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Anal. Chem, 1001, 63, 1676-1679
Anion-Selective Electrodes Based on Electropolymerized Porphyrin Films Sylvia Daunert, Shelley Wallace, Antonio Florido,’ and Leonidas G. Bachas* Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055
Ionselective membranes were prepared by electropdymerization of cobalt( I I ) tetrakh( 0-amln0phenyl)porphyrln on a glarsy-carbon surface for the development of potentiometric sensors. The resulting electrodes demonstrate a nearNernstlan response to anions and have submicromolar detection lknlts and good sdecthmy properties. n~ seiecthrlty pattern observed is thlocyanate > perchlorate > lodlde > nltrlte salicylate bromide > chloride > blcarbonate > phosphate. The parameters that affect the reeponse of them electrodes, Including the electmpolymerizatkn condltkns, are also described.
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INTRODUCTION Anion-selective electrodes prepared with conventional ion exchangers, such as quaternary ammonium salts, respond to anions in the following order: C 1 0 > ~ SCN- > I- > NO3- > Br- N3- > NO2- > C1- > HC03- acetate (I). Since this is the order of decreasing hydrophobicity (Hofmeister selectivity series), these electrodes are essentially “nonselective”. Rather recently, new ionophores have been employed in the design of “truly” anion-selective electrodes. These include metalloporphyrins, corrins, and complexes of tin (2). A wealth of information exists on the coordination and electron-transfer properties of both naturally occurring and synthetic porphyrins (3). Although metalloporphyrins have been extensively used as redox mediators or catalysts, it has only been in the last few years that their potential as ionophores in polymer-membrane ion-selective electrodes (ISEs) has been exploited. Indeed, electrodes based on poly(viny1 chloride) (PVC) containing Co(III), Sn(IV), Mo(IV), and Mn(II1) porphyrins have demonstrated selectivity toward specific anions (4-10). In addition, the above studies indicated that the central metal and the bulkiness of the side chains that surround the porphyrin ring control the selectivity of the ISEs (4-7, 9, 10). Electropolymerization has emerged as a useful technique for the preparation of polymer-modified electrodes (11). Theae electrodes have been used in electrocatalysis as well as in the development of electrochemical (amperometric) biosensors. The basis of this polymerization method is the electrodeposition of a polymeric film on the surface of an electrode that is immersed in a solution of an appropriate monomer. The monomers need to contain groups that can be electrochemically oxidized or reduced to form a polymer. Heineman et al. first realized that certain polymer-modified electrodes can be used in a potentiometric mode as sensors for ions (12). In addition, it was found that electrodes modified with polyaniline (13), polydiaminobenzene (12, 14), polydiaminodiphenyl ether (13), and polyphenol (13) can function as pH electrodes. This is a result of an association between H+and exchange sites on the films, and it appears to be independent of the nature of the electrode itself (i.e., this behavior was seen
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Permanent address: Departament #Enginyeria Quimica, Universitat Polt6cnica de Catalunya, 08028 Barcelona, Spain. 0003-2700/91/0383-1676$02.50/0
with platinum, glassy-carbon surfaces, etc.) (14). Further, several metallomercaptide-modified electrodes have been reported that use a conductive redox film of poly(mercaptop-benzoquinone). These electrodes presented potentiometric response to heavy-metal ions (15). Finally, an electrochemically generated polypyrrole film has been used to develop a chloride-selective electrode (16). Electropolymerized metalloporphyrins have been extensively used in voltammetric and amperometric studies (In, but to date there are no reports on their use to develop potentiometric sensors. The electrochemistry of polymeric films prepared by electrooxidation of metal-free, as well as Zn(I1) (18,19),Co(I1) (18), Ni(I1) (18,20), Mn(I1) (18),Fe(I1) (I@, and Fe(III) (19),porphyrin derivatives onto electrode surfaces has been described. The effect of different ring substituents such as amino, dimethylamino, hydroxy, and pyrrolyl groups on the electrochemical properties of metal complexes of tetraphenylporphyrin has been reported by Bettelheim et al. (21). In view of the ability to electropolymerize metalloporphyrins, coupled with their successful use in PVC-based ISEs, it appears feasible that electropolymerized porphyrin films should result in functional potentiometric anion-selective electrodes. An inherent advantage of these electrodes should be the retention of the ionophore in the polymer membrane. Indeed, one of the drawbacks of PVC-based membrane electrodes is the limited lifetime due to the leaching of the plasticizer or the ionophore from the membrane to the aqueous sample solution. ISEs based on electropolymerized films present the advantage that they do not need any PVC or plasticizer, and therefore, the ionophore is firmly attached to the electrode surface. This should result in an increase in the lifetime of the electrodes. In this report, cobalt(I1) tetrakis(o-aminophenyl)porphyrin, [Co(o-NH,)TPP], is electropolymerized on a glassy-carbon electrode surface by cyclic voltammetry. The development and the investigation of the potentiometric behavior of sensors based on poly[Co(o-NH2)TPP] films is the main focus of this study.
