499
Anal. Chem. 1988, 60, 499-502
In Situ Electrochemical Renewal of Glassy Carbon Electrodes Joseph Wang* and Meng Shan Lin Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A rapid electrochemlcalmethod for In sltu renewal of glassy carbon electrodes Is descrlbed. A short, but “sever(B”,electrochemlcal treatment applled repetnhrely between succ8%8hre vottammetrk scans Is shown to effectively remove passhratlng fllms. Hence, each voltammogram Is recorded on a freshly actlvated surface. Preventlon of electrode passlvatlon Is 11lustrated for materials, such as phenol, dlhydronlcotlnamlde adenlne dlnucleotlde (NADH), chlorpromazlne, uric acid, butylated hydroxyanisole (BHA), or dodecyl sodium sulfate, known to produce severe loss of electrode actlvlty. Relatlve standard devlatlons of the peak current In the presence of these compounds range from 3 % to 5 % , compared to 24 % to 110% at untreated electrodes. Pretreatment Is characterlzed wlth respect to potentlal Ilmlts, frequency, duratlon, deactlvatlng materlal, solutlon, and other variables. Dlfferent deactlvatlng materlals dlctate use of slightly dlfferent treatment condltlons. Thls simple procedure Is carrled out whlle the electrode Is set up as usual for voltammetrlc measurements and yields much more usable electrodes for analytical appllcatlons.
In recent years, the use of solid electrodes for electroanalysis has gained popularity, one of the primary reasons being their applicability to anodic oxidations. Such electrodes thus permit important analytical applications not amenable to the dropping mercury electrode (DME). The effective utility of solid electrodes for voltammetric analysis is often hampered by a gradual fouling of the surface. Such loss of electrode activity is usually attributed to adsorption of reaction products, of the analyte itself, or of large electroinactive organic surfactants. For example, the voltammetric response in samples of clinical origin is rapidly degraded by the adsorption of proteins. The passivating layer hinders the redox process of interest, resulting in poor stability. Complete inhibition of the electron-transfer process may be observed in extreme cases. Unlike the renewable surface of the DME,the behavior of solid electrodes is thus strongly dependent on their history. There is no fundamental reason, however, to expect an inferior stability at solid electrodes, provided that an electrochemically “equireactive”surface is maintained. Appropriate protection of solid electrodes or periodic in situ regeneration of their activity is thus highly desirable. The development of new schemes for maintaining a stable response, based upon the above strategies, has received growing attention recently. For example, films of cellulose acetate or irradiated poly(ethy1enimine) cast on the electrode surface, have been used to prevent fouling effects due to various organic surfactants (1,2). This protection strategy, however, does not address stability problems associated with adsorption of reactants or produds of the electrode reaction. An in situ renewal procedure, that effectively removes the passivating film (independent of its source), appears to be a more desired strategy. Methods needed for such reactivation should be simple, rapid, and reproducible. Poon and McCreery (3, 4 ) described recently an in situ method for effective renewal of solid electrodes using a short laser pulse delivered to the electrode directly in solution. This laser0003-2700/88/0360-0499$01.50/0
assisted voltammetric approach provides substantial enhancement of the electrode stability, and circumvents the need for tedious ex situ treatments. While relatively low-cost lasers can be used (4), such activation scheme requires an additional optical arrangement, that is not available in the majority of electroanalytial laboratories. In addition, the laser-activated voltammetric concept may not be feasible in several relevant applications, such as in vivo clinical monitoring or in situ environmental surveillance, where surface reactivation is highly desirable. This paper describes an electrochemical approach for in situ renewal of glassy carbon electrodes. The development of surface treatment methods for the creation of active glassy carbon electrodes has been the subject of numerous recent studies (5-12). Electrochemical pretreatment schemes have proven useful for enhancing the electron-transfer kinetics of various solution species (8, 10). “Mild” (8,9) and ”severe” (10) treatments, using direct and alternating current (dc and ac) waveforms, respectively, have been examined. The reduction in overpotential is commonly attributed to an increase in surface-boundoxygen groups and cleaning of the glassy carbon surface of contaminants. Similar observations were reported using nonelectrochemical activation procedures. In particular, Kuwana’s group (6, 7) illustrated the important role of surface cleanliness in the activation of glassy carbon electrodes. While various electrochemical treatments have been used to achieve reproducible initial activity, none of them was considered for repetitive cleaning of the surface. Electrochemicaltreatment should be ideal for the in situ reactivation of gradually fouling surfaces, as it requires no additional instrumentation and can be utilized in situations where laser irradiation is not possible. A short ac waveform is applied in the present study between successive voltammetric scans, so that each voltammogram is recorded on a freshly activated surface. Such treatment effectively removes passivating layers, thus bringing a high state of eiectrode surface cleanliness. Hence, a repetitively renewed surface, free of memory effects, is obtained in a manner analogous to the renewable DME (or laser-reactivated solid electrodes). The improved stability is accompanied by enhanced rates of electron transfer. The scope of this study is represented by results obtained for numerous compounds known to foul solid electrodes. The characteristics and advantages of in situ electrochemicalreactivation of glassy carbon electrodes are discussed below.
