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Voltammetry: Dynamic Electrochemical Techniques

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Voltammetry: Dynamic Electrochemical Techniques Chapter 18b Voltammetry: Dynamic electrochemical techniques Abul Hussam 18b.1 INTRODUCTION Voltammetry is a part of the repertoire of dynamic electrochemical techniques for the study of redox (reduction–oxidation) reactions through current–voltage 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 form of voltammetry where redox reactions are studied with a dropping mercury electrode (DME). Polarography was the first dynamic electro- chemical technique developed by J. Heyrovsky in 1922. He was awarded the Nobel Prize in Chemistry for this discovery. The characteristic shape of i–E curve depends on the nature of the redox couple in the condensed phase, its thermodynamics, kinetics, mass transfer, and on the voltage–time profile (EÀt). In this section we will discuss various voltammetric techniques and their applications in modern chemistry. With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various 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 to monitor the activity of neurotransmitter in a single living cell in sub- nanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now 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 miniaturization and mass production. This is evident in the development of lab-on-chip Comprehensive Analytical Chemistry 47 S. Ahuja and N. Jespersen (Eds) Volume 47 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)47028-8 r 2006 Elsevier B.V. All rights reserved. 661 A. Hussam technologies with applications ranging from capillary electrophoresis chips with integrated electrochemical detection [3], chemical plume tracking [4], micro-electrophoresis system for explosive analysis [5], and clinical diagnostic devices [6–8]. There are hundreds of chemical and biochemical electroactive species, which are amenable to the volt- ammetric detection system. Table 18b.1 shows selected applications in three 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 DNA tags could be potentially used for genotyping with significant clinical potential [10]. 18b.2 TOOLS OF THE TRADE All electrochemical reactions are carried out in a suitable cell with electrodes connected to a programmable voltage or current source. For analytical work the dimension of the electrodes are in the range of micrometers to millimeters. Almost in all electrochemical studies, one also 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 electrodes Any container or a flow system with three electrodes closely placed can be used for electrochemical studies. Some electrochemical cells are shown in Fig. 18b.1. Most electrochemical cells contain three elec- trodes. These are the working electrode (W), counter electrode (C), and the 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 most common shapes—button as planar electrodes and wire as cylindrical electrodes. Metal and carbon fibers are also used to make dot-shaped ultramicroelectrodes with few micrometers in diameter. Mercury is the classical electrode for polarography. In polarography, a glass capillary is used to deliver the liquid mercury in drops known as the DME. This is the only electrode where the surface of the electrode is renewed with each new drop. Hanging mercury drop electrode (HMDE) and static mercury drop electrode (SMDE) are also used but are less common. 662 TABLE 18b.1 Analytical applications of selected reactions studied by voltammetric techniques Analyte Redox reactions Eo (V vs. ref) Applications and comments Voltammetry: Dynamic electrochemical techniques Probe molecules for general cell characterization (CA and CV, SWV) 3À 3À À 4À Fe(CN)6 Ferricyanide Fe(CN)6 +e 3Fe(CN)6 0.200 V vs. Ag/AgCl Reversible redox couple to test cell performance + À R2Cp2Fe Ferrocene R2Cp2Fe3R2Cp2Fe +e Cp: 0.200 mV vs. Ag/ Reversible test candidate R ¼H, ÀCOOH Cyclopentadiene AgCl in non-aqueous solvents and a redox mediator Toxic metals (ASV, SWASV) + À As H3AsO3+3H +3e 3As(s)+3H2O 0.247 V vs. SHE Trace inorganic arsenic species in groundwater and biological samples Pb Pb (II)+2eÀ3Pb(s) À0.126 V vs. SHE Trace Pb (II) in water, blood, paint, etc. Cd Cd (II)+2eÀ3Cd(s) À0.402 V vs. SHE Trace Cd (II) in water and biological samples