2280
Anal. Chem. 1981, 53,2280-2283
(12) Figura, P.; McDuffie, B. Anal. Cbem. 1980, 52, 1433-1439. (13) Tuschall, J. R., Jr.; Brezonik, P. L. Limnol. Oceanogr. 1980, 25, 495-504. (14) Sekerka, I.; Lechner, J. F. Anal. Lett., 1978, A l l , 415-427. (15) Buffle, J.; Greter, F.-L.; Haerdi, W. Anal. Cbem. 1977, 49, 216-222. (16) Shuman, M. S.; Crorner, J. L. Environ. Scl. Tecbnol. 1979, 13, 543-545. (17) Shuman, M. S.; Woodward, G. P., Jr. Envlron. Scl. Tecbnol. 1977, 7 1 , 809-813. (18) Shuman, M. S.; Woodward, G. P., Jr. Anal. Cbem. 1973, 45, 2032-2035. (19) Weber, J. H.; Wilson, S.A. Wafer Res. 1975, 9 , 1079-1084. (20) Delahay, P. "New Instrumental Methods in Electrochemistry"; Interscience: New York, 1954; Chapter 5. (21) Brainina, Kh. 2 . "Stripping Voltammetry in Chemical Analysls"; Wiiey: New York, 1974; Chapters 11, 111. (22) Bond, A. M. "Modern Polarographic Methods in Analytical Chemistry"; Marcel Dekker: New York, 1980; Chapters 2-6, 9.
(23) Davison, W. J. €lecfroanal. Cbem. 1978, 87, 395-404. (24) Greter, F. L.; Buffle, J.; Haerdi, H. J . Nectroanal. Cbem. 1979, 101, 2 11-229. (25) Crow, D. R. "Polarography of Metal Complexes"; Academic Press: New York, 1969; Chapter 4. (26) Lukaszewski, Z.; Pawiak, M. K. J. Electroanal. Cbem. 1979, 703, 225-232. (27) Srna, R . F.; Garrett, K. S . ; Miller, S. M.;Thurn, A. B. Envlron. Sc/. Tecbnol. 1980, 14, 1482-1486. (28) Hansen, E. H.; Lamm, C. G.; Ruzicka, J. Anal. Cbim. Acta 1972, 59, 403-426. (29) Hansen, E. H.; Ruzicka, J. Anal. Cbim. Acta 4974, 72, 365-373.
RECEIVED for review May 6, 1981. Accepted September 2, 1981. National Science Foundation Grant No. OCE 79-10571 provided partial funding for this research.
Epoxy-Bonded Graphite Microelectrodes for Voltammetric Measurements Joseph Wang Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A mlcroelectrode composed of a commercial expoxy bonded graphite, which appears well suited for voltammetric measurements, is discussed. The electrode ls easy and inexpensive to fabricate and can be used for numerous voltammetric measurements. The technlques used In evaluating the electrode are linear scan voltammetry, dlfferentlal pulse voltammetry, pulsed-stlrringvoltammetry (at the submilllmolar concentratlon level) and dlfferentlal pulse anodlc strlpplng (for the nanomolar level). Well-defined current-potential curves are reported. Background currents are low and the usable potential range Is wide. Ascorbic add, dopamine, ferroeyanlde, lead, and cadmium were used as test systems.
