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Assembly of Electroactive Ordered Multilayer Films of Cobalt Phthalocyanine Tetrasulfonate and Polycations
Journal of Colloid and Interface Science 212, 570 –575 (1999) Article ID jcis.1998.6083, available online at http://www.idealibrary.com on
Assembly of Electroactive Ordered Multilayer Films of Cobalt Phthalocyanine Tetrasulfonate and Polycations Yuri M. Lvov, Geoffrey N. Kamau, 1 De-Ling Zhou, and James F. Rusling 2 Department of Chemistry, U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received October 23, 1998; accepted December 30, 1998
growth technique we have been exploring for ultrathin electrode coatings. In this paper, we report layer-by-layer assembly and electrochemistry of organized ultrathin films of cobalt tetrasulfophthalocyanine (Co IIPcTS 42) with precisely known numbers of molecular layers. The films were made by alternate adsorption of Co IIPcTS 42 with polycations. Alternate adsorption was first invented for oppositely charged colloidal particles by Iler (12) and later used for assembly of films of linear polycations and polyanions, proteins, and nanoparticles (13–16). Charged dye molecules can also be used to make multilayer films by the same method (17). Electrochemical studies of multilayers containing polyoxometallate (18), polybutanyl viologen/polystyrene sulfonate (19), ferrocyanide/polyvinyl methyl pyridinium (20), and glucose oxidase/polyallyl amine ferrocene (21) were reported recently. The procedure is as follows: an electrode with a negatively charged solid surface is immersed into a solution containing a cationic polyelectrolyte, and a layer of polycation is adsorbed. Since adsorption is carried out at relatively high concentrations of polyelectrolyte, a number of ionic groups remain exposed to the interface with the solution, and the surface charge is effectively reversed. After rinsing in pure water, the electrode is immersed in the solution of negatively charged dye. A new layer is adsorbed, but now the original surface charge is restored. By repeating both steps, alternating multilayer assemblies are obtained (Fig. 1).
Films with alternating layers of the anion cobalt phthalocyanine tetrasulfonate (Co IIPcTS 42) and cationic polydimethyldiallylammonium chloride (PDDA) were prepared by electrostatic layer-bylayer adsorption. Quartz crystal microbalance and optical studies demonstrated formation of smooth ultrathin films with a linear increase in thickness with the number of deposition steps. Films containing 1, 2, 3, 4, and 5 bilayers of Co IIPcTS 42/PDDA on a gold electrode gave reversible, reproducible steady state cyclic voltammetry for the Co II/Co I redox couple with midpoint potential at 20.28 V vs a saturated calomel reference electrode. Voltammetry was controlled predominantly by charge transport processes in the film, even for films containing only a bilayer of PDDA/Co IIPcTS 42. The peak reduction current increased with the number of layers and showed a tendency to saturation after a deposition of 4 –5 bilayers. © 1999 Academic Press Key Words: cobalt phthalocyanine tetrasulfonate; layered films; electrochemistry; polyions; electrostatic adsorption.
INTRODUCTION
Electrochemical reactions mediated (catalyzed) by cobalt macrocyclic complexes are applicable to a wide variety of synthetic manipulations (1, 2). Recently, we demonstrated advantages of microemulsions (3) in selectivity or stereochemistry for several cyclization reactions using dissolved cobalt complex catalysts. If such reactions could be done using catalytic films attached to electrode surfaces (4), additional benefits including reusability, economy of catalyst, and improvements in efficiency might accrue (5–7). Previous work in our laboratory reported ordered films of water-insoluble dialkylammonium surfactants in which metal phthalocyanine tetrasulfonates are intercalated between cationic surfactant bilayers (8, 9). Association of metal phthalocyanine tetrasulfonates within the film was important for film stability. Cobalt phthalocyanine tetrasulfonate (Scheme 1) is a complex suitable for electrochemical catalysis of important carbon-bond forming reactions (10, 11). We felt that association might also be a stabilizing factor in a layer-by-layer film
EXPERIMENTAL
Materials
1 Permanent address: Department of Chemistry, University of Nairobi, Box 30197, Nairobi, Kenya. 2 To whom correspondence should be addressed.
