18b Voltammetry-electrochemical Techniques

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Transcript of 18b Voltammetry-electrochemical Techniques

Chapter 18bVoltammetry: Dynamicelectrochemical techniquesAbul Hussam18b.1 INTRODUCTIONVoltammetry is a part of the repertoire of dynamic electrochemicaltechniques for the study of redox (reductionoxidation) reactionsthrough currentvoltage relationships. Experimentally, the current re-sponse (i, the signal) is obtained by the applied voltage (E, the exci-tation) in a suitable electrochemical cell. Polarography is a special formof voltammetry where redox reactions are studied with a droppingmercury electrode (DME). Polarography was the rst dynamic electro-chemical technique developed by J. Heyrovsky in 1922. He was awardedthe Nobel Prize in Chemistry for this discovery.The characteristic shape of iE curve depends on the nature of theredox couple in the condensed phase, its thermodynamics, kinetics,mass transfer, and on the voltagetime prole (Et). In this section wewill discuss various voltammetric techniques and their applications inmodern chemistry.With the introduction of modern electronics, inexpensive computers,and new materials there is a resurgence of voltammetric techniques invarious branches of science as evident in hundreds of new publications.Now, voltammetry can be performed with a nano-electrode for the de-tection of single molecular events [1], similar electrodes can be used tomonitor the activity of neurotransmitter in a single living cell in sub-nanoliter volume electrochemical cell [2], measurement of fast electrontransfer kinetics, trace metal analysis, etc. Voltammetric sensors arenow commonplace in gas sensors (home CO sensor), biomedical sensors(blood glucose meter), and detectors for liquid chromatography. Volt-ammetric sensors appear to be an ideal candidate for miniaturizationand mass production. This is evident in the development of lab-on-chipComprehensive Analytical Chemistry 47S. Ahuja and N. Jespersen (Eds)Volume 47 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)47028-8r 2006 Elsevier B.V. All rights reserved. 661technologies with applications ranging from capillary electrophoresischips with integrated electrochemical detection [3], chemical plumetracking [4], micro-electrophoresis system for explosive analysis [5],and clinical diagnostic devices [68]. There are hundreds of chemicaland biochemical electroactive species, which are amenable to the volt-ammetric detection system. Table 18b.1 shows selected applications inthree categories encompassing analytical, environmental, and biomedi-cal sensing applications. In biosensing, the p-PAP probe for immuno-assay is used for clinical diagnostic testing [9]. The redox active DNAtags could be potentially used for genotyping with signicant clinicalpotential [10].18b.2 TOOLS OF THE TRADEAll electrochemical reactions are carried out in a suitable cell withelectrodes connected to a programmable voltage or current source. Foranalytical work the dimension of the electrodes are in the range ofmicrometers to millimeters. Almost in all electrochemical studies, onealso needs an inert supporting electrolyte to carry most of the charges.The electrodes, their properties, and the working principle of the poten-tiostat is described below.18b.2.1 Cells and electrodesAny container or a ow system with three electrodes closely placed canbe used for electrochemical studies. Some electrochemical cells areshown in Fig. 18b.1. Most electrochemical cells contain three elec-trodes. These are the working electrode (W), counter electrode (C), andthe reference electrode (R). Table 18b.2 shows the materials and prop-erties of W, R, and C.The working electrode (W) is the substrate on which the redox re-action takes place. Generally, working electrodes are made of platinum,gold, mercury, and carbon. Solid working electrodes come in two mostcommon shapesbutton as planar electrodes and wire as cylindricalelectrodes. Metal and carbon bers are also used to make dot-shapedultramicroelectrodes with few micrometers in diameter. Mercury is theclassical electrode for polarography. In polarography, a glass capillary isused to deliver the liquid mercury in drops known as the DME. This isthe only electrode where the surface of the electrode is renewed witheach new drop. Hanging mercury drop electrode (HMDE) and staticmercury drop electrode (SMDE) are also used but are less common.A. Hussam662continuedTABLE 18b.1Analytical applications of selected reactions studied