Anal. Chem. 2000, 72, 2914-2918
Electrochemiluminescence of Ruthenium(II) Tris(bipyridine) Encapsulated in Sol-Gel Glasses Maryanne M. Collinson,* Brian Novak, Skylar A. Martin,† and Jacob S. Taussig
Department of Chemistry, 111 Willard Hall, Kansas State University, Manhattan, Kansas 66506-3701
The electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ and tripropylamine, tributylamine, triethylamine, trimethylamine, or sodium oxalate encapsulated within sol-gel-derived silica monoliths have been investigated using an immobilized ultramicroelectrode assembly. The major purpose of this study was to investigate the role of the reductant on the magnitude and stability of the ECL in this solid host matrix. For gel-entrapped Ru(bpy)32+/ tertiary amines, the shape and intensity of the ECLpotential curves were highly dependent on scan rate. At 10 mV/s, the ECL intensity was ca. 6-fold higher relative to that observed at 500 mV/s. When the ECL acquired at low scan rates was normalized by that obtained in solution under similar conditions, a value of 0.03-0.06 was obtained. In direct contrast, the ECL of the Ru(bpy)32+oxalate system showed little dependence on scan rate, and the ECL was ca. 65-75% of that measured in solution. These differences can be attributed to differences in rotational and translational mobility between the reductants (amines vs oxalate) trapped in this porous solid host. For both systems, the ECL was found to be stable upon continuous oxidation or upon drying the gels in a highhumidity environment for over 10 days. Electrogenerated chemiluminescence (ECL), the production of light from electrochemically generated reagents,1 has received considerable attention during the past several decades due to its versatility, simplified optical setup, and good temporal and spatial control. Applications in the general areas of chemical sensing,2-4 imaging,5-7 lasing,8 and optical studies9 and as detectors for chromatography10,11 have been widely reported. The entrapment or covalent attachment of ECL precursors in a polymer host † Current address: Department of Chemistry, Truman State University, Kirksville, MO 63501. (1) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry, Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol 10, pp 1-95. (2) Knight, A. W. Trends Anal. Chem. 1999, 18, 47-62 and references therein. (3) Lee, W.-Y. Mikrochim. Acta 1997, 127, 19-39 and references therein. (4) Gerardi, R. D.; Barnett, N. W.; Lewis, S. W. Anal. Chim. Acta 1999, 378, 1-41 and references therein. (5) Wightman, R. M.; Curtis, C. L.; Flowers, P. A.; Maus, R. G.; McDonald, E. M. J. Phys. Chem. 1998, 102, 49, 9991-9996. (6) Engstrom, R. C.; Pharr, C. M.; Koppang, M. D. J. Electroanal. Chem. 1987, 221, 251-255. (7) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987, 59, 67073. (8) Kozlov, V. G.; Bulovic, V.; Burrows, P. E.; Forrest, S. R. Nature 1997, 389, 362-64. (9) Fan, F.-R. F., Cliffel, D.; Bard, A. J. Anal. Chem. 1998, 70, 2941-2948.
