Anal. Chem. 1991, 63,653-656
ACKNOWLEDGMENT We thank Rosalind Mitchell and Nancy Harmony for assistance in the preparation of the manuscript.
LITERATURE CITED Prabhu, S.;Baldwin. R. Anal. Chem. 1989, 61, 852-856. Prabhu, S.;Baldwin, R. Anal. Chem. 1989, 61, 2258-2263. Santos, L. M.; Baldwin, R. P. Anal. Chem. 1987, 59, 1766-1770. Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A-390A. Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1988, 6 0 , 2379-2384. Kost, K . M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1990, 62, 151-157. Hoyer, B.; Florence, T. M.; Batley, G. Anal. Chem. 1987, 59, 1608-16 14. Nagy, G.; Gerhardt, G. A.; Oke, A.; Rice, M. E.; Adams, R. N.; Moore, R. B.;Szentirmav. M. N.: Martin. C. R. J. Electroanal. Chem. 1985. 188, 85-94. Wang, J.; Tuzhi, P.; Golden, T. Anal. Chim. Acta 1987, 194, 129-138 . -. . -.
Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 1639- 1641. Martin, C. R.; Dollard. K. A. J. Electroanal. Chem. 1983, 159, 127-1 35. Kristensen. E. W.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1987, 59, 1752-1757.
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(13) Brazell, M. P.; Kasser, R. J.; Renner, K. J.; Feng, J. X.; Moghaddam, B.;Adams, R. N. J. Neurosci. Methods 1987, 2 2 , 167-172. (14) Harrison, D. J.; Turner, R. F. B.; Baltes. H. P. Anal. Chem. 1988, 6 0 , 2002-2007. (15) Luo, p.; Prabhu, s. V.; Baldwin, R. P. Anal. Chem. 1990, 6 2 , 752-755. (16) Miller, B. J . Electrochem. SOC. 1969, 116, 1675-1680. (17) Pyun, C . H.; Park, S. M. J. Nectrochem. SOC. 1986. 133, 2024-2030. (18) CRC h'andbook of Chemistry and Physics, 64th ed.; CRC Press: Boca Ration, FL, 1983-1984; Table 1 of Electrochemical Series, D-156. (19) Latimer, W. M. The Oxidation States of the Elements and their Potentials in Aqueous Solutions: Prentice-Hall: New York, 1938. (20) Lange's Handbook of Chemistry, 13th ed.; Dean, J. A.. Ed.; McGrawHill Book Co.: New York, 1985; Table 6-2. (21) Burke, L. D.; Ahern, M. J. G.; Ryan, T. G. J . Electrochem. SOC.1990, 137, 553-561. (22) Kok, W. Th.; Hankamp, H. B.; Bos, P.; Frei, R. W. Anal. Chim. Acta 1982, 142, 31-45.
RECEIVED for review April 27,1990. Accepted December 10, 1990. This work was supported by the Shimadzu Corporation, Kyoto, Japan, and the Kansas Technology Enterprise Corporation under their Applied Research Matching Grants Program. Juan Marioli acknowledges the support of the National Research Council of Argentina.
CORRESPONDENCE Enhanced Poly(chlorotrifluoroethy1ene) Composite Electrodes Sir: Poly(chlorotrifluoroethy1ene)composite electrodes were first introduced in 1978 (I). The first such electrode, affectionately known as the Kelgraf electrode, consisted of graphite compression molded with the inert binder poly(ch1orotrifluoroethylene) or Kel-F (3M Company tradename). These composite electrodes have been characterized electrochemically, as well as with electron microscopy and surface chemical methods, and have been shown to behave as microelectrode ensembles ( 2 , 3 ) .The improved signal-to-noise ratios observed with these electrodes are caused by the enhanced current densities due to the contribution of nonlinear diffusion. These electrodes have proven to be most useful as voltammetric detectors for liquid chromatography and flow injection analysis (4-7) where an additional enhancement effect contributes to the analytical signal, namely the depletion layer recharging effect (8). In more recent work, Kel-F has been used as an inert binder with other conductors. The Kelsil, Kelgold, and Kelplat electrodes are made with the precious metals silver, gold, and platinum, respectively (9, IO). Although work has been done to optimize the performance of these composite electrodes by controlling the weight percent of the conductor, little attention has been given to the active site radii, which have the potential of drastically affecting the behavior of these electrodes. Previous studies have controlled the size of the Kel-F resin and the graphite particles used prior to the compression molding step. However, independent studies have yielded similar results for the active site radii that are on the order of 25 wm ( 2 , I I ) . Note that, in all instances, the conductor particles have been considerably smaller than that of the plastic. Therefore, the active sites have been considered to be made of consolidated conductor between the particles of the inert binder. In fact, these electrodes have been classified as consolidated composites consisting of a network of conducting particles within the inert polymer (IO). In this work, we described a procedure for reducing the
active site radii of the Kel-F graphite. As will be shown, a reduction in the active site radii for an electrode with a given percentage of graphite significantly enhances the current. The grinding procedure used to accomplish this particle size reduction has the additional advantage of providing a more easily polished electrode and allows electrodes to be made with higher percentages of graphite than have been possible in the past.