The application of electroanalysis to the in vivo monitoring (1) or microliter volume batch analysis (2)requires miniature voltammetric electrodes due to the sample limitation. Microelectrodes based on various forms of carbon (mounted in glass or Teflon capillaries) have been used in connection with these applications. Most recent in vivo studies have been carried out with carbon paste microelectrodes (3). An epoxy can be added to this graphite-Nujol mixture, providing a better mechanical strength (4). Carbon fibers have been used to fabricate voltammetric microelectrodes with satisfactory response for the oxidation of catecholamines (5, 6). While significant progress has been made in this direction, the preparation of sensitive and reproducible miniature electrodes remains one of the major problems associated with in vivo electroanalysis and microelectroanalysis (3, 4). The present paper describes the relative merits of different analytical techniques using microelectrodes. Anodic stripping voltammetry (ASV) and hydrodynamic modulation voltammetry (HMV) are incorporated, for the first time, with the operation of microelectrodes for the measurement of very low concentrations of electroactive species. The WMV response is compared with that of differential pulse voltammetry (DPV) and linear scan voltammetry. The microelectrode is made of a commercial epoxy bonded graphite (Grade RX, Dylon, 0003-2700/81/0353-2280$0 1.25/0
Cleveland, OH). It is a two-component, graphite-filled, epoxy resin bonded adhesive filler. Specifications and characteristics are available along with several suggested nonelectrochemical applications (7). However, as was suggested recently by Jwtice (8) it appears to be well suited as an electrode material for various analytical applications. I t has high electrical conductivity, high mechanical strength, low residual currents, wide operating voltage range, and reproducible performance. I t is inexpensive and may be machined into various shapes. Various combinations of carbon and epoxy resin have been exploited for fabricating conventially sized voltammetric electrodes, mainly for ASV (9-12). Large and nomeproducible background currents limit their application to other forms of voltammetry. In addition, their preparations involve relatively cumbersome procedures (e.g.,grinding, centrifugation, mixing, overnight curing, etc). In contrast, microelectrodes made of commercial graphite epoxy are easily fabricated within less than 4 h. Their voltammetric characteristics and applications are described in the following work.
EXPERIMENTAL SECTION Electrode Fabrication. Two parts (by volume) of the epoxy bonded graphite resin were mixed thoroughly with one part of the correspondingaccelerator. Mixing continued for 5 min after all indications of the two individual components were gone. The end of a 3 cm length of Teflon tube (0.3 or 0.5 mm i.d., No. 24009 or 24005, respectively, Pierce Chemical Co., Rockford, IL) was then dipped in the epoxy. The epoxy filled the tip t o a height of about 3 mm. The electrode was then cured at 70 "C for 3 h. Electrical contact to the cured epoxy was made by filiing a portion of the Teflon tube with graphite powder and pushing a copper wire into it. The epoxy face was polished with a 0.1 ym alumina slurry and then washed with a stream of deionized water. Electrodes fabricated in this manner typically had resistances of approximately 80 Q. Since the currents measured in this work were in the nanoampere range, such a resistance had a negligible effect on the electrochemical response. A completed graphite f i e d epoxy microelectrode is shown in Figure 1. Apparatus. The electrochemical cell was a 22-mL (2.4 cm diameter, 5.0 cm high) glass container (Model VC-2, Bioandytical Systems, West Lafayette, IN). The cell was joined to the working electrode, reference electrode (Ag/AgCl, Model RE-1, Bioana0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14,DECEMBER 1981
2281
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lytical Systems),platinum wire auxiliary electrode, and the nitrogen delivery tube through four holes in its Teflon cover. The cell was placed on a magnetic stirrer (Sargent-Welch,No. 76490) and a 2.2 cm long magnetic bar was placed in its center. The three electrodes were connected to a Princeton Applied Research Model 364 polarographic analyzer, the output of which was displayed on a Houston Oinniscribe strip-chart recorder. %agents. Deionized water was used to prepare all solutions. Supporting electrolytes were 0.1 M phosphate buffer (pH 7.4), and 0.1 M KNOB. Millimolar stock solutiions of ascorbic acid, ferrocyanide (analyticalgrade, Baker Chemical Co.), and dopamine (reagent grade, Sigma Chemical Co.) were made up fresh every day. Stock solutions, 2.5 mM in Pb(N03)2,Cd(N03)2and Hg(NO&, were prepared and stored in polyethylene bottles. All studies were made by adding aliquots of the stock solution to the supporting electrolyte solution to give the desired concentration. Procedure. For studies at the bare graphite filled epoxy microelectrode thLe following procedure was employed. A 10-mL aiiquot of the