EXPERIMENTAL SECTION Reagents. Cobalt(I1) tetrakis(o-aminoph4nyl)porphyrinwas purchased from Porphyrin Products (Logan,UT). TetraethylammoNum perchlorate (TEN’)was from Southwestem Analytical Chemicals (Austin, TX) and was recrystallized before use. Acetonitrile was obtained from Burdick and Jackson (Baxter; Muskegon, MO) and was stored over 3-A molecular sieves. 1,3Bis[(tris(hydroxymethyl)methyl)amino]propane (BTP) was obtained from Calbiochem (La Jolla, CA). Tris(hydroxymethy1)aminomethane (Tris) was purchased from Research Organics (Cleveland,OH). 2-(N-Morpholino)ethanesulfonicacid (MES), sodium salicylate, sodium acetate, and all inorganic salts used were purchased from Fisher (Fair Lawn,NJ) or Sigma (St. Louis, MO). Deionized (Milli-Q water purification system; Millipore; Bedford, MA) distilled water was used to prepare all buffers and standard solutions. Apparatus and Electrodes. Differences in potential were obtained between the electropolymerized fh-coated glassy-carbon electrode and a double-junction reference electrode (Fisher; Model 13-620-47),whose inner compartment was filled with a 4 M KCl solution saturated with silver chloride (Fisher, SP 135). The outer compartment of the reference electrode contained the same buffer 0 1991 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 03, NO. 17, SEPTEMBER 1, 1991
Table I. Electropolymerization Conditions (Cyclic Voltammetry)o electrode A1 A6
A12 A3 A9
A15
starting pot., V
switching pot., V
final pot., V
no. of scans
0.0 0.0 0.0
1.1 1.1 1.1 1.2 1.2 1.2
0.0 0.0 0.0
200 200 200
-0.1 -0.1
100 100 100
-0.1 -0.1 -0.1
-0.1
300
280 -
E 260lLi
240 220 6""
RESULTS AND DISCUSSION Although liquid polymeric membranes are used extensively in the preparation of IS&, the resulting electrodes may have short lifetimes as a result of leaching of either the plasticizer or the ionophore from the membrane (22). Several alternatives, such as grafting of the ionophore to the backbone of the polymer have been proposed to overcome this problem (23). Even though grafting is not suitable for use with every ionophore, in certain cases it provides a substantial improvement of the characteristics of the electrode. For example, it has been recently reported that electrodes obtained by covalent immobilization of ionophores onto carboxylated PVC present extended lifetimes and have good selectivity and sensitivity
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OThe scan rate in all cases was 200 mV s-l. used in the sample solution. A pH/mV meter (Model 245) from Instrumentation Laboratory was employed to monitor the voltages, which were registered on a Linear (Model 1200, Reno, NV) strip-chart recorder. Electropolymerization was carried out with an EG & G Princeton Applied Research potentiostat/galvanostat (Model 273) in conjunction with an X-Yrecorder (Houston Instrument; Model 200). A conventional three-electrode cell was used with a glassy-carbon rod electrode (Bioanalytical Systems; West Lafayette, IN Model MF2012) as the working electrode, and silver and a platinum (coiled wire) were used as reference and counter electrodes, respectively. All the glassy-carbon electrodes were polished with e 2 0 0 mesh alumina adsorption powder (Fisher) and sonicated before use. Electropolymerization. The solution used to prepare the electropolymerized film was 0.10 M TEAP in acetonitrile containing 1.0 mM [Co(o-NH2)TPP]monomer. The electrolytic solution was deoxygenated by bubbling nitrogen through the solution for approximately 10 min and then by allowing the Nz to flow over the solution during the experiment. [Co(o-NHJTPP] was polymerized onto the glassy-carbon electrodes from the electrolytic solution by cycling the working electrode potential repetitively between a set starting potential and a set positive switching potential (Table I). The potential scan rate remained constant throughout the electropolymerization and was equal to 200 mV 5-l. The total number of scans was one of the variables used to optimize the prepared electrodes. After [Co(o-NHz)TPP] was polymerized onto the glassy-carbon electrode, the potenticetat/galvanostat was set to a constant f i i potential for a 2-min period. The newly formed poly[Co(o-NHJTPP]-coatedelectrodes were thoroughly washed with acetone and deionized distilled