The tetrasodium salt of cobalt(II) 4,49,40,4--tetrasulfophthalocyanine 2-hydrate (Co IIPcTS 42Na 4) was prepared by a published method (22). Aqueous sodium polystyrene sulfonate (PSS, MW 70000, Aldrich) was used at a concentration of 3 mg/mL. Polydimethyldiallylammonium chloride (PDDA, Aldrich) at a concentration of 2 mg/mL was dissolved in water purified with a Barnstead Nanopure system to a specific resistance, .15 MV cm. Electrolyte solutions were 0.1 M NaCl, 0.01 M acetate buffer, pH 5.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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Voltammetry A BAS-100B/W electrochemical analyzer (Bioanalytical Systems) was used for cyclic voltammetry. The three electrode cell contained the gold-coated electrode as working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). The ohmic drop of the cell was compensated to 96 –98% by the BAS-100B/W system, and uncompensated resistance was typically ,7 V. Prior to voltammetry solutions were purged with purified nitrogen for at least 5 min, and a nitrogen atmosphere was maintained over solutions during experiments. Measurements were performed at ambient temperature of ca. 22–23°C. RESULTS SCHEME 1
Film Assembly The multilayer films were assembled on gold-coated resonator electrodes (0.016 cm 22) of a quartz crystal microbalance (QCM) or on quartz plates, by repeating alternate adsorption of PDDA and Co IIPcTS 42. Fused quartz plates (1-mm thick) were washed with sonication in 59% alcohol 1 40% water 1 1% NaOH for 20 min to induce negative charges on the surface, then rinsed thoroughly. Gold electrode resonators were washed in aqua regia for 20 s (Caution: Aqua regia is corrosive to all body tissues and must be handled with care), then in water and alcohol. A negatively charged surface was prepared by immersing the cleaned gold electrodes in 1 mM ethanolic 3-mercapto-1-propanesulfonic acid (MPS, sodium salt, Aldrich) for 12 h, followed by rinsing in pure ethanol and water (23). To grow multilayers the solid substrates were alternately immersed for 15 min in 1 mg/ml aqueous solutions of Co IIPcTS 42 and solution of polyions with intermediate water washing. The process was periodically interrupted and samples were dried under a stream of nitrogen for UV spectroscopy, QCM, and electrochemical measurements. Monitoring Film Growth We monitored the assembly process by UV absorption spectroscopy (Perkin–Elmer l6 UV-vis spectrometer) for films made on quartz plates. For films made on gold-coated quartz crystal microbalance (QCM, USI System, Japan) resonators, the films were removed from solution and dried in a stream of nitrogen before QCM frequency measurements. Optical studies were done with a Nicon–Methaphot (Japan) microscope in reflection mode. After film preparation, electrochemical studies of the films were done directly on the gold QCM resonators. Steel wires of the resonators were isolated by painting with silicon paint (Toray Dow, PRX 30). No peaks on cyclic voltammograms from such bare gold electrodes were detected in the 20.7 to 10.4 V range.
Assembly of Co IIPcTS 42/PDDA Monitored by QCM QCM frequency changes were measured on dry films to obtain the weight of each layer. The long-term stability (several hours) of the quartz resonator frequency was 62 Hz. The resonators used are coated by vapor-deposited gold electrodes on both faces and resonance frequency is 9 MHz (AT-cut). Typically, a QCM resonator was immersed into a polyelectrolyte or dye solution for a given period, removed, and dried in a nitrogen stream, and the frequency change was measured. For our resonator electrodes, calibration using SEM cross-sections of dry polyion films (16) independently confirmed the Sauerbrey equation (24), DF 5 21.83 3 10 8 M/A (mass/area) for dry films. Comparative studies of the same films by QCM and SEM, using film density 1.3 g cm 23, and a measured Au surface area 20% larger than a geometric area, showed that layer thickness (d) and QCM frequency shift (DF) are related by d (nm) ' 20.016 DF (Hz). Assembly of films of anionic Co IIPcTS 42 and cationic PDDA (Co IIPcTS 42/PDDA multilayer) was done by 15 min alternate adsorption from their respective solutions at neutral pH. QCM monitoring of dry films showed linear changes of the frequency with increasing number of adsorption steps (Fig. 2). The frequency shift is proportional to the mass of each ad-
FIG. 1. Schematic diagram of multilayer architectures prepared by sequential adsorption of PDDA and Co IIPcTS 42 layers. Additional layers are added as necessary for films of desired thickness.