EXPERIMENTAL SECTION Reagents and Materials. All chemicals used were of reagent grade. The Kel-F 81 resin was obtained from the 3M Commercial Chemicals Division, St. Paul, MN. The powdered graphite (UCP-2-325) was obtained from Ultra Carbon Corp., Bay City, MI. The Kel-F rod used in the electrode fabrication was from Plastic Profiles Inc., East Hanover, NJ. Instrumentation Procedures. The Kel-F graphite electrodes were prepared with an apparatus and procedure described previously (11). The only change in the procedure involved grinding the particular Kel-F graphite mixtures prior to the compression molding step. The mixtures were ground in a micronizing mill obtained from McCrone Accessories and Components, Chicago, IL, for 60 min using corundum grinding elements. The mixture was then compression molded. The resulting Kel-F graphite pellets were then machined to a diameter of 3 mm and press fit into a hole in a pure Kel-F rod. Epoxy was placed around the Kel-F graphite pellet prior to the press fitting to ensure that no seepage of solution occurred between the Kel-F rod and the electrode material. Electrical contact was made with a brass rod through a hole at the opposite end of the Kel-F rod. The electrodes were polished with successively finer suspensionsof alumina down to 0.05-gm particle size. It was found that less time was required to obtain a mirrorlike surface than with the electrodes fabricated without grinding. The potential step experiments were performed by using a homemade potentiostat interfaced to an Apple IIe computer. The interface consisted of a 12-bit analog-to-digital converter board (TM-AD213) and a 12-bit digital-to-analog converter board (TM-DAlOl), both from TecMar,
0003-2700/91/0363-0653$02.50/00 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
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I / t'/2 Flgure 1. Theoretical chronoamperometric response for a hexagonal microelectrode array with various active site radii ((-) 20 pm, (---) 2.5 pm, (--) 1 pm, (-.) 0.5 pm, (---) 0.1 pm). The current is normalized to the geometric area of the array. Each response is for an electrode that has a 10% active area. Parameters used include n = 1, D = 0.76 X cm2/s,and C = 1 X M.
Inc. The software was written in machine language and Applesoft basic. The simplex optimization were performed on either the Apple IIe in Applesoft basic or on an IBM PS Model 20-286 in Quick basic.
RESULTS AND DISCUSSION Various theoretical expressions have been reported over the years to predict the chronoamperometric behavior of microelectrode ensembles (12-14).We have chosen to use the expression given by Shoup and Szabo (13)that predicts the current response in a potential step experiment at an ordered hexagonal array of microelectrodes. In addition, the theory given by Scharifker (14)was also used, which is an approximate expression for the chronoamperometric response a t a random array of microelectrodes. Figure 1 illustrates the chronoamperometric response of a 10% active electrode with various active site radii. The theory of Shoup and Szabo was used to produce this figure and the parameters used are given in the caption. It is immediately clear that a reduction in the active site radii leads to a significant enhancement in the current for the time domain used (0.01-1 s). This is due to the increased contribution of nonlinear diffusion. Note that the 20-pm active site radii given corresponds to that of typical present day Kel-F composite electrodes. The most interesting result illustrated in Figure 1 is that when the active site sizes are reduced to 0.1 pm, the microelectrode array behaves as though the entire geometric area is electrochemically active. This results from the complete overlap of adjacent diffusion layers on this time scale. This situation would be advantageous since the theory for the electrodes behavior then becomes the same as that of a solid electrode while the charging current and/or noise is still related to the active area of the electrode (l/lOth of the geometric area in this example). T o reduce the active site radii of the Kel-F graphite electrodes, we considered a number of approaches. Since graphite can be purchased with particle sizes less than 1 pm, the problem appeared to lie with the Kel-F resin. Although the resin has been sieved in the past, this is a nuisance because of the buildup of the static charge. In addition, the yield of particles less than 45 pm in diameter is very low when using the resin as it comes from the manufacturer. Attempts to grind the Kel-F resin have been unsuccessful because the Kel-F plastic tends to reweld itself together during the grinding process. Therefore, we chose to grind the Kel-F and graphite together prior to the compression molding step. The grinding procedure and the apparatus used are given in the Experimental Section. Microscopic examination of a 10% graphite mixture after grinding indicated that no distinction could be made between particles of Kel-F and graphite. The
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Flgure 2. Chronoamperometricresponse of two 10% Kel-F graphite electrode fabricated with the grinding process ( X , A)and one fabricated without grinding (0).The s o l i lines are the theoretical fits. The electroactive species was 2 mM ferricyanide in 1.0 M KCI (pH 2.8). Potential step +0.6 to -0.2 V v s AgCl (2 M KCI).