EXPERIMENTAL SECTION Apparatus. An EG&G PAR Model 264A voltammetric analyzer, in conjunction with an EG&G PAR Model 0073 X-Y recorder, was used for cyclic voltammetry and differential pulse voltammetry. A Bioanalytical Systems Model VC-2 electrochemical cell was employed in most experiments. The glassy carbon working electrode (Model MF2012, BioanalyticalSystems, 2.5 mm diameter), reference electrode (Model RE-1, Bioanalytical Systems),and the platinum wire auxiliary electrode joined the cell through holes in its cover. A function generator (Wavetek, Model 182) was used for the electrochemical cleaning treatment of the surface. Potentials were monitored with a digital multimeter (Dynascan Co., Model 2830). Reagents. Deionized water was used to prepare all solutions. Stock solutions of uric acid, chlorpromazine,dihydronicotinamide adenine dinucleotide (NADH), and butylated hydroxyanisole 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
Table I. Optimum Parameters of the ac Reactivation
Waveform
frequendeactivating NADH
phenol uric acid BHA chlorpromazine dodecyl sodium sulfatea
cy, Hz
duration,bs Ei, V 15 (30) 60 (60) 60 (60) 5 (5) 30 (300) 60 (300)
10 30
50 50 50 50
Ef,V -1.8 -1.5 -2.0 -0.9
+1.8 +1.5 +2.0 +0.9 +0.4
-1.4
+2.5
a Response to potassium ferrocyanide. *Theduration of the initial pretreatment, prior to the first scan, is given in parentheses.
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1
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3
5
7
9
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-0.2 0
0.2 OA 0.6
Potential
Measurement No.
(v)
M NADH at unFlgure 2. Differential pulse response for 2 X treated (A) and ektr-lty renewed (B) glassy carbon electrodes: (a)first voltammogram, (b) tenth voltammogram; scan rate, 10 mV/s; amplitude, 50 mV; electrolyte, 1 M sodium chloride: electrochemical treatment, as in Table I.
J
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Figure 1. Successive (a-d) cyclic voltammograms for 2 X M NADH (A, B) and 1 X M phenol (C, D) at untreated (B, D) and electrochemically renewed (A, C) glassy carbon electrodes: Scan rate, 50 mV/s; electrolyte, 1 M sodium chlorlde; electrochemical treatment, as In Table I.
