Anal. Chem. 1986, 58, 1787-1790
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Enhanced Stability of Glassy Carbon Detectors following a Simple Electrochemical Pretreatment Joseph Wang* and Peng T u z h i Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
Glassy carbon amperometrlc flow detectors are shown to exhlblt a substantial Improvement In their stablllty followlng a slmple preanodlzatlon procedure. The effects of the preanodizatlon potentlal and duration and of other variables are discussed. Optimal preanodlzatbn conditions are 1.50 V vs. Ag/AgCI for 30 mln. The stabllhy enhancement Is attrlbuted to the formation of an oxlde layer on the surface. Preventlon of electrode passlvatlon Is observed for repetitive Injections of dlhydronlcotlnamlde adenlne dlnucleotlde (NADH), chlorpromazine, dlnltrophenol, albumln, and serum solutlons. Some deactlvatlon Is observed wlth phenol, chlorophenol, and deslpramlne solutions. The precondltlonlng effect Is long-llved, as lndkated from 360 repettllve 1n)ectlons of a 1 mM NADH solution. For certaln analytes, e.g., NADH, a concomltanl catalytk effect Is observed. The pretreatment procedure Is shnple and carried out wMle the electrode Is set up as usual for amperometrlc detection. The data also provlde new lnslghts Into surface processes assoclated with electrochemlcal pretreatment procedures.
Amperometric detection has been proved extremely useful for monitoring flowing streams such as in liquid chromatography and flow injection systems. Glassy carbon is the most widely used electrode material in such a detection scheme. Unfortunately, glassy carbon (as well as other solid electrodes) shows a decrease in the activity of the surface when certain analytes or samples are employed. This is primarily due to adsorption of electroinactive surface-active organic materials or of the analyte (or the product of its redox reaction). A gradual decrease in the response characterizes these deactivation processes. Such behavior is commonly referred to as "fouling" or "poisoning". The detector then has to be dismounted, the surface is cleaned, and after reassembling a long time period is required to allow the decay of transient currents. A suitable potential pulse train can also be used for restoring the response after deactivation (I). An elimination (or minimization) of the electrode deactivation problem is preferred over such reconditioning procedures. Various approaches have been suggested recently to maintain the electrochemical activity of glassy carbon electrodes. These include electrode coverage with a nonelectroactive polymeric film (2,3) or the use of a pulsed laser light (4). In this paper, we will demonstrate how an extremely simple electrochemical treatment can be employed to substantially enhance the stability of glassy carbon detectors. A variety of electrochemical pretreatment procedures have been proposed recently to improve the electrochemical reversibility at glassy carbon electrodes (5-8). Similar improvements in the reversibility can be achieved also by heat treatments of glassy carbon electrodes (9,lO). Electrochemical pretreatment procedures are based on stepping (5-7) or scanning (8) the potential between positive and negative values at different frequencies or rates, over different time periods. Such pretreatment procedures have been applied at other carbon electrodes, such as graphite-epoxy (II), carbon fiber (12,13), 0003-2700/86/035&1787$01.50/0
or carbon paste (12). The enhanced electrochemical reversibility associated with these procedures resulted in improvement in the analytical performance, including selectivity and sensitivity enhancements of electrochemicaldetection for liquid chromatography (7, 14, 15), enhanced response of differential pulse voltammetry (71, as well as voltammetric differentiation between two analytes with similar redox potentials (11). However, the utility of electrochemical pretreatment procedures for improving the stability of electroanalytical measurements has not been demonstrated. Such improvements in the amperometric response of glassy carbon detectors are described in the following sections. EXPERIMENTAL SECTION Apparatus. The flow injection system has been described previously (3). A 20pL sample loop and a glassy carbon thin-layer detector (Model TL-5, Bioanalytical,Systems) were used. All potentials are reported vs. a Ag/AgCl reference electrode (Model RE-1,Bioanalytical Systems). An EG&G PAR Model 174 polarographic analyzer was used in conjunction with a Houston Omniscribe chart recorder. Scanning potential measurements were performed with Model VC-2 voltammetric cell and Model MF2020 glassy carbon disk electrodes (Bioanalytical Systems). Reagents. All solutions were prepared with double-distilled water. Sample solutions of dihydronicotinamide adenine dinucleotide (NADH), chlorpromazine,albumin (bovine),desipramine, dinitrophenol (Sigma Chemical Co.), chlorophenol (Aldrich), and phenol (Fisher) were made up fresh each day by dissolving the compound in the supporting electrolyte solution. Abnormal control serum (Ortho Diagnostics Systems, Product Code 9691)was prepared according to the manufacturer's reconstitution procedure and diluted (1+ 3) with the supporting electrolyte solution. The latter was a 0.05 M phosphate buffer (pH 7.4)solution. Procedure. Before