J. Phys. Chem. 1992, 96, 1388-1394
1388
therms lie in problems associated with measurement and interpretation of the surface tension of CI4TABsolutions. To some extent this could have been concluded from earlier measurements. Thus, Okuda et al.” found that glass or R Wilhelmy plates gave unreliable results with cationic surfactants, Jones and Ibbotsonls were unable to obtain reproducible tensions by the ring (or the drop volume) method for the alkyl-TABS, and Ftijnbo~t’~ obtained a value of 105 A* for CI6TABusing a F’t plate. Venable and Nauman5 purified their C14TABsample extremely carefully but obtained a value of 61 A2for A at the cmc, although their value of the cmc itself is within error of ours. Finally Miyajima et a1.,20 who used the ring method and fitted a curve to their data, obtained an adsorption isotherm which shows reasonable agreement with the neutron data for concentrations greater than 1.5 X M. The measurements of Miyajima et aL20and Sokolowski and WilkI6 agree with the neutron data in that they show a continuous change in the area per molecule right up to the cmc. There is, of course, no easy way of using surface tension data to measure A above ~
~
(17) Oh&, H.; Ozeki, S.;Ikeda, S. Bull. Chem. Soc. Jpn. 1984,57, 1321. (18) Jones, M. N.; Ibbotson, G. Trans. Faraday SOC.1970, 66, 2394. (19) Rijnbout, R. B. J. Colloid Interface Sci. 1977, 62, 81. (20) Miyajima, K.; Yoshida, H.; Maetani, J.; Nakagaki, M. Bull. Chem. SOC.Jpn. 1980, 53, 1523.
the cmc. It is well-known that cationic surfactants tend to render surfaces hydrophobic, and it can be anticipated that the determination of areas for other types of surfactant will be less problematic. Conclusions
The main purpose of this work was to establish the value of neutron reflection as a technique for measuring the absolute surface concentration and to show that there is no isotope effect in the thermodynamic properties of C14TAB. The process of having to compare surface excesses determined by neutrons and surface tensions has highlighted some problems in the surface tension experiment. In the past, discrepancies of the sort that we have described have usually been attributed to impurities. This cannot be done in the present case because the same sample was used for the two techniques. The anomalies we have observed in the different surface tension measurements highlight the p i b f i t y that there may be a systematic, and not easily explained, error in any surface tension determination of the surface excess using ring or plate.
Acknowledgment. We thank RhonePoulenc for a studentship to P.C. and Unilever, Port Sunlight, for funding to A S . and E.A.S. Registry NO. TTAB, 1119-97-7.
Characterlzatlon of Carrageenan Hydrogel Electrode Coatlngs with Immobilized cationic Metal Complex Redox Couples A. L.Crumbliss,* S. C. Perine, Department of Chemistry, Duke University, Durham, North Carolina 27706
A. Kirk Edwards, and D. P. Rillema* Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223 (Received: July 12, 1991)
The redox behavior of cationic metal complexes immobilized in a K-carrageenan hydrogel matrix, which acts as a cation-exchange polymeric electrode coating, is described. Ru(bpy):+, R~(en)~’+, Ru(NH&’+, and C~(bpy),~+ (bpy = 2,2’-bipyridine; en = ethylenediamine) were immobilized singly and in pairs (Ru(bpy)3Z+and Co(bpy)?+) on the surface of a Pt electrode and were characterized by cyclic voltammetry. The redox couples were selected on the basis of their structural similarity and wide range of electron self-exchangerate constants (101-109M-I s-l). The surface-modifiedcarrageenan hydrogel electrode was found to exhibit superior electrolyte diffusion properties when compared with more commonly used cation-exchange immobilization matrices such as Nafion, and to be stable with respect to leakage of cations into the solution. The carrageenan hydrogel film was also found to be permeable to anionic redox couples such as Fe(CN),’/4-. All immobilized redox couples exhibited quasi-reversibleelectrochemical behavior. Evidence supporting a dual-modemechanism involving physical diffusion and electron hopping for charge propagation through the carrageenan hydrogel is presented.
Introduction
There has been considerable interest in the development of chemically modified electrodes for use as chemical sensors and other practical devices.’ Various methods have been used to modify electrode surfaces. One common method used is the application of polymer films on electrode surfaces. Such polymer (1) (a) Bard, A. J. J . Chem. Educ. 1983, 60, 302. (b) Murray, R. W. Electmnal. Chem. 1984,13, 191. (c) Murray, R. W. Annu. Rev. Mater. Sci. 1984,14,145. (d) Murray, R. W. Proc. Roberi A. Welch Found. Conf. Chem. Res. 1986, 30, 169. (e) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A. ( f ) Surridge, N. A.; Jernigan, J. C.; Dalton, E. F.; Buck, R. P.; Watanabe, M.; Zhang, H.; Pinkerton, M.; Wawter, T. T.; Longmure, M. L.; Facci, J. S.; Murray, R. W. Faraday Discuss. Chem. Soc. 1989, 88, 1 . (8) Faulkner, L. R. Chem. Eng. News 1984, 62, 28. (h) Faulkner, L. R. Electrochim. Acta 1989,341699. (i) Johnson, D. C.; Ryan, M. D.; Wilson, G. S. Anal. Chem. 1986,58, 33R. Redepenning, J. G. Trends Anal. Chem. 1987,6,18. (k) Abrufia, H. D. Coord. Chem. Rev. 1988, 86, 135.
u)
films can be divided into two categories: redox polymers in which the redox active sites are included in the polymer backbone or pendant side chains and ion-exchange polymers in which the redox sites are immobilized by electrostatic attraction. The first ionexchange polymer coated electrodes were introduced a decade ago? Since that time ion-exchange polymer films have been widely studied, partly due to the ease of preparation of the coated electrodes and the numerous ionic redox species which can be immobilized by this method. Polymeric cation-exchange materials used for electrode surface modification include poly(styrene ~ulfonate),~ Nafion: clays,’ and (2) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 247. (3) (a) Majda, M.; Faulkner, L. R. J. ElectroanaI. Chem. 1982, 137, 149. (b) Majda, M.; Faulkner, L. R. J . Electroanal. Chem. 1984, 169, 77. (c) Majda, M.; Faulkner, L. R. J . Electraanal. Chem. 1984,169,97. (d) Jones, E. T. T.; Faulkner, L. R. J . Elecrroanal. Chem. 1987, 222, 201. (e) Chen, X.; He, P.; Faulkner, L. R. J . Electroanal. Chem. 1987, 222, 223.