Table I. Active Site Radii and Percent Active Area Determined by Using the Simplex Optimizationa
hexagonal array theory active site % graphite
radii, pm
70 active area
5 10 15 20
4.3 4.0 (28) 6.5 (30) 5.4 (41)
14 17 (9) 36 (22) 36 (24)
random array theory active site % active radii, pm area 1.9 3.3 1.5 2.4
12 19 22 32
All values are the average from at least two electrodes. Terms in parentheses were determined by using electrodes prepared without the grinding process.
average diameter of the particles was about 25 pm. The graphite appears to be coating the Kel-F particles or is compacted into them. Therefore, conductivity of the material after molding can still be explained in terms of a network of conducting graphite particles. A series of electrodes was prepared as described in the Experimental Section with weight percentages of graphite of 5%, 1070,1570,and 20%. In the past, it has not been possible to fabricate these electrodes with percentages above approximately 25% because the mechanical integrity of the material deteriorates above this value. However, since the grinding procedure produces a more intimate mixture, we attempted to use higher percentages. T o our surprise, it was possible to produce electrodes with up to 90% graphite that could still be machined in a lathe. However, we have found that electrodes that are greater than 50% graphite are somewhat POrow as was evident from memory effects and high background currents. In addition, it will be seen that there is no need to use percentages of graphite above 40% or 50%. The results on 30% and 40% graphite electrodes are also included in this work. To characterize the electrodes with the low percentages of graphite (5%-20%), chronoamperometric experiments were performed with 2 mM ferricyanide as the electroactive species in 1 M KC1 (pH 2.3). The theory of Shoup and Szabo was then fit to the experimental data by using a simplex optimization to determine the size of the active sites and the percent active area. Figure 2 shows the experimental chronoamperometric responses of two 10% graphite electrodes fabricated with the grinding procedure. The response of a 10% electrode fabricated without the grinding procedure is also shown for comparison purposes. Also shown are the theoretical fits for the three electrodes (solid lines). It is immediately obvious that the grinding procedure provides a significant enhancement in the current, reflecting a reduction
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991 1600
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an electrode that has active site radii of 1 pM. Parameters used in cm2/s, and C = 1
the calculations include n = 1, D = 0.76 X x 10-3 M.
in the size of the active sites. Table I summarizes the results of the theory-experiment fits for the low-percent graphite electrodes. Note that the theory of Scharifker was also used with the simplex optimization to obtain estimates of the active site radii and the fraction of the surface that is active. For three of the percentages, results are also given for electrodes fabricated without the grinding process. As may be seen from the results, the size of the active sites has been reduced by approximately an order of magnitude, which explains the enhanced currents. Although good fits were obtained, we view these results as estimates of these parameters since neither of the theories used are based on accurate models of the electrodes surface. Examination of the simplex surface indicated that the minimum deviation between theory and experiment lies along a long valley regardless of the theory used. This is probably a result of the limited data used in the optimization (0.01-1 s). As a result, small changes in the experimental data can cause significant changes in the position of minimum deviation (determined values of active site size and fraction active). Furthermore, the data in Table I suggest that the active area of the electrodes is increased by the grinding process, which is unlikely. This apparent increase in the active area is undoubtedly a result of both the models used and the limited amount of data. However, preliminary examination using electron microscopy confirms that the active sites are on the order of 1pm. Finally, it is interesting to note that a simple closest packing model of uniform Kel-F spheres indicates that the active site size should decrease as the square of the radius of the spheres. In previous work, the Kel-F particles used were on the order of 75 pm in diameter. Using the 25-pm diameter observed here for the Kel-F graphite particles, we expect the active site sizes to be decreased by a factor of about 9 as a result of the grinding process. This is in agreement with the approximately 10-fold reduction observed. Given that we have not reduced the size of the active sites to the point a t which the surface behaves 100% electrochemically active, we turn our attention to the higher percent graphite electrodes. Figure 3 illustrates what we might expect for various percentages of graphite assuming that the active sites are 1 pm in diameter. The theory of Shoup and Szabo was used to produce this figure, and the parameters are given in the caption. Note that as the percentage of the surface that is active increases, the electrodes electrochemically approach the limit of 100% active geometric area. In this particular case (1-pm active sites), the electrodes have essentially 100% active geometric areas for percentages above 30%. This is significant as mentioned previously since the theory reverts to that of a solid electrode while the noise or background is
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Chronoamperometric response of four Kel-F graphite electrodes with increasing percentages of graphite ((0)10, (0)20,(V) 30, (0) 40). The lines are the theoretical fRs. The potential step was from +0.6 to -0.2 V vs Ag/AgCI (2 M KCI). The electroactive species was 2 mM ferricyanide in 1.0 M KCI (pH 2.8). Onehundred data points were collected at 10-ms intervals. Figure 4.