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structure has also been investigated as a means to reduce the consumption of reagents, probe the nature of charge and mass transfer within polymers, and develop solid-state electrochemical devices for imaging applications.12-23 ECL, for example, has been observed from polymer films of diphenylanthracene and polyphenylenevinylene derivatives,17,18 electropolymerized films of ruthenium tris(bipyridine) derivatives,16,19-20 and ruthenium derivatives trapped in an ionic polymer or bound to a polymer framework.15,21-23 Sol-gel-derived materials prepared by the hydrolysis and condensation of alkoxysilanes (i.e., tetramethoxysilane) are an attractive alternative to many polymeric materials for solid-state applications due to the ease at which they can be prepared, modified, and doped with various reagents.24-27 In their hydrated state, sol-gel-derived materials can be quite porous, thus providing a good matrix to entrap chemiluminescent reagents that must diffuse together to react.28 Recently we have shown the ECL can be observed from ruthenium(II) tris(bipyridine), Ru(bpy)32+, trapped in a silica gel host using gel-entrapped tripropylamine (TPA) as the reductant.29,30 The Ru(bpy)32+-TPA system was chosen because it has been well studied in solution and shown to give rise to ca. 10-fold higher ECL compared to other commonly used reductants such as oxalate.31,32 Relative to that obtained in (10) Noffsinger, J. B.; Danielson, N. D. J. Chromatogr. 1987, 387, 520-24. (11) Lee, W.-Y., Nieman, T. A. J. Chromatogr. 1994, 659, 111-118. (12) Rao, N. M.; Hool, K.; Nieman, T. A. Anal. Chim. Acta 1992, 266, 279-286. (13) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195-201. (14) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781-2787. (15) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 6642-44. (16) Abruna, H. D.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 2641-2642. (17) Fan, F. R. F.; Mau, A.; Bard, A. J. Chem. Phys. Lett. 1985, 116, 400-404. (18) Richter, M. M.; Fan, F.-R. F.; Klavetter, F.; Heeger, A. J.; Bard. A. J. Chem. Phys. Lett. 1994, 226, 115-120. (19) Maness, K. M.; Terrill, R. H.; Meyer, T. J.; Murray, R. W.; Wightman, R. M. J. Am. Chem. Soc. 1996, 118, 10609-10616. (20) Elliott, C. M.; Pichot, F.; Bloom, C. J.; Rider, L. S. J. Am. Chem. Soc. 1998, 120, 6781-6784. (21) Downey, T. M.; Nieman, T. A. Anal. Chem. 1992, 64, 261-268. (22) Lyons, C. H.; Abbas, E. D.; Lee, J.-K., Rubner, M. F. J. Am. Chem. Soc. 1998, 120, 12100-12107. (23) Wu, A.; Lee, J.; Rubner, M. F. Thin Solid Films 1998, 327-9, 663-667. (24) Brinker, C. J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1989. (25) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (26) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A-1127A. (27) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67, 22A-30A. (28) Collinson, M. M.; Zambrano, P. J.; Wang, H.; Taussig, J. S. Langmuir 1999, 15, 662-668 (29) Collinson, M. M.; Taussig, J.; Martin, S. A. Chem. Mater. 1999, 11, 25942599. (30) Collinson, M. M.; Martin, S. A. Chem. Commun. 1999, 10, 899-900. 10.1021/ac9913208 CCC: $19.00
© 2000 American Chemical Society Published on Web 04/29/2000
solution, however, the ECL intensity in the solid-state host was low. To increase the intensity of the ECL, it is first necessary to understand some of the factors that influence light production in these porous solids. In this work, we have examined the ECL of gel-entrapped Ru(bpy)32+ in the presence of reductants of different sizes and charge to investigate the role of the reductant on the magnitude and stability of the ECL in this solid host matrix. Specifically, tertiary amines with different alkyl chain lengths (methyl, ethyl, propyl, butyl) and sodium oxalate were used to provide insight into surface confinement and interfacial interactions in this solid host structure. Such interactions have been shown to be important in regulating molecular motion in silica glasses33-35 and will inevitably influence the performance and usability of these materials in solidstate electrochemical applications. A simplified reaction mechanism for the reaction of Ru(bpy)32+ with these reductants is given below. In this reaction, oxalate is oxidized to an intermediate radical anion (e.g., CO2•) which acts as a strong reducing agent.3 For the tertiary amines, the intermediate is believed to be the deprotonated amine radical.2-4
