Anal. Chem. 1992, 64, 1706-1709
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Screen-Printed Stripping Voltammetric/Potentiometric Electrodes for Decentralized Testing of Trace Lead Joseph Wang’ and Baomin Tian Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
The utility of rcrwn-printed electrodes for stripping voltam metrk and potentbmetrlcmeasurements of trace metalshave been evaluated toward their exploitation of singlause decenlralredtdng. The“yaated8crwwprIntedcarbon e M r o d ~on , a plastlc strip, performin a manner comparable to conventlonal hanging mercury drop and mercury-coated giamy carbon electrodes. Well-defined peaks are thus obtalnedfor microgram per liter (ppb) concentratbnsof heavy metals foliowlng very short preconcentration times. The morphology of the resulting mercury f l h , as characterized by rcanlng oioctron mic~o~copy, faclYtatestrace measurements in unstirrud and nondeaeratud sohdkns, as desired in many decentralized applications. Reproducible measurements of ppb had In 100-pL drops are thw obtained. A detection ihit of 90 ng/L (ppt) had k estimated following 10min preconcentratbn. C~venhtqutntlta~ofieadinwknanddrhkhg B.dder havlnggreat potentlai watw sample8b for rlnglww dwentrallzedclinlcai or envkomental testing, a highlystable response of the rcreen-prlntedekctrode makes them very attractive for routine, lowcost, centralized operatlons.
INTRODUCTION The development of low-cost, mass-produced, electrochemical sensors based on screen-printing technology has attracted considerable attention in recent years.14 Most of the activity in this direction has focused on the development of disposable enzyme electrode strips for decentralized clinical testing of important substrates such as glucose or salicylate.1~4 In addition to biosensors,screen-printedchemically modified2 electrodes and microelectrodes3 have been fabricated for anodic voltammetric measurements of biomolecules (such as ascorbic acid or reduced glutathione). Applications of screenprinted electrodes for trace measurements of metals-which hold an enormous promise for decentralized environmental monitoring, clinical testing, or food assays-have not been reported. In this paper we report on the characteristics of screenprinted electrodesfor w e in Stripping analysis of trace metals. Stripping analysis is the most powerful electroanalytical techniques for trace-metal measurements.5~6The technique offers remarkable sensitivity, coupled with multielement, speciation, in situ, and on-line capabilities, all with low-cost instrumentation. It is thus extremely suitable for meeting the new challenges and demands of environmental field screening or decentralized clinicaltesting. Two basic electrode (1)Hilditch, P.; Green, M. Analyst 1991,116, 1217. ( 2 ) Wring, S.; Hart, J.; Bracey, L.; Birch, B. Anal. Chim. Acta 1990, 231, 203. (3) Craston, D.; Jone, C.; Williams, D.; Murr, N. Talanta 1991,38,17. (4) Green, M.; Hilditch, P. Anal. Proceed. 1991,28, 374. (5).W v g , J. Stripping Analysis: Principles, Instrumentation, and Application; VCH Publishers: Deerfield Beach, FL, 1985. (6) Florence, T. M. J.Electroanal. Chem. Interfacial Electrochem. 1970,27, 273. 0003-2700/92/0364-1706$03.00/0
systems, the hanging mercury drop electrode (HMDE) and the mercury-coated (glassy carbon or carbon fiber) electrodes, have been commonly employed in stripping analysis.6-7 The successful adaptation of screen-printed electrodes for stripping analysis is illustrated in the following sections, using extremely inexpensive carbon and reference electrode strips (of less than $1.00 per pair) printed on a poly(viny1 chloride) (PVC) substrate. Particular attention is given to the monitoring of trace lead due to growing concerns about levels of this metal in drinking water or children’s blood.