in principle only 30% of that of a solid electrode. It should be noted that the random array theory does not predict 100% active geometric areas until the surface is above approximately 40% active for the 1-gum sites on this time scale. Figure 4 shows the chronoamperometric responses of electrodes with compositions from 10% to 40% graphite. The solid lines are once again the theoretical curves based on the theory of Shoup and Szabo. The responses of the 30% and 40% electrodes are all quite similar in that they are all linear, indicating 100% active geometric areas. The theoretical fitting parameters determined for the 30% and 40% graphite electrodes are all about 1-gm active sites and 43% active surface area presumably reflecting the limit at which 100% active response occurs for this time domain. This may or may not reflect the true situation at the surface, which we are continuing to investigate. However, the fact that the electrodes behave 100% active is attractive and provides an easily machined alternative to glassy carbon electrodes. In addition, one might expect that there may always be some moderate variation in the average active site size and percent active area from electrode to electrode. However, if the active sites are small enough (the electrode behaves 100% active), the electrode's response will not be affected by this variation. It is interesting to note that glassy carbon has long been considered to have active sites on its surface that are presumably small enough and in sufficient numbers to yield 100% active behavior a t very short times. This in fact explains the relatively low capacitance of glassy carbon compared to other solid electrodes such as platinum. CONCLUSIONS We believe that the preliminary work reported here provides a procedure for significantly enhancing the behavior of Kel-F graphite electrodes. We are presently continuing the characterization of these electrodes and examining their performance as voltammetric detectors. Initial work in this area indicates that the fabrication procedure used here leads to substantial increases in the analytical signal when used in thin-layer cells. The work presented also points the way to further studies in this area. For example, if the active site radii can be reduced by another order of magnitude, 10% graphite electrodes should behave as though they are 100% active on the millisecond time scale. The extension of this work to the precious metal composites would obviously be useful if one envisions a low-percent platinum electrode that acts as though it were 100% active. We are continuing work in these directions. LITERATURE CITED (1) Tallman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051-1056. (2) Welsshaar, D. E.: Tallman, D. E. Anal. Cbem. 1983, 55, 1146-1151.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991 Petersen, S. L.: Weisshaar, D. E.; Tallman, D. E.; Schuize, R . K.: Evans, J. F.; DesJariais, S. E.; Engstrom, R . C. Anal. Chem. 1988, 6 0 , 2385-2392. Chesney. D. J.; Anderson, J. L.: Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981. 724, 321-331. Weisshaar, D. E.;Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981, 53, 1809-1813. Tallman, D. E.; Weisshaar, D. E. J. Liq. Chromatogr. 1983, 6, 2 157-2 172. Anderson, J. L.; Whiten, K. K.; Brewster, J. D.: Ou, T. Y.; Nonidez, W. K. Anal. Chem. 1985,5 7 , 1366-1373. Cope, D. K.: Tallman, D. E. J. Nectroanal. Chem. InterfaclalNectrochem. 1886, 205, 101-123. Petersen, S . L.; Tallman, D. E. Anal. Chem. 1988, 6 0 , 82-86. Petersen, S. L.; Taliman, D. E. Anal. Chem. 1990, 6 2 , 459-465. Anderson, J. E.: Hopkins, D.: Shadrick, J. W.: Ren, Y. Anal. Chem. 1989. 67, 2330-2332. Gueshi, T.: Tokuda, K.: Matsuda, H. J. Electroanal. Chem. Interfacial Electrochem. 1978, 89, 247-260.
(13) Shoup, D.; Szabo, A. J. Ektrmnal. Chem. InterfacialElectrochem. 1964, 760, 19-26. (14) Scharifker, B. R. J. Nectroanal. Chem. 1988, 240, 61-76.
Jeffrey E. Anderson* Jeffrey B. Montgomery Ren Yee
Department of Chemistry Murray State University Murray, Kentucky 42071
RECEIVED for review October 23, 1990. Accepted December 27,1990. We acknowledge financial support of this work by the Committee for Institutional Studies and Research a t Murray State University.