Ru(bpy)32+ f Ru(bpy)33+ + 1eRu(bpy)33+ + reducing agent f [Ru(bpy)32+]* [Ru(bpy)32+]* f Ru(bpy)32+ + hν (610 nm)
These results show that the ECL in these solid host structures strongly depends on the nature of the entrapped reagents and the physicochemical properties of the host material. EXPERIMENTAL SECTION Reagents. Tetramethoxysilane (99%), tributylamine (98.5+%), tripropylamine (98%), triethylamine (99.5%), trimethylamine (40% in water), sodium oxalate (99.5%), and ruthenium(II) tris(bipyridine) (98%) were purchased from Aldrich. All reagents were used as received. Potassium hydrogen phosphate and potassium dihydrogen phosphate were purchased from Fisher Scientific. Deionized water was purified to type I with a LabConco water purification system. Procedures. The microdisk electrodes (r ) 13.3 µm Pt) and the silica sols were prepared as previously described.29,30 In brief, tetramethoxysilane (TMOS) was mixed with water and hydrochloric acid (0.1 M) and the solution stirred for several hours. An aliquot of the silica sol was combined in a 2:1 volume ratio with the precursor (e.g., sodium oxalate) previously dissolved in phosphate buffer (either 0.3 or 0.1 M, pH 6.2) in a silanized glass vial containing a r ) 13.3 µm Pt electrode and a silver chloride coated silver wire reference/auxiliary electrode. The sol gelled within a few minutes. The pH of the gels was estimated using indicator dyes. For those gels prepared with 0.3 M, pH 6.2 phosphate buffer, the pH of the sol was ca. 6. For those gels prepared with 0.1 M, pH 6.2 buffer, the pH was estimated to be ca. 5-5.5. The gel was aged and dried for typically 2-3 days under (31) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127-3131. (32) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865-868. (33) Koone, N.; Shao, Y.; Zerda, T. W. J. Phys. Chem. 1995, 99, 16976-16981. (34) Sieminska, L.; Zerda, T. W. J. Phys. Chem. 1996, 100, 4591-97. (35) Watson, J.; Zerda, T. W. Appl. Spectrosc. 1991, 45, 1360-1365.
Figure 1. Current-potential curves for gel-encapsulated Ru(bpy)32+ (a) and gel-encapsulated Ru(bpy)32+ (10 mM) and oxalate (5 mM) (b) acquired at 10 and 500 mV/s. Corresponding emission potential curves are also shown for gel-encapsulated Ru(bpy)32+ and oxalate. The gels were prepared with 0.1 M, pH 6.2 phosphate buffer.
a 60-70% relative humidity environment. The glass vial containing the gel was placed within 4-5 cm of the photocathode of a Hamamatsu 4632 photomultiplier tube and the signal amplified with a EGG VT 120A fast preamplifier and collected with an EGG Ortec T-914 multichannel scaler. A BAS 50 potentiostat modified to trigger the multichannel scaler or a Pine AFRDE5 bipotentiostat was used to apply the excitation waveform.29,30 RESULTS AND DISCUSSION Voltammetry and ECL of Gel-Encapsulated Ru(bpy)32+. Ru(bpy)32+ and/or the reductant (oxalate or amine) along with a r ) 13 µm Pt working electrode and a Ag/AgCl reference electrode were encapsulated into the silicate host by simply adding the reagents and the electrodes to the sol prior to gelation. The sols gelled quickly after addition of phosphate buffer, thus entrapping the reagents into a porous, hydrated solid. Figure 1 shows cyclic voltammograms (CVs) of gel-encapsulated Ru(bpy)32+ and Ru(bpy)32+-oxalate at a r ) 13.3 µm Pt electrode. In the absence of oxalate, the CVs at slow sweep rates (e.g., 10 mV/s) for gel-encapsulated Ru(bpy)32+ are steady state in nature and superimposed on a large rising background due to solvent oxidation. As the scan rate is increased, the voltammograms become more peak shaped in appearance as expected for (36) Wightman, R. M.; Wipf, D. O. Voltammetry at Ultramicroelectrodes. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15.
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Figure 3. Scan rate dependence of the maximum ECL intensity for gel-encapsulated Ru(bpy)32+ and (b) TPA, (3) TEA, (9), TMA, and (]) oxalate. The gels were prepared with 0.1 M, pH 6.2 phosphate buffer.