(BHA) (Sigma), potassium ferrocyanide and dodecyl sodium sulfate (Baker), and phenol (Fisher) were prepared daily. Unless specified otherwise, the supporting electrolyte was a 1M sodium chloride solution. River water (Rio Grande) was collected at Las Cruces, NM. Procedure. Before each experimentthe glassy carbon electrode was hand-polished with alumina slurries of 3, 1,and 0.05 pm for time periods of 2, 1, and 1min, respectively. Residual polishing material was removed from the surface by sonication in a water bath for 1min after each polishing process. An electrode prepared by this procedure will be considered as untreated glassy carbon. The electrochemical treatment involved cycling a square-wave potential in a 1M sodium chloride solution (or, in some instances, in phosphate buffer solution (pH 7.4)), prior to recording the first voltammogram and between successive voltammograms. The exact parameters of such an ac waveform depend upon the deactivating substance under investigation (see Table I). The electrode was held at the initial potential for 15 s prior to each voltammetric scan. The sequence activation-measurement-activation proceeded throughout the experiment. RESULTS AND DISCUSSION To illustrate the electrochemical renewal of glassy carbon electrodes, three common cases of passivation will be considered adsorption of electroactive reactants, adsorption of reaction products, and adsorption of electroinactive surfactants. In each case, the exposure to the working solution and/or the voltammetric measurement results in a growing blockage of untreated electrodes and, hence, progressive diminution of the response. Figure 1 shows typical cyclic voltammograms for NADH and phenol, recorded repetitively at the treated (A, C ) and untreated (B, D) glassy carbon electrodes. The voltammetric analysis of these important analytes is commonly complicated by surface fouling by adsorbed reaction products. These
Potential (v)
Flgure 3. Dlfferentiil pulse voltammograms for 1 X M phenol obtained at electrochemically renewed (A) and untreated (B) glassy carbon electrodes. Voltammograms (a-d) are part of a series of 16 successive scans, with every fifth voltammogram shown. Pretreatment (A) was as described in Table I. Other conditions are given in Figure 2.
electrogenerated species couple to form an inhibitory layer that results in a continually diminished current response a t untreated electrodes (compare a-d). In contrast, no such degraded response is observed a t the electrochemically renewed surface, indicating that it is possible to remove a passivating layer in situ and repetitively. The resistance to electrode fouling is accompanied by enhancement of electron-transfer rates, as indicated from the improved reversibility of the NADH voltammogram. The extent of surface fouling increases when the commonly used differential pulse waveform is employed. The slower potential scan rates characterizing this waveform increase the time availabile for reactants or their products to adsorb. For example, Figure 2 shows the differential pulse response for 2 X lo4 M NADH at the untreated (A) and electrochemically treated (B) glassy carbon electrode. Repetitive scans a t the untreated electrode results in a 70% reduction of the peak height (compare A, a, and b). No evidence of electrode fouling is apparent a t the electrochemically renewed surface. Notice again, the simultaneous activation effect, as indicated from the lowering of the overvoltage for the oxidation process by ca. 200 mV. Even more severe is the passivation observed in differential pulse measurements of phenol (Figure 3). In this case, a complete inhibition of the redox process occurs very rapidly, as indicated from the disappearance of the peak after five successive scans (B, a vs b). The in situ treatment, in
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
501
Table 11. Reproducibility of Voltammetric Data at the Untreated and Electrochemically Renewed Electrodesa % re1 std
dev electrochemically
'I
r
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analyte
1I
BHAb chlorpromazineb uric acidb NADH'
potassium ferrocyanided
untreated electrode
renewed electrode
51 35 24 45 110
5 3 5 3 4
For a series of ten successive measurements. Conditions: as in Figure 5. Conditions: as in Figure 2. In the presence of 50 mg/L dodecyl sodium sulfate, as in Figure 4. Measurement NO.
Figure 4. Response for 1 X
M K,Fe(CN), In the presence of 50 mg/L dodecyl sodium sulfate at untreated (A) and electrochemically renewed (E) glassy carbon electrodes: electrochemical treatment, as in Table I; differential pulse waveform and electrolyte,as In Figure 2.
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6
AI 100
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50
1
2
5
7
9
1
3
5
7
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Measurement No.
Figure 5. Response for 5 X M uric acid (A), 5 X M chlorpromazine (E), and 2 X M BHA (C) at untreated (a) and electrochemically renewed (b) glassy carbon electrodes. Electrochemical pretreatment was as described in Table I.
