ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2015, том 70, № 4, с. 339-358
= ОБЗОРЫ
УДК 543
VOLTAMMETRIC TECHNIQUES AT CHEMICALLY MODIFIED ELECTRODES © 2015 Rakesh R. Chillawar, Kiran Kumar Tadi, Ramani V. Motghare1
Department of Chemistry, Visvesvaraya National Institute of Technology Nagpur 440010 (M.S.), India 1E-mail: rkkawadkar@chm.vnit.ac.in Received 08.05.2013; in final form 30.09.2014
Voltammetric and amperometric techniques are powerful analytical tools that are widely used in chemical analysis. This article reviews a summary of some important types of modifying agents and their application in voltammetric and amperometric sensors for clinically and biologically important target species reported from the period 2003 to 2013. In this review, different modifiers such as molecularly imprinted polymers (MIPs), polymers, metal complexes, nanomaterials and composite films are discussed. Under the heading of each modifier, method of fabrication, properties, and applications of chemically modified electrodes (CMEs) are given. Tables that give analyte, modified electrode, measurement technique, measuring medium, linear detection range (LDR) and limit of detection (LOD) referenced from original work are also provided.
Keywords: voltammetry, amperometry, chemically modified electrodes, voltammetric sensor.
DOI: 10.7868/S0044450215040180
Electroanalytical chemistry is a branch of chemical analysis in which electroanalytical techniques are employed to obtain information about chemical species with respect to their quantities, properties and environment. According to Issac Maurits Kolthoff, electroanalytical chemistry is the application of electrochemistry to analytical chemistry. It is appropriate to consider electroanalytical chemistry as an area of chemical analysis in which the electrode is involved as a probe to determine various chemical species directly or indirectly [1].
Nowadays, the demand for improved analytical techniques is increasing, especially for pharmaceutical drugs and their metabolites, environment pollutants, food additives, etc which widely affect human health. So the development of simple, reliable, rapid, sensitive and accurate analytical methods is essential. Elec-troanalytical chemistry provides such electrochemical methods like voltammetric techniques. These electrochemical methods are characterized by simplicity, low cost instrument, high sensitivity, good stability, environmental friendliness and on-site monitoring [2].
Voltammetry was discovered by Czech scientist Jaroslav Heyrovsky in the 1920s. It is a form of electrochemistry, based on the measurement of current flowing through the working electrode immersed in a solution containing electro-active sample to be analyzed by varying potential in some systematic manner. Hey-rovsky also discovered polarography for which he was awarded the Nobel Prize in chemistry in 1959 [3]. Vol-
tammetric techniques are now becoming increasingly important because of its simple, cost-effective and quick way of determining biologically and environmentally important species. In the last few decades, voltammetric techniques have become a promising analytical tool in the study of electrochemical reactions [4], model studies of enzymatic catalysis [5] and free radicals [6], co-ordination chemistry [7], environmental monitoring [8], industrial quality control [9], solar energy conversion [10] and determination of trace concentrations of biologically and clinically important species [11, 12].
Voltammetric techniques show excellent selectivity and sensitivity because the analyte can be readily identified by its voltammetric peak potential. However, the major problem encountered in these techniques is the interference of concomitant compounds. Owing to their very similar oxidation peak potentials, overlapped voltammetric responses are obtained. To overcome this problem, electrodes modified using various materials and techniques are employed [13, 14]. The voltammetric techniques involve reactions at different electrodes such as glassy carbon electrode (GCE), pencil graphite electrode (PGE), carbon paste electrode (CPE), gold and platinum electrodes, etc. However, application of these electrodes is limited due to fouling of the electrode surface by several species, thereby decreasing sensitivity and accuracy of the methods. The performance characteristics of these electrodes can be increased by using CMEs.
^^ Functional monomers Cross-linker ^^^ Self-assembly _^
Template Copolymerization
(analyte)
Pre-polymerization complex
Extraction Rebinding
*
T
Moleculary imprinted polymer Fig. 1. General principle involved in molecular imprinting.
