670
Anal. Chem. 1987, 5 9 , 670-673
(8) Luque de Castro, M. D.; ValcBrcel, M.; Albahadily, F. N.; Mottola, H. A. J . Electroanal. Chern., in press. (9) Albahadily, F. N.; Mottola, H. A,, Anal. Chem., in press.
RECEIVED for review June 30, 1986. Accepted October 27,
1986. This work was part of Project No. 84-063 supported by the US-Spain Joint Committee for Scientific and Technological Cooperation. Such a support is gratefully acknowledged here.
Characterization of Electrode Heterogeneity with Electrogenerated Chemiluminescence Royce C. Engstrom,* Kirk W. Johnson,’ and Scott DesJarlais
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069
Eledrogenerated chem#wninescence(ECL) was used to form Images showlng the distrlbutlon of electrochsinlcal actMty at mkroheterogemous electrode surfaces. ECL was generated by the reactlon of lumlnol, rubrene, or Ru(bpy):+ at carbon electrodes. Images of the luminescence patterns were recorded photographlcaHy afler sutflclent maflcatlon to show the locatlon, size, and shape of lndlvlduai actlve regions. Epoxy-impregnated retlculated vitreous carbon electrodes served as test electrodes, since they possess active reglons that are vbible under magniflcatlon and external Illumlnatlon. Quantitative Information obtalned from the photographs lncluded the percent actlve area of the electrode surface and histograms showing the dlstributlon of actlve M e dknendons. Those parameters were also evaluated on a carbon paste electrode.
Electrodes with microscopically heterogeneous surfaces are frequently used by electroanalytical chemists in the form of composite electrodes. For example, the carbon paste electrode, prepared by mixing graphite powder with an organic binder such as mineral oil, has been used for years (1, 2 ) . More recently, carbon composites have been prepared with epoxy (3-6) or Kel-F (7-10) as the binder. The surfaces of electrodes prepared from composite materials contain regions of electrochemically active graphite as well as regions of the inactive binder, creating a heterogeneous surface with regard to electron transfer. A heterogeneous electrode with well-defined and visible active regions has been prepared from reticulated vitreous carbon with the void spaces filled with epoxy, and the surface sanded and polished to a smooth finish (11). Besides deliberate electrode design, microscopic heterogeneity has been reported as a result of adsorption processes (12,13) and the anodization of graphite (14). Microscopic heterogeneity has several implications to electroanalytical chemistry. First, the active area of a heterogeneous electrode is obviously less than that of a uniformly active electrode of the same geometric area. Second, the dimensional relationships between the active and inactive regions may result in significant nonlinear diffusion. Third, a signal-to-noise advantage has been demonstrated (9, 10). Information about heterogeneity on electrode surfaces has come from two general categories of techniques, those based on measurement of the total current through an electrode with the application of mathematical modeling, and those based Present address: Eli Lilly Co., Indianapolis, IN.
on microscopic probing of the electrode surface or the solution immediately adjacent to the electrode surface. In the first category, a number of theoretical treatments have appeared that attempt to relate the degree of coverage and dimensions of active and inactive regions to the results of various electrochemical experiments (15-22). Those theories have been used to estimate the dimensions of active and inactive regions on carbon paste electrodes (15)anodized graphite electrodes (14) and Kel-F graphite electrodes (9, 10). The theories generally rely on the assumption that active regions have uniform size, shape, and distribution. As a result, the information obtained from the use of those theories presents an average description of the electrode surface, without information regarding the distribution of active site dimensions or geometry. In the second category of techniques, “maps” of electrode surface activity have been generated with the use of various types of microprobes. Isaacs and co-workers have located centers of corrosion on metal surfaces with the use of scanning microreference electrodes or microreferenceauxiliary combinations (23-25). We have adapted a physiological technique known as iontophoresis to the spatial resolution of electrode activity and have applied it to studying the distribution of activity on graphite-epoxy surfaces (26, 27). We have also used carbon fiber or platinum ultramicroelectrodes to detect species generated a t microscopically localized regions of another electrode surface, with the two electrodes being part of a bipotentiostat circuit (28). In all of these microprobe-based techniques, the spatial resolution that has been achieved has been of the order of tens of micrometers. Lui and co-workers have obtained substantially better resolution in an adaptation of scanning electron-tunn e b microscopy, carried out in solution with the probe being a microelectrode that was electropolished to submicrometer dimensions (29). Butler has reported