0022-365419212096-1388$03.00/0 0 1992 American Chemical Society
Redox Behavior of Immobilized Cationic Metal Complexes
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1389
mlita.6 The most extensively studied is Nafion, a perfluorinated polysulfonate material. Of relevance to this paper is the number of metal complexes (e.g., Ru(bpy)2+,4a-c Os(bpy),Z+,"Ve Co(bpy),*+,4' R U ( N H ~ ) ~4g,h) , + and other redox couples (e.g., methylvi~logen~~) which have been immobilized with Nafion on an electrode surface. Much of this work has been directed toward understanding the mechanism of charge transport in the Nafion polymer film and elucidating the structure of the film. Evidence for the existence of three "phases" within the Nafion film has been rep~rted:~~J a hydrophobic fluorocarbon phase; a hydrophilic ion cluster phase (sulfonate sites); and an interfacial region between the two. A problem associated with many cation-exchange polymers, including Nafion, is that they yield small diffusion coefficients for the immobilized species, typically 4 orders of magnitude smaller than for the same species free in solution. For certain applications, such as electrocatalysis where rapid charge transfer is essential, these small diffusion coefficients are undesirable. The subject of this paper is the use of a cation-exchange hydrogel as a polymeric electrode coating and redox couple immobilization matrix which overcomes many of the previous difficulties. Hydrogels may be characterized as a single-phase "aqueous" matrix with excellent diffusional characteristics. Hydrogels have a number of dharacteristics that make them desirable electrode coatings: high water content; biocompatibility; low interfacial tension between hydrogel surface and aqueous solution; excellent diffusion characteristics for small molecules and ions; and optical transparency.' Also, they can be tailored for specific applications by chemical modification of the polymer backbone. Hydrogels are excellent immobilization matrices and have been used for the immobilization of cells and e n ~ y m e s . ~ . ~ A number of nonionic hydrogels have been used to modify electrode surfaces: po1yacrylamide;'O poly(hydroxyethy1methacrylate);" poly(viny1 alcohol);12and agarose.13 These hydrogel electrodes have been used in sensory devices for glucose,'"" ascorbic acid,lDCand urea,12 in the development of an in vivo reference electrode," and recently as a physically rigid, solid-state voltammetry so1~ent.l~Agarose hydrogel has been impregnated (4) (a) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980,102,6641. (b) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1982, 104,4824. (c) White, H. S.;Lcddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982,104,4811. (d) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817. (e) He, P.; Chen, X. J. Elecrroanal. Chem. 1988, 256, 353. (f) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1983,105,685. (g) Martin, C. R.J. Chem. Soc., Faraday Trans. I 1986, 82, 1051. (h) Penner, R. M.; Martin, C. R. J. Electrochem. Soc. 1985. 132. 514. (i) Gaudiello. J. G.:Ghosh. P. K.: Bard. A. J. J. Am. Chem. Soc. 1985,'107,3027. 6) Viing, W. J.; Meyer, T. J. Elecrroanal. Chem. 1987, 237, 191. (5) (a) Bard, A. J.; Ghosh, P. K. J. Am. Chem. Soc. 1983,105,5691. (b) Ghosh,P. K.; Mau, A. W.; Bard, A. J. J. Elecrroanal. Chem. 1984,169, 315. (c) Exe. D.: Ghosh. P. K.: White. J. R.: h u e v . J. F.: Bard. A. J. J. Am. Chem. S&. id,107,5644. (d) Ru&ki, W. E.; Bard, A.J. J.'E/ectroana/.Chem. 1986, 199, 323. (e) King, R. D.; Nocera, D. G.; Pinnavaia, T. J. J. Elecrroanal. Chem. 1987,236,43. (f) Itaya, K.; Bard, A. J. J. Phys. Chem. 1985, 89, 5565. (8) White, J. R.;Bard, A. J. J. Electroanal. Chem. 1986,197, 233. (h) Villemure, G.;Bard, A. J. J. Elecrroanal. Chem. 1990, 283, 403. (i) Villemure, G.; Bard, A. J. J. Electroanal. Chem. 1990, 282, 107. (6) (a) Murray, C. G.; Nowak, R.J.; R o l i n , D. R.J. E/ectroanal. Chem. 1984,164,205. (b) De Vismes, B.; Bedioui, F.; Bid-Charreton, C.; Devynck, J. J. Elecrroanal. Chem. 1985,187, 197. (c) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986, 208, 95. (7) Ratner, B. D.; Hoffman, A. S.In Hydrogelsfor Medical and Related Application$; Andrade, J. D., Ed.; ACS Symposium Series 31; American Chemical Society: Washington, DC, 1976; p 1. (8) Tosa, T.; Sato, T.; Mori, T.; Yamamoto, K.; Takata. I.; Nishida, Y.; Chibata, I. Biorechnol. Bioeng. 1979, 21, 1697. (9) Kierstan, M. P.; Coughlan, M. P. In Immobilized Cells and Enzymes; Woodward, J., Ed.;IRL Press: Washington, DC, 1985; p 39. (IO) (a) Liu, C. C.; Wingard, L. B., Jr.; Wolfson, S. K., Jr.; Yao, S.J.; Drash, A. C.; Schiller, J. G. J. Elecrroanal. Chem. 1979, 104, 19. (b) Liu, C. C.; Weaker, J. P.; Chen, A. K. J. E/eCtrOaM/. Chem. 1981. 128, 379. (c) Lange, M. A.; Chambers, J. Q.Anal. Chim. Acra 1985,175,89. (d) Lange, M. A.; Chambers, J. Q. Anal. Chem. 1986,58,2872. (e) Van Koppenhagen, J. E.; Majda, M. J. Electroanal. Chem. 1985, 189, 379. (f) Oliver, B. N.; Murray, R. W. J. Am. Chem. SOC.1988, 110, 2321. Egekeze, J. 0.; (11) Margules. G.S.; Hunter, C. M.; MacGregor, D. C. Med. Biol. Eng. Compur. 1983, 21, 1.
(12) Kirstein, D.; Scheller, F. Anal. Chim. Acra 1985, 171, 345. (13) Moran, K. D.; Majda, M. J. Electroanal. Chem. 1986, 207, 73.