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because of the superposition of PDDA and Co IIPcTS 42 absorbance maxima at 225 nm. At this wavelength, there is an increase of optical density at both PDDA and Co IIPcTS 42 adsorption steps. At 677 nm there is no absorbance for PDDA, and the film optical density increased only at the Co IIPcTS 42 adsorption steps. Thus, the incremental increase in A is significantly smaller at this longer wavelength. To check the necessity of polycation layers for film assembly, we attempted step-wise Co IIPcTS 42 adsorption without PDDA interlayers. No multilayer growth was observed by QCM with such an approach, and no UV-VIS absorbance
FIG. 2. Influence of number of adsorbed layers on QCM frequency demonstrating linear growth for a dry {(PDDA/PSS) 2 1 (PDDA/Co I42 I PcTS ) 1–9 } multilayer film; (PDDA/PSS) 2 is the precursor.
sorbed layer and indicates a linear increase of film mass on the electrodes with increasing number of deposition cycles. In four independent experiments, we found that the QCM frequency shift for Co IIPcTS 42/PDDA bilayers was 80 Hz (65%): 40 Hz for Co IIPcTS 42 layers and 40 Hz for PDDA layers. Thus, dye monolayer coverage on every other deposition step is 110 ng cm 22. Assuming that the layer is smooth and no microcrystals are formed by dye molecules, we used these frequency shifts to estimate a layer thickness of 0.6 nm for both Co IIPcTS 42 and PDDA. The dye layer thickness is less than the maximum dimension of the Co IIPcTS 42 molecule in the plane of the PcTS ligand of ca. 1.2 nm (25). Thus, it is possible that dye molecule planes are stacked face-to-face and tilted in the layer. Optical Properties Optical microscopy of films on quartz slides revealed lightpurple coverage at a resolution of 0.5 mm, corresponding to the microscope resolution. No color fluctuations in different parts of the film were detected by eye. Dye multilayers of the best optical quality were prepared on slides coated initially by a precursor polycation/polyanion layer of {(PDDA/PSS) 2 1 PDDA}. The thickness of this precursor layer is ca. 6.1 nm, as calculated from corresponding 2DF 5 380 Hz measured for precursor layers assembled on a QCM resonator. The precursor provides a smooth and permanently charged surface, which apparently encourages uniform distribution of the dye layer. Figure 3a shows absorption spectra at different stages of the assembly of the film {(PDDA/PSS) 2 1 (PDDA/Co IIPcTS 42) 1– 8}. A regular increase of absorbance after deposition of every bilayer was found. UV-VIS absorbance at 677 and 225 nm demonstrates a linear increase of the amount of dye in the film with a number of adsorption cycles (Fig. 3b). The increments of these dependencies are different. At 225 nm, A increases by 0.0065 A/step, and for 677 nm 0.0035 A/step. This is partly
FIG. 3. UV monitoring of film assembly: (a) Spectra for individual bilayers of PDDA/Co IIPcTS 42 during {(PDDA/PSS) 2 1 (PDDA/Co IIPcTS 42) 1–8} multilayer assembly on a quartz slide and along with spectrum of aqueous 0.14 mM Co IIPcTS 42; (b) dependence of film absorbance at 225 and 677 nm on a number of layers.
