Electroanalytical Technique (Voltammetric techniques) critical - Literature review Example

? Electroanalytical Technique (Voltammetric techniques Critical Review 22/04 Introduction Electrochemistry entails association ofelectrical properties and chemical substances in a reaction. Electrical properties commonly measured include conductance, resistance, current and voltage. In voltammetry, various electrochemical techniques congregate such as linear sweep voltammetry (LSV), pulse techniques, among others. Voltammetry analysis samples by measuring current as a function of applied electrical potential. When compared to methods such as chromatography which uses relatively large amounts of sample with low sensitivity, voltammetry techniques will require few amounts of sample while affording high sensitivity. In addition, wide range of concentration can be used in voltammetry. Voltammetric techniques reduces ions at the electrodes which are usually three the working electrode, auxiliary electrode and reference electrode. Given such wide capabilities, voltametric techniques have been amenable to use in biochemical analyses, environmental studies, plating analysis, pharmaceuticals as well as waste water analyses among other useful applications. Instrumentation In voltammetry basic instruments include the electrochemical cell composed of a three electrodes configuration; a voltamperomeric analyser formed by the potentiostat coupled to a computer system. Working electrode provides the response of the analyte response due to the applied potential. Examples of electrode include mercury electrodes such as Hanging drop mercury electrode (HDME), static drop mercury electrode (DME) and rotating disk electrodes (RDE). Auxiliary electrode is tasked with completing the circuit between the potentiostat and working electrode and examples include platinum and glassy carbon electrodes. Reference potential to the auxiliary and working electrodes is provided by the reference electrodes e.g. Ag/AgCl in KCl and Hg/HgCl in KCl(l). Drugs analysis applies various separation-based techniques such as chromatography and electrophoresis. These techniques are important when following-up metabolism of the drug in complex samples such as urine. However, electroanalytical methods such as voltammetry become vital when samples being analysed contain a single physiologically active component such as the active ingredient in a particular drug. Electroanalytical methods offer advantages such as simple handling of the procedures, high sensitivity, better accuracy and faster speed relative to separation techniques – chromatography and electrophoresis. They also utilise cheaper instrumentation and low cost chemicals. Two electroanalytical techniques exist with the first one being where the surfaces of electrodes are continuously renewed for instance in the dropping mercury electrode. Techniques employing this type of electrode are called polarography and they mainly differ in the approach of application of voltage to the dropping mercury electrode (DME). The second class of techniques belong to techniques which use electrodes whose surfaces are not renewed. Common electrodes in this group include the hanging mercury drop electrode (HMDE). Techniques that record current-voltage curves with these electrodes are referred to voltammetry. Voltammetric Techniques Voltammetric methods have been used in the determination of trace levels of platinum group metals which have traditional employed spectroscopic methods in their determination (Locateli, 2007). Such metals include Osmium, Ruthenium and Iridium. One of the approaches used in determination of these metals is the differential pulse voltammetry (DPV). The electrodic processes associated with platinum group metals (PGMs); however, present challenges such as the low reversibility degree thereby limiting detection of these trace metals (Kritsotakis et al., 1985). Presented with such challenge of detection limits, some methods have been worked out to concentrate these trace metals onto the working electrode. One technique which has been developed is the adsorptive stripping voltammetry (AdSV) which involves formation, adsorptive accumulation and reduction of a surface-active complex of a metal (Locatelli, 2007). Voltammetric response of this complex is associated to its surface concentration which is described by an isotherm function. This function provides a linkage/relationship between the surface and adsorbate bulk concentration (Bard and Faulkner, 1980; Bond, 1980). However AdSV suffers from interferences attributed to competitive adsorption by surface-active organic compounds and in the case of PGMs, they are surfacants, fulvic acids and humic acids. Electrochemical methods have also gained use in drug discovery where they have been used to assess the mechanisms of action. This capability