with Nafion to study the electrochemistry of methylvi01ogen.l~ This was an attempt to combine the ion-exchange properties of Nafion with the permeability of the agarose hydrogel. However, with the exception of a paper from our laboratory,lS hydrogels with pendent charged groups that can act as ion exchangers have not previously been used to modify an electrode surface. K-Carrageenan (I) is an anionic polysaccharide hydrogel that is extracted from seaweed. It consists of repeating units of
1 L HO' I
~-galactose-4-sulfateand 3,5-anhydro-~galactoe.~ The use of K-carrageenanhydrogel as an electrode coating can provide both ion-exchange properties and hydrogel permeability. K-Carrageenan hydrogel is very hydrophilic, containing more than 80% water, and is transparent to visible light. A fm,rigid gel is formed when K-carrageenan is cured with metal ions, amines, amino acid derivatives, and water-miscible organic solvents.* The properties of K-carrageenan hydrogel make it an ideal electrode coating for applications in electrocatalysis,biosensors, l5 and photoelectrodes.16 The purpose of this paper is to illustrate the effectiveness of K-carrageenan hydrogel in immobilizing cationic redox couples, singly or in pairs, on an electrode surface, and to provide electrochemical characterization that relates to the mechanism of charge transport within the film. A subsequent paper will discuss its use as a photoelectrode coating.16
Experimental Section Materials. K-Carrageenan powder was obtained as a gift from FMC (research grade, 25% sulfate groups). Tris(2,2'-bipyridine)ruthenium(II) chloride hexahydrate ( [Ru(bpy),]C126H20), silver triflate (AgCF,SO,), ethylenediamine dihydrochloride, and potassium nitrate were purchased from Aldrich. Sodium perchlorate was prepared by neutralization of HClO, (Fisher) with Na2C03(Fisher) and was recrystallized from water prior to use. Potassium ferricyanide (K,Fe(CN),) was obtained from Fisher. Hexaammineruthenium(II1) chloride (Ru(NH3),C13) was obtained from Strem Chemicals. Analysis for RU(NH,)~CI,% N: found, 27.18; calcd 27.13. All analyses were performed by MHW Laboratories, Phoenix, AZ. Preparation of Compounds. The trifluoromethanesulfonate (triflate) salt of Ru(bpy),Z+ was prepared by metathesis. An aqueous solution of silver triflate was slowly added to an aqueous solution of the dichloride salt in a 2:l mole ratio. The resulting AgCl precipitate was removed by filtration, and the filtrate was concentrated in vacuo until the triflate precipitate began to form. After the solution was oooled to 0 OC to effect further precipitation, it was filtered, and the precipitate was washed with cold ether and dried in a vacuum desiccator. Elemental analysis (calculated values in parentheses): R U ( ~ ~ ~ ) , ( C F , S O %, C ) ~44.36 (44.25); % H 3.28 (2.77); % N 9.78 (9.68). Tris(2,2'-bipyridine)cobalt(III) chloride (Co(bpy),Cl,) was prepared according to the following method. The bis(2,2'-bi~~
(14) Oliver, B. N.; Coury, L. A.; Egekeze, J. 0.;Sosnoff, C. S.;Zhang,
Y.; Murray, R.W.; Keller, C.; Umafia, M. X. In Biosensor Technology;Buck, R.P., Hatfield, W. E., Umafia, M., Bowden, E. F., Eds.;Dekker: New York, 1990; p 117. (15) Crumbliss, A. L.; Henkens, R. W.; Perine, S.C.; Tubergen, K. R.;
Kitchell, B. S.;Stonehuerner, J. In Biosensor Technology; Buck, R. P., Hatfield, W. E., Umafia, M., Bowden, E. F.. Eds.; Dekker: New York, 1990, p 187. (16) Rillema, D. P.; Edwards, A. K.; Perine, S.C.; Crumbli, A. L. Inorg. Chem. 1991, 30,4421.
1390 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
pyridine)cobalt complex [Co(bpy),CI,] C1.2H20 was prepared according to a previously published method." A 10-mL methanol solution containing 0.0380g of bipyridme (0.243"01) was added to a 20-mL methanol solution of [C0(bpy)~Cl~]C1*2H~O (0.104 g; 0.203 mmol). The mixture was refluxed under Nz for 2 h, during which time the solution changed color from purple to yellow. The solvent was reduced to -2-3 mL by rotary evaporation and was then added to swirling ether to precipitate the complex. The complex was isolated by vacuum filtration and dried at reduced pressure. The yield was 0.105 g (82%). The product was recrystallized from hot ethanol followed by slow cooling at 4 OC. The crystals were filtered off, washed with ether, and dried under vacuum. Elemental analysis (calculated values in parentheses): Co(bpy),Cl34H20% C 51.18 (51.04);% H 4.87 (4.54); % N 12.01 (11.90). Tris(ethylenediamine)ruthenium(III) chloride (Ru(en),C13) was prepared according to a previously published method.'* Elemental analysis (calculated values in parenthesis): Ru(en),Cl, % C 18.33 (18.58); % H 6.22 (6.19); % N 21.70 (21.67). prepantioa of Camgemm-Coated Electrodes. Platinum disk (BAS, 0.020 cm2area) and gauze (Fisher, 52 mesh, 0.26 cm2area) working electrodes were cleaned by applying a potential of +1.2 V (vs Ag/AgCl) for 30 min in 1.0 M H2S04. Hydrogel-coated electrodes were prepared from a 4% (w/v) carrageenan aqueous solution (sulfate group concentration 10.1 M) in which carrageenan was dissolved by warming. The Pt gauze was then dipped into the 4%solution, withdrawn, and allowed to stand in a curing solution for 30 min. The curing solution contained a positively charged cation (e.g., 100 mM ethylenediamine dihydrochloride, 20 mM Ru(bpy):+, 20 mM Co(bpy)?+, 20 mM R u ( N H ~ ) ~ ,or + , 20 mM R~(en)~'+).The R gauze/cured carrageenan was then rinsed with water and used as a polymercoated electrode. The coatings on the Pt gauze electrodes were ca.2 mm thick for a total hydrogel volume of ca. 0.05 mL. Immobilizationof Cationic Redox Active Complexes. Divalent and trivalent redox active complexes of ruthenium and cobalt were immobilized in the carrageenan hydrogel on an electrode surface by two methods. In method 1 an ethylenediamine dihydrochloride cured hydrogel coated on a Pt gauze electrode was placed in an aqueous solution of the cationic metal complex containing 0.10 M KNO,and repeatedly cycled through the 2+/3+ redox couple until a steady-state cyclic voltammogram was achieved. In method 2 a hydrogel-coated Pt gauze electrode was cured in a solution containing the cationic metal complex, followed by current/voltage cycling in a solution containing only background electrolyte until a steady-state cyclic voltammogram was obtained. Curing times from 10 to 180 min were tried. No improvement was found for curing times over 30 min, which was considered optimal and used throughout this study. The concentration of cationic redox species in the hydrogel was higher than its concentration in the cyclic voltammetry solution (method 1) or curing solution (method 2) for both methods of immobilization. Furthermore, the concentration of the redox couple in the carrageenan hydrogel may be changed by changing the metal complex concentration in the solution used in immobilization method 1 or 2. The hydrogelcoated electrode with an immobilized cationic redox species can be removed from a solution containing the cationic redox species and placed in a solution containing only background electrolyte without any loss in the steady-state redox signal after continuous use for over 6 h. These observations provide evidence for ion exchange of the cationic redox species into the carrageenan matrix as opposed to simple diffusion in and out of the matrix. When any of the cationic complexes, MLsn+,were immobilized in the hydrogel matrix by ion exchange over repeated current/voltage scans (method l), 100 mM K N 0 3 was used as the background electrolyte to obtain optimum results. When method 2 was used to cure the hydrogel, more stable and well-defined current/voltage (17) Hancock, M. P.; Josephson, J.; Schaffer, C. E. Acta Chem. Scand. A 1976, 30, 79. (18) Smolenaers. P. J.; Beattie, J. K. Inorganic Syntheses XIX, Shriver, D. F., Ed.;Wiley and Sons: New York, 1979; p 118.