COBALT PHTHALOCYANINE TETRASULFONATE AND POLYCATIONS
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increase was found. Thus, alternation of oppositely charged dye and polyion layers was necessary to obtain an ordered nanocomposite. It is informative to compare absorbance of Co IIPcTS 42 in the film and in dilute solutions (Fig. 3a). In solution, aggregation of Co IIPcTS 42 is known to occur (22), with the peak near 665 nm assigned to a P–P* transition of the phthalocyanine ligand, characteristic of monomers (26). Peaks or shoulders at about 610 – 630 nm arise from face-to-face aggregates because of extensive coupling between the P-electron systems of adjacent rings. This leads to a blue shift in the P–P* band (27). In our films, the maximum at 665 nm and shoulder at about 620 nm suggest the presence of both monomers and aggregates of Co IIPcTS 42 in the films (Fig. 3a). An additional sharp maximum at 225 nm also appears. These maxima grow linearly with the number of the film layers. Matching of the integrated optical absorbance in the 550 – 750 nm region of dye solutions with those of the multilayer film gives a rough estimate of 15 ng cm 22 for a dye monolayer, much less than measured by QCM at 110 ng/cm 2. The lower value from absorbance is most likely connected with negative deviations from Beer’s law in the condensed films containing aggregated dye molecules. This also suggests Co IIPcTS 42 dimerization or aggregation in the films. Monomer to dimer conversion decreases the monomer concentration, and extinction coefficients for aggregates are roughly 1/3 to 1/4 less compared with the monomer (22). These two combined factors could result in the observed 1/7 decrease of optical absorbance for each layer compared to the measured mass. Voltammetry of Co IIPcTS 42/PDDA Multilayers In all experiments with Co IIPcTS 42/PDDA films, reproducibile pairs of reduction– oxidation peaks were found by cyclic voltammetry. Fig. 4a shows voltammograms for (PDDA/ Co IIPcTS 42) 1–5 films on MPS-treated gold electrodes. A pair of nearly reversible reduction– oxidation peaks was observed centered at about 20.280 V vs SCE. Peak current increased with the number of layers, and peaks all had the unsymmetric shapes characteristic of charge transport diffusion control (4). When an additional polycation–polyanion layer of 1-nm thickness was deposited underneath the first Co IIPcTS 42 layer, resulting in {(PDDA/PSS) 1 1 (PDDA/Co IIPcTS 42) 1–5} films, an increase in reduction peak current was also found with an increasing number of layers, but the peak separation was much larger (Fig. 4b). We measured the layer mass with QCM before and after every electrochemical experiment. About 50% of the initially adsorbed dye-layer was washed out during CV scanning, resulting in a stable coverage at every dye adsorption step of ca. 50 ng cm 22, considerably less than the layer growth step of 110 ng cm 22 found during assembly of the film.
FIG. 4. Cyclic voltammograms at 1.0 V s 21 of Co IIPcTS 42 films on MPS-gold electrode in a pH 5 buffer: (a) (PDDA/Co IIPcTS 42) 1,3,5; (b) {PDDA/ PSS 1 (PDDA/Co IIPcTS 42) 1–5}.
DISCUSSION
Layer-by-layer assembly of films of Co IIPcTS 42 and oppositely charged polycations provided films with multiple layers of the electroactive dye. QCM monitoring (Fig. 2) demonstrated a linear increase of the film mass with the number of adsorption steps, which corresponds to monolayer formation at every adsorption step for polyion and Co IIPcTS 42. UV-visible absorbance (Fig. 3) was also consistent with a regular layerby-layer film growth and suggested that Co IIPcTS 42 is partly aggregated in the films. Similar aggregation was found in ordered cast films of M IIPcTS 42 complexes and DDAB (8, 9). The layered Co IIPcTS 42/PDDA films gave reduction– oxidation peaks that increase in height with the number of Co IIPcTS 42 layers. For (PDDA/Co IIPcTS 42) 1–5 films, nearly reversible pairs of reduction– oxidation peaks were observed (Fig. 4a) with average midpoint potential of 20.28 V vs SCE at pH 5. These peaks appear at similar potentials to those of the
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FIG. 5. Influence on scan rate on peak current for {PDDA/PSS 1 (PDDA/ Co IIPcTS 42) 1,2,4} films in a pH 5 buffer.