on a photoelectrochemical imaging technique, in which photocurrents were induced on microscopically localized regions of semiconductor electrode surfaces with a focused laser (30). The position of the electrode surface with respect to the laser beam was controlled with a stepper-motor driven stage in 1-km increments, so that photocurrents were plotted as a function of position or displayed as a video image (31)with a resolution of 3 pm. Rubinstein located nonconducting inclusions on metal surfaces by electrodepositing a polymer film on the surface. Inclusions of a t least 30 wm diameter showed up as regions devoid of the polymer coating (32). We report here a technique for obtaining an image of surface electrochemical activity based on the observation of electrogenerated chemiluminescence (ECL), a phenomenon that has been described extensively in the literature (33,34,
0003-2700/87/0359-0670$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 59, NO. 4. FEBRUARY 15. 1987
and references therein). In this experiment, the potential of the electrode being studied in stepped to a value that initiates the ECL reaction sequence. The light generated by the reaction originates at the electrochemically active regions of the electrode only, forming an image of activity distribution with the location, size, and geometry of individual active regions clearly visible. After magnification the image is recorded photographically. This technique has several important features: information gained is related directly to electrochemical activity and not structure; by careful choice of the ECL system, spatial resolution can be high since it is determined by the distance the excited species diffuses before releasing a photon; a relatively large fraction of the electrode or the entire electrode can be observed in a single experiment; the results are largely unaffected by surface topography. In this paper. the ECL imaging technique w a characterized ~ with model heterogeneous electrodes prepared from reticulated vitreous carbon and then used to image a carbon-paste surface. EXPERIMENTAL SECTION Apparatus. In experiments where photographic images of the ECL reactions were recorded, the electrode being studied was mounted in a cell machined from cast acrylic so that the electrode surface faced upward. The surface was covered to a depth of approximately 2 mm with a solution containing the compounds required for the ECL reaction (see Results). The tip of a silversilver chloride reference electrode and a platinum wire auxiliary electrode were immersed in the solution. The cell potential was applied with a conventional three-electrode potentiostat, usually a Bioanalytical Systems Model CVlA (West Lafayette, IN). Photographic images were recorded with a Nikon N2000 camera attached to a bellows and a 55-mm "micro" lens. The entire apparatus was kept in a darkroom, so that only the light generated by the ECL reaction reached the film. The equipment was mounted on a vibrationally isolated table 80 the pictures were not blurred by room vibrations. Electrodes. Glassy carbon electrodes were used 88 examples of uniformly active surfaces and were obtained from B i d y t i c a l Systems. The epoxy-impregnated reticulated vitreous carbon electrode (RVC-epoxy) used as a model heterogeneous electrode was prepared as described in the literature (11.26). Carbon paste electrodes were prepared by mixing graphite powder with squalane in a 2 1 weight ratio, packing some of the paste into a length of 5 mm diameter glass tubing, and inserting a piece of copper wire to make electrical contact with the paste. The surfaces of the wbon paste electrodes were made smooth by rubbing on a manila folder. Reagents. Luminol (5-amino-2,3-dihydro-1,4phthalazinedione)was obtained from Eastman Kodak (Rochester, NY); rubrene (5,6,11,12-tetraphenylnaphthacene)was obtained from Aldrich (Milwaukee,WI);tris(2,2'-bipyridyl)ruthenium(ll) chloride was obtained from Strem Chemicals, h e . (Newburyport, MA). All of the above were used as received. All supporting electrolytes and buffers were prepared from reagent grade chemicals without further purification. Procedure. Based on the initial investigations of the three ECL systems, the luminol system was used to obtain the images presented here. A solution containing 0.20 mM luminol, 0.10 mM hydrogen peroxide, and 0.05 M sodium borate, adjusted to pH 10.0, was placed in the cell. The lights were turned off in the darkroom, and the camera shutter was opened. A potential of 0.6 V vs. SCE was then applied to the working electrode, resulting in visible luminescence. After a predetermined length of time, the potential to the cell was turned off and the camera shutter closed. Usually, several exposure durations were taken for each experiment in the range of 5-15 min. RESULTS AND DISCUSSION Initial Characterization of ECL Systems. Three ECL systems were evaluated hy use of glassy carbon electrodes to determine which was most suitable for the characterization of electrode heterogeneity. The important criterion was to find a system that gave an emission intensity under nonconvective conditions that would be strong enough to photograph.