Crumbliss et al. scans were obtained in background electrolyte solutions consisting of 50 mM NaC104/50 mM KNO,. ConcentrltionOf H y W - h d ~* ~ i RedoxActivecompkx h d The concentrations of redox active species in the hydrogel were determined as follows. The Pt gauze electrode was weighed before and after hydrogel modification. The difference between these the weight of the hydrogel (precise to fl%). two weights repr-ts Since the weights of solid carrageenan, redox active species, and electrolytes in the hydrogel are negligible (less than 4%)compared to the weight of the hydrogel, this measured weight also represents the weight of water, from which the volume of water was derived. The electrode was then immersed in warm water to dissolve the hydrogel. After cooling, this solution was diluted to a known volume. The absorbance of the solution was obtained at a wavelength corresponding to the & for the redox active complex. The concentrations of metal complexes were determined using the following A and c values: (1) Ru(bpy),2+,I9A = 450 nm, t = 1.3 X lo4 M-' cm-I; A = 317 nm, t = 1.1 X lo4 M-' cm-I; A = 305 nm, t = 1.4 X lo4 M-' cm-I; (2) C ~ ( b p y ) , ' + ,A~ ~= 453 nm, t = 70 M-' cm-I; A = 317 nm, t = 2.6 X lo4 M-' cm-'; A = 305 nm, c = 2.9 X lo4 M-' cm-'; (3) RU(NH&3+,2' A = 275 nm,t = 630 M-' cm-';(4) Ru(en),9+,2' A = 306 nm, t = 290 M-' cm-'; (5) R~(en)~diim'+?~ A = 447 nm, t = 6.9 X lo3 M-I cm-I; and (6) Fe(CN)63-,23A = 420 nm, t = 1.0 X lo3 M-' cm-'. The number of moles of metal complex was calculated from the known volume of the absorbance test solution and then divided by the volume of water in the hydrogel to yield a concentration in the hydrogel (precise to &5%). In contrast to all of the other redox species studied, R ~ ( e n ) ~ , + is oxygen sensitive. The species Ru(en)?+ undergoes air oxidation to yield R ~ ( e n ) ~ d i i m(diim ~ + = NH=CHCH=NH).Z2 This was not a problem for solution work where Ar degassing was efficient, but for hydrogel electrodes there was a problem with the determination of R ~ ( e n ) ~ ,concentrations + in the hydrogel by UVvisible spectroscopy. This problem was solved by air oxidizing solutions of dissolved hydrogel containing Ru(en)?+ (A = 305 nm) until the absorbance for the R ~ ( e n ) ~ d i i m(A~ += 445 nm) was no longer increasing. The concentration of R ~ ( e n ) ~in~the + hydrogel was then determined. Physical Measurements.UV-visible spectra were determined with a Beckman Acta I11 spectrophotometer. Cyclic voltammetry measurements were carried out with a BAS CV-27 potentiostat and recorded using a Houston Instruments Model 100 X-Y recorder. The electrochemical experiments were conducted in a threeelectrode cell with both modified and unmodified Pt disk and Pt gauze working electrodes, a Ag/AgCl reference electrode, and a Pt wire counter electrode. All potentials are reported relative to the Ag/AgCl electrode. Solutions containing the appropriate electrolyte were degassed with N2 or Ar prior to use in electrochemical measurements. The polarographic redox potential, was taken as the average between the anodic and cathodic peak potentials, (E, E,)/2. The difference, E - E was taken as AE,. Hectrode kinetics and diffusion of the ions were determined by analyzing the scan rate dependence of the cyclic voltammograms over the range 5-250 mV/s according to published procedure^.^^*^^ Plots of peak current (i,) vs v ' / ~pass through the origin and are linear with R 1 0.99. The experimental diffusion coefficients (Dexp)were calculated from the slopes of these As a test of our methods, some diffusion coefficient determinations were also made by chronocoulometryand found to agree with values obtained by cyclic voltammetry. Furthermore, diffusion coefficient data obtained for a bare Pt disk electrode
+
kL6zq'+
(19) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617.
(20) Martin, B.; Waind, G . M. J . Chem. SOC.1958, 4284. (21) Meyer, T. J.; Taubc, H. Inorg. Chem. 1968, 7, 2369. (22) Mahoney, D. F.; Beattie, J. K. rmrg. Chem. 1973, 12, 2561. (23) Strojeck, J. W.; Kuwana, T. J . Elecrroanal. Chem. 1968, 16,471. (24) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods: F u d u mentals and Applications; Wiley and Sons: New York, 1980; p 218. (25) Nicholson, R. S. Anal. Chem. 1965, 37, 1351.
Redox Behavior of Immobilized Cationic Metal Complexes
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1391
TABLE I: Electrochemical Parameters for Ru(bpy)3Z+/J+/Camgeenan Hydrogel Electrodeso immob [W~PY)~”+I, mM electrode methodb hydrogel solution Pt disk 1 .o Pt disk Pt disk
5.6 25.0 1 .o 5.1 5.6 25.0 1 .o 5.6 15.0 25.0
cm2/s
k. cmls
lod lod lod
very large 7.6 x 10-3 8.1 x 10-3
3.0 X 2.4 X 3.1 X 4.7 x 6.6 X 9.5 x 1.2 x 1.1 x 9.3 x 2.6 X 1.8 X 1.1 x 4.1 x 1.4 X