Co(II)/Co(I) redox couple of Co IIPcTS 42 dissolved in dimethylformamide (E 0 5 20.32 V) and in a microemulsion (20.25 V) (10, 11). Thus, the CV results suggest that the catalytically active Co(I) form of the complex (10) can be produced reversibly in these films. With an additional 1-nm polymer barrier between the gold surface and the dye layer in {(PDDA/PSS) 1 1 (PDDA/Co I42 I PcTS ) 1–5 } films, the peak separation increases and the voltammetry is less reversible (Fig. 4b). Nevertheless, integrations of the reduction peaks suggest about the same amount of charge is passed, i.e., 6 mC/cm 2 (1 V s 21) for 5 bilayers of both types of films. This suggests that roughly the same amount of electroactive material is being addressed in both types of films. Plots of reduction peak current (I p) vs scan rate (n) gave nonlinear dependencies which fit I p 5 n 0.65 for films with Co IIPcTS 42 monolayer and I p 5 n 0.5 for four-layer films, with linear regression correlation coefficients 0.99. (Fig. 5). The latter square root dependence of I p on scan rate suggests a diffusional limitation of charge transport (4) in the Co IIPcTS 42/ PDDA multilayers. This view is also consistent with the characteristic unsymmetric diffusional CV peak shape. Thus, as additional Co IIPcTS 42/PDDA layers are added above the first PDDA/Co IIPcTS 42 bilayer, peak current shifts from mixed surface– diffusion control to full diffusion control at four layers. While we have not investigated this point in detail, the I p 5 n 0.65 dependence for films with a single monolayer of electroactive dye (i.e., Au-MPS-PDDA/Co IIPcTS 42) consistent with partial diffusion controlled charge transport probably does not reflect actual physical diffusion of the dye molecules within the films. This suggestion is further supported by the larger peak separation and decreased reversibility of the CVs for {(PDDA/ PSS) 1 1 (PDDA/Co IIPcTS 42) 1–5} films. Here, with a 1-nm layer of polymer between the electrode surface and the first dye
layer, and all other properties assumed the same, electron transfer would seem to be slower than in the (PDDA/ Co IIPcTS 42) 1–5 films, which have only one PDDA layer separating the electrode and the first dye layer. If the dye could diffuse readily through the polymer layers to the electrode, we would expect the steady state CV reversibility and charge transport after multiple scans to be similar for both types of films, but this was not the case. The larger peak separations in the films with the 1 nm “polymer barrier” between dye and electrode seem to give slower electron transfer. Figure 6 shows the dependence of the CV reduction peak current on the number of dye layers. The increase in peak current is linear for the first three layers, then begins to bend off at the 4th and 5th layers of Co IIPcTS 42. This coincides with the loss of surface control of current as the number of film layers increases. Schlenoff et al. (19) found for polybutanyl viologen/PSS multilayer films that the CV peak heights increased with the number of layers up to 10 bilayers. For films of the iron heme protein myoglobin and polystyrene sulfonate, only two layers of the electroactive protein were electrochemically addressable (23). In the present case, the CV peak heights at the 4th layer begin to show a tendency to saturation, but the data suggest that all five layers in the thickest films studied communicate with the electrode. Charge transport in polyion films may be limited by electron diffusion and/or transport of electroinactive counterions into and out of the films (4). Transport of charge involving the outermost phthalocyanine layers in our films may become difficult because the rate of electron transfer decreases exponentially with distance from the electrode and distance between redox partners (28) and/or because of decreasing rates of counterion transport in the increasingly thicker films. Electron self-exchange rates between electroactive layers could also be an important feature of charge transport in these films. The I p 5 n 0.65 dependence (Fig. 5) for films with one monolayer of electroactive dye shows that even for these
FIG. 6. Dependence of peak current at 1.0 V s 21 in a pH 5 buffer on a number of dye layers in PDDA/Co IIPcTS 42 films.
