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Metal-Dispersed Carbon Paste Electrodes Joseph Wang,' Najih Naser, Lucio Angnes,t Hui Wu, and Liang Chen Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
has prompted us to explore such preparations. These metalThe preparatlon of motal-dlspersedcarbon paste electrodes, dispersed (1-5 % ) graphites are characterized by a strong b a d on mbdng an organlc Mnder wlth metallzed graphlte, adhesion of the metal center to the graphite substrate. k doscrlbed. Such electrodes combine the efflclent elecPowerful electrocatalytic CPEs have thus been fabricated by trocatalytlc actMty of metal mlcropartlcleswlth the attractlvo simply mixing the metalized graphite powder with the organic propmtkr of carbon paste matrlces. The ablllty to catalyze pasting liquid. Such use of metalized graphite for preparing tho dow electrod. reactionsof hydrogenor organk p.roxld08, modified CPEs offers a homogeneous and stable introduction hydrazlno compounds, arcorblc acld, and dlhydronlcotlnaof the catalytic metal microparticles into the bulk of the mldo adenlno dlnuckotld. k Illwtratod. Carbon pastesdopod electrode matrix. The resulting electrodes thus combine the wtth dmwent metals (Pt, Pd, and Ru) exhlblt dlffwent ole+ inherent advantages of CPEs with the efficient catalytic trocataiytk actMtles, backgroundcurrant contrlbutb, and activity of metalized sites. Hence, the need for incorporating homo analytlcal performances. CohnmoMlkatlon of an oxredox mediators for preparing catalytic CPEs is eliminated. Maso enzyme allows coupllna of a Mocatalytk reactlon wlth The catalytic properties of metalized CPEs toward several tho oioctrocatalytlc detection of the Ilbwatedperoxide. The compounds of environmental and clinical significance, as well decrease In oporatlng potentlal greatly bendlts the amperas their overall analytical utility, will be described in the onntrlc monltorlngof fbwlng stream. The attractlvocouplng following sections. ol~alcadytlc~~wnhcarbonpcrrtefonn~knr~ bo very valuable for routlne qualltatlve work. EXPERIMENTAL SECTION
INTRODUCTION Carbon paste electrodes (CPEs), composed of a matrix of graphite powder and an organic pasting liquid, have been widely and successfully employed over the past three decades in electroanalyeis.1-4 Such electrodes offer the advantages of very low background current, a wide potential window, miniaturization, and easily prepared, renewed, and modified surfaces. The organic binder, which is responsible for many of the attractive properties of CPEs, accounts also for the slow rates of electron transfer and hence to the substantial overvoltages for certain important analytes.2 The present paper describes the characterization, performance, and advantages of metal-dispersed CPEs. The modification of carbon electrodes with catalytic metal microparticles is receiving considerable attention.+lO While early work focused on depositing the metal microparticles onto the surface of carbon electrodes,+* recent activity demonstrated the ability to dope the bulk of carbon materials with a catalytic metal.gJ0 Doped glassy carbon9 and carbon film10 electrodes, with high catalytic activity, have thus been prepared. Analogous preparations of CPEs have not been reported. The recent introduction of metalized graphite powder, including platinum, palladium, or ruthenium on graphite, Permanent address: Inetitutode Quimicada USP,SHo Paulo, Brazil. (1) Adams, R. N. Anal. Chem. 1968,30, 1576-1578. (2) Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chen. Interfocial Electrochem. 