Hg Hg (II)+2eÀ3Hg(s) 0.852 V vs. SHE Ultratrace Hg (II) in the environment (sub-ppb) Gas sensing (CA) + À H2 (g) 2H +2e 3H2 (g) 0.0 Hydrogen gas sensor + À AsH3 (g) AsH3+4H2O3H3AsO4+8H +8e 0.247 V vs. SHE Inorganic arsenic in the environment and biological sample through 663 hydride generation continued 664 Table 18b.1 (continued ) Analyte Redox reactions Eo (V vs. ref) Applications and comments + À CO (g) CO(g)+H2O3CO2+2H +2e 0.0 vs. Ag/AgCl Basis of CO gas sensor Molecules for bio-sensing (CA, SWV) + À H2NC6H4OH p- H2NC6H4OH3HN ¼ C6H4O+2H +2e 0.25 V vs. Ag/AgCl Probe molecule for ELISA aminophenol p-PAP immunoassay sensor for antigen and antibody Glucose Glucose+O23Gluconic acid+H2O2 0.60 V vs. Ag/AgCl Blood glucose monitor. (needs glucose oxidase as catalyst) H2O2 is the redox species. 3 + À H2O2 O2+2H +2e O2 is often replaced by a A. Hussam mediator molecule e.g., Ferrocene 2+ 2+ 3+ À M(bpy)x M(bpy)x 3M(bpy)x +e 0.3–0.7 V Reversible molecular tag M ¼ Fe, Co, Ru X ¼ 2, 3, 5, 6 vs. Ag/AgCl for DNA detection CA—chronoamperometry, CV—cyclic voltammetry, SWV—square wave voltammetry, ASV—anodic stripping voltammetry, SHE—standard hydrogen electrode. Voltammetry: Dynamic electrochemical techniques (a) (b) (c) (d) (e) Fig. 18b.1. Electrochemical cells and representative cell configurations. (a) Schematic diagram of a cell–potentiostat system. (b) Typical laboratory cell with Hg-drop electrode and drop knocker. (c) Voltammetric cell as detector at the end of a high-performance liquid chromatographic column. (d) A two- electrode (graphite) chip cell for biosensor development. (e) Three-electrode chip cells on a ceramic substrate for bioanalytical work. Owing to the toxicity of mercury and its disposal problem, solid elec- trodes are now very popular. In particular, electrodes made of carbon such as glassy carbon, graphite, carbon paste, and carbon fibers have gained popularity. Mercury, gold, bismuth, and other metals can be deposited as thin metal films on carbon and serves as thin metal film electrodes (TMFE) with excellent analytical advantages in trace metal analysis. The choice of working electrode is determined by the redox 665 666 TABLE 18b.2 Electrode materials and properties Type of electrode Material Properties Comments W: Working or Pt—Platinum in 1 M sulfuric acid À0.25–1.2 V Ultramicroelectrodes are indicator electrode Pt—Platinum in 1 M NaOH À1.0–0.6 V made of metal or carbon Hg—Mercury in 1 M sulfuric acid À1.2–0.50 V fibers Hg—Mercury in 1 M KCl or NaOH À2.0–0.20 V C—Carbon in 1 M HclO4 À0.2–1.5 V C—Carbon in 0.1 M KCl À1.5–1.0 V (Glassy carbon, carbon paste, graphite, (shows effective diamond film). potential window) A. Hussam R: Reference Ag/AgCl, satd. KCl (SSC) 0.197 V vs. NHE At 25 1C. NHE—normal electrode Hg (l)/Hg2Cl2 (s), satd. KCl (SCE) 0.241 V vs. NHE hydrogen electrode Ag wire (Pseudo reference) 0.200 V vs. NHE C: Counter or Platinum, carbon or steel Similar to the Must be larger than W auxiliary electrode working electrode electrode except for steel Voltammetry: Dynamic electrochemical techniques potential of the analyte and the potential window within which the solvent and the supporting electrolyte remains electrochemically inert. Table 18b.2 shows the effective potential window for most common working electrodes. Most common reference electrodes are silver–silver chloride (SSC), and saturated calomel electrode (SSC, which contains mercury). The reference electrode should be placed near the working electrode so that the W-potential is accurately referred to the reference electrode. These reference electrodes contain concentrated NaCl or KCl solution as the inner electrolyte to maintain a constant composition. Errors in electrode potentials are due to the loss of electrolytes or the plugging of the porous junction at the tip of the reference electrode. Most problems in practical voltammetry 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 (0.1 M) in solvent//, is used. There are situations where a conventional reference electrode is not usable, then a silver wire can be used as a pseudo-reference electrode. The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelec- trode where the maximum current demand is of the order of few micro- amperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode.
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