phosphate buffer was pipetted into the cell and the electrode was pretreated by cycling the applied potential between + L O and -1.0 V for 8 min, allowing 2 min at each potential. Following this, measurements were made on the blank solution and the analyte solution. Linear scan and differential pulse experiments were performed at a stationary electrode without stirring the solution. Voltammogramswere Iecorded at scan rates of 50 mV/s (linear scan) and 5 mV/s (DPV). Pulses of 50 mV were applied in the DPV experiments. Pulsed-stirring experiments (13) were done by switching manually between low and high stirring speeds 15-45 s after the working potential was applied. Stirring speeds were measured with a digital tachometer. Pulsed-stirring voltammograms were developed pointwise by making 100-mV changes in applied potential and waiting about 30 s before applying the stirring pulse. For ASV experiments, 0.1 mL of the mercury stock solution was added to 9.9 mL of 0.1 M KNO9 The mixture was deaerated for 6 min, while the working electrode was kept at 0.0 V. Thie nitrogen delivery tube was then raised above the solution, and a potential of -1.0 V was applied at the working electrode while the solutioin was stirred at 450 rpm. After 6 min, the potential was switched to 0 0 V and held there for 2 rnin. Following this conditioning,the electrode was ready for use in an analytical run. Background and sample measurements were carried out successively by applying the deposition potentid for a selected time determined by the sought-for concentration level. The solution stirring was then stopped, and after a 15-s rest period the metalr, were stripped from the mercury by applying a differential pulse anodic potential ramp with 5 mV/s scan rate and 50 mV am. plitude. The scan cvas stopped at -0.05 V and after 45 s the system, was ready for the next deposition-stripping cycle. The mercury film was removed at the end of the day by holding the electrode at +0.5 V for 5 min and wiping of the electrode face with a wet laboratory tissue. Between analyses, the working electrode was stored in the empty cell.
RESULTS AND DISCUSSION Background Voltammograms. Figure 2 shows a comparison of background current-potential curves of the phosphate buffer obtained in a conventional scanning mode and a pulsed-stirring mode. The anodic potential limit of the epoxy bonded graphite electrode, as determined by the rise
Figure 2. Background current-potential curves for 0.1 M phosphate cm; linear scan voltambuffer, pH 7.4: electrode radius, 1.5 X metry (-), 5 mV/s; pulsed-stirring voltammetry (0);stirring rates, 0 (low) and 450 (high) rpm; pulsing frequency, 15 s.
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Figure 3. Electrochemical oxidation of 0.6 mM ferrocyanide (A, B)end 15 pM dopamine (C, D) in 0.1 M phosphate buffer, pH 7.4: differential pulse voltammetry (A, C), scan rate 5 mV/s; amplitude, 50 mV; linoar scan voltammetry (B, D), scan rate, 50 mV/s; electrode radius, 2.5 X
IO-' cm. Dotted lines represent blank solution.
of the current in the vicinity of f1.0 V (vs. Ag/AgCl electrode) is comparable to that of other carbon electrodes (14,15). The cathodic current at - 4 . 4 V is due to the reduction of oxygen present in the solution. The scanned residual current over most of the anodic range is less than 2 nA a t 5 mV/s, and less than 5 nA for scanning at 50 mV/s (not shown). The nonlinear increase of the residual current with the scan rate indicates that the double layer charging current is accompanied by other background components, e.g., surface-controlled processes, as expected for carbon-based electrodes. The pulsed-stirring modulation procedure (dotted line) shows an extremely low background, indicating good correction for currents arising from sources other than convective transport of electroactive species (Le., double layer charging, surface reactions, and solvent decomposition), Oxidation of Ferrocyanide, Dopamine, and Ascorbic Acid. Linear scan and differential pulse voltammograms for the oxidation of 15 pM dopamine and 0.6 mM ferrocyanide (obtained under identical conditions) are illustrated in Figure 3. Both species produced defined and readily quantifiable anodic peaks. The DPV peak potentials for ferrocyanide and dopamine are a t I-0.15 and f0.10 V, with widths a t half-height of 140 and 89 mV, respectively. The linear scan oxidation peaks for ferrocyanide and dopamine are characterized by their peak potentials a t +0.18 and +0.13 V and widths a,t half-height of 210 and 84 mV, respectively. Voltammograms with similar shape and potential regions were obtainable throughout this study. Similar peak potentials have been observed for dopamine and ferrocyanide a t various carbon microelectrodes ( 2 , 5 ) . Repeatability of the ferrocyanide and dopamine peak height from scan to scan i s approximately 5%. In the case of dopamine this is true only when the consecutive scan is initiated immediately after recording the previous one. A waiting period a t the initial potential (-0.2 V) has resulted with increased peak height. Work is currently under way to investigate this phenomenon.