contrast, repetitively removes the passivating phenoxy film, resulting a stable response over the entire series of 16 scans (A, a vs d). An electrochemical removal of phenolic films from a platinum electrode by anodic polarization in an acidic solution of ferric chloride was reported by Koile and Johnson (13). In addition to the minimization of electrode fouling associated with adsorbed reaction products (e.g., Figures 1-3), the in situ potentiostatic treatment can be extremely useful for circumventing deactivation processes due to the presence of electroinactive surfactants. For example, Figure 4 compares the stability of the response for successive measurements of potassium ferrocyanide in a solution containing 50 mg/L dodecyl sodium sulfate at the untreated (A) and electrochemically reactivated (B) electrodes. The adsorption of this surfactant at the untreated electrode results with a rapid loss of the electrode activity (with up to 86% depression of the response). The electrochemically treated electrode, in contrast, exhibits an excellent resistance toward the surfactant interference, as indicated from the highly stable response during a 30-min period of direct exposure. The utility of the repetitive electrochemical treatment for removing passivating layers was evaluated in the presence of other oxidizable compounds. Results obtained for ten succesaive measurements of uric acid, chlorpromazine,and BHA are shown in Figure 5 (A, B, and C, respectively). These compounds were chosen to illustrate the case of electrode passivation due to reactant adsorption. A rapid decay of the
response, with up to 50%, 6070,and 70% depressions of the uric acid, chlorpromazine, and BHA peak height, respectively, is observed at the untreated electrode (curves a). A dramatic improvement in the stability is obtained in analogous measurements at the electrochemically renewed electrode (curves b). Table I1 illustrates the substantial improvement in the reproducibility achieved by the ac treatment for these and other deactivating materials tested in this study. Relative standard deviations of the peak height for successive measurements at the treated and untreated electrodes are compared. These range from 24 to 110% a t the untreated electrode and from 3 to 5% at the repetitively treated one. The continual cleaning obtained under a variety of conditions is obvious. The various variables of the polarization (cleaning) waveform were systemically evaluated. Optimum conditions may differ from solute to solute, due to the different nature of their interaction with the surface. Table I summarizes the optimum treatment conditions that yield the most reproducible response for the deactivating materials tested in this study. Note, for example, that the reactivation in the presence of NADH requires amildernconditions, compared tQ other compounds. For some compounds, high stability was observed over a wide range of treatment conditions. For example, an active surface was maintained when successive measurements of 5 X lo4 M uric acid were coupled to ac treatments of increasing frequencies, from 25 tQ 150 Hz (together with 60-s duration and k2-V potential limits). In contrast, the frequency had a profound effect upon the behavior of NADH, with a stable response at 10 and 20 Hz and progressively larger ones at 30, 45, and 60 Hz. In addition, a stable NADH response was obtained by using square-wave amplitudes of k1.6, *1.8, or f2.0 V, but not at f1.2 or f1.4 V. Measurement of phenol, in particular, requires a careful attention to the polarization conditions given in Table I. Table I indicates also that the optimum duration of the ac reactivation process may range from 5 to 60 s. The surface renewal is thus substantially shorter than the time required for recording the differential pulse voltammogram (90-180 s). It is, however, not as fast as the laser activation of Poon and McCreery (4, 5). The polarization conditions specified in Table I were optimized for measuring submillimolar concentrationsof six deactivating materials. Measurements at different concentration levels, or of other deactivating solutes, may require readjustment of the polarization waveform to accommodate for the changes in the extent of passivation. The fact that different solutes dictate use of slightly different treatment conditions is attributed to different fouling mechanisms and/or interaction with the surface. The supporting electrolyte may have also a profound effect upon the efficiency of the surface renewal. Early ac pretreatment schemes of glassy carbon electrodes have been carried out in a chloride media, with the chlorine (generated
502
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
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I
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0.7
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Flgure 8. Successive (a-c) measurements of phenol in river water (8 mL of river water 4- 2 mL of 1 M sodium chloride -t 2 X M phenol), at untreated (A) and treated (6)electrodes. Conditions are given in Figure 3.