EXPERIMENTAL SECTION Apparatus. An EG&G PAR 264A voltammetric analyzer, a PAR 303A static mercury drop electrode, and a PAR 0073 X-Y recorder were used to obtain the differentialpulse stripping voltammograms. A TraceLab potentiometric stripping unit (PSU 20, Radiometer), with SAM 20 sample station (Radiometer)and an IBM PS/2 55SX,were used to obtain potentiograms. Squarewave stripping voltammogramswere obtained with a BAS l00A electrochemial analyzer. Most voltammetric and potentiometric stripping experiments were carried out in 10-and 20-mL cella (BAS and Radiometer), with the electrode being introduced through holes in the cover. The screen-printed electrodes (ExacTechBlood Glume Strips, Medisense Inc.) were purchased from a local drugstore. These strips consist of the planar working and reference electrodes printed on a PVC substrate (with carbon contacts on the oppoeite side). One printed carbon contact, of rectangular (2 X 8 mm) shape, served as a substrate for the mercury film electrode (since the original working-electrodetarget area is covered with enzyme/ mediator layers). The printed reference electrode(Ag/AgCl) from another strip served as reference during the voltammetric experiments. Potentiometric stripping work employed the conventional Ag/AgCl electrode of the TraceLab unit. Most experiments employed a platinum wire auxiliaryelectrode. Some experiments involved a two-electrodesystem and 1WpL sample drops. For this purpose, the strip was cut in the center, to allow placing of the carbon contact in direct proximity to the printed reference electrode (on a microscope slide). Reagents. All solutionswere prepared with double-distilled water. The metal atomic absorption standard solutions (lo00 mg/L) were purchased from Aldrich. The supporting electrolyte was an acetate buffer solution (0.02 M, pH 4.8). Drinking water samples were collected at this laboratory. The urine samples were obtained from a healthy volunteer. Fumed silica waa obtained from Sigma (No. S-5005). Procedures. Anodic stripping voltammetry (ASV) and potentiometric stripping analysis (PSA) were performed in the following manner. The mercury film waa preplated at the beginning of each day from a nondeaerated, stirred, 80 mg/L mercury solution (in 0.02 M HCl), by holding the carbon strip electrode at the deposition potential (-1.15 V for ASV or -0.90 V for PSA) for 15min. The potential was then switched to -0.20 V (ASV) or -0.05 V (PSA) for a 2-min “cleaning” period. Subsequent ASV and PSA cycles involved the common metal deposition and stripping steps. Experiments were performed with both stirred and unstirred solutions (duringthe deposition), as well as in the presence and absence of dissolved oxygen. The (7) Peterson, W. M.; Wong, R. W. Am. Lab. (Fairfield, CT) 1981, 13 (ll), 116.
0 1992 Amerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
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Flgure 1. Stripping voltammograms for 25 Fg/L Cd2+,40 Fg/L Pb2+, and 35 pg/L Cu2+ at the mercurycoated carbon strip (A) and glassy carbon (6) electrodes, as well as at the HMDE (C). Three-mlnute prewncentratlonat-l.l5V, wlthstirred(400rpm),deaeratedsolutions. Dlfferential pulse wave form with 10 mV/s scan rate and 50-mV amplitude. Electrolyte, 0.02 M acetate buffer (pH 4.8).
stripping step was performed with a quiescent solution. In ASV the potential was scanned (usually with a differential pulse wave form) and stopped at -0.20 V. This potential was maintained for 60 s before the next measurement was performed. Potentiometric stripping was carried out by applying a constant oxidation current of +1.0 HA;the electrode was conditioned for 15 s at -0.05 V before the next deposition-stripping cycle. The mercury film was removed at the end of the day’s work by holding it at +0.40 V (vs the printed reference electrode) for 5 min.
RESULTS AND DISCUSSION Screen-printed carbon and silver/silver chloride electrodes of disposableglucose strips4 were employed, as they are readily available, at very low cost (from any drugstore) in connection with the ExacTech blood glucose meter. Since the target working electrode area for glucose testing is covered with the enzyme/mediator layer, the carbon contact-on the opposite side of the strip-was successfully used as substrate for the mercury film. Figure 1compares stripping voltammogramsfor a solution containing25Fg/L cadmium,40 pg/L lead, and 35 Fg/L copper, obtained under identical conditions at the mercury-coated carbon strip (A) and glassy carbon (B)electrodes, as well as at the hanging mercury drop electrode. The mercury-coated screen-printed electrode exhibits well-defined and sharp stripping peaks, good resolution between neighboring signals, low background current, and a wide potential window. Hence, the relatively short (3 min) preconcentration period allows convenient quantitation of microgram per liter (ppb) concentrations. Comparison to the traditional hanging mercury drop or glassy carbon based electrodes indicates that the sensitivity and overall signal-to-background properties are not compromised by the use of the low-cost, screen-printed carbon substrate. Note also that the use of a screen-printed reference electrode (A, B)results in a ca. 200-mV negative shift in peak potentials, as compared to the use of a conventional silver/silver chloride reference (C). (Such shift was observed also using the same working electrode). Examinationof the bare and mercury-coated screen-printed carbon electrodes by scanning electron microscopy is shown in Figure 2. Some roughness and discontinuity is observed at the bare carbon strip (A). The mercury deposition resulted in numerous individual spherical microdroplets (of 1-2-pm diameter) (B).About 20% of the geometric area is covered