2282
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER t98f
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Flgure 4. Current-potential curves for 0.2 mM ascorbic acid in 0.1 M phosphate buffer, pH 7.4: linear scan (A) and differential pulse voltammetry (B), as in Figure 3; pulsed-stirring voltammetry (C), stirring rates, 100 (L) and 600 (H) rpm; pulsing frequency, 15 s; electrode
radius, 2.5 X IO-* cm. Dotted lines represent blank solution. -10
Voltammograms showing the oxidation of 0.2 mM ascorbic acid, obtained in linear scan, DPV, and pulsed-stirring voltammetry, are shown in Figure 4. Because of the high degree of irreversibility the linear scan (A) and DPV (B) signals appear drawn out, resulting with difficulties in measuring submillimolar concentration levels of ascorbic acid using these procedures. This is consistent with the irreversible nature of ascorbic acid oxidation a t other carbon surfaces (16). The peak potentials for the ascorbic acid oxidation are +0.11 V (linear scan) and +0.03 V (DPV), which are more negative than those observed a t various carbon surfaces (5, 16) (apparently due to the dependence of the ascorbic acid peak potential on the surface state of the carbon (16)). Detection of ascorbic acid may be better accomplished by hydrodynamic modulation procedures. The pulsed-stirring voltammogram (Figure 4C) is characterized by its well-defined wave and plateau regions, and it is easily quantifiable. This is probably due to the extremely low background current (dotted line) and the steady-state nature of the measurement (Le., the effect of its longer time scale upon the observed reversibility). The pulsed-stirring half-wave potential is +0.037 V, with E3l4giving a value of 183 mV. Hydrodynamic modulation voltammograms of ascorbic acid with similar shape and half-wave potential have been reported for a pulsed-rotation procedure at a glassy carbon disk (17). Also shown in Figure 4C is a reproduction of a single pulsed-stirring current amplitude in the plateau region. The noise level, associated mainly with the stirrer operation, is about 5 nA. On the basis of a signal-to-noiseratio of 2, the limit of detection for ascorbic acid would be around 40 pM. Similar experiments with dopamine have yielded a limit of detection of about 8 pM. These values are higher than the submicromolar detectability reported for the pulsed-stirring operation of conventially sized electrodes (13). This is due to the fact that when using electrodes of small size the analytical current amplitudes are being reduced much more than the stirrer noise level. The pulsed-stirring response time is around 5 s, which is similar to that observed at macroelectrodes (13). A pulsed-rotation operation (17) of a disk microelectrode, that may result with lower limit of detection, is presently under investigation. A series of 12 successive pulsed-stirring measurements of 45 WMdopamine gave current pulses similar to those of Figure 4C, which permitted measurement of the precision (conditions: stirring rates, 200 (low) and 700 (high) rpm, I-0.85 V applied potential, 10 s pulsing period, 2.5 X cm electrode radius). The mean current difference found was 55.75 nA with a range of 53.6-57.6 nA. The relative standard deviation over the complete series was 2.5%. Of the three compounds examined above, none undergoes totally reversible electron transfer. Dopamine is more reversible than ferrocyanide, whereas ascorbic acid is totally
-08
-06
-04
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Flgure 5. Differentialpulse anodic stripping voltammograms obtained after two standard additions of Cd and Pb to 0.1 M KNOBsolution: (a) blank; (b, c) concentration increments of 4.0 ppb Cd and 7.5 ppb Pb; deposition, 2 min at -1.1 V; stirring rate, 450 rpm; scan rate, 5 mV/s; amplitude, 50 mV; electrode radius, 2.5 X lo-' cm.