each experiment the ghsy carbon electrode was hand-polished for 1 min with a 0.05-pm alumina slurry. The electrode was then rinsedwith double-distilled water and sonicated in a water bath for 2 min to remove residual polishing material from the surface. Experiments were usually performed on a freshly polished electrode and then again after the electrode was pretreated. The electrode was polished again prior to the pretreatment. In the pretreatment, the working electrode was held at +1.50 V for 30 min while the phosphate buffer blank solution was flowing at 1.0mL/min. Following pretreatment,the working potential was applied and transient currents were allowed to decay. Samples were injected at a rate of 120 per hour. The solutions for voltammetric experiments were purged with nitrogen. RESULTS AND DISCUSSION Analytical Performance. In an effort to develop an in situ cleaning-measurement scheme, based on reverse-pulse amperometry, we have surprisingly observed that a simple electrochemical pretreatment-preanodization at 1.50 V vs. Ag/AgCl for 30 min-yields a substantial improvement in the stability of subsequent dc amperometric detection. For example, Figure 1shows typical anodic peaks for various Samples, at freshly polished (a) and pretreated (b) glassy carbon electrodes. The samples used in Figure 1represent common practical situations involving the formation of adsorbed films by an electroinactive surfactant (albumin, A), the analyte (chlorpromazine, B), or its reaction product (dinitrophenol, 0 1986 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58,NO. 8,JULY 1986
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Flgure 2. Effect of preanodization time (A) and potential (B) on flow injection measurements of 1 X M NADH over a 60-min time period. Preanodiition times are 5 (a), 10 (b), 20 (c), and 30 (d) min for part A and 20 min for part B. Preanodization potentials are 1.50 V for part A and 0.90 (a), 1.10 (b), 1.30 (c), and 1.50 (d) V for part B. Applied potential is 0.70 V. Other conditions are as in Figure 1.
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Figure 1. Detection peaks at freshly polished(a) and electrochemically treated (b) glassy carbon electrodes: samples, 1 X lo-‘ M uric acid and 300 mg/dL bovine albumin (A), 1 X lo4 m chlorpromazine (e), 2 X lo-‘ M dinitrophenol (C);electrolyte (sample and carrier), 0.05 M phosphate buffer (pH 7.4); applied potential, 0.80 V (A), 0.90V (B,C); flow rate, 1.0 mL/min; injections of 20-pL samples at a rate of 120 per hour.
C). Accordingly, a gradual loss in electrode activity is observed when the freshly polished electrode is used up to 13,26, and 40% decrease in the response for uric acid (in the presence of albumin), chlorpromazine, and dinitrophenol, respectively, for the 30 repetitive injections shown. This behavior is indicative of a passivating film, gradually blocking the access of solutes to the surface. In contrast, no loss of electrode activity is observed for similar injections at the electrochemically pretreated electrode. Clearly, the “poisoning” effect appears to have been eliminated. Approximately similar sensitivity is observed at both electrodes for the analtested in Figure 1, with the pretreated electrode exhibiting some increase in the chlorpromazine response and some decrease in the uric acid and dinitrophenol peak heights. Changes in surface conditions, which may be responsible for this behavior, are discussed below. In order to demonstrate the electrode passivation and its prevention at the untreated and treated electrodes,respectively, relatively high concentrations (0.1-1.0 mM) of analyks, known to cause such deactivation, were used throughout this study. The utility of the preanodization procedure in improving the stability of the response of additional analytes is described below. To optimize the parameters of the pretreatment on the stability of the amperometric response, an experiment was performed by preanodizing the freshly polished electrode for different times or potentials. The results are given in Figure 2. The oxidation of the enzymatic cofactor NADH was used in this, and many of the following experiments, because this process is utilized in amperometric sensing of many biological compounds (as in various enzymatic or immunoassay tranducers) and is known to be complicated by electrode fouling by adsorbed product. The stability of the NADH detection peaks-over a series of 120 injections-is improved as the preanodization period is increased (A): up to 35,8,0, and 0% decrease in peak current for 5,10,20, and 30 min, respectively. The preanodization potential has a drastic effect on the elimination of electrode fouling (B). Preanodizations at 0.9, 1.1, and 1.3 V result in 33, 28, and 21% losses of electrode activity, respectively (a-c). In contrast, uniform level of activity is observed following preanodization at 1.5 V. The pretreatment conditions selected as “optimum”were 1.5 V and 30 min. Such preanodization potential has been shown to yield
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Flgure 3. Response of the freshly polished (a) and electrochemically treated (b) electrodes to a 1 X lo4 M dopamine solution before and after eight successive injections of a diluted (1 -t 3) serum sample. Applied potential is 0.80 V. Other conditions are as in Figure 1.