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by the mercury. Such behavior is similar to that of mercury deposition on common glassy carbon substrates.8 However, under the same plating conditions, different microdistributions of the droplets occur at the carbon strip and glassy carbon electrodes. While the data of Figure 1were obtained using common stripping conditions (deaerated solution that was stirred during the deposition step), decentralized stripping applications will usually require the elimination of the nitrogen purge and the convection transport. Figure 3 shows the voltammetric (A) and potentiometric (B)stripping response of the screen-printed electrode for quiescent, nondeaerated, solutions of increasing lead concentration, 20-100 pg/L (ae). Despite these conditions, and the use of a short (2 min) deposition period, well-defined peaks are observed. The sharper peaks and lower background response of the PSA operation makes it more attractive when quiescent, nondeaerated solutions are concerned. These 5 peaks were part of a series of 10 pg/L concentration increments. The resulting calibration plots were linear over the entire range, with slopes of 17 nA.L/pg (A) and 1.37 ms.L/pg (B)and correlation coefficients of 0.999. The microdistribution of the mercury droplets (shown in Figure 2) enhances the deposition efficiency from quiescent solutions. Relativelyhigh ratios of current peaks in quiescent and stirred solution (iP,,JipJ can thus be obtained. For example, an i,,q/ip,svalue of 0.25 was estimated from the voltammetric stripping response for 30 pg/L lead following 3min deposition. Analogous measurements at the mercurycoated glassy carbon surface yielded a value of 0.10. Apparently, the carbon strip possess less surface sites for the mercury plating, resulting in enhanced microarray character (during the initial part of the deposition period). Different rates of natural convection may also account for the different preconcentration efficiencies. As expected for a stripping operation, the response of the screen-printed electrodes is strongly dependent upon the preconcentration time. For example, Figure 4 shows voltammograms (A) and potentiograms (B)for 50 pglL lead, in the presence of dissolved oxygen, after different preconcentration times (0-2408). The longer the preconcentration period the larger is the response. Yet, even very short periods of 40-80 s (b,c) result in well-defined peaks. The resulting response vs preconcentration time plots (also shown) exhibit linear and nonlinear dependences for the voltammetric and potentiometricstripping operations,respectively. Longer preconcentration periods allow convenient quantitation of subppb concentrations. Detection limits of 30 and 50 ng/L (ppt) lead and cadmium, respectively, were estimated from voltammetric stripping measurements of 1.0 and 0.5 pg/L of these metals, following 10-minpreconcentration (stirred and deoxygenatedsolution). Analogous potentiometric stripping measurements of a nondeaerated solution yielded detection limits of 0.3 and 0.4 pg/L lead and cadmium, respectively. The effect of the deposition potential was evaluated over the -0.60 to -1.20 V range (50 pg/L lead; 2-min deposition for unstirred nondeaerated solution). The PSA response increased gradually with the potential between -0.60 and -1.1 V, and then it started to level off. It is known that the use of a square-wave voltammetric wave form can facilitate stripping measurements in the presence of dissolved oxygen.9 Such capability is attributed to the electrolytic depletion of oxygen at the surface prior to the fast scan. Figure 5 illustrates square-wavestripping voltammograms at the mercury-coated carbon strip (A) and (8)Stulikova,M. J . Electroanul. Chem.InterjaciaIElectrochem. 1973, 48,33. (9) Wojciechowski, M.; Balcerzak, J. Anal. Chem. 1990, 62, 1325.
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Flgure 2. Scanning electron micrographs of the bare (A) and mercury-coated (B) screen-printed carbon electrodes. e
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TIME (sec) Flgure 4. Voltammetric(A) and potentiometric(B) strippingresponse to 50 pg/L lead after different preconcentrationtimes: 0 (a), 40 (b), 80 (c), 120 (d), 160 (e), 200 (f), and 240 s (g), from a stirred, nondeaerated solution. Other conditions as in Figures 1 and 3.
glassy carbon (B)electrodes for nondeaerated and deaerated solutions containing 30 pg/L lead. Using the screen-printed electrode, the square-wave response in the presenceof oxygen
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POTENTIAL (v> Flgure 5. Square-wave strippingvoltammograms for 30 pg/L lead in nondeaeratedsolutionat carbon strip(A) and glassy carbon(B)mercurycoated electrodes. The corresponding response without oxygen is shown as dotted lines. Three-minute preconcentratlonsat -1.15 V with a stirred solution. Square-wave amplitude, 30 mV; step, 4 mV; frequency, 30 Hz. Electrolyte, as in Figure 1.