irreversible. Because of the decreased sensitivity associated with increased irreversibility, the DPV limits of detection are around 3 pM (dopamine), 60 pM (ferrocyanide), and 150 pM (ascorbic acid). A similar value has been reported for dopamine using normal pulse voltammetry a t a carbon fiber electrode (5). The linear scan limits of detection are around 5 pM (dopamine), and 100 pM (ferrocyanide and ascorbic acid). Anodic Stripping Voltammetry. ASV can be used in conjunction with the epoxy bonded graphite microelectrode coated with a thin mercury film, for the trace analysis of amalgam-forming heavy metals. Such application is of great interest for developing in vivo monitoring procedures for trace metals such as lead, zinc, cadmium, or copper. Differential pulse anodic stripping voltammograms, obtained after two standard additions of Cd and P b to a 0.1 M K N 0 3 solution, are shown in Figure 5. Each addition effects a 4.0 ppb increase in Cd concentration and 7.5 ppb increase in Pb concentration. The peak potentials are observed at 4 . 6 5 V (Cd), -0.49 V (Pb), and -0.13 V (Cu). The peaks are well-defined (peak half-width of 59 mV) and peak height is proportional to metal ion concentration. On the basis of a signal-to-noise ratio of 2, the limit of detection for cadmium (using 2-min deposition) would be around 0.4 ppb. Lower detectability is obtainable by using longer deposition periods. The precision of DPASV measurements is indicated by the repetitive determination of 14 ppb lead in 0.1 M K N 0 3 (conditions as in Figure 5, except that deposition potential and time were -1.0 V and 1min, respectively). The mean peak current found was 33.5 nA with a range of 31-35 nA (n = 8). Thus at the 10 ppb level, the relative standard deviation was 4%.
The present results compare favorably with differential pulse ASV data reported for various conventionalIy sized carbon based mercury film electrodes (e.g., ref 10 and 11). Future work will include a more detailed characterization of stripping analysis a t various mercury-coated graphite filled epoxy electrodes, as well as at various bare carbon microelectrodes. In conclusion, the epoxy bonded graphite microelectrode appears to be a viable alternative to other carbon-based electrodes for microanalysis, in vivo monitoring, and various larger scale voltammetric applications. The electrode performs in a manner consistent with what might be expected of an
Anal. Chem.
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1981, 53, 2283-2287
effective carbon electrode. Full realization of its properties depends1 upon the proper incorporatioin of sensitive voltammetric techniques in conjunction with its operation. All of the techniques discussed above may easily be adapted for batch microanalytical applications. Om the other hand, the feasibility of niodulating the rate of mass transport at the surface of an indicator electrode implanted in vivo or of employing An ASV procedure (with a mercury-film and oxygen removal) for in vivo monitoring seems to be a difficult task; however, due to the inherent sensitivity of hydrodynamic modulation voltammetry and ASV, work is currently under way to modify these techniques for these important applications.
ACRNO WLEDGMENT The assistance of L. Sarin in fabricating the electrodes is gratefully acknowledged.
(4) Cheng, H. Y.; Schenk, J.; Huff, R.; Adams, R. N. J. Hectroanal. Chem. 1979, 100, 23-31. (5) Ponchon, J. L.; Cespuglio, R.; Gonon, F.; Jouvet, M.; Pujol, J. F. Anal. Chem. 1979, 51, 1483-1486. (6) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. And. Chem. 1980, 52, 946-950. (7) “Dylon Grade RX Epoxy Bonded Graphite”; Dylon: Cleveland, OH, 1979. (8) Justlce, J. B., Jr. Chem. Biomed. Environ. Instrum. 1980, 10, 31 1-330. (9) . . Swofford. H. S.. Jr.: Carman. R. L., 111 Anal. Chem. 1988, 38, 966-989. (IO) Anderson, J. E.; Tallman, D. E. Anal. Chem. 1978, 48, 209-212. (11) McLaren, K. G.; Batley, G. E. J. Nectroanai. Chem. 1977, 79, 169-178 . - - . . -. (12) Sykut, K.; Cukrowski, I.: Cukrowska, E. J . Eiectroanal. Chem. 1981, 115, 137-142. (13) Wang, J. Anal. Chim. Acta 1981, 129, 253-257. (14) Adams, R. N. “Electrochemistry at Solid Electrodes”; Marcel Dekker: New York, 1969; Chapter 2. (15) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 51, 353-357. (16) Dayton, M. A.; Ewing, A. G.; Wightman, R. M. Anal. Chem. 1980, 52, 2392-2396. (17) Blaedel, W. J.; Wang, J. Anal. Chim. Acta 1980, 716, 315-322.