1983, 143, 89-102. (3) Kalcher, K. Electroanalysis 1990,2, 419-433. (4) Wang, J.; Wu, L. H.; Lu, Z.; Li, R.; Sanchez, J. Anal. Chim. Acta 1990,228,251-257. (5) Gorton, L. Anal. Chim. Acta 1986, 178, 247-253. (6) Cox, J. A.; Jaworski, R. K.; Kulesza, P. J. Electroanalysis 1991,3, 86*877. (7) Shimazu, K.; Weisshaar, D.; Kuwana, T. J. Electroanal. Chem. Interfacial Electrochem. 1987,223, 223-234. (8) Cox, J. A.; Jaworski, R. K. Anal. Chem. 1989, 61, 2176-2178. (9) Callstrom, M. R.; Neenan, T. X.; McCreery, R. L.; Alsmeyer, D. C. J . Am. Chem. SOC.1990,112,4952-4956. (10) Ingersoll, D.; Huskieson, D. H. J. Electroanal. Chem. Interfacial Electrochem. 1991,307, 281-285. 7
0003-2700/92/0364-1285$03.00/0
Apparatus. Batch experiments were performed with a 10mL cell (ModelVC-2, BioanalyticalSystems (BAS)). The carbon paste working electrode, the Ag/AgCl reference electrode (Model RE-1, BAS), and the platinum wire auxiliary electrode joined the cell through holes in its Teflon cover. The flow injection system was described earlier." A thin-layer amperometric detector (ModelTL-4, BAS) and a 20-pL sample loop were used. All experimentswere performedwith an EG&GPAR Model 264A voltammetric analyzer, with Houston Omniscribe X-Y or strip chart recorders. Electrode Preparation. Metal-dispersed CPEs were prepared by thoroughly hand mixing the desired amounts of metalized graphite powder and mineral oil (Aldrich). The metalized graphite powders included platinum on activated carbon (3% Pt content) and ruthenium on carbon (5% Ru content) from Aldrich and palladium in graphite (1%Pd content, Graphimet Pd-1) from Alfa. Pastes containing71 % ,58% ,and 30 % mineral oil were employed for Ru-, Pt-, and Pd-CPEs, respectively, as they offered the most convenient preparation and packing and yielded the most successful data. Unmodified CPEs were prepared in a similar manner using Acheson 38 graphite powder (Fisher) or activated carbon (from Aldrich). The latter was employed in comparison to Pt-CPEs. A portion of the metalized CPE was packed into the end of a glass tube (3-mm i.d., 5-mm o.d.), and ita inner end was connected to a copper wire. A platinum disk electrode (1.5-mm diameter, BAS) and a cobaltphthalocyanine (1%(w/w))-modified CPE were also used for comparative work. For flow measurements, the modified paste was packed into the electrode cavity of a thin-layer detector. New surfaces were smoothed on a weighing paper. Reagents and Procedure. All chemicalswere reagent grade, and solutions were freshly prepared with double-distilled water. Hydrogen peroxide, ascorbicacid, dihydronicotinamideadenine dinucleotide(NADH),glucoseoxidase(EC 1.1.3.4., 138 OOOunits/ g), and glucose were received from Sigma. Methylhydrazine, 1,2-dimethylhydrazine, cumene hydroperoxide, peracetic acid, and 2-butanone peroxide were obtained from Aldrich, while hydrazine sulfate and potassium ferrocyanide were obtained from Baker. A 0.05 M phosphate buffer solution (pH 7.4) served as the supporting electrolyte. Amperometric detection was proceeded under batch and flow injection conditions, with 300 rpm stirring or 1.0 mL/min flow, respectively. The desired working potential was applied, and (11) Wang, J.; Hutchins, L. A w l . Chem. 1985,57, 1536. 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
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Cycllc voltammogram for 5 X M hydrogen peroxlde Tecorded at the ordinary CPE (A) and at Ru- (B), Pt- (C), and Pd-CPEs (D). Scan rate, 10 mV/s; supporting electrolyte, 0.05 M phosphate buffer (pH 7.4). I
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Cycllc voltammograms for (A) 1 X 1O3 M ascorblc acld and M NADH, recorded at the Pt-CPE (solld Ilnr) and ordlnary platinum electrode (dotted line). Condltbns, as in Figure 1.
Flguro 1.