by voltammetric techniquesAnalyte Redox reactions Eo(V vs. ref) Applications andcommentsProbe molecules for general cell characterization (CA and CV, SWV)Fe(CN)63Ferricyanide Fe(CN)63+e3Fe(CN)640.200 V vs. Ag/AgCl Reversible redox couple to test cellperformanceR2Cp2Fe FerroceneR H, COOHR2Cp2Fe3R2Cp2Fe++e Cp:Cyclopentadiene0.200 mV vs. Ag/AgClReversible test candidatein non-aqueous solventsand a redox mediatorToxic metals (ASV, SWASV)As H3AsO3+3H++3e3As(s)+3H2O 0.247 V vs. SHE Trace inorganic arsenicspecies in groundwaterand biological samplesPb Pb (II)+2e3Pb(s) 0.126 V vs. SHE Trace Pb (II) in water,blood, paint, etc.Cd Cd (II)+2e3Cd(s) 0.402 V vs. SHE Trace Cd (II) in water andbiological samplesHg Hg (II)+2e3Hg(s) 0.852 V vs. SHE Ultratrace Hg (II) in theenvironment (sub-ppb)Gas sensing (CA)H2 (g) 2H++2e3H2 (g) 0.0 Hydrogen gas sensorAsH3 (g) AsH3+4H2O3H3AsO4+8H++8e 0.247 V vs. SHE Inorganic arsenic in theenvironment andbiological sample throughhydride generationVoltammetry:Dynamicelectrochemicaltechniques663Table 18b.1 (continued)Analyte Redox reactions Eo(V vs. ref) Applications andcommentsCO (g) CO(g)+H2O3CO2+2H++2e 0.0 vs. Ag/AgCl Basis of CO gas sensorMolecules for bio-sensing (CA, SWV)H2NC6H4OH p-aminophenol p-PAPH2NC6H4OH3HN C6H4O+2H++2e 0.25 V vs. Ag/AgCl Probe molecule for ELISAimmunoassay sensor forantigen and antibodyGlucose Glucose+O23Gluconic acid+H2O2(needs glucose oxidase as catalyst)H2O23O2+2H++2e0.60 V vs. Ag/AgCl Blood glucose monitor.H2O2 is the redox species.O2 is often replaced by amediator molecule e.g.,FerroceneM(bpy)x2+M Fe, Co, RuM(bpy)x2+3M(bpy)x3++eX 2, 3, 5, 60.30.7 Vvs. Ag/AgClReversible molecular tagfor DNA detectionCAchronoamperometry, CVcyclic voltammetry, SWVsquare wave voltammetry, ASVanodic stripping voltammetry, SHEstandardhydrogen electrode.A.Hussam664Owing to the toxicity of mercury and its disposal problem, solid elec-trodes are now very popular. In particular, electrodes made of carbonsuch as glassy carbon, graphite, carbon paste, and carbon bers havegained popularity. Mercury, gold, bismuth, and other metals can bedeposited as thin metal lms on carbon and serves as thin metal lmelectrodes (TMFE) with excellent analytical advantages in trace metalanalysis. The choice of working electrode is determined by the redox(e)(a) (c)(b)(d)Fig. 18b.1. Electrochemical cells and representative cell congurations.(a) Schematic diagram of a cellpotentiostat system. (b) Typical laboratorycell with Hg-drop electrode and drop knocker. (c) Voltammetric cell as detectorat the end of a high-performance liquid chromatographic column. (d) A two-electrode (graphite) chip cell for biosensor development. (e) Three-electrodechip cells on a ceramic substrate for bioanalytical work.Voltammetry: Dynamic electrochemical techniques665TABLE 18b.2Electrode materials and propertiesType of electrode Material Properties CommentsW: Working orindicator electrodePtPlatinum in 1 M sulfuric acid 0.251.2 V Ultramicroelectrodes aremade of metal or carbonbersPtPlatinum in 1 M NaOH 1.00.6 VHgMercury in 1 M sulfuric acid 1.20.50 VHgMercury in 1 M KCl or NaOH 2.00.20 VCCarbon in 1 M HclO4 0.21.5 VCCarbon in 0.1 M KCl 1.51.0 V(Glassy carbon, carbon paste, graphite,diamond lm).(shows effectivepotential window)R: ReferenceelectrodeAg/AgCl, satd. KCl (SSC) 0.197 V vs. NHE At 25 1C. NHEnormalhydrogen electrode Hg (l)/Hg2Cl2 (s), satd. KCl (SCE) 0.241 V vs. NHEAg wire (Pseudo reference) 0.200 V vs. NHEC: Counter orauxiliary electrodePlatinum, carbon or steel Similar to theworking electrodeexcept for steelMust be larger than WelectrodeA.Hussam666potential of the analyte and the potential window within which thesolvent and the supporting electrolyte remains electrochemically inert.Table 18b.2 shows the effective potential window for most commonworking electrodes.Most common reference electrodes are silversilver chloride (SSC),and saturated calomel electrode (SSC, which contains mercury). Thereference electrode should be placed near the working electrode so thatthe W-potential is accurately referred to the reference electrode. Thesereference electrodes contain concentrated NaCl or KCl solution as theinner electrolyte to maintain a constant composition. Errors in electrodepotentials are due to the loss of electrolytes or the plugging of the porousjunction at the tip of the reference electrode. Most problems in practicalvoltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene car-bonate, etc., the half-cell, Ag (s)/AgClO4 (0.1M) in solvent//, is used.There are si