water. Procedure. The calibration of the electrodes was accomplished by adding, under stirring, known volumea of the different standard solutions to a beaker containing 5.00 mL of one of the following buffer solutions: 0.100 M Trii-HC1, pH 7.00; 0.100 M BTP-HCl, pH 7.00; 0.100 M MES-NaOH, pH 6.50,0.100 M BTP-HCl, pH 6.00; 0.100 M MES-NaOH, pH 6.00; 0.100 M NaH2P04-NaOH, pH 6.00,0.100 M citrate-NaOH, pH 6.00; 0.100 M acetate-NaOH, pH 6.00; 0.100 M MES-NaOH, pH 5.50. The response of the electrodes was monitored by the pH/mV meter and recorded by the strip chart recorder. Calibration curves were constructed by plotting the potential, E, vs the logarithm of the concentration of the anion present in the buffer solution. When the coated electrodes were not in use, they were stored in either 1.00 X 1V2 or 1.00 X M KSCN at room temperature.
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,
-4.0
-3.0
-2.0
log [Thiocyanate] Figure 1. Effect of the pH on the response of electrode A3 to t h b cyanate. The buffers used were (0)0.100 M MES-NaOH, pH 0.50; (0)0.100 M MES-NaOH, pH 6.00; and (+) 0.100 M MES-NaOH, pH 5.50.
properties (24). These results suggest that a stronger, permanent immobilization of an ionophore in a matrix should result in electrodes with longer lifetimes. If, in addition, the problem of the plasticizer leaching out of the membrane could be avoided, electrodes with even longer service lives should be obtained. Electropolymerizationof appropriate ionophore monomers results in polymer films with confined ionophores and, therefore, should provide electrodes with extended lifetimes. In this report, Co(II) tetrakis(o-aminophenyl)porphyrinwas electropolymerized by cyclic voltammetry on a glassy-carbon surface. On the basis of previous electrochemicalstudies, the polymerization of this monomer appears to proceed through a radical cation of the porphyrin with a mechanism similar to that of the oxidative electropolymerization of aniline (13). It has also been reported that during the polymerization the essential porphyrin monomer structure is preserved (17,18, 21, 25).
It has been found previously that it is possible to alter the ion-exchange properties of the polypyrrol-based chloride-selective electrode by changing the electropolymerization conditions (16). Consequently, the effect of the electropolymerization conditions on the properties of the poly[Co(oNHdTPP] f i was investigated initially. By changing several parameters such as the number of scans, the starting, switching, and final potentials, etc., ISEs with different behaviors were obtained. For the electropolymerizationto occur, it was necessary to scan the potential through values positive of 1 V (1.1or 1.2 V). Table I shows the conditions that lead to the preparation of the electrodes that performed best as anion-selective sensors in our study. All the electrodes listed in Table I presented near-Nernstian slopes for thiocyanate, their preferred anion, and had good detection limits. The response time of these electrodes was typically less than 25 s. The response time is defined as the time elapsed from spiking the sample solution until the signal reaches 95% of the steady state. Five different buffers, 0.100 M MES-NaOH buffered at pH 5.50,6.00, and 6.50,0.100 M Tris-HC1, pH 7.00, and 0.100 M BTP-HC1, pH 7.00, were used to study the effect of the pH on the response of the electrodes to thiocyanate. Figure 1 illustrates the results obtained when electrode A3 was exposed to the different MES-NaOH buffers. The detection limits (determined according to IUPAC recommendations (26))of the calibration curves for thiocyanate obtained with both pH 5.50 and 6.00 buffers were quite similar. Likewise, there was essentially no difference in the slopes of the calibration curves for these two buffer systems. The calibration curve that corresponds to the pH 6.50 buffer starts leveling off at con-
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
> E
J
220 -140
j
L V V .
-4.0
-2.0
-3.0
I
-5,O
log [Thiocyanate1
I
7
-4,O
I
- 3,O
'
I
-2,O
'
0
log [Anion]
Figure 2. Calibration curves of electrode A 3 for thiocyanate in the following buffers: )(. 0,100 M BTP-HCI, pH 6.00; (0)0.100 M MESNaOH, pH 6.00; (0)0.100 M NaH,PO,-NaOH, pH 6.00; (A)0.100 M citrate-NaOH, pH 6.00; (+) 0.100 M acetate-NaOH, pH 6.00.