at the upper potential limit) "attacking" the surface (10). For the repetitive reactivation process the presence of chloride is essential for reproducible measurement of uric acid, but not when NADH or BHA are concerned; for these compounds high stability is obtained also in phosphate buffer (pH 7.4) solutions. During the treatment in such media the surface becomes oxidized and more hydrophilic, and thus less susceptible to adsorption (10). Freshly polished (untreated) glassy carbon surfaces, in contrast, are very reactive and thus highly suceptible to adsorption and blocking of the electrode. To obtain reproducible results, it is essential also to apply a short ac waveform prior to the recording of the first voltammogram. Such pretreatment yields an initial reproducible state of surface activity and "stabilizes" the response when concomitant activation occurs. However, the initial pretreatment alone does not prevent the deactivation process during the course of the voltammetric measurement. The parameters of such pretreatment are usually similar to those of the repetitive one (Table I), except that longer durations of 90,300, and 300 s are required in the presence of NADH, chlorpromazine, and dodecyl sodium sulfate, respectively. The surface roughness, associated with the potentiostatic treatment, is indicated from the increase in the cyclic voltammetric background current envelop (e.g., Figure 1). The initial electrochemical pretreatment serves also to "stabilize" this charging current component, as no cumulative effects are observed after repetitive reactivation steps. The change in the double-layer capacitance has no effect on the analytical data commonly obtained by pulse voltammetry (that corrects for the charging current). Because of the concomitant enhancement of the electron-transfer kinetics, improved signal-to-background characteristics may actually be obtained in both differential pulse and linear sweep experiments (e.g.,
Figures 2 and 1,respectively). Surface redox peaks were not observed (in background voltammograms) as a result of the potentiostatic treatment. Figure 6 illustrates the applicability of the electrochemical renewal scheme to real samples. Successive measurements of 2 x 10" M phenol in river water yielded a stable response at the treated electrode (B) and a rapid depression of the peak (up to 70%) at the untreated electrode (A). These three measurements are part of a series of eight successive runs. At the untreated electrode, the phenol peak disappeared after the fifth scan; in contrast, no apparent change in the peak height was observed at the treated electrode.
CONCLUSION The present study illustrates that repetitive electrochemical treatment can be used in situ to prevent solid electrode fouling in the presence of various deactivating materials. Hence, the needs for frequent ex situ metallographic procedures are eliminated. The electrochemical approach provides an effective alternative to the recently introduced laser-activation concept, with each reactivation scheme offering certain advantages. The potentiostatic scheme is not as fast as laser activation but requires simpler instrumentation and may be used for applications where laser-irradiation may not be feasible. The results are consistent with early suggestions (6, 7) that surface cleanliness plays a major role in the function of various electrochemical and nonelectrochemical treatments of glassy carbon electrodes. Registry No. NADH, 58-68-4; BHA, 25013-16-5; K,Fe(CN)6, 13943-58-3;phenol, 108-95-2;uric acid, 69-93-2; chloropromazine, 50-53-3;sodium dodecylsulfate, 151-21-3;carbon, 7440-44-0. LITERATURE CITED Wang, J.; Hutchins, L. D. Anal. Chem. 1985, 5 7 , 1536. DeCastro, E. S.; Huber, E. W.; Villarroel, D.; Galiatsatos, C.; Mark, J. E.; Heineman, W. R.; Murry. P. T. Anal. Chem. 1987, 5 9 , 134. Poon, M.; McCreery, R. L. Anal. Chem. 1988, 5 8 , 2745. Poon, M.; McCreery, R. L. Anal. Chem. 1987, 5 9 , 1615. Rusling, J. F. Anal. Chem. 1984, 5 6 , 578. Hu, I. F.; Karweik, D. E.; Kuwana, T. J . Electroanal. Chem. 1985, 188, 59. Fagan, D. T.; Hu, I. F.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2759. Nagaoka, T.; Yoshino, T. Anal. Chem. 1988. 58, 1037. Hoogvliet, J. C.;Van Den Beld, C. M. B.; Van Der Pod, C. J.; Van Bennekom, W. P. J . Electroanal. Chem. 1988, 201, 11. Wang, J.; Hutchins, L. D. Anal. Chim. Acta 1985, 167, 325. Stutts, K. J.; Kovach, P. M.; Wightman, R. M. Anal. Chem. 1983, 5 5 , 1632. Taylor, R. J.; Humffray, A. A. J . Electroanal. Chem. 1973, 42, 347. Koile, R. C.; Johnson, D. C. Anal. Chem. 1979, 5 1 , 741.
RECEIVED for review July 24,1987. Accepted November 16, 1987. This work was supported in part by the National Institutes of Health Grant No. GM 30913-04.