COBALT PHTHALOCYANINE TETRASULFONATE AND POLYCATIONS
systems there is significant charge diffusion control operating in the system. Note that the electroactive layer in these particular films lies on a bed of polymer, which could slow charge diffusion. For films with four and five layers, the data clearly show a square root dependence of peak current on scan rate, suggesting nearly complete charge diffusion control. Using the weights of materials estimated by QCM to estimate film volume and the concentration of Co IIPcTS 42 in the films (assuming full electroactivity), the slope of the I p vs. n 0.5 plots, and the Randles Sevcik equation (4), we estimated a lower limit of 5 3 10 213 cm 2 s 21 for the charge transport diffusion coefficient (D ct) in films with five layers of Co IIPcTS 42. Actual D-values of 10 215 cm 2 s 21 were found for watersoluble rhodamine and 10 213 cm 2 s 21 for the smaller tetramethyl-4-piperedinol through layered polyion films (29). These values are slightly smaller than the lower limit D ct in our films. D-values in the polyion films are significantly smaller than those of films of Co IIPcTS 42 and the cationic surfactant didodecyldimethylammonium bromide (DDAB). D ct values were 3 3 10 28 cm 2 s 21 for a liquid crystal DDAB film into which CuPcTS 42 had been ion exchanged and 5 3 10 29 cm 2 s 21 for a solid film cast from a CuPcTS 42/DDA salt in pH 12 phosphate buffer (30). Spectroscopic data for Co IIPcTS 42/polycation films suggest that face-to-face dimers are present in the films. QCM mass measurements are consistent with a face-to-face, tilted orientation in the layer. However, neutron reflectivity of polyion films made by the layer-by-layer method showed considerable mixing between adjacent oppositely charge layers, but no mixing between layers separated by two intervening layers (31). It is possible that partial mixing of adjacent layers also occurs in the Co IIPcTS 42/polycation films, and this might indeed facilitate charge transport by bringing the redox centers in adjacent bilayers closer together. In summary, electroactive films of Co IIPcTS 42 and polycations were prepared by a layer-by-layer growth method. Reversible steady state cyclic voltammograms of the Co(II)/Co(I) redox couple were obtained and were controlled by charge transport diffusion at 0.1–1 V s 21 even for films containing only a monolayer of electroactive dye. While charge transport diffusion coefficients were smaller than for films of metal phthalocyanine tetrasulfonates and cationic surfactants, the layer-by-layer method provides advantages of precise control of film thickness and the possibility of higher concentrations of electroactive species in the films.
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ACKNOWLEDGMENT This work was supported by Grant CTS-9632391 from the National Science Foundation. G.N.K. thanks UNESCO Paris/Nairobi for a grant to travel to the University of Connecticut.
REFERENCES 1. Scheffold, R., in “Modern Synthetic Methods” (R. Scheffold, Ed.), Vol. 3, pp. 355– 439. Wiley, New York, 1983. 2. Scheffold, R., Abrecht, S., Orlinski, R., Ruf, H.-R., Stamouli, P., Tinembart, O., Walder, L., and Weymuth, C., Pure Appl. Chem. 59, 363 (1987). 3. Rusling, J. F., and Zhou, D.-L., J. Electroanal. Chem. 439, 8 (1997). 4. Murray, R. W., Ed., “Molecular Design of Electrode Surfaces,” Techniques of Chemistry, Vol. 22, Wiley, New York, 1992. 5. Bessel, C. A., and Rolison, D. R., J. Am. Chem. Soc. 119, 12673 (1997). 6. Tollner, K., Popovitz-Biro, R., Lahav, M., and Milstein, D., Science 278, 2100 (1997). 7. Mbindyo, J. K. N., and Rusling, J. F., Langmuir 14, 7027 (1997). 8. Hu, N., Howe, D. J., Ahmadi, M. F., and Rusling, J. F., Anal. Chem. 64, 3180 (1992). 9. Zelina, J. P., and Rusling, J. F., Microporous Mater. 5, 20 (1995). 10. Zhou, D.-L., Gao, J., and Rusling, J. F., J. Am. Chem. Soc. 117, 1127 (1995). 11. Zhou, D.-L., Carrero, H., and Rusling, J. F., Langmuir 12, 3067 (1996). 12. Iler, R., J. Colloid Interface Sci. 21, 569 (1966). 