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POTENTIALN M hydrazine, (B) Flgurr 2. Cyclic voltammograms for (A) 1 X 1 X 10" M ascorbic acld, (C) 2.8 X M 2-butanone peroxide, and (D) 5 X 10-4 M NADH, recorded at the plain CPE (dotted line)and the W-CPE (solld he). Other condltbns are as In Figure 1.
transient currents were allowed to decay to a steady-statevalue. All measurements were performed at room temperature.
RESULTS AND DISCUSSION The use of metalized graphite (rather than plain graphite) for fabricating CPEs greatly promotes catalytic activity toward important analytes with a sluggish redox process. Figure 1 shows typical cyclic voltammograms for hydrogen peroxide at the plain (A), Ru-doped (B),Pt-doped (C), and Pd-doped (D) CPEs. At the conventional electrode, redox activity is observed only above +0.9 V. The overvoltage for the oxidationprocess is greatly lower at the metalized surfaces (with the Pt-dispersed graphite yielding the most efficient catalytic activity). The anodic peak potentials at the Ru-, Pt-, and Pd-CPEs are + O M , +0.62, and +0.90V,respectively. Similar lowerings of the overvoltage are observed in Figure 2. The oxidation of hydrazine (A), ascorbic acid (B),butanone peroxide (C), and NADH (D) at the ordinary CPE
takes place at considerable overpotentials (dotted lines). The palladium-based carbon paste exhibits a dramatic improvement in the response for all reactants. The anodic peak potential values are 0.16 (A), 0.48 (B),0.80 (C), and 0.62 (D) V. Note, in particular, the several hundreds millivolts lowering of the potential for the oxidation of hydrazine. The catalytic response for NADH (ca. 200-mV shift), as well as hydrogen peroxide (Figure l),are of great significance in connection with the development of carbon paste based biosensors (see discussion below). Catalytic response was observed also for other organic peroxides (cumene hydroperoxide and peracetic acid) and hydrazine species (methylhydrazine and 1,2-dimethylhydrazines). In contrast, no catalytic activity was observed toward oxalic acid, cysteine, or potassium ferrocyanide. Such differences in the activity toward different species can be exploited for improving the selectivity. The unique coupling of metalized redox centers and a carbon matrix makes the metalized carbon paste electrode attractive even when compared to ordinary metal surfaces. Figure 3 displays cyclic voltammograms for ascorbic acid (A) and NADH (B)at the Pt-CPE (solid line) and an ordinary platinum electrode (dotted line). Unlike the defied anodic peaks observed at the metalized electrode (at +0.45 (A) and +0.70 (B)V), the ordinary metal electrode exhibits a poorlyshaped response. The metalized CPEs compare favorably also with common mediator-containingcarbon paste matrices. For example, the widely used cobalt-phthalocyanine CPE yielded anodic peaks for hydrazine and ascorbic acid at +0.52 and +0.55 V (not shown), as compared to +0.16 and +0.48 V at the Pd-CPE. The main advantage of metalized CPEs over mediator-based carbon pastes is that the metal centers are integral parts of the graphite particles, thus eliminating the need for an additional modification step and possible stability (leaching) problems. The background current of metalized CPEs is strongly affected by the nature of the incorporated metal. Figure 4 compares background scans obtained in a phosphate buffer solution using metalized CPEs with different dopants (Pd (b), Pt (c), and Ru (d)), as well as ordinary carbon paste (a) and platinum (e) surfaces. The Pd-CPE exhibits a relatively narrow potentialwindow (4.15 to 1.05V)andalargecharging current envelope. A wider potential window (4.30 to 1.20 V) is observed at the Pt- and Ru-CPEs. The former is characterized also by small charging current contributions, approaching those observed at the ordinary carbon paste surface. Note also the marked decrease in the background current in comparison to the pure platinum surface (c vs e). Coupled with ita efficient electrocatalytic behavior, the PtCPE offered the most favorable signal-to-background char-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1902
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POTENTIALlV Flem 4. Cyck voitammoqams obtained forthe supporting electrolyte sdutlon wing ordinary CPE (a)and W- (b), Pt- (c),and Ru-CPEs (d), as well as an ordinary platinum electrode (e). Scan rate, 50 mV/s. Solution, as in Figure 1.