Pt gauze 10-6 very large Pt gauze lod 1.1 x 10-2 Pt gauze 10-6 3.1 x 10-3 Pt gauze 10-6 5.7 x 104 Pt gauze/hydrogel 1 1.6 10-6 5.8 x 10-3 Pt gauze/hydrogel 1 13.0 10-7 6.8 X lo4 Pt gauze/hydrogel 1 23.0 lo-’ 4.4 x 10-4 Pt gauze/hydrogel 1 39.0 10-1 4.6 X lo4 Pt disk/hydrogel 2 19.0 0.0 10-6 C Pt gauze/hydrogel 2 24.0 0.0 10-7 6.9 X lo4 Pt gauze/hydrogel 2d 38.0 0.0 10-1 1.4 X lo4 ‘Except where noted, background electrolyte is 0.10 M KN03 for no hydrogel and in the presence of hydrogel using immobilization method 1; background electrolyteis 0.05 M KN03/0.05 M NaClO, for hydrogel coating using immobilization method 2. bImmobilizationmethods 1 and 2 are defined under Experimental Section. Not determined. Background electrolyte is 0.10 M NaC10,. TABLE II: Electrochemical Parameters for C o ( b ~ y ) ~ * RU(NH~)~~+/%, +/~, and R~(en)~*+’~/Carrageenan Hydrogel Electrodes4 redox couple immob [ML6”+],mM electrode ~ ~ ~ 2 + / 3 + methodb hydrogel solution Dexplcm2/s k,, cm/s Pt disk Co(bpy), 5.0 2.0 x 10“ 6.3 x 10-3 Pt gauze CO(bPY)3 5.0 3.3 x 10-6 3.3 x 10-3 Pt gauze/hydrogel Co(bpy), 1 1.6 1 .o 4.2 X 2.2 x 104 Pt gauze/hydrogel CO(~PY)~ 1 16.4 5.0 4.7 x 10-8 9.6 x 10-5 Pt gauze/hydrogel Co(bpy13 2 18.0 0.0 1.9 X 8.8 X Pt gauze/hydrogel C ~ ( ~1 3P Y 2c 19.0 0.0 3.1 x 10-9 2.1 x 10-5 Pt disk Ru(NHd6 5 .O 3.6 X 10“ very large Pt gauze Ru(NH316 5.0 3.0 X lod 2.3 x 10-3 Pt gauze/hydrogel RU(NHJ)~ 1 20.0 5.0 1.3 X 4.6 x 10-5 Pt gauze/hydrogel Ru(NH3)6 2 0.0 6.2X lo4 3.8 x 10-5 25.0 Pt gauze/hydrogel Ru(NH316 2c 46.0 0.0 5.5 x 10-9 2.1 x 10-5 Pt disk Ru(en13 5.0 3.9 x 10-6 very large Pt gauze wen13 5.0 1.6 X lod 8.9 X lo4 Pt gauze/hydrogel Wen13 1 11.0 5.0 1.2 x 10-7 1.1 x 104 Pt gauze/hydrogel Ru(en)3 2 14.0 0.0 6.8 X 1.5 X lo4 Pt gauze/ hydrogel Wen13 2e 14.0 0.0 1.4 X 10-1 2.9 X lo4 ‘Except where noted, background electrolyte is 0.10 M KN03 for no hydrogel and in the presence of hydrogel using immobilization method 1; background electrolyteis 0.05 M KNOJ0.05 M NaClO, for hydrogel coating using immobilization method 2. bImmobilizationmethods 1 and 2 are defined under Experimental Section. cBackground electrolyte is 0.10 M NaC10,.
were found to be consistent with values reported in the literature for R ~ ( b p y ) ~ ~ + / ’Dcxp + . ~values ~ were used to calculate heterogeneous electron-transfer rate constants (k,).25The influence of uncompensated resistance on our results was judged to be insignificant on the basis of the following observations. In cyclic voltammetry experiments the observed peak to peak separations were found to be independent of the redox couple concentrations and electrode area, and the heterogeneous electron-transfer rate constant independent of scan rate. The experimental diffusion coefficients (flap) did not change with electrode area, and values obtained from cyclic voltammetry and chronoamperometry experiments agree to within experimental error.
ReSUltS R u ( b p ~ ) ~ ~ + Electrochemistry. /’+ The R ~ ( b p y ) cation ~ ~ + was immobilized in carrageenan hydrogel on an electrode surface by both methods 1 and 2. A representative cyclic voltammogram for a hydrogel-immobilized Ru(bpy)32+modified Pt gauze electrode (method 1) is shown in Figure 1A. A Ru(bpy),Z+l3+redox couple with = +1.09 V (vs Ag/AgCl) and AE, = 0.105 V (at 10 mV/s scan rate) was obtained. This illustrates that quasi-reversible electrochemical behavior is exhibited by Ru(bpy):+ immobilized in a carrageenan hydrogel matrix. Diffusion coefficients and heterogeneous electron-transfer rate constants are given in Table I for Ru(bpy),*+ immobilized in a (26)
Buttry, D. A.; Anson, F. C. J. Electroanal. Chem. 1981, 130, 333.
carrageenan hydrogel at a Pt electrode by both methods 1 and 2. Comparison of Dcxpvalues with and without the hydrogel coating emphasizes the excellent diffusion properties exhibited by these hydrogel electrodes. Diffusion coefficients for Ru(bpy),Z+ immobilized in carrageenan do not exhibit any trends with concentration over the range 2-40 mM. [ C ~ ( b p y ) ~ p +E /l ~e+ctrochemistry. The Co(bpy)?+ cation was immobilized in a carrageenan hydrogel on an electrode surface by methods 1 and 2. A representative cyclic voltammogram for a hydrogel-immobilized Co(bpy)?+ modified Pt gauze electrode (method 1) is shown in Figure 1B. A C~(bpy),Z+/~+ redox couple with El12= +0.128 V (vs Ag/AgCl) and AE, = 0.130 V (at 10 mV/s scan rate) was obtained. The electrode kinetics and diffusion of the C ~ ( b p y ) , ~ +ion /~+ within the carrageenan hydrogel coating were determined, and values for the diffusion coefficients and heterogeneous electrontransfer rate constants are given in Table 11. These data show that quasi-reversible electrochemical behavior is observed for C ~ ( b p y ) ’ ~ + immobilized /~+ in a carrageenan matrix. Diffusion coefficients show facile diffusion within the hydrogel. However, in contrast to Ru(bpy),Z+/’+ immobilized in the carrageenan hydrogel, diffusion coefficients obtained for C ~ ( b p y ) , ~ + im/~+ mobilized in carrageenan are significantly lower than those obtained in bulk solution (see Table 11). An order of magnitude variation in immobilized C ~ ( b p y ) ~ ~concentration +/~+ did not produce a significant change in the measured diffusion coefficient for the complex immobilized in the hydrogel matrix (see Table 11).
1392 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Crumbliss et al.
TABLE III: Electrochemid Parameters for Ru(bpy),’+’” and C ~ ( b p y ) ~ * + /Coimmobilized ” in Carrageenan Hydrogel Electrodes” [Ru(bpy)3*1, mM [CO(~PY)~*I, mM gel soln gel soln ([Ru]/[Co]),,, os’x”,, cm2/s D2.p, cm2/s kp,cm/s kp,cm/s 3.4 1.o 18.0 5.O 0.19 6.8 X 1.1 X 3.1 X 6.0 X lW5 9.0 5.0 12.0 5.O 0.75 6.4 X 5.1 X 1.2 X 1.6 X lo4 14.0 5.0 2.9 1.o 4.8 2.3 X lo-’ 3.6 X lo4 5.0 X lo4 1.3 X 17.0 7.0 1.9 0.5 9.0 4.1 x 10-7 9.7 x 10-9 7.7 x 104 2.3 x 10-5 “Immobilizationmethod 1 using a Pt gauze electrode; 0.10 M KN03 background electrolyte. TABLE I V Electrochemical Parameters for Fe(CN),*IC”
electrode D..,. cm2/s k.. cmls Pt disk 3.3 x 10-6 1.1 x 1OT 7.8 X 10” 1.2 x 10-3 Pt gauze 4.8 X 10” 1.6 x 10-3 Pt gauze/hydrogelb 5.0 mM Fe(CN):-/O.l M KN03. bEthylenediamine dihydrochloride cured carrageenan hydrogel (immobilization method 1); [Fe(CN):-] in hydrogel = 6.4 mM.