13. Decher, G., and Hong, J-D., Ber. Bunsen Ges. Phys. Chem. 95, 1430 (1996). 14. Lvov, Y., Decher, G., and Moehwald, H., Langmuir 9, 481 (1993). 15. Fendler, J. H., and Meldrum, F., Adv. Mater. 7, 607 (1995). 16. Lvov, Y., Ariga, K., Ichinose, I., and Kunitake, T., J. Am. Chem. Soc. 117, 6117 (1995). 17. Araki, K., Wagen, M., and Wrighton, M., Langmuir 12, 5393 (1996). 18. Ingersoll, D., Kulesza, P., and Faulkner, L., J. Electrochem. Soc. 141, 140 (1994). 19. Laurent, D., and Schlenoff, J., Langmuir 13, 1552 (1997). 20. Lowy, D., and Finklea, H., Electrochim. Acta 42, 1325 (1997). 21. Hodak, J., Etchenique, R., Calvo, E., Singhal, K., and Bartlett, P., Langmuir 13, 2708 (1997). 22. Yang, Y.-C., Ward, J., and Seiders, R., Inorg. Chem. 24, 1765 (1985). 23. Lvov, Y. M., Lu, Z., Schenkman, J. B., and Rusling, J. F., J. Am. Chem. Soc. 120, 4073 (1998). 24. Sauerbrey, G., Z. Physik 155, 206 (1959). 25. Ariga, K., Lvov, Y., and Kunitake, T., J. Am. Chem. Soc. 119, 2224 (1997). 26. Stillman, M. J., and Nyokong, T., in “Phthalocyanines, Properties and Applications,” Vol. 3, p. 133, VCH Publishers, New York, 1993. 27. Leznoff, C. C., and Lever, A. B. P., in “Phthalocyanines, Properties and Applications,” Vol. 1, VCH Publishers, New York, 1989. 28. Closs, G., and Miller, J., Science 240, 440 (1988). 29. Klitzing, R., and Mohwald, H., Thin Solid Films, 284, 352 (1996). 30. Rusling, J. F., and Howe, D. J., Inorg. Chim. Acta. 226, 159 (1994). 31. Decher, G., Science 227, 1232 (1997).
Assembly of Electroactive Ordered Multilayer Films of Cobalt Phthalocyanine Tetrasulfonate and Polycations Yuri M. Lvov, Geoffrey N. Kamau, 1 De-Ling Zhou, and James F. Rusling 2 Department of Chemistry, U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received October 23, 1998; accepted December 30, 1998
growth technique we have been exploring for ultrathin electrode coatings. In this paper, we report layer-by-layer assembly and electrochemistry of organized ultrathin films of cobalt tetrasulfophthalocyanine (Co IIPcTS 42) with precisely known numbers of molecular layers. The films were made by alternate adsorption of Co IIPcTS 42 with polycations. Alternate adsorption was first invented for oppositely charged colloidal particles by Iler (12) and later used for assembly of films of linear polycations and polyanions, proteins, and nanoparticles (13–16). Charged dye molecules can also be used to make multilayer films by the same method (17). Electrochemical studies of multilayers containing polyoxometallate (18), polybutanyl viologen/polystyrene sulfonate (19), ferrocyanide/polyvinyl methyl pyridinium (20), and glucose oxidase/polyallyl amine ferrocene (21) were reported recently. The procedure is as follows: an electrode with a negatively charged solid surface is immersed into a solution containing a cationic polyelectrolyte, and a layer of polycation is adsorbed. Since adsorption is carried out at relatively high concentrations of polyelectrolyte, a number of ionic groups remain exposed to the interface with the solution, and the surface charge is effectively reversed. After rinsing in pure water, the electrode is immersed in the solution of negatively charged dye. A new layer is adsorbed, but now the original surface charge is restored. By repeating both steps, alternating multilayer assemblies are obtained (Fig. 1).
Films with alternating layers of the anion cobalt phthalocyanine tetrasulfonate (Co IIPcTS 42) and cationic polydimethyldiallylammonium chloride (PDDA) were prepared by electrostatic layer-bylayer adsorption. Quartz crystal microbalance and optical studies demonstrated formation of smooth ultrathin films with a linear increase in thickness with the number of deposition steps. Films containing 1, 2, 3, 4, and 5 bilayers of Co IIPcTS 42/PDDA on a gold electrode gave reversible, reproducible steady state cyclic voltammetry for the Co II/Co I redox couple with midpoint potential at 20.28 V vs a saturated calomel reference electrode. Voltammetry was controlled predominantly by charge