LITERATURE CITED (1) Adam, R. NI. Anal. Chem. 1978, 48, 1128 A-1138 A. (2) Messner, J. L.; Engslrom, R. C. Anal. Chem. 1981, 53, 128-130. (3) Heineman, W. R.; Kissinger, P. T. Anal. Chem. 1980, 52, 1383151R.
RECEIVED June 30,1981. Accepted September 8,1981. J.W. is the recipient of the 1981 Starter Grant (award) from the Society of Analytical Chemists of Pittsburgh.
Bis(2,2 ’-* biqu ino1ine)copper(I) Nitrate as an in Situ Spectrophotometric Probe of Cadm um Sulfide/Cuprous Su Het eroj LInct ion Formation Connie J. Schllesener, Holger H. Streckert, and Arthur 6. Ellis” Departmelit of Chemistry, University of Wisconsin- Madison, Madison, Wisconsin 53706
Amyl alcohol sailutions of the bls( 2,2’-bilquinoline)copper( I) cation, I, react with single-crystal, n-type CdS to yield nCdS/p-Cu,S heterojunctions. The Intense visible absorptlon band of I (A,, 545 nm; E -6300 M-’ cm-’at 25 “C) provides an in situ spectrophotometric probe of heterojunction formation: spectral changes can be correlated with the extent of reaction. Kinetic studies using the nitrate salt of I reveal that lnltial reaction rates are directly proportional to CdS macroscopic surface area, first order in the concentration of I, and temperatlure dependent with an apparent activation energy of 13 f 2 kcal/rnol between 60 irnd 90 OC. Mechanistlc aspects of the reactlon are dlscuased.
halide complexes (3). Kinetic studies have typically required the removal of the reacted CdS sample from the reaction medium and determination of the quantity of Cu present by electrochemical stripping ( 4 , 5 )or by increase in weight (6). We were interested in finding an analytical technique to monitor heterojunction formation which would permit in situ kinetic and mechanistic studies. The sensitivity afforded by certain bidentate chelating ligands for Cu(1) detection suggested their use as spectroscopic probes of heterojunction formation in nonaqueous media. In this paper we report that amyl alcohol solutions of the nitrate salt of bis(2,2‘-biquinoline)copper(I)cation, I, can be used to prepare CdS/Cu@ r
The cadmium sulfide/cuprous sulfide (CdS/Cu2S)photovoltaic cell is a leading candidate for widespread use as an optical to electrical energy converter. Among its most attractive features are ease of preparation and a solar energy conversion efficiency of about 10% ( I , 2). The key element of this cell. is a heterojunction formed between n-type Cd!3 and p-type C u a . A common synthetic route to heterojunction formation is the “dip” method, which consists of simply exposing CdS to a hot aqueous cuprous halide solution, eq 1.
CdS(s) + 2Cu+(solvat,ed)
-
Cu2S(s) -1 Cd2+(solvated) (1) Conducted in this manner the heterogeneous reaction does not readily lend ihelf to characterization. Mechanistic studies, for example, are complicated by equilibria involving cuprous
+,
I heterojunctions. More importantly, the complex provides an in situ spectroscopic handle on the progress of the reaction leading to heterojunction formation. In sections below we describe synthetic, stoichiometric, kinetic, and mechanistic aspects of this system. EXPERIMENTAL SECTION Materials. All. solvents were reagent grade and used without purification. Preparation of the nitrate salt of I, [ (biq)2Cu]N0,, was based on Jardine’s method (7). Our procedure differed in that the complex was crystallized from the acetone reaction mixture by concentrating to -20 mM, heating to reflux, filtering,
0003-2700/81/0353-2283$01.25/00 1981 American Chemical Society