maximum lowering of overvoltage where the catalytic utility of dc electrochemical treatment is concerned (6). The effect of these parameters on surface processes, which may be responsible for the enhanced stability, are discussed below. Notice also the increase of the NADH response under these conditions. This is attributed to a simultaneous electrocatalytic effect that is coupled to the prevention of electrode deactivation. Such behavior is observed for analytes with slow heterogeneous electron transfer, e.g., NADH. It is well-known (8, 16) that anodization of glassy carbon electrodes produces quinoidal-likesurface functionalities that have been proposed as electron transfer mediators for the oxidation of NADH. The achievement of larger, or similar, detection peaks represents an advantage of the electrochemical pretreatment procedure over the use of polymeric coatings (2,3).In the latter case, the electrode protection is coupled with concomitant loss in sensitivity, particularly when large species such as NADH are monitored. Reproducible peak height can be obtained at the freshly polished electrode when the NADH concentration is maintained below lo4 M (17). To further investigate the practical utility of the preanodization procedure, the glassy carbon detector was exposed to serum samples. The response of the electrode to a 1 X lo4 M dopamine solution was recorded before and after eight successive injections of a serum sample. Figure 3 illustrates the dopamine detection peaks in such alternate detection scheme (six cycles totaling 48 serum injections). At the freshly polished electrode (a), a 24% reduction of the dopamine response is observed following such series. A stable dopamine detection peak is observed when the electrochemically pre-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Figure 4. Detection peaks for repetitive injections of a 1 X M NADH solution at the freshly polished (A) and electrochemicailytreated (B) electrodes. Applied potential is 0.70 V. Other conditions are as in Figure 1. For a series of 360 successive injections, every tenth peak is shown.
treated electrode is used (b). Thus, a high level of electrode activity is maintained, even after exposure to complex biological samples containing a variety of proteins and other components that strongly adsorb on the surface of carbon electrodes. In order to be practical for routine applications, the enhanced stability should be maintained over a long time period. Figure 4 compares the stability of the response for 360 repetitive injections of samples containing 1 x M NADH at the freshly polished (A) and electrochemically treated (B) electrodes. The untreated electrode (A) exhibits a gradual loss of activity; 43 and 53% reductions of the NADH oxidation peaks are observed following 90 and 180-min periods (180 and 360 injections), respectively. In contrast, no observable decrease of detector current is observed at the treated electrode (B). Thus, the elimination of electrode fouling is quite long-lived. However, in view of the simplicity and speed of the preanodization step, it is recommended to carry out the pretreatment at the beginning of each day. The above data do not imply that electrode passivation is eliminated in all situations. Repetitive injections of solutions containing chlorophenol, phenol, or desipramine resulted in 3ome decrease in detection peaks. For example, at the treated electrode we observed a minor (-4%) loss of electrode activity following 30 repetitive injections of a 2 X M phenol solution, whereas a gradual decrease in the response, of up to 70%, was observed in a similar experiment at the fresh (electrode(applied potential, 0.8 V). Chlorophenol (2 X M) exhibited a rapid loss of electrode activity at both electrodes (96 and 83% at the untreated and treated electrodes, respectively, for a series of 30 injections; applied potential, 0.9 V). As was shown earlier (Figure l C , part b), a uniform (electrode activity was obtained a t the treated electrode in idmilar measurements of dinitrophenol. It is well-known that (differentphenolic compounds exhibit electrode deactivation by a similar mechanism (coupling of phenoxy radicals to form
