefore low limits of detection), provide good stability and reproducibility while their hard and chemically inert surface is less prone to fouling. A notable additional feature of BDD electrodes is that they can be successfully coupled to ultrasound or microwave radiation for signal enhancement (the review by Professor P. Grundler discusses in detail these techniques). Nanoparticle-modified electrodes are another alternative and are dealt with in the second review by Professor R.G. Compton's group. Nanoparticle-modified electrodes are fabricated by covering the surface of a (usually carbonaceous) substrate with randomly distributed metal (e.g. gold, silver, platinum and bismuth) nanoparticles. The electrodes are characterised by a high effective surface area, catalytic activity, enhanced mass transport properties and reduced cost and can act as disposable sensors in Stripping Analysis. Dr. A. Economou's group review the field of bismuth and bismuth-modified electrodes. Bismuth electrodes have attracted much attention since their introduction in 2000 since they have negligible toxicity and their analytical features are analogous to their mercury counterparts. In addition, the fact that metallic bismuth is solid at room temperature offers wide scope for the fabrication of bismuth-modified sensors. The paper by Professor A. Bobrowski reviews the combination of metal film electrodes with Catalytic Adsorptive Stripping Analysis. This Variant of Adsorptive Stripping Analysis exploits catalytic effects to further enhance the sensitivity, leading to the lowest limits of detection yet obtained by any electroanalytical technique. Replacing the pure liquid mercury electrodes traditionally used in Catalytic Adsorptive Stripping Analysis with solid metal (mercury, bismuth or lead) film electrodes reduces mercury waste and provides more robust sensors. Amalgam and alloy electrodes for environmental analysis is the subject of the review by Professor O. Mikkelsen and his group. Alloy electrodes are made of noble metals (mainly silver) in which a second metal (which possesses a high hydrogen overvoltage, such as mercury or bismuth) is added in small amounts. These electrodes are easy to fabricate and maintain, have a wide cathodic potential window, exhibit low toxicity and can be used for on-site monitoring of trace metals by Stripping Analysis. In their paper, Professor C.M.A. Brett's group present an overview of polymer modified electrodes in Stripping Analysis. These electrodes are prepared by modifying the surface of a solid electrode with an ion-exchange, size-exclusion or conductive polymer film. The main utility of such coatings is to improve the selectivity of the determination by preventing fouling of the electrode surface by surface-active macromeolecules. However, polymeric coatings may additionally improve the sensitivity and long-term stability of the underlying electrodes. The general trend towards miniaturization has also affected research in Stripping Analysis. Microlectrodes, nanoelectrodes and arrays of microelectrodes are comprehensively reviewed in the papers authored by the teams of Professor S. Daniele and Dr. D. Arrigan.

Boron-doped diamond (BDD) is one of a number of solid electrode materials that have been developed as alternatives to mercury electrodes for electroanalysis. Electrodes made from boron-doped diamond possess many attractive properties including a wide potential window, low background currents, high resistance to electrode fouling, ease of chemical modification, long term stability and the hard and flat nature of the electrode surface. This review covers work on a wide range of analytical applications and enhancement techniques for stripping voltammetry at boron-doped diamond electrodes including the use of power ultrasound, microwave radiation, lasers and microelectrode arrays.

In recent years significant work had been carried out on the fabrication and application of nanoparticlemodified electrodes for trace heavy metal detection with the view to producing sensitive and versatile sensors. Much has focused on the use of gold nanoparticles for the detection of arsenic and other toxins, however silver, platinum and bismuth nanoparticles have also been researched. This review covers a range of different nanoparticle fabrication and electrode modification methods for the production of electrodes which have been applied using stripping voltammetry. Particular attention is paid to their applications and success as sensors.

This article is intended as an overview of bismuth-based electrodes as applied to electrochemical stripping analysis. These electrodes rely on a bismuth or bismuth-modified active surface and, while they have analytical performance comparable to mercury electrodes, are characterized by negligible toxicity in comparison to their mercury counterparts. These properties make them ideally suited as sensors for trace monitoring by electrochemical stripping analysis. Topics covered in this review are the main types of bismuth electrodes and their fabrication, the analytical characteristics and common interferences of bismuth-based electrodes in stripping analysis and typical applications of these sensors in industrial, food, clinical and environmental analysis by stripping techniques. Finally, the future prospects of bismuth electrodes in conjunction with stripping analysis are critically discussed.

The recent tendency to replace liquid mercury by less toxic electrode materials has led many analysts to investigate and design various thin film electrodes generated ex-situ or in-situ by electroplating the selected metals at different supports, and to develop new stripping and catalytic adsorptive stripping voltammetric procedures that would use them. The catalytic adsorptive stripping voltammetry (CAdSV) coupling of very efficient adsorptive accumulation of the electroactive species on the electrode surface with the catalytic reaction provides a significant amplification of the analytical response, and, consequently, a considerable decrease of the detection limit, as well as an improvement in the selectivity of the determination. The present article presents an overview of recent progress in the research on catalytic adsorptive stripping voltammetry, with a focus on procedures in which film electrodes were utilized.