1.2
0.5
0.4
0
-0.4
Volts (vs Ag/AgCI)
Figure 1. Cyclic voltammograms using Pt gauze electrode coated with carrageenan containing the following redox ions immobilized in the hydrogel (method 1): (A) 13.0 mM Ru(bpy),Z+; (B) 16.4 mM Co(bpy),S+; (C) 9.0 mM Ru(bpy)32t and 12.0 mM Co(bpy),’+; (D) 11.0 mM Ru(en),)’; (E) 20.0 mM Ru(NH,)~~+; (F) 5.3 m M Fe(CN)63-. Dotted vertical lines correspond to E , values for each couple. Solid vertical lines define current scale for 5d PA. Conditions: 100 mM KN03, scan rate 10 mV/s except for (C), where it is 50 mV/s.
Electrochemiptryof [R~(bpy)#+/~+ end [C~(bpy)#+/~+ Cobmowlizprd in & ”,Both Ru(bpy)z+ and C~(bpy),~+ were
coimmobilized in carrageenan hydrogel on an electrode surface by method 1. This was accomplished by using an ethylenediamine dihydrochloridecured carrageenan hydrogel on a Pt gauze electrode for cyclic voltammetry experiments in a solution containing 5.0 mM Ru(bpy)?+, 5.0 mM Co(bpy)J+, and 0.1 M KNO,. The hydrogel electrode was repetitively scanned from +1.4 to -0.30 V (vs Ag/AgCl) until the currents for both the R~(bpy),~+/,+ and the C~(bpy),~+/~+ couple remained constant. A representative cyclic voltammogram for this system is shown in Figure 1C. The Ellz and hE,values for Ru(bpy)3n+(1.10 and 0.085 V, respectively, at 10 mV/s scan rate) and Co(bpy),”+ (0.120 and 0.102 V, respectively, at 10 mV/s scan rate) are comparable to those observed in Figure 1A,B for the ions immobilized individually. Electrochemical parameters (Dcnp,k,) and the concentration of each redox species in the hydrogel are given in Table 111. The excellent diffusion properties found for each individual redox species w i t h the hydrogel (Tables I and 11) are retained in this combination R~(bpy),~+/Co(bpy),~+/carrageenan hydrogel electrode. This illustrates that the two complexes behave independently when coimmobilized in the carrageenan matrix. Studies were also performed to determine if there is any competition between Ru(bpy)z+ and C~(bpy),~+ for anionic sites in
the carrageenan hydrogel. Values for Dnp were determined for various concentration ratios of Ru(bpy)32+and Co(bpy),,+ immobilized in the hydrogel simultaneously (method 1). Regardless of the [Ru]/[Co] concentrationratios (column 5 , Table 111), the values of Dcxpfor Ru(bpy)?’ are essentially the same. While the values of Dexpfor Co(bpy)?+ cover a larger range, they most likely represent experimental uncertainty since the DCv value for 2 mM Co(bpy),,+ in the hydrogel is virtually identical to that for 18 mM C ~ ( b p y ) , ~in+ the hydrogel. Thus, it appears that there is no competition between Ru(bpy)32+and Co(bpy),,+ for anionic sulfate sites in the carrageenan hydrogel. R u ( N H ~ ) ~ ~and + / R~(en),~+/’+ ~+ Electrochemistry. The Ru(NH3)63+and Ru(en)?+ cations were independently immobilized in separate carrageenan hydrogels on electrode surfaces by both methods 1 and 2. A representative cyclic voltammogram for a R ~ ( e n ) ~ ~redox + / ~ couple + immobilized in carrageenan is shown in Figure 1D with E l I 2= -0.015 V (vs Ag/AgCl) and AE, = 0.146 V (at 10 mV/s scan rate). A representative cyclic voltammogram for a R U ( N H , ) ~ ~ + redox ~ , + couple immobilized in carrageenan is shown in Figure 1E with = -0.14 V (vs Ag/AgCl) and AE, = 0.152 V (at 10 mV/s scan rate). The electrode kinetics and diffusion of the R~(en),~+/,+ and Ru(NH,),~+/~+ ions within the carrageenanhydrogel coating were determined, and values for the diffusion coefficients and heterogeneous electron-transfer rate constants are given in Table 11. Quasi-reversible electrochemical behavior was observed for both metal complexes independently immobilized in a carrageenan matrix. Diffusion coefficients show facile diffusion within the hydrogel. Although the data are more limited for these two ions, there is no evidence for the observed diffusion coefficients being dependent on concentration. Fe(CN)63-/4-Electrochemistry. Figure 1F shows a cyclic voltammogram for an ethylenediamine-cured carrageenan hydrogel on a Pt gauze working electrode (method 1) in 5 mM Fe(CN)63 solution with 0.1 M KNO,supporting electrolyte. El and AE, values of 0.27 and 0.105 V (at 10 mV/s scan rate(, respectively, were obtained and are consistent with data obtained without a carrageenan hydrogel coating. Values of De, and ks for Fe(CN)63-/eobtained using this carrageenan-coated eiectrode are shown in Table IV along with corresponding data obtained using a Pt disk and unmodified Pt gauze electrode in the same solution. The presence of the carrageenan hydrogel on the Pt electrode had practically no effect on the electrode kinetics for Fe(CN)63-/e. In this case, the concentration of Fe(CN)63 in the hydrogel was the same as its concentration in solution. Therefore, this negatively charged redox species was not ion-exchanged into the hydrogel but simply diffused into and out of the hydrogel matrix.