Flgve 3. Selectivtty pattem of electrode A9 in 0.100 M MES-NaOH, pH 6.00. The electrode was exposed to (1) thiocyanate, (2) perchlorate, (3)Iodide, (4) nitrite, (5) salicylate, (6) bromide, (7) chloride, (8) bicarbonate, and (9) phosphate.
centrations of thiocyanate less than lo4 M. This is accompanied by a decrease in the potentiometric base line potential (i.e., potential of the cell before any addition of the standard solutions) compared to those of pH 6.00 and pH 5.50. The latter observation can be explained by OH- interference. As a consequence,the detection limit of the electrode at pH 6.50 is equal to 2 X loa M. The detection limits of the electrodes deteriorate even further when either the 0.100 M Tris-HC1, pH 7.00, or the 0.100 M BTP-HC1, pH 7.00, buffers were used (data not shown). These detection limits were 2 X 10"' and 4 X 10"' M, respectively. A study was undertaken to identify the best buffer solution at pH 6.00 for this type of electrode. For that, the performance of electrode A3 was tested in the following buffers: 0.100 M BTP-HCl, pH 6.00; 0.100 M MES-NaOH, pH 6.00; 0.100 M NaH2P04-NaOH, pH 6.00; 0.100 M citrate-NaOH, pH 6.00; 0.100 M acetate-NaOH, pH 6.00. As shown in Figure 2, both 0.100 M MES-NaOH, pH 6.00, and 0.100 M NaH2P04-NaOH, pH 6.00, are suitable buffers for the determination of thiocyanate. However, the experiments done with the 0.100 M MES-NaOH, pH 6.00, buffer presented slightly better slopes and detection limits for thiocyanate. As a result of the above studies, and unless otherwise stated, the rest of the experiments were performed with the 0.100 M MES-NaOH, pH 6.00, buffer. A typical selectivity pattern toward a series of anions presented by electrode A9 is illustrated in Figure 3. The ISE responded in the following order of preference to anions: thiocyanate > perchlorate > iodide > nitrite salicylate bromide > chloride > bicarbonate > dihydrogen phosphate. This anion-selectivity pattern deviates from that of the Hofmeister series (I),with the major deviations presented by the thiocyanate and nitrite ions. This suggests a selective interaction of the immobilized porphyrin with these two anions. With this particular electrode, experiments performed in the 0.100 M MES, pH 6.00,buffer yielded a detection limit for thiocyanate equal to 5 X lo-' M and a slope of -43 mV/decade. After 2 months the electrodes still had good slopes and detection limits. As mentioned earlier, several investigators have developed PVC-based electrodes by using Co(II1) porphyrins as ionophores (4,6,7). The selectivity observed by Hodinar and Jyo (6, 7) with electrodes that incorporated a Co(II1) tetraphenylporphyrin was as follows: thiocyanate > iodide > nitrite > perchlorate > bicarbonate > hydrogen phosphate > chloride bromide > nitrate sulfate. The slopes for thiocyanate were near-Nernstian (-49 to -55 mV/decade). The lifetimes of these electrodes were short, as demonstrated by the fact that their slopes deteriorated in just a few weeks. Similarly,
Ammann et al. (4) found that electrodes prepared with a lipophilic ester of Co(II1) tetraphenylporphyrin were selective for thiocyanate with slopes in the range from -49 to -63 mV/decade. Although both the electropolymerized films and the PVC electrodes that use cobalt porphyrins as ionophores respond primarily to thiocyanate, their selectivity properties are quite different. Specifically, based on data obtained from refs 4 and 6, the k&tN,r calculated for the ionophores of Ammann et al. and Hodinar and Jyo were 0.78 and 0.13, respectively. This indicates that in both cases there is only a slight discriminationfor thiocyanate over iodide. On the other hand, the electropolymerized [Co(o-NH2)TPP]films demonstrate a much larger discrimination between these two anions, as shown by a value of the k&tN,r of 5.1 X lo4 (calculated by the matched-potential method (27)). This may be due to differences in the ion-recognition properties of the porphyrin imposed by its immobilization in the matrix. Indeed, the porphyrin monomers are held in the membrane in a three-dimensional cross-linked structure. Such an arrangement should provide an additional control on selectivity by functioning as a "sieve" for ions. This molecular sieving property of electropolymerized porphyrin films has been confirmed by Murray and co-workers by obtaining permeability data in ultrathin (15-4600 A) poly[Co(o-NH2)n?P]films (28). As a consequence, in the potentiometric sensors reported here bulkier anions may be precluded from an effective interaction with the ionophore. There have been several reports on the use of quaternary ammonium salts for the development of thiocyanate-selective electrodes. Although these electrodes respond better to thiocyanate than to iodide, their preferred anion is perchlorate. This is consistent with the order expected from the Hofmeister series. A "truly" thiocyanate-selective electrode has been reported recently by Brown et al. (29). This electrode uses a PVC membrane that is impregnated with the ionophore