-
071
Flgure 1. ECL image of 3 mm diameter glassy carbon electrode a1 0.6 V vs. SCE in a luminol solution (see text for details);magnilicalii.
approximately lox.
A solution of 1.0 mM Ru(bpy)sz+,0.10 mM sodium oxalate, and 0.1 M acetate buffer at pH 6.0 yielded a faint orange luminescence a t an applied potential of 0.95 V. T h e luminescence in this system has been reported to result from the oxidation of Ru(hpy)?+ to Ru(bpy),,+ at the electrode, followed by chemical reduction of Ru(bpy)3z' to R u ( b p y ) j by the radical anion, COz'-, and the subsequent annihilation reaction between Ru(hpy)?* and Ru(bpy),* to produce Ru(bpy),*+ in the excited state (35-37). In our hands, the intensity generated hy this system faded rapidly to a level that was not usable. possibly due to fouling of the electrode. A solution of 0.43 mM rubrene. 0.41 mM benzoyl peroxide, and 0.1 M tetraethylammonium perchlorate, in dimethylformamide emitted a yellow light intense enough to be seen with the room lights on at an applied potential of -1.4 V. Deaeration of the solution was necessary to observe the emission. In this system, luminescence arises from the cathodic production of the anion radical of rubrene, which is then chemically oxidized by the presence of benzoyl peroxide, producing an excited state of ruhrene (3s). While the ruhrene system gave satisfactory results, it was not used for the work reported here since the nonaqueous solvent was not compatible with carbon paste electrodes. T h e solution of luminol described in the Experimental Section yielded a moderately intense blue luminescence at an applied potential of 0.6 V. T h e luminol reaction has been studied fairly extensively (39-42) and the luminescence is thought to result from the oxidation of luminol to a radical, which reacts with a superoxide radical to form 3-aminophthalate in an excited state. Of the three systems, the luminol system was chosen for the work reported here. Figure 1 shows a photograph of an ECL image generated at a 3 mm diameter glassy carbon electrode. The electrode was bathed in a solution containing the ingredients of the luminol ECL system, a t an applied potential of 0.6 V v8. SCE. As seen in the figure, the light intensity was quite uniform over the entire surface, indicating a uniformly active electrode as expected for highly polished glassy carbon. The small region of light below the large circle is a reflection from a bubble next to the electrode. Reticulated Vitreous Carbon Electrodes. To characterize the ability of ECL to provide information about electrode heterogeneity, a model electrode system was needed that had well-defined active and inactive regions that were visible under magnification. Figure 2A shows a photograph of an RVC-epoxy electrode under external illumination. In that photograph, the light-colored regions are the active regions of carbon in the darker epoxy matrix. As pointed out by Slesynski et al. ( 2 0 , the RVC-epoxy surface contains an array of irregularly shaped and sized active regions which tend to
672 ANALYTICAL CHEMISTRY. VOL. 59, NO. 4. FEBRUARY 15. 1987
Active Site S m . ym
Fbwe 3. Hbtosram of active regmdimensions for an RVCepoxy electrode taken fmm E U image.