Discussion We have immobilized several cationic redox active metal complexes individually and in pairs in a carrageenan hydrogel
Redox Behavior of Immobilized Cationic Metal Complexes
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1393
2,the diffusion coefficient for Ru(bpy)?+I3+is ca. 30 times greater matrix at a Pt electrode by two methods. In method 1 the hydrogel than that for C ~ ( b p y ) ~ ~ +This / ~ +suggests . that the mechanism was cured with ethylenediaminedihydrocblorideand the cationic for charge transport involving metal complex camers immobilized redox couples ion-exchanged into the hydrogel by repeated curwithin the carrageenan matrix is different for R ~ ( b p y ) ~ ~and +/~+ rent/voltage cycles until a steady-state cyclic voltammogram was Co(bp~),Z+/~+. Charge transport is more efficient within a carobtained. In method 2 the cationic redox active complex ML6“+ rageenan matrix that contains R ~ ( b p y ) ~ ~than + / ~w+ithin one that was used to cure the hydrogel. For both methods of immobilicontains C ~ ( b p y ) ~ ~ +This / ~ +point . is also illustrated by the Dex zation, we observed quasi-reversible electrochemicalbehavior for data listed in Table I11 for the carrageenan hydrogels whicg the hydrogel-immobilizedspecies when the current/voltage scans were obtained in solutions that did or did not contain M4*. In contain coimmobilizad R u ( b p ~ ) ~ ~and + / ~C+~ ( b p y ) ~ ~ +Further /~+. evidence of the relative diffusion efficiencies of R u ( b ~ y ) t + / and ~+ either cape the measured diffusion coefficients, Dw are a measure C ~ ( b p y ) ~ ~when + / ~ +coimmobilized may be seen in Figure lC, of diffusion within the carrageenan hydrogel matrix since the where the ratio of cathodic peak currents, ip/igis 1.4 but the thickness of the hydrogel coating was greater than that of the ratio of M ( b ~ y ) t + / concentrations ~+ in the carrageenan hydrogel, diffusion layer. [Ru(b ~ y ) ~ ~ /+[Co( / ~ +b ]~ y ) ~ ~ + is / ~0.75. +], We have also demonstrated excellent stability and diffusion Additional insight into the mechanism of charge transport properties for these hydrogel-coated electrodes. Dcxpvalues for within the carrageenan hydrogel containing an immobilized redox all of the cationic redox couples Wrrc in the carrageenan hydrogel couple may be obtained by noting that measured diffusion ranged from lo4 to lo4 cm2/s, compared to those obtained in water in the vicinity of lo4 cm2/s. This represents excellent coefficients are relatively independent of the concentration of immobilized complex. Furthermore, coimmobilization of Codiffusion characteristics associated with the carrageenan hydrogel ( b ~ y ) t + /and ~ + R ~ ( b p y ) , ~ + does / ~ + not influence their individual matrix and is a significant improvement over previously reported charge transport behavior, nor does the ratio of their concentracation-exchange matrix electrode coatings. The Dwrp values for tions. Apparently each species behaves independently of the other R ~ ( b p y ) ~and ~ +Co(bpy),O+ immobilized in carrageenan are 2-3 and there is no competition for anionic sites within the carrageenan orders of magnitude larger than those found previously for the hydrogel. same redox species in a Nafion cation-exchange matrix“J and A number of years ago Kaufman and Engler34proposed that are 4-5 orders of magnitude larger than those found in montcharge propagation through polymeric films with redox sites morillonite claysM R~(bpy),~+ has also been immobilized in anchored to the backbone occurs by electron hopping between sulfonated polystyrene” where the value of Dcxpis approximately adjacent oxidized and reduced sites. It has subsequently been 2 orders of magnitude smaller than that observed in a carrageenan S ~ O W ~that ~ Vthis J ~electron-hopping process may also contribute hydrogel matrix. to charge propagation when charged redox species are electroPrevious mearchers have found that Nafion-coated electrodes statically bound within polyelectrolyte polymer films. The results do not permit a current/voltage res nse for negatively charged of previous charge transport studies have often been discussed in redox species such as Fe(CN)?-.2? They proposed that the the context of an earlier analysis of redox ions in solution by negatively charged complex is repelled by the sulfate groups of D a h m ~ ’and ~ Ruff and co-worker~.~~ These authors proposed the Nafion and thus does not permeate the matrix. In contrast, parallel paths involving physical diffusion and electron hopping. data in Figure 1 and Table IV demonstrate carrageenan to be permeable to Fe(CN)63- and to exhibit a much larger diffusion This is mathematically presented in coefficient than that observed for NafionF8 These data suggest (1) Dexp = Do + Det that Fe(CN),> is free to diffuse into and out of the carrageenan where De, is the experimentally observed diffusion coefficient hydrogel matrix. In summary, we have demonstrated excellent diffusion prop(cm2/s), is the contribution from physical diffusion (cm2/s), erties for both cationic and anionic redox species within the and Da is the contribution due to self-exchange electron transfer carrageenan hydrogel. The carrageenan matrix provides an open, (cm2/s). For aqueous solutions, Do >> D,. In polymer films Det macroporousenvironment for the immobilization of cationic redox can become significant. This approach, which proposes a dual mode for charge transport,has been applied to polymer films. The species. Both preparation methods 1 and 2 provide a stable electrode with an immobilized ML6* species, which does not leak electron-hopping mechanism for charge transport has received into solution and which provides a quasi-reversible electrochemical wide acceptance for polymers with discrete immobilized redox response. couples.lf For such polymer films the Del term in eq 1 has been The mechanism of charge transport in electroactive polymer expressed as an electron diffusion coefficient parameter which films has received significant attention in the l i t e r a t ~ r e . ~ ~ J A + is ~ ~related ~ ~ by eq 2 to an electron self-exchange rate constant, k, A consideration of the electrochemical parameters obtained for D, = 103~2kCxcT (2) the various metal complexes ML6”+immobilized in the carrageenan hydrogel provides information about the process of charge (M-l s-l), the distance between reaction pairs when electron expropagation in this electroactive polymer film. Aqueous solution change occurs, 6 (cm), and the redox site concentration CT(mol cm-3) .lf,37,38 diffusion coefficients for R~(bpy),~+/’+ and Co(bpy),2+13+determined at a bare Pt disk or gauze electrode are essentially Application of this dual-mode theory described by eqs 1 and identical (ca. 3 X 10-6 cm2/s; Tables I and 11), which is consistent 2 to charge transport in carrageenan hydrogel films containing with literature report^.^^,^^ However, inspection of the data in immobilized cationic complexes ML6”+suggests that the electron Tables I and I1 reveals that when either redox couple is indeself-exchange rate constant for ML6“+is a measure of the relative pendently immobilized in a carrageenan matrix by method 1 or efficiency of the electron-hopping component. Consequently, we would expect the electron-hopping pathway to be much more efficient for R~(bpy),2+/~+ than for C~(bpy)$+/~+, since kcxfor (27) Krishnan, M.; Zhang, X.; Bard, A. J. J . Am. Chem. Soc. 1984,106, the former is lo9 M-l s-l 39 and for the latter is 10’ M-l This
8,
7371. (28) De Wulf, D.; Bard,A. J. J. Mucromol. Sci., Chem. 1989, A26.1205. (29) (a) Fritsch-Faules, I.; Faulkner, L. R. J. ElecrroaMl. Chem. 1989, 263, 237. (b) Oh, S. M.; Faulher, L. R. J. ElecrrooMI. Chem. 1989, 269, 77. (30) (a) Pinkerton, M. J.; Le Mest, Y.; Zhang, H.; Watanabc, M.; Murray, R. W. J. Am. Chem. Soc. 1990,112,3730. (b) Dalton, E. F.; Surridge, N. A.; Jernigan, J. C.; Wilbourn, K. 0.; Facci, J. S.; Murray, R. W. Chem. Phys. 1990,141, 143. (31) Bowden, E. F.; Dautortas, M. F.; Evans, J. F. J . Elecrroanul. Chem. 1987, 219,49. (32) Doblhofer, K.; Dum,W.; Jauch, M. Electrochim. Acra 1982,27,677. (33) Rubenstein, I.; Bard, A. J. J . Am. Chem. Soc. 1981, 103, 5007.