In voltammetry, the choice of working electrode is of outmost importance to fulfil needed demands and criteria. Heyrovsky's invention of the polarographic method started in 1922, for which he was granted the Nobel Prize in 1959, was a pioneer work within the field of electroanalytical chemistry. In polarography, the working electrode is normally a liquid mercury drop electrode, which possesses several advantageous properties for this purpose. Voltammetric methods have recently proved to have a great potential within off-laboratory environmental monitoring, which has created a demand for additional electrodes specifically for such purposes. This review gives a short overview of the solid silver amalgam electrode and some additional alloy electrodes, which during the last 10 years have been reported to be suitable alternative electrodes to liquid mercury, especially for environmental monitoring.

Recent progress in polymer modified electrodes for stripping analysis is surveyed, mainly concerned with ionexchange polymer and conducting polymer coatings on mercury thin film electrodes and other solid electrode substrates. Film preparation methods are discussed and applications involving different types of polymer film or polymer film composites illustrated, with emphasis on recent literature published in the last five years.

This review surveys the main aspects of the combination of microelectrodes and stripping analysis, including recent developments in working microelectrodes, analytical methodologies, instrumentation, and applications in both synthetic and real matrices. Emphasis is given to papers published since 2000.

In this article a comprehensive overview of the developments in the field of electrochemical stripping analysis with microelectrode arrays and microfabricated devices is presented. Due to the vastness of the topic, this mini-review deals only with the use of regular microelectrode arrays. After the description of the main fabrication methodologies employed, a large part of the review is dedicated to applications, categorised by the electrode material. Microelectrode arrays have found application in several areas of electroanalytical science including clinical and environmental analysis. They have been used for the detection of heavy metals in waters, soil extracts and blood, proving to be reliable analytical devices and bringing the advantages of low-cost, simplicity of use and easy adaptability to field measurement. In many applications, limits of detection are sub-parts per billion. Finally a short section of the review discusses miniaturised potentiostats.

This review describes our recent results regarding adsorptive stripping voltammetric determination of submicromolar and nanomolar concentrations of various environmentally important chemical carcinogens using both traditional (hanging mercury drop electrode, carbon paste electrode) and non-traditional types of electrodes (solid amalgam electrodes, glassy carbon paste electrodes, carbon ink film electrodes, solid composite electrodes). The review concentrates on our own results in the context of the general development in the filed.

The ability of nucleic acids (NA) and their components to accumulate at electrode surfaces and electrochemical properties of these species are closely related. This review is devoted to electrochemical stripping techniques applied in NA studies. Cathodic or anodic stripping voltammetry have been used for a highly sensitive determination of nucleobases, nucleosides, nucleotides or acid-hydrolyzed NAs, based on formation of sparingly soluble complexes of the NA constituents with electrochemically generated mercury or copper(I) ions. DNAs, RNAs and their synthetic analogues, either unmodified or labeled with electroactive markers, have been analyzed by adsorptive stripping (AdS) techniques with mercury, mercury film, amalgam and carbon-based electrodes. Strong adsorption of NAs at the electrode surfaces has been utilized in adsorptive transfer stripping (AdTS) techniques. In AdTS, a NA-modified electrode is prepared by adsorptive accumulation of the NA at the electrode surface, followed by transfer into background electrolyte not containing any NA. NA-modified electrodes can be used as simple electrochemical NA sensors. Recent applications of AdS and AdTS in NA microanalysis, in detection of DNA damage as well as in studies of DNA hybridization or DNA-protein interactions are reviewed.

Stripping analysis is a two-step procedure. In the first step, the analyte is accumulated at an electrode surface either by electrolysis or by adsorption. In the second step, the deposited material is removed, typically by electrolysis. The second step is the source of the analytical signal. Sensitivity of the method mainly depends on the amount of substance which is transferred to the electrode surface during accumulation. The methods reviewed make use of unconventional techniques to enhance efficiency of accumulation. By ultrasound, a combined action of thermal and mechanical impacts, partially with extreme energetic states, causes improvement of analytical results. Heating of electrodes or of a nearelectrode area induces laminar electrolyte flow acting as stirring, combined with increased surface temperature that is helpful to lower kinetic hindrances. Magnetic fields in a cell with arbitrarily increased deposition current enhances convection without additional thermal effects. Most of the techniques reviewed are able to improve the efficiency of stripping analysis much more than could be done by driving classical stirring up to the limits.