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Practical Problems in Voltammetry: 4. Preparation of Working Electrodes

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Practical Problems in Voltammetry: 4. Preparation of Working Electrodes Practical Problems in Voltammetry: 4. Preparation of Working Electrodes Adrian W. Bott, Ph.D. The condition of the surface of the working electrode can have a Bioanalytical Systems, Inc. West Lafayette, IN significant effect on the current response in both voltammetric and 47906-1382 amperometric experiments. The methods most commonly used for the E-mail: preparation of working electrodes (polishing, electrochemical [email protected] pretreatment, and heat pretreatment) are discussed in this article. The fundamental process in electro- with different particle sizes sus- with distilled water and diamond chemical reactions is the transfer of pended in solution (BAS supplies polishes with methanol or ethanol. electrons between the electrode sur- 0.05 µm alumina polish and 1, 3, 6, The rinsing solution should be face and molecules in the inter- and 15 µm diamond polishes). The sprayed directly onto the electrode facial region (either in solution or pad used for polishing also depends surface. After the surface has been immobilized at the electrode sur- on the material being used for pol- rinsed, electrodes polished with face). The kinetics of this heteroge- ishing—Texmet pads are used with alumina should also be sonicated in neous process can be significantly alumina polish, and nylon pads distilled water for a few minutes to affected by the microstructure and should be used with diamond pol- ensure complete removal of the alu- roughness of the electrode surface, ish. Working electrodes supplied by mina particles. If more than one the blocking of active sites on the BAS have first been lapped to pro- type of polish is used, then the elec- electrode surface by adsorbed mate- duce a flat surface, and have then trode surface should be thoroughly rials, and the nature of the func- beenextensivelypolishedtoa rinsed between the different polishes. tional groups (e.g., oxides) present smooth, mirror-like finish at the As discussed above, the effect on the surface (1, 2). Therefore, factory. Therefore, they typically of any surface pretreatment can be there has been considerable effort only require repolishing with 0.05 determined by its effect on the rate devoted to finding methods that re- µmor1µm diamond polish by the of electron transfer. This can be move adsorbed species from the user in between experiments. Mate- judged qualitatively by examining electrode and produce an electrode rials that have a rougher surface the separation of the peak potentials surface that generates reproducible (e.g., electrodes which have been in a cyclic voltammogram of a results. Some of these methods scratched) must first be polished molecule whose electron transfer have also resulted in the activation using a larger-particle polish in or- kinetics are known to be sensitive of the electrode surface (as judged der to remove the surface defects. to the state of the surface; a more by an increase in the rate of elec- After the defects have been re- quantitative determination can be tron transfer). These methods are moved, the polishing should con- made by calculating the value of ks the subject of this paper, and in- tinue with successively smaller-par- from this peak potential separation. µ clude mechanical polishing, heat ticle-size polish (e.g., 15 m, then 6 For example, ks for potassium ferri- pretreatment, and electrochemical µm, then 3 µm,andthen1µm). cyanide at glassy carbon surface pretreatment. Once polishing has been com- following a simple polishing proto- The most common method for pleted (this can require from 30 s to col was found to lie in the range surface preparation is mechanical several minutes, depending upon 0.01 - 0.001 cm s-1 (3,4) (this polishing. The protocol used for the state of the electrode), the elec- should be compared with the values polishing depends on the applica- trode surface must be rinsed thor- measured for ks for a platinum elec- tion for which the electrode is being oughly with an appropriate solvent trode, which are at least one order used and the state of the electrode to remove all traces of the polishing of magnitude larger). The strong surface. There are a variety of dif- material (since its presence can af- dependence of the electron transfer ferent materials available (e.g., dia- fect the electron transfer kinetics). kinetics of ferricyanide on the state mond, alumina, silicon carbide), Alumina polishes should be rinsed of the electrode surface means that F1 functional groups which may cata- Voltammetry at a lyze electron transfer reactions (the HO (A)OH (B) cylindrical carbon fi- composition of the functional ber electrode (1) be- HO O fore and (2) after HO NH2 O groups in this layer is sensitive to electrochemical pre- the pretreatment conditions and de- treatment: (A) 0.1 1 HO OH mM dopamine and pends on the solution pH as well as (B) 1.0 mM ascor- 2 bic acid, pH 7 solu- the potentials used for the pretreat- tions, scan rate = ment (19)). The oxide layer can also 0.1 v/s. (Adapted adsorb and/or exchange ions from from reference 11.) 