(34) Kaufman, F. B.; Engler, E. M. J . Am. Chem. Soc. 1979, 101, 547. (35) Dahms, H. J. Phys. Chem. 1968, 72, 362. (36) (a) Ruff, I. Electrochim. Acra 1970, 15, 1059. (b) Ruff, I.; Friedrich, V. J. J. Phys. Chem. 1971, 75, 3297. (c) Ruff, I.; Friedrich, V. J.; Demeter, K.; Csillag, K. J . Phys. Chem. 1971, 75, 3303. (37) Andrieux, C. P.;Saveant, J. M. J. E l e c t m m l . Chem. 1980,111,377. (38) Laviron, E. J . ElecrroaMI. Chem. 1980, 112, 1. (39) Young, R. C.; Kecne, F. R.; Meyer, T. J. J . Am. Chem. Soc. 1977, 99, 2468. (40) Baker, B. R.; Basolo, F.; Neumann, H. M. J . Phys. Chem. 1959,63, 371.
Crumbliss et al. C~(bpy),'+'~*
. j _o l 2
R~(bpy),~*'~*
RU~),*''~*
-I
I
Pf disk
-
hydrogel
108 107
109
101
io2
103
104
105
108
107
108
10s
1010
kex
Figure 2. Log experimentally determined diffusion coefficient, Dcxp. obtained in aqueous solution (top curve) and method 1 cured carrageenan hydrogel (bottom curve) plotted as a function of log electron transfer self-exchange rate constants, k,,, obtained in aqueous solution for Co(bp~),~+/'+,Ru(NH,)~*+/~+,R ~ ( e n ) , ~ + / ' +and , R ~ ( b p y ) , ~ + / ) + Dcxp . values are from Tables I and 11, and k,, values are from refs 29 and 39-41.
is consistent with experimental observation since for the carrageenan hydrogel DexpRU > DexpCo(Tables 1-111). The fact that the difference in k , is 8 orders of magnitude while the difference in De,,, is less than 2 orders of magnitude suggests that physical diffusion accounts for the major component of charge transport. This is consistent with the fact that the carrageenan hydrogel structure is mainly water. In addition, it is possible that numerical values for electron self-exchange rate constants obtained in aqueous solution cannot be directly applied to eq 2 in a carrageenan matrix. The other two ruthenium complexes investigated, Ru(NH3)62+/3+ and R ~ ( e n ) ~ ~ +have / ~ +electron , self-exchange rate constants that are intermediate between the extremes represented by R ~ ( b p y ) ~ ~ and + / ~ +C ~ ( b p y ) ~ ~ + /The ~ + . value for Ru(NH3)62+/3+ is 4 X lo3 M-' P121 and for R ~ ( e n ) ~ ~is+4/ ~X +lo4 M-'':'-S According to eqs 1 and 2 we would expect the relative importance of electron hopping for these two complexes to be intermediate between that for Ru(bpy)$+ and Co(bpy)?+. Figure 2 shows the Dexpvalues obtained for all of the ML6"+species in the carrageenan matrix relative to Dcxpvalues obtained in aqueous solution. This comparison suggests a high physical mobility for ML6* in the carrageenan hydrogel. Figure 2 also shows a general trend relating Dexpfor ML6"+in the carrageenan hydrogel and the electron self-exchange rate constant, k,,. This variation in Dcxpvalues with k,, suggests that electron hopping, in addition (41) Beattie, J. K. Private communication. In Geselowitz, D.; Taube, H. Ado. Inorg. Bioinorg. Mech. 1982, 1, 391.
to physical diffusion, is a contributing process for charge propagation in these electroactive carrageenan hydrogel films. The concentration dependence on De, predicted by eqs 1 and 2 was not observed for any of our i m m o b h redox couples. This result, however, is not unusual. The dependence of De, on the concentration of redox sites within a conducting polymer hlm was considered by several a ~ t h o r s . ' ~ f * ~Values ~ for Dap were found to decrea~e,4~~".~~ and remain as the concentration of redox species within a polymer film increased. There may be several factors which contribute to the propagation of charge in a polymer film. These include physical diffusion, electron hopping between redox sites, short- and longrange internal mobility of redox centers, and bamers to the motion of charge-compensating counter ion^.^^^ If the redox species has a large k,, value, then an increase in concentration of the redox species in the polymer film should increase the possibility of electron hopping, thereby enhancing Dexpas predicted by eqs 1 and 2. On the other hand, as the concentration of redox sites in a polymer film increases, Coulombic repulsion between likecharged species and higher electrostatic cross-linking of the polymer backbone could serve to decrease diffusion within the film. All of these processes may be operative in our carrageenan hydrogel matrix, resulting in a relatively flat dependence of Dexp on redox complex concentration.
Acknowledgment. We thank the North Carolina Biotechnology Center, the Office of Naval Research, the Duke University Research Council, the Office of Basic Energy Science of the Department of Energy under Grant DE-FG05-84ER13263, and the National Science Foundation under Grant CHE-87 19709 for financial support. S.C.P. gratefully acknowledges support provided by the Electrochemical Society and the Department of Energy for an Energy Research Summer Fellowship. We thank R. W. Henkens, D. Turner (deceased), and F. Bedioui for helpful discussions and D. Cooke for checking some of the experiments. This is a contribution from the North Carolina Biomolecular Engineering and Materials Application Center, of which A.L.C. and D.P.R. are members. ~~~~~~
~~
~
~
(42) Facci, J. S.; Schmehl, R. H.; Murray, R. W. J . Am. Chem. Soc. 1982, 104,4959. (43) (a) Shigehara, K.; Oyama, N.;Anson, F. C. J . Am. Chem. Soc. 1981, 103, 2552. (b) Oh, S.-M.; Faulkner, L. R. J . Am. Chem. Soc. 1989, 111, 5613. (44) (a) Oyama, N.; Yamaguchi, S.; Nishiki, Y.; Tukuda, K.; Matsuda, H.; Anson, F. C. J . Electroanal. Chem. 1982, 139, 371. (b) Facci,J.; Murray, R. W. J . Phys. Chem. 1981, 85, 2870. (c) Kuo, K. N.; Murray, R. W. J . Electroanal. Chem. 1982,131, 37. (d) Facci, J. S.; Murray, R. W. J . Electroanal. Chem. 1981, 124, 339. (45) (a) Oyama, N.; Takeo, 0.;Kaneko, M.; Sato, K.; Matsuda, H. J.Am. Chem. SOC.1983, 105,6003. (b) Ohsaka, T.; Takahira, Y.; Hatozaki, 0.; Oyama, N. Chem. Soc. Jpn. 1989, 62, 1023.