of electrochemical techniques is based on the fact that most physiological processes in the body depend on oxidoreduction reactions. One such application is in the antihypertensive oral drug, Captoril (CAP). In the determination of CAP amounts in samples, a suitable homogeneous mediator such as chlorpromazine (CPZ) is used in the electrooxidation of CAP (Bahramipur and Jalali, 2011). Then, electrochemical behaviour of CPZ is studied by cyclic voltammetry. Cyclic voltammetry closely resembles linear sweep voltammetry (LSV) where in LSV a fixed potential charge is employed such as in potential measurements with voltage being scanned from a lower to an upper limit. However in cyclic voltammetry, voltage swept between two values is at a fixed rate. Result of the electrocatalytic oxidation of CAP in presence of CPZ demonstrates that CAP is not electroactive. The evidence of the electrocatalytic oxidation mediated by CPZ is substantially evident since very little amounts of CAP are found at the anodic current against a substantial increase in CPZ. Thus, though electrochemical methods may have setbacks such as low sensitivity and reproducibility, chemical modifications with redox-active methods may provide, in the coming future, a method of designing and developing electrochemical sensors (Bahramipur and Jalali, 2011). Recent trends have made it possible to use direct potentiometry to the detection of nanomolar concentration levels. A common technique used in voltammetry for trace substances measurements has been the anodic stripping voltammetry (ASV). In ASV target metal ions are electrodeposited onto the working electrode. However, recent advances in ion-selective electrodes (ISEs) have even made direct potentiometry even better. Due to the low detection limits, ISEs use in direct potentiometry has made this technique the method of choice in the detection of trace metals thereby overtaking methods such as stripping voltammetry (Chumbimuni-Torres et al., 2011). A study set to compare stripping voltammetry and direct potentiometry demonstrated that both techniques have similar sensitivity but ISEs (potentiometric measurements) may offer better selectivity in the case where there are interfering metals (Chumbimuni-Torres et al., 2011). In this experiment bismuth-coated glassy carbon electrodes are used in voltammetry whereas in potentiometric measurements the recently developed solid-contact cadmium and lead ion selective electrodes (Cd2+ -ISE and Pb2+ -ISE) are used. Albeit the two methods have similar sensitivity, potentiometry possesses a high selective capability in the presence of excess metal ions which may interfere with stripping voltammetry. It also addresses specific analytical problems for instance in measurement of copper, a Cu ISE could be used against antimony and bismuth ions which usually affect anodic stripping voltametric measurements. Voltammetric techniques have also been in use in the pharmaceutical industries. Techniques applied here include the non-stripping voltammetric techniques such as cyclic voltammetry, differential pulse voltammetry (DPV), square-wave voltammetry (SWV) and LSV. Cyclic voltammetry has the capacity of rapidly providing the observation for redox behaviour over a wide potential range. The DPV use is significant in the determination of trace amount of an electroactive compounds in pharmaceutical compounds (Jain et al., 2010). It presents advantages such as its easy applicability and the availability of low-cost instrumentation. Square wave voltammetry (SWV) is a large amplitude differential technique where the waveform is composed of symmetrical square waves. Speed is guaranteed in SWV as well as an excellent sensitivity achieved from the fact that the net current is large in comparison to either forward or backward current. In addition, it has a high discrimination against charging current (Jain et al., 2011). Voltammetric techniques have been applied in the analysis of polyphenols. These polyphenols possess antioxidant properties which enable them to donate electrons and this ability has been used for their detection. In wine two voltammetric techniques have been applied in the detection of polyphenols – differential pulse voltammetry (DPV) and cyclic voltammetry (CV). Apart from their use in detection of trace metals in the environment, they have seen increased use also in environment in the detection of organophosphates which are remnants of pesticides. Voltammetric methods have been amenable to use in detection of neurotransmitters in vivo which help monitor neurochemical changes in the intact central nervous system (CNS). In vivo voltammetry (IVV) with implanted amperometric electrodes is used in the in situ detection of substances such as neurotransmitter, dopamine, in the brain extracellular fluid (Mulla et al., 2009). Among the voltametric techniques