(solid line) is similar to that observed without oxygen (dotted line). In contrast, a significant oxygen contribution-that affects the quantitation of lead-is observed at the coated glassy carbon surface. The exact reason for this improvement is not clear at the present time. A similar observation was reported recently for mercury-coated carbon composite electrodes.lO While most work was carried out in 10-mL solutions, we examined the feasibility of using the strips for microliter stripping analysis. This was accomplished by placing the screen-printed carbon and silver/silver chloride electrodes in direct contact (on a microscope slide) and covering the resulting two-electrode system with the 100-pL samples. Figure 6 displays repetitive stripping potentiograms for 50 pg/L lead obtained on the same sample drop (A) and using (10) Wang,J.; Brennsteiner, A.; Angnes, L.; Sylwester, A.; La Gasse, R.; Bitsch, N. Anal. Chem. 1992,64,151.
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POTENTIAL (V) Microliter volume analysis. Repetitivepotentlometric stripping measurements of 50 pg/L In the same 100-pL drop (A) and In different drops (B). Drops were piaced on the two-eiectrode systems of the strip. Fivaminute preconcentratlon at -1.15 V wlth quiescent, unstirred solution. Other conditions as in Figure 3. Flgwe 6.
POTENTIAL CV) Figure 8. Voltammograms (A) and potentiograms (B) for urine and drinking water samples, respectively. Fivamlnute preconcentratlon at -1.15 (A) and -0.80 V (B) with deaerated (A) and nondeaerated (B), stirred solutions. (a)Voltammograms for the sample; (b, c)successive concentration increments of 10 (A) and 5 (B) pg/L. Samples: (A) urine sample treated in accordance to ref 11; (B) 19.6 mL of drinking water +0.4 mL of the 1.0 M acetate buffer (pH 4.8) solution. Pulse amplitude (A), 25 mV. Other conditions as In Figure 3.
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POTENTIAL (V) Voltammograms (A) and potentiograms (B) for repetitive strlpplng measurements of 100 and 50 pgIL lead, respectively. Preconcentrationfor 120 s with unstlrred, nondeaerated solution. Other conditions as in Figure 3. Figure 7.
different drops (B). Well-defined peaks are observed despite the very small (nondeaerated/unstirred)samples and the low analyte concentration. The relative standard deviations over these series were 5.2% (A) and 3.9% (B). No apparent difference in the response was observed for two- and threeelectrode systems (not shown). Besides having great promise for single-use applications, the screen-printed electrodes hold a great promise for use as reusable stripping devices. Figure 7 displays voltammograms (A) and potentiograms (B) recorded during a long run of 20 successive measurements of lead. For both stripping schemes, the lead peak remained unchanged throughout these prolonged operations. Ita relative standard deviationsover these complete series are 2.4% (A) and 3.2% (B). Such performance indicates that the mercury deposit on the strip is very stable. Notice again that the potentiometricstripping approach offers a more favorableresponse in the presence of dissolved oxygen. The highly stable response, coupled with the extremely low cost, make the screen-printed electrodes an attractive alternative to electrodes commonly used in routine (centralized) stripping operations. ~~~
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The applicability of the screen-printed electrodes to the analysis of urine and drinking water samples is demonstrated in Figure 8. No sample preparation was used, except of the addition of fumed silica to the urine sample to “collectnthe organic surfactants.11J2 With 5-min preconcentration time the voltammetric (A) and potentiometric (B) stripping schemes yielded well-defined lead peaks (a), which allow convenient quantitation following standard additions (b, c). Lead samplevalues of 10.2 (A) and 4.7 (B)pg/L were calculated from the resulting standard addition plots (also shown). In conclusion, the above results demonstrate for the first time that screen-printed electrodes are suitable for stripping measurements of trace metals. These extremely low cost electrodes function in a manner comparable to traditional stripping electrodes, with no compromise in the stripping performance. Since neither deoxygenation nor stirring is required, such electrodes hold a great promise for decentralized (clinical,environmental, or industrial) testing. Such applications will require the developmentof smallinexpensive portable stripping analyzers. Single-use applications will require complete stripping of the mercury prior to disposal of the strip. Certain applications (e.g. decentralized testing for blood lead level) will require the adaptation of simple and rapid sample preparation (e.g. acidification) and/or coverage with a suitable permselective/protectivelayer. Work along these lines is proceeding in this laboratory.
ACKNOWLEDGMENT This work was supported by Sandia NL (contact DE-AC0476DP00789). We wish to thank Radiometer America Inc. for the gift of the TraceLab system used in this work. T.B. acknowledges a fellowship from Sichuan University (PR China).
RECEIVED for review February 24, 1992. Accepted May 1, 1992.
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(11) Kubiak, W.;Wang, J. J. Eleetroanal. Chem. Interfacial Electrochem. 1989,258, 41. (12) Stauber, J.; Florence, T.M. Anal. Chim. Acta 1990,237, 177.
Registry No. Pb,7439-92-1; Hg,7439-97-6; C,7440-44-QHz0, 7732-18-5.