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POTENTIAL/V Flguro 5. Hydrodynamic voltammograms for hydrogen peroxide (A) and hydrazine (8)at ordinary (a)and R-CPEs (b). Batch experiments wlth a 400 rpm solution stirring. Electrolyte, 0.05 M phosphate buffer
(pH 7.4).
acteristics and was thus chosen for all subsequent analytical work. Examination of the Pt-CPE surface by scanning electron microscopy revealed spherically-shaped metal microparticles, with an average size of 10nm, that are randomly dispersed on the “large” (-20-rm diameter) graphite particles (not shown). Onlyaverysmallfractionofthegraphitearea(-5%) is covered by the metal centers. Such coverage accounts for the attractive (low) background current of Pt-CPEs. Since the metal centers are dispersed throughout the bulk of the paste matrix, the outer catalytic layer can be easily removed and renewed, as needed for routine analytical applications. Such applications are described below. Analytical Performance. Batch and flow forcedconvection systemswere utilized to investigate the analytical utility of metalized CPEs and to demonstrate the advantages accrued from their catalytic activity. Figure 5 shows hydrodynamicvoltammetric profiles for batch additions of hydrogen peroxide (A) and hydrazine (B), at the ordinary (a) and Ptdoped (b) CPEs. The conventional electrode exhibits redox activity at potentials higher than +0.6 V. In contrast (and in agreement with the cyclic voltammetric data) distinct voltammograme, with oxidations starting at +0.1 V,are observed at the Pt-CPE. Hence, the metalized surface allows con-
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ventient amperometric monitoringof hydrogen peroxide and hydrazine at low operating potentials. Figure 6 compares the amperometric response of the ordinary (a) and platinized (b) CPEs to successive additions of hydrogen peroxide (A), 2-butanone peroxide (B), and hydrazine (C). The conventionalelectrode respondsvery poorly (at the relatively low operating potentials) to these concentration changes. The metalized surface, in contrast, exhibits a very sensitive response, as expected from its catalytic activity. The slopes of the initial linear portions of calibration data correspond to 1.86 (A), 0.34 (B), and 6.4 (C) pA/mM. Detection limits of 2 X 10-8M hydrazine, 5 X 1VM hydrogen peroxide, and 2 X 10-6 M 2-butanone peroxide can be estimated on the basis of the signal-to-noise characteristics (S/N = 3). The platinized surface is characterized also by a fast achievement of steady-state currents (5-10 8 ) . CPEs are also widely used for amperometric monitoring of flowing streams. However, the detection of many important solutes (with irreversible redox process) is complicated by the need for using high operating potentials. The electrocatalytic nature of metalized CPEs can greatly benefit such flow operation, as it allows the use of lower detector potentials, and hence enhances the sensitivity, selectivity, and scope of on-line amperometry. For example, Figure 7 displays the flow injection response of the conventional (a) and Pt (b) carbon paste thin-layer detectors to repetitive injections of 1X 10-3 M hydrogen peroxide (A), 2-butanone peroxide (B), and hydrazine (C) solutions. When poised at relatively low operating potentials (0.3-0.7 V), the ordinary CPE is not responsive to injections of these solutions. In contrast, the metalized CPE exhibits a very sensitive and reproducible response, a low base-line noise level, and a fast increase and decrease of the current. These 12-16 measurements are a part of series of 24 repetitive injections which yielded relative standard deviations of 2.6 (A),2.1 (B), and 1.7% (C). Hence, the catalytic response is quite stable (as expected for the strong adherence of the metal centers to the graphite), as compared to “leaching”problems that may occur with CPEs containing redox mediators. CPEs are extremely versatile in that numerous modifiers can be easily (and controllably) incorporated into the paste matrix with no need to devise individualized attachment scheme