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-
-
(5,10,15,20-tetrakis(2,4,6-triphenylphenyl)porphyrinato)-
manganese(II1) chloride. The slopes of the PVC-based electrodes are closer to Nernstian than those of the electropolymerized film electrodes. However, the kEtNT of the electrodes reported by Brown et al. is lo9, which is slightly higher than the one reported here (i.e., it has worse selectivity). It should be noted that electropolymerization of the derivative employed by Brown et al. may yield electrodes with a lower k&tN-,r than the one obtained with the poly[Co(o-NH,)TPP] electrode. The reproducibility in the preparation of these ISEs was investigated by preparing several electrodes under the optimum electropolymerization conditions stated in Table I. Two different sets of electrodes were tested. Both families, com-
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
posed of electrodes Al, A6, A12, and A3, A9, A15, respectively, responded in a near-Nernstian fashion to thiocyanate. Electrode A3 had average slopes of -53 and -40 mV/decade over a period of 2 months when the experiments were performed in 0.100 M BTP-HCl, pH 6.50, and in 0,100 M MES-NaOH, pH 6.00, respectively. The other electrodes, such as A6 and Al, showed average slopes of -51 and -45 mV/decade, respectively, over the same approximate period of time in the BTP and MES buffers mentioned above. The family of the A3, A9, A15 sensors had better detection limits (e.g., 5 X 10-7M for thiocyanate with electrode A9 in the 0.100 M MES-NaOH, pH 6.00, buffer), making them more attractive for the analysis of thiocyanate in real samples. In both families of electrodes there is little deterioration of the slopes of the calibration curves as a function of time. In addition, the potentials before any addition of standard solutions (Le., base line potentials) were highly reproducible over this period of time; for electrode A3 the standard deviation of the starting potential was 6.2 mV (11experiments performed), indicating that this type of sensor may have a long serve life. Typically, the slopes of the electrodes improved during the first week and then remained steady to within f 2 mV/decade for the remainder of the testing period (2 months). Then, the electrodes were stored dry at room temperature and were tested after 9 months from their preparation date. Although the electrodes still showed response to thiocyanate, the slopes decreased substantially and the detection limits became worse. The above experiments indicate that the lifetime of these electrodes is at least 2 months, which is a significant improvement over the PVC-based electrodes that used Co(II1) tetraphenylporphyrin as the ionophore (6, 7). This may be attributed to the covalent fixation of the ionophore in the polymeric matrix of the poly[Co(o-NHz)TPP]electrodes;i.e., there is a reduced leaching of the ionophore out of the membrane. The above observations also suggest that the type of buffer used in the sample solution affects the slopes and detection limits of the ISEs. As mentioned above, the electrodes have the best detection limits in 0.100 M MES-NaOH, pH 6.00. However, by employing 0.100 M BTP-HC1, pH 6.50, we were able to achieve higher slopes. These observations were consistent with all the electrodes studied and allow for a choice of either one of the buffers depending on whether, in a particular case, a better slope or a lower detection limit is desired. In summary, the feasibility of using electropolymerized porphyrin films for the development of a highly selective thiocyanate electrode has been demonstrated. The obtained ISEs presented lifetimes of a t least 2 months. Since it has been reported that by changing the metal center or the side chains in the porphyrin ring different selectivity toward anions can be obtained (3-61, the electropolymerization of a variety of porphyrins could lead to the development of new, robust
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ISEs for anions that have unique selectivity properties.