CMEs have attracted considerable interest in the study of the electro-catalytic reactions of various ana-lytes such as pharmaceuticals, explosives, food contaminants, metal ions, etc. According to IUPAC, a CME is defined as "an electrode made up of conducting or semiconducting material and coated with a film of chemical modifier, shows chemical, electrochemical or optical properties of a film by means of faradaic reactions or interfacial potential differences." CMEs offer various advantages over unmodified electrodes: they catalyze the oxidation or reduction of species that shows high overvoltages at unmodified electrodes, i.e. they lower the peak potentials. They also enhance sensitivity and improve selectivity in the application of pharmaceutical analysis [15—18].
During past decades, a wide electrochemical research has been devoted to the development and application of different types of CMEs. Various materials such as carbon nanotubes (CNTs), gold nanoparticles (AuNPs), platinum nanoparticles (PtNPs), MlPs, molecularly imprinted conducting polymers, L-cys-teine (L-cys), Prussian blue, graphene (GR), metal complexes, zeolites, etc. were used for the modification of electrode surface. Many reviews on the development of modified electrodes and their applications in different fields of analysis have been reported. Blan-co-Lopez et al. [19] reviewed the electrochemical sensors based on non-covalent MIPs, giving details of different recognition elements, electrochemical trans-duction and strategies for integration. A review on voltammetric techniques and their various applications to pharmaceutical analysis were reported by Vin-od K. Gupta et al. [20]. Eder Tadeu et al. [21] reviewed recent developments in the bioelectroanalysis of pharmaceuticals with respect to carbon-based materials, new potentiometric sensors and electrochemical biosensors.
The objective of this review is to present a summary of some important types of modifying agents and their application in voltammetric and amperometric sensors for clinically and biologically important target species reported from the period 2003 to 2013. In this review, different modifiers such as MIPs, polymers, metal complexes, nanomaterials and composite films are discussed. Under the heading of each modifier, method of fabrication, properties, and applications of CMEs are given along with the table containing ana-lytes, modified electrodes, measuring techniques, measuring media, LDR and LOD. It is our hope that this will help researchers to know the merits and demerits of CMEs and facilitate them to plan their own strategies to develop selective, sensitive, reproducible, anti-fouling and cost-effective CMEs for voltammet-ric detection of various types of clinically and biologically important target species.
MOLECULARLY IMPRINTED POLYMERS
During the last decade, MIPs have become important recognition elements in the electrochemical sensing. These are synthetic polymers capable of mimicking natural systems like enzymes, antibodies, etc. In MIP synthesis, the template and functional monomers are mixed to allow self assembly between them. This is then followed by thermal or photo-polymerization in the presence of a cross linker. After completion of polymerization, the template is extracted from the polymeric matrix, leaving cavities which are exactly complementary in the size, shape and functionality to the original template molecule. Figure 1 shows the general principle involved in molecular imprinting. MIPs are lower price materials than antibodies with higher thermal and chemical stability. Due to their stability, MIP modified sensors are easy to store and operate and have a long shelf life [22—24]. Various polymeric systems used in sensor fabrication are acrylic or
vinylic type of polymers, electro polymerized films, self-assembled monolayers (SAMs), sol—gel polymers, pre-formed polymers, etc. [19].
When the electrode is modified with non-conductive polymers such as the highly cross-linked MlPs, detection of an analyte molecule in a polymeric layer is possible if it contains an important concentration of well-connected recognition sites through a network of pores of appropriate size and density close enough to the electrode surface. The measurement is carried out in a medium which favours the release of the analyte. If the thickness of polymeric layer is excessive, strong diffusion impediments would occur due to recognition sites being far away from the electrode surface. Therefore, for successful MIP modified sensor, porosity of the polymer should be controlled. This can be achieved by means of nature and volume of porogenic solvent used in the monomer mixture [25].
Acrylic MIP modified GCE was suggested by Blanco-Lopez et al. for the voltammetric determination ofvanillylmandelic acid (VMA). A 1 : 8 : 40 molar ratio of the template molecule (VMA), functional monomer methacrylic acid (MAA) and cross-linking agent divinylbenzene or ethylene glycol dimethacry-late (DVB or EGDMA) along with porogenic solvent acetonitrile (ACN) and initiator 2,2-dimethoxy-2-phenylacetophenone (DPP) were taken to prepare prepolymerization mixture. 10 ^L of the mixture were dropped onto the electrode surface. Thin layers of MIPs were formed by spin coating (1000 rpm) followed by in-situ photopolymer
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