for each modifier.12 The metalized CPEs maintain this modification(by-mixing) advantage, and thus couple the catalytic activity of the dispersed metal with additional function(8) of the incorporated moiety. For example, mixedenzyme CPEs have received growing attention in past years in connectionwith the development of reliable biosensors.4J3 By incorporatingthe enzyme into the metalized carbon paste matrix, it is possible to couple the biocatalytic and electrocatalytic functions and hence to obtain additional sensing advantages. Figure 8 displays the amperometric response of glucose-oxidase/carbon paste (a) and glucose-oxidase/Ptcarbon paste (b) electrodes for successive additions of 2 X 1 W M glucose. The metalized carbon paste biosensor rapidly responds to these changes in the substrate concentration. Because of the inherent electrocatalytic activity of the platinum microparticles toward the oxidationof the liberated hydrogen peroxide (e.g. Figure 11,the sensitivity of the metalized carbon paste biosensor is about 14-fold higher than that of the conventional carbon paste enzyme electrode (slopea of linear portions 0.888 and 0.0625 pA/mM, respectively). In addition to oxidase-basedbiosensors, similar advantages are expected for dehydrogenase electrodes (based on the electrocatalytic oxidation of NADH, e.g. Figure 2D). (12)Ravichandran,K.;Baldwin,R.P.J.Electroonul. Chem.Interfacial Electrochem. 1981,126, 293-285.
(13)Bremle, G.; Persson, B.; Gorton, L. Electroanulysis 1991,47786.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
TIME Flgurr 6. Current-time recordings at the ordinary (a) and platinized (b) CPEs upon increasing the hydrogen peroxide (A), 2-butanone peroxide (B), and hydrazine (C) concentrations In 1 X 5X and 2 X M steps, respectively. Operating potentials, +0.8 (A), +0.6 (B), and +0.3 V (C). The resulting calibration plots are also shown (inset). Other conditions are as in Figure 5.
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solutions obtained at conventional carbon paste (a) and Pt-carbon paste (b) electrodes. Flow rate, 1.0 mL/min; operating potential, 0.7 (A),0.6 (B), and 0.4 V (C). Carrier, 0.05 M phosphate buffer (pH 7.4).
In conclusion, metalized CPEs have been shown to couple the high catalytic efficiency of metal microparticles with the surface regeneration/modification (and other attractive) features of carbon paste matrices. The decrease in operating potential associatedwith the use of metalized graphite greatly enhances flow injection and biosensing operations. The overall analytical performance is dependent on the type of doped metal. Even though the concept is presented in terms of Pt-, Pd-, or Ru-CPEs, it could be extended to other catalytic metals (e.g. Ni, Ir, Os,Ag), combinations of metals, and coupling with other modifiers. For example, the carbon paste preparation approach permits convenient mixing of different metalized graphites to achieve multimetal-CPEs. Increased use of metalized CPEs for routine quantitative work is expected in view of the wide range of catalytic activities exhibited by these surfaces and the versatility of CPEs. Analogous preparations of metalized-graphite epoxy composites should be advantageous for applications (e.g. chro-
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Flgure 8. Current-time recordings, obtained on lncreaslng the glucose concentration in 0.2 mM steps, using the giucoseoxidase modifled (10 % w/w) carbon paste (a)and Pt-carbon paste (b) electrodes. Also shown are the resuiting calibration plots (Inset). Operating potentlai, +0.80 V. Other conditions as in Figure 5. matographic detection) requiring improved rigidity and chemical stability.
ACKNOWLEDGMENT This work was supported by the U S . Environmental Protection Agency (Grant No. CR-817936-010). Mention of trade names or commerical products does not constitute endorsement or recommendation by the US EPA. L.A. acknowledges a fellowship from Fundacb de Amparo 6 Pesquisa do Estado de SBo Paulo (FAPESP), Brazil.
RECEIVED for review December 23, 1991. Accepted March 5, 1992.