20 nA 50 nA 2 1 the solution, which leads to im- iOX proved detection limits. However, electrochemical pretreatment of 0.6 0.2 -0.2 0.60.2 -0.2 electrodes increases the background E (V vs. SCE) current of the electrode relative to there can be significant variations described above is adequate (for ex- that of a polished electrode, which in the peak potential separation af- ample, when using non-aqueous may be disadvantageous for some ter each polishing. Polishing alters electrolytes, since blocking of ac- applications. the microstructure, roughness, and tive sites by adsorbed species is less Some of the specific effects functional groups of the electrode common in such electrolytes than and applications of electrochemical surface in addition to removing ad- in aqueous solutions). pretreatment can best be illustrated sorbed species (for example, it has Another method for prepara- by a number of examples. been shown that the oxygen-to- tion of the electrode surface that is carbon ratio is increased by polish- becoming more widely used is elec- Discrimination Between ing (5)). It has also been reported trochemical pretreatment (ECP), Ascorbate and Dopamine that materials used for the polishing particularly for electrodes which Using Pretreated Carbon Fiber Electrodes can affect the value of ks (1,4,6). cannot readily be polished (e.g., For example, the electrode surface carbon fiber cylinder electrodes). In vivo determination of neuro- can be contaminated by the ag- ECP consists of applying condi- glomerating agents required to keep tioning potentials to the electrode transmitters such as dopamine is the alumina particles suspended in surface before the experiment. As hindered by the ubiquitous presence solution and by the components of for polishing, this has the effect of of ascorbate, since dopamine and the polishing pad. The presence of removing adsorbed species and al- ascorbate are oxidized at similar these species can have a deleterious tering the microstructure, rough- potentials. However, it has been effect on the electron transfer kinet- ness, and functional groups of the found that pretreatment of carbon ics by blocking the active sites for electrode surface. The precise ECP fiber electrodes using a triangular the electron transfer reaction. For protocol depends upon the applica- waveform (cycling between about 0 the most exacting studies, it was tion and varies considerably. The V and +3 V at a frequency of 70 Hz suggested that the alumina suspen- potential waveforms typically are for 20 s, followed by holding at a sion be freshly made with ultrapure held at, or cycle to, a large positive constant potential of +1.5 V for 20 water and that the electrode should or negative potential, either using s) increases the rate of electron transfer for both ascorbate and be polished on glass (a ks value of steps or sweeps (constant potential 0.14 cm s-1 for ferricyanide was re- (6), potential scan (7,8), triangular dopamine (F1) and changes the ported following polishing under wave (9-15) and square wave (16, sensitivity of the electrode to these these stringent conditions (4)). 17)). Although the development of two analytes (9-12). The shift in the However, it should be noted that the preconditioning protocols has peak potentials allowed resolution such pronounced dependence on been largely empirical, the pre- of the peaks due to ascorbate and the state of the electrode surface is treated electrode surface has been dopamine measured using differen- only observed for certain systems characterizatized in order to eluci- tial pulse voltammetry. The relative (the most well characterized exam- date the reasons for the activation sensitivities for dopamine and as- ples are the reduction of ferricy- of the electrode surface (6,7,17,18). corbate for pretreated electrodes anide, the oxidation of ascorbate, For glassy carbon electrodes, in ad- were about 1000:1, which provides and the adsorption of dopamine). dition to the removal of adsorbed further discrimination against inter- For such systems, polishing is often species, the preconditioning poten- ference by ascorbate. used in combination with another tial leads to the formation of an The effect of this pretreatment pretreatment (e.g., heat or electro- oxygen-rich layer on the carbon on carbon fiber electrodes was fur- chemical). However, for many surface. This layer contains oxides ther examined by studying the be- other systems, the simple polishing as well as other oxygen-containing havior of a range of molecules at F2 500 dopamine, while maintaining a Peak current variation good response time. with increasing con- y = -40.76 + 9.338x R = 0.99 centration, (A) without electrochemical pre- 375 Analysis of Lead(II) by treatment and (B) Square Wave Voltammetry with electrochemical (b) pretreatment.
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