in use, stripping voltammetry has proven to be a versatile and powerful analytical tool and as such has been applied in food and nutrition analysis. This technique has a preconcentration step which is achieved by oxidation of the analyte as an insoluble film on the electrode. The concentrated analyte is then reduced and measured during the negative scan. However since not all analytes may be concentrated by electrolysis a variant of this technique is the adsorptive stripping voltammetry (AdSV) where accumulation the analyte is via adsorption rather than electrolysis. Though stripping voltammetry used mercury electrode their continued use has come to a halt due to their environment concerns and in their place more environmentally friendly electrodes such as carbon paste, polymer film electrode among other environmentally friendly electrodes have come into use. The accuracy and precision of stripping voltammetry coupled to low cost instrumentation has seen this method being increasingly applied in food analysis. Such areas include determination of toxic metals, pesticides, veterinary residues and fertilizers residuals in food (Alghamdi, 2009). Stripping voltammetry may be amenable to determination of vitamins such as ascorbic acid via the use of glassy carbon electrode. Thus, stripping voltammetric techniques may become candidate analytical tools to compete with traditional analytical methods such as chromatography and spectroscopic in food sample analysis. Future trends There as been an increased use of voltammetry in detection of pesticides and future trends are directed towards development of some innovative technique which will provide disposable tools with high sensitivity and accuracy based on voltammetric techniques. One such development is the miniaturization of the working electrode to at least a one dimension not greater than 25 µm (Wang, 2006). This small-sized electrode will have will be amenable to the study of reactions and electrodic processes. Moreover, the availability of these microelectrodes may be used in assessing the local concentration profiles usually in vivo of samples such as pesticides/trace metals accumulation in organism. Therefore, current research effort is geared towards development of voltammetric sensors which may be designed to curb environmental pollution and detection of pollutants in the food supply. One major advantage of electrochemical biosensors is that they may not require sophisticated and heavy equipment in signal processing (Pijanowska and Torbicz, 2005) and in addition they are easy to calibrate thereby providing cost-effective and portable analytical tools for real time result taking. In conclusion, voltammetry provides a wide range of applications due to the assortment of electrodes which allows for specific analysis of a wide range of different chemicals. The detection limits of this technique spans from parts per trillion to part per million. In addition, electroanalytical techniques are gaining ground currently in their use due to their ease of automation and miniaturization that affords them the capacity for portable instrumentation. References Alghamdi A.H., 2010. Applications of stripping voltammetric techniques in food analysis. Arabian Journal of Chemistry 3, 1-7 Bahramipur H. and Jalali F., 2011. Voltametric determination of Captoril using Chlorpromazine as a homogenoes mediator. International journal of electrochemistry, 2011 (864358). Bard A.J. and Faulkner L.R., 1980. Electrochemical Methods - Fundamentals and Applications, Wiley, New York Bond A.M., 1980. Modern Polarographic Methods in Analytical Chemistry, Marcel Dekker Inc., New York Chumbimuni-Torres K., Calvo-Marzal P. and Wang J., 2009. Comparison between Potentiometric and Stripping Voltammetric Detection of Trace Metals: Measurements of Cadmium and Lead in the Presence of Thalium, Indium, and Tin. Electroanalysis, 21 (17-18):1939-43. Jain R., Gupta V.K., Jadon N., Radhapyari K., 2010. Voltammetric determination of cefixime in pharmaceuticals and biological fluids, Anal. Biochem. 407:79-88 Kritsotakis K., Tobschall H.J., Fresenius Z., 1985. Anal. Chem., 320: 156 Locatelli C., 2007. Review: Voltammetric analysis of trace levels of platinum group metals – principles and applications Electroanalysis 19: (21): 2167-2175 Mulla I., Lowry J.P., Serra P.A. et al., 2009. Development of a voltammetric technique for monitoring brain dopamine metabolism: compensation for interference caused by DOPAC electrogenerated during homovanillic acid detection. Analyst, 134: 893-898. Pijanowska D. G. and Torbicz, W., 2005. Biosensing for analytical applications. Bulletin of the Polish Academy of Sciences, 53 (3): 251-260. Wang J., 2006. Controlled-potential techniques, In: Analytical electrochemistry (3rd Edition), John Wiley & Sons, 67-114, Wiley-VCH, ISBN 10 0-471-67879-1, New Jersey. Read More