Figure 2. Current-potential curves for gel-encapsulated Ru(bpy)32+ (a) and gel-encapsulated Ru(bpy)32+ (10 mM) and triethylamine (TEA) (5 mM) (b) acquired at 5 and 250 mV/s. Corresponding emission potential curves are also shown for gel-encapsulated Ru(bpy)32+ and TEA. The gels were prepared with 0.1 M, pH 6.2 phosphate buffer.
an ultramicroelectrode.36 In the presence of oxalate, the oxidation current for gel-entrapped Ru(bpy)32+ is significantly larger than that observed for Ru(bpy)32+ alone, and the half-wave potential is shifted negatively, consistent with an electrocatalytic reaction mechanism. The corresponding ECL-potential curves are also shown in Figure 1. The increase in luminescence is contiguous with the oxidation of gel-encapsulated Ru(bpy)32+ and oxalate. At low sweep rates, the ECL-potential curves are steady state in appearance analogous to the current-potential curves observed at the microdisk electrodes. At higher scan rates, the forward scan in the current-potential curve becomes more peak shaped as does the resultant ECL due to the onset of planar diffusion.36 In the absence of either Ru(bpy)32+ or oxalate, no ECL was observed. The current-potential and ECL-potential curves for Ru(bpy)32+ in the presence of TMA, TEA, TPA, or TBA are considerably different. Figure 2 shows the CVs and corresponding ECL-potential plots for gel-encapsulated Ru(bpy)32+ and Ru(bpy)32+-TEA. In contrast to that observed for the Ru(bpy)32+oxalate system, the current for the Ru(bpy)32+-TEA system is only slightly higher than that observed for Ru(bpy)32+ alone even at 5 mV/s. The shape and intensity of the ECL-potential curve are also highly dependent on scan rate. At low sweep rates (5 or 10 mV/s), the ECL observed on the forward scan is higher than that observed on the reverse scan, and it drops at large overpotentials. This drop is likely due to concentration polarization resulting from the reduced diffusion of the amines in the gel (see below). As the scan rate is increased, the ECL observed on the forward scan becomes significantly less than that observed on the reverse scan until no ECL is observed at all (>0.75 V/s). This drop cannot be the result of an increase in ohmic drop (iR) at 2916 Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
the higher scan rates because under the conditions used in this work (ca. 30-100 mM supporting electrolyte, 5-750 mV/s), the r ) 13 µm Pt microelectrode will be essentially immune from such effects.36 The observed scan rate dependence of the ECL can more clearly be seen in Figure 3, which shows a plot of the maximum ECL intensity acquired from either the forward or reverse scan vs scan rate (ν) for the Ru(bpy)32+-oxalate and Ru(bpy)32+-amine systems. The ECL intensity for the Ru(bpy)32+-oxalate system was nearly invariant with scan rate up to ca. 1 V/s, whereas the ECL of the Ru(bpy)32+-amine systems showed a significant drop at scan rates greater than 100 mV/s. The slight increase in ECL with scan rate for the Ru(bpy)32+-oxalate system may be due to the slight adsorption of oxalate on the electrode surface at low sweep rates.37 The overall rate at which this system responds to changes in electrode potential will depend on the chemical kinetics of the ECL system as well as on the rate of diffusion in the solid host. Previous work using a stopped flow method has shown the Ru(bpy)32+-TPA system to be of intermediate reaction kinetics in pH 6 buffer relative to that observed for the Ru(bpy)32+-oxalate (fastest) and Ru(bpy)32+-proline systems.38,39 The diffusion of molecules trapped within sol-gel-derived glasses has been shown to strongly depend on the structure of the gel as well as the size and charge of the molecules trapped within.28,33-35 Both surface interactions and surface confinement effects are important.33-35 The translational and rotational mobility of the alkylamines and oxalate entrapped within this porous host matrix will be dissimilar due to their different sizes and charges. Specific surface interactions between the amine and the walls of the silicate host coupled with their larger size could significantly inhibit their diffusion in this host matrix. To better evaluate the differences in ECL for the different reagents in the solid host, a parallel set of experiments were conducted in solution with the r ) 13 µm Pt microelectrode. In these experiments, a freshly prepared Ru(bpy)32+ stock solution in water was mixed with the reductant dissolved in phosphate (37) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007-5013. (38) Shultz, L. L.; Stoyanoff, J. S.; Nieman, T. A. Anal. Chem. 1996, 68, 349354. (39) Lee, W.-Y.; Nieman, T. A. Anal. Chem. 1995, 67, 1789-96.
Figure 4. Normalized ECL versus reductant. The ECL intensity obtained at low sweep rates in the gel was normalized with that obtained in solution under similar conditions. The gels and solutions contain 10 mM Ru(bpy)32+ and 5 mM reductant, pH 6. Error bars are standard deviations from 3-5 experiments.
buffer (pH 6.0, 0.3 M) in a 2:1 volume ratio as described in the Experimental Section. The final concentration of Ru(bpy)32+ was 10 mM, and that of the reductant (TPA, TMA, TBA, TEA, or oxalate) was 5 mM. In solution, the ECL of the Ru(bpy)32+alkylamine systems was more than an order of magnitude larger than that observed in the gel. In addition, the ECL intensity was essentially invariant with scan rate, indicating that the ECL reaction kinetics are not a limiting factor under these conditions. Also, no significant drop in the ECL intensity at large overpotentials at low scan rates was observed as noted in the solid host (see Figure 2). For the Ru(bpy)32+-oxalate system, the ECL observed in solution and the gel were very similar in terms of both magnitude and scan rate response. To better compare the ECL in solution versus the gel, the ECL obtained at slow sweep rates in the gel was normalized by that obtained in solution under similar conditions. In these experiments, the gels were prepared with 0.3 M, pH 6.2 phosphate buffer to increase the buffer capacity so that the pH of the gel better matched that in solution (estimated to be ca. 6 with indicator dyes). Figure 4 shows a plot of normalized ECL versus reductant. The ECL of the Ru(bpy)32+-oxalate systems is ca. 65-75% of that measured in solution, whereas the ECL of Ru(bpy)32+-alkylamines is ca. 3-6%. It is apparent that ECL reaction of Ru(bpy)32+ with the alkylamines is more hindered in the gel compared to that observed for oxalate. These large differences can be attributed to differences in the mobilities of the gel-entrapped reagents. The fact that the normalized ECL for the different alkylamines is nearly the same regardless of their sizes suggests that the surface interactions with the matrix play a more important role than size effects. At pH 6, the silicate matrix will have a net negative charge (the pKa of silica is ca. 240) and a large population of unreacted silanols. The oxalate anion being negatively charged will likely not adsorb or otherwise interact with the pore walls and will diffuse more freely in the matrix. In previous work we have shown that the diffusion coefficient of a negatively charged analyte (Fe(CN)63-) in an aged gel is nearly the same as it is in the sol state. In contrast, ferrocenemethanol, a molecule that will likely interact with the walls of the matrix, exhibits a diffusion coefficient that is 4-10 times smaller when entrapped in an identical matrix.28 The tertiary (40) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
Figure 5. (A) Intensity-time curve following a potential step to 1.2 V at t ) 30 s for gel-encapsulated Ru(bpy)32+ and oxalate. (B) ECL intensity as a function of drying time for gels that were reused during the two-week period (b) and gels that were used only once and then discarded ([). Error bars are standard deviations from three gels. Gels contain 10 mM Ru(bpy)32+ and 5 mM oxalate and were prepared with 0.1 M, pH 6 phosphate buffer.
amines used in the current study would be able to hydrogen bond or otherwise interact with the silicate framework, reducing its translational and rotational mobility. Such effects have been observed for pyridine and acetonitrile incorporated in sol-gel glasses.41,42 In chromatography, tertiary amines are known to strongly interact with residual silanol groups on silica-based reversed-phase columns to cause broadened and skewed peaks. It is also possible that the amine groups could function as local base-type catalysts for the condensation reactions between adjacent silanol groups in the gel, thus diminishing its porosity and reducing the mobility of the entrapped reagents.43 ECL Stability. Two viable concerns in the development of practical ECL display materials based on sol-gel-derived solids are the long-term stability of the ECL upon repetitive oxidation and the reusability (long-term stability) of the solid-state host. As a silica monolith dries, the solid shrinks in size and mass and becomes more cross-linked and less porous, potentially influencing the reactivity of the entrapped reagents.25-27 In this work, these two issues were addressed by (1) examining the ECL upon application of an oxidizing potential and monitoring the ECL intensity as a function of time and (2) evaluating the ECL as a function of the length of time the gel was dried. Figure 5A shows the ECL of gel-encapsulated Ru(bpy)32+oxalate after the microelectrode potential was stepped from 0.6 to 1.2 V. The ECL increased rapidly (less than 1 s) after application (41) Nikiel, L.; Hopkins, B.; Zerda, T. W. J. Phys. Chem. 1990, 94, 7558-7464. (42) Nikiel, L.; Zerda, T. W. J. Phys. Chem. 1991, 95, 4063-69. (43) Harris, T. M.; Knobbe, E. T. J. Mater. Sci. Lett. 1996, 15, 132-133.