6 Flov.2.
Epoxyimpregnated reticubtsd vlreow cabon ekhoa): (a)e m 1 i-tbn of d a c e . (bl E a image. CarmOns are
in Flgue 1,
rn
have a greater length than width, the former typically measuring in the hundreds of micrometers and the latter in tens of micrometers. In the presence of the luminol solution and a t an applied potential of 0.6 V vs. SCE,the active regions generated chemiluminescence. Figure 28 shows a photograph of the ECL pattern of the same electrode as in Figure 2A and under the same magnification. Upon careful inspection, it can be seen that the lumineseence image faithfully reproduces the pattern of active regions of the externally illuminated surface. Each active region on the electrode surface is represented in the ECL image by a region of exposure having the =me size. shape, and relative position as the active region. The ECL photograph was taken with a 5-min exposure. over which time the products of the electrochemical reaction would diffuse a considerable distance. However, at the magnification level used here the light orginates only at the active regions, indicating that either the duration of the ECL reaction sequence is short compared to the time needed for diffusion to occur over a deteetahle distance or that the emitting species is surface bound to the carbon. The latter was ruled out by gently agitating the solution and observing that the image blurred as the emitting species was convectively removed from the active regions. Of interest in an electroanalytical sense is the fraction of the geometric electrode area that is electrochemically active. On the RVC electrode, that fraction was evaluated by enlarging the image through projenion onto a sheet of graph paper. Each point of intersection of the graph paper w a s inspected to see if it fell within or outside a region of activity. The percent active area was calculated by dividing the numher
of intersections within active regions by the total number of intersections inspected. The total number of intersections used was 600,and the areas obtained by this method agreed within 1%of areas determined by a more tedious "cut-andweigh" method. (In the latter, regions of exposure on the enlarged image were traced onto a sheet of preweighed paper and cut out with a scapel. The residual paper was weighed and the percent active area calculated from the weight difference.) The active area of the RVC electrode as determined from the externally illuminated picture (Figure 2A) was 26.7% of the total area. From the ECL picture (Figure 2B)the active area was found to be 21.3%. The discrepancy suggests that not all of the carbon that is visible on the RVC surface is electrochemically active. Perhaps during fabrication or polishing the edges of the carbon regions become covered with a thin layer of epoxy that blocks the surface. Another quantitative descriptor of the electrode surface in concerned with the dimensions of the active regions. Indeed, one of the primary reasons for development of the technique was to provide detailed information about heterogeneous surfaces to replace the assumptions about active region size and shape used in existing theories of heterogeneous surfaces. While there could be several descriptors of active site size (linear dimension, area, "boundary density"), it is not certain which is the most appropriate with respect to electrochemical behavior. We have chosen to use a linear dimension to characterize the surface. A transparent grid was placed over the projected ECL image. with 12 evenly spaced vertical and horizontal lines. The distances between the point where a line entered a region of ECL exposure and where it left the same region was measured. Every line was used, and each line intersected a number of exposed regions. The distances were plotted in the form of a histogram which is shown in Figure 3 for an RVC-epoxy electrode. Carbon-Paste Electrodes. Electrodes prepared by mixing graphite powder with an inert binder p~isesselectmchemidy active regions of graphite and inactive regions of binder. Visual observation of the composite electrode surface, even with a microscope, does not allow the identification of active and inactive regions as with the RVC-epoxy electrodes. The ECL technique provides a visual image of the surface activity on a carbon paste electrode as shown in Figure 4. The active regions comprise 46.9% of the geometric area, determined by using the procedure described above for the RVC-epoxy electrode. The histogram of active region dimensions is shown in Figure 5, which shows that most active region dimensions fall under 100 pm, with a few larger than that. A detailed study of the relationship between bulk composition. surface activity distribution, and electrochemical behavior of carbon-paste electrodes is in progress. Detailed information about heterogeneity such as that generated by the ECL technique should help describe the behavior of composite (and other heterogeneous) electrodes in electroanalytical situations. For example, histograms of active region size combined with histograms of distance between active regions should make it possible to define the time
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY
15. 1987 073
(5) cheng.H. Y.: Fakl. L. Anal. Cham. 1982. 5 4 . 2111.
(6) chsng. H. Y.; Fakl, L. J . EkZnwnaI.ann.1903. 157.393. (7) A&-. J. E.: Tallman. D. E.: chrmey. D. J.; Andaaon. J. L. Anal. chem. 1918. 5 0 . 1051. (8) Weisshaar. D. E.: Tallman. D. E.; Andsraon. J. L. Anal. Unn. 1981. 5 3 . 1809. (9) We16ShBar. D. E.: Tallnun. D. E. Anal chan.WIS. 55. 1148. (IO) l a man. D. E.: WdoohBar. D. E. J . L q . ChaMmg.W81, 6.2157. (11) Seszynrkl. N.: Cmleryoung. J.: Caner. M Anal Urrn lW4. 56. 12"
Lindsmann. J.; Landriberg. R. J . Eb2WMnal. Uunn. 1971. 31. 107. Povarov. Y.;~ O V S ~ O P. Y . Elecboblm. Acta 1973. 18. 13. Scheller. F.: Landriberg. R.; WM. H. ElscfrocNm. Acta 1970. 15.525. Landsberg. R.; Thlele. R. Ek9mchh. Acta 1968. 11. 1243. Scheller. F.; Muller. S.: Landsberg. R.: SpIWer. H. J. J . E ~ V M M I .
."