LITERATURE CITED Schulthess, P.; Ammann, D.; Krautkr, B.; Caderas, C.; S t e p h k , R.; Simon, W. Anal. Chem. 1985. 57, 1397-1401. Chang. Q.; Park, S. 8.; Kim, D.; Cha, G. S.; Yhn. H.; Meyerhoff, M. E. Am. Blotechno/. Lab. 1990, 8(15), 10-21. Hambright, P. In Pwphvrtns and Meta/bporphyr/ns; Smith, K. M.. Ed.; Elsevier: Amsterdam, 1976; pp 233-278. Ammann, D.;Huser. M.; KrButler. B.; Rusterholz. E.; Schulthess, P.; Lindemann, B.; Ham, E.; Simon. W. Heh. Chlm. Acta 1988, 69, 849-854. Chaniotakis. N. A.; Chassef, A. M.; Meyerhoff, M. E.; Groves, J. T. Anal. Chem. 1988, 60, 185-188. Hodlnar, A.; Jyo, A. Chem. Lett. 1988, 993-996. Hodlnar, A.; Jyo, A. Anal. Chem. 1989. 67, 1169-1171. Chang, Q.; Meyhoff, M. E. Anal. Chlm. Acta 1988, 186, 81-90, Chanlotakis, N. A.; Park, S. B.; Meyerhoff, M. E. Anal. Chem. 1989, 67, 566-570. Abe, H.;Kokufuta, E. Bun. Chem. Soc. Jpn. 1990, 63, 1360-1364. Helnre, J. Top. Curr. Chem. 1990, 752, 1-47. Heineman, W. R.; Wleck, H. J.; Yacynych, A. M. Anal. Chem. 1980, 52, 345-346. Ohnuki, Y.; Matsuda, H.; Ohsaka, T.; Oyama, N. J . Ekboanal. (2”. InWacial E l e c t ” . 1983, 758, 55-67. Cheek, 380-381.0.; Wales, C. P.; Novak, R. J. Anal. Chem. 1983, 55, Aral, G.; Ishii, T.; Yamamoto. S.; Yasumori, 1. Bull. Chem. SOC. Jpn. 1988, 61, 787-791. Dong, S.; Sun, 2.; Lu, 2. Analyst 1988, 113, 1525-1528. White, B. A.; Raybuck, S. A.; Bettelheim, A.; Pressprich, K.: Muray, R. W. In Iwganlc and &ganometa& Polymers; Zeldin, M., Wynne, K. J., AUcock, H. R., Eds.; American Chemical Society: Washlngton, DC, 1988; pp 408-419. White, B. A.; Mwray, R. W. J . Ektfasnal. Chem. Interfacial Elecb.0& e m . 1985, 789, 345-352. Macor, K. A.; Spiro, T. 0. J . Elecboanal. Chem. Interfacial Electrochem. 1984, 763, 223-236. Maiinskl. T.; Cisrewski, A.; Fish, J. R.; Czuchajowski, L. Anal. Chem. 1990. 62, 909-914. Bettelhelm, A.; White, B. A.; Raybuck, S. A.; Murray, R. W. Inorg. Chem. 1987, 26, 1009-1017. Lawton, R. S.; Yacynych, A. M. Anal. Chlm. Acta 1984, 760, 149-158. Moody. 70, 71-106. 0. J.; Saad. B. B.; Thomas, J. D. R. Sei. Elec@o& Rev. 1988, Daunert, S.; Bachas, L. 0. Anal. Chem. 1990. 62, 1428-1431. Bettelheim, A.; White, B. A.; Murray, R. W. J . Electfasnal. Chem. Interfacial E l e c t ” . 1987, 277. 271-286. 48, 129-132.on Analytical Nomenclature. Pwe Appl. Chem. 1975, Commission Attlyat, A. S.; Kadry, A. M.; Badawy, M. A.; Hanna, H. R.; Ibrahim, Y. A.; Chrlstlan, 0. D. Electroanalysis 1990, 2, 119-125. Pressprlch, K. A.; Maybury, S. 0.;Thomas, R. E.; Linton, R. W.; Irene, E. A.; Murray, R. W. J. Phys. Chem. 1989, 93, 5568-5574. Brown. D. V.; Chanlotakls, N. A.; Lee, H. I.; Ma, S. C.; Park, S. B.; Meyerhoff, M. E.; Nick, R. J.; Groves, J. T. Elecboanalysls 1989, 7 , 477-484.
RECEIVED for review January 14,1991. Accepted May 6,1991. This work was supported in part by a grant from the National Science Foundation (DMR-9000782). Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. A.F. thanks CIRIT, Generalitat de Catalunya, for a research grant to support his stay at the University of Kentucky.