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of 1.2 V, dropped slightly, and reached a near-steady-state value for over 2 h. As reported previously, the ECL was also found to be stable for the Ru(bpy)32+-TPA system with less than a 1020% drop in intensity observed for over 24 h.29 The enhanced stability of the ECL can be attributed to the small size of the ultramicroelectrode, which results in a decreased consumption of reductant, and also to the steady-state flux of reagents to the electrode surface.29,36 The initial drop in ECL for the Ru(bpy)32+oxalate system observed immediately after application of the electrode potential may reflect adsorption of oxalate on the platinum electrode surface.37 This drop has not been observed for the Ru(bpy)32+-amine systems. The time it takes to reach the maximum ECL intensity is much faster for the Ru(bpy)32+oxalate system than for the Ru(bpy)32+-amine system, consistent with that observed in the scan rate dependence data shown earlier. The ECL was also examined as the gels were dried for different lengths of time under ca. 60-70% relative humidity to evaluate what effect changes in the structure of the gel will have on the ECL intensity. Over the two-week period, the gels lose ca. 1012% in mass. The results obtained from the Ru(bpy)32+-oxalate system are shown in Figure 5B. In this figure, the diamond symbols represent the average ECL acquired from three different gels run only once and then discarded, whereas the circles represent the average ECL acquired from three gels continuously examined over a two-week period. For either experiment, no significant difference in the ECL was observed when the gels were dried for 1 day versus 9 days. After ca. 10 days, the gels that were handled on a regular basis showed a significant drop in the ECL intensity. This drop may be due to a disruption in the gelelectrode interface. Because the inorganic gel is macroscopically rigid, the gel can crack and/or pull away from the electrode surface during drying (and especially during handling), thus creating an air-solution gap at the electrode-gel interface that can support voltammetry.28 For the gels that were not handled, no significant change in the ECL was noted until the gels were approximately 15 days old, when two of the three electrodes examined gave an ECL intensity lower than average and consistent with the gels that were constantly handled. Similar results were
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also observed for the Ru(bpy)32+-TPA system. It should be possible to extend the stability of the ECL for a longer length of time by sealing the vials completely so the gel does not dry out. CONCLUSIONS Chemiluminescent precursors can be trapped in sol-gelderived solids and electrochemically excited using an immobilized electrode assembly. The ECL thus produced is very stable and can be used to probe diffusion in constrained environments and assess surface interactions between the entrapped reagents and the walls of the silicate host. When Ru(bpy)32+ is trapped into the silicate host with alkylamines such as tripropylamine or trimethylamine, the ECL intensity is considerably lower than that observed in solution. The ECL is also highly scan rate dependent. In contrast, when sodium oxalate is encapsulated with Ru(bpy)32+ in the silica gel host, the ECL is similar to that measured in solution and no scan rate dependence is observed. These differences can be attributed to differences in rotational and translational mobility between the reductants trapped in this porous solid host. Surface interactions between the alkylamines and the surface of the silicates and/or local changes in the degree of condensation in the gel significantly reduce the mobility of gel-entrapped amines relative to oxalate. These results suggest that it may be possible to increase the ECL intensity and improve the response time of the device via modification of the chemical and physical microenvironment of the silicate host. Future studies will be directed in this manner. ACKNOWLEDGMENT We gratefully acknowledge support of this work by the National Science Foundation through the NSF CAREER program (CHE) and Research Experiences for Undergraduates program (Grant CHE-9732103) and by the Office of Naval Research.
Received for review November 17, 1999. Accepted March 8, 2000. AC9913208