C b m ...a.I._. 1 z . .07 10,. LWart. E.; Sdumann. D.; Conkmln. 0.; Elman. M. J. Ek!manaI. Chem . 1978. 70. 117. K.: Matsuds. H. J. EWtrcaml.chan.1978.89. OUBShi,, T.; T&&.
Flgure 4. ECL Image of
a carbon paste electrode.
Conditions are
given in Figure 1.
747. (19) cues~. T.: ~okuda.K.; ~ 0 ~ 6 H. 8 J.. E-I. ann.ien.i o i . 29. (20) Reller. H.: KlrowaIimw.E.; Glsadl. E. J . EbCVMnaI. chsm.1982. 138. 65. (21) Amatore. C.: Saveant. J. M.: Tesslrrr. D. J . Eb2WMml. chan.1983. ."7 ,., , a,."-. Contarnln. 0.:Lsvsn. E. J. E W M M I . auWn. 1982. 136. 259. Isaacs. H. S.; K W . 0. J . Ehlrc&sm. Soc. 1972. 119, 1628. Isaacr. H. S.; Kendig. M. W. cmosion 1980. 36. 269. Isaaacs. H. S. A S T M S p c . Tech. Publ. 1972. No. 518. 158. Engslrm. R. C.A w l . chan.1884.56. 890. Engslrorn. R. C.: Weber. M. ; Werih. J. Anal. Chsm. 19% 57. 933. Engstrorn. R. C.: Weber. M.; Wunder. D. J.; Bugess. R.: Wlnqubl. S. Anal. Uam. 1986. 54. 644. Lui. H.-Y.:Fan. F.4. F.:. Lin.. C. W.:. Bard.. A. J. J . Am. h m . SOC.
iois. ion. 3838.
A. J . E h @ o b s m . Soc. 1983. 130. 2358. (31) Butler. M. A. J . E!emocbm. Soc. 1984. 131. 2185. (32) Rubinstein. 1. J . A w l . Elscbochsm. 1985. 13. 689. 133) . . Bard. A. J.: Fadkner. L. R. Elecnochem&calM8fhcds: WiW: New York; I980 p 621. (34) Tachlkawa. H.: Faulkmr. L. R. Laborefay Tedn~!quesh E l s c b O e ~ (30) B u t k , M.
Active Site Sire. urn Ftpun 5. m l ~ m of active region dimensions for a carbonpasta elemode.
tytkal ChstWsby: K l s l ~ P. . 1..Heinaman. W. R.. E&.; Marcel Dek-
a t which the electrode makes the transition from behaving as an array of independent microelectrodes to an electrode that appears to be uniformly active.
ACKNOWLEDGMENT We wish to thank Simon Spicer and Elise Sudbeck for help with the photography and Timothy Nieman for his helpful discussions in the initial phases of the work.
LITERATURE CITED (1) Adam. R. N. Anal. chan.1958.30, 1576. (2) Adam. R. N. E k e d m n ! s O a1 sasd Ebcbc&s: Marc4 Dekka: New Ymk. 1989 pp 26. 280. l9M. 38. 966. (3) Swottad. H. S.: Carman. R. L. Anal. (4) Andsrm. J. E.: Tanman. 0. E. Anal. chem. 1978. 48. 209.
ker: New Ymk. 1964 p 880. (35) RUblNlein. 1.: Martin. C. R.: Bard. A. J. Anal. &m. 1983.55. 1580. (36) Ege. 0.; M e r . W. G.; Bard. A. J. AmI. Mam. 1984. 5 6 . 2413. (37)Rubinstein. 1.: Bard. A. J. J . Am. chsm.Soc. 1981. 103. 512. (38) Akinr. 0. L.: ~kke.R. L. ann.mp, Len. 1974.29. 428. (39) Haapakks. K. E.: Kankus. J. J. Anal. CMll. Acta 1982. 138. 263. (40) Merenyi. G.: L i d . J. S. J . Am. cylem. Soc. 1980. 102. 5830. (41) Epslein. 0.; Kuwana. T. PhOlOdWm. ~ I M1987. . 6 . 605. (42) Kuwana. 1.;Eprleln. E.: Seo. E. T. J . Am. Chsm. Soc. 1964. 8 7 . 2243.
RECEW for review August 25,1986. Accepted October 27, 1986. This work was supported in part by the National Science Foundation, Grant No. CHE-8411000.K.W.J.received a Grant-in-Aid of Research from Sigma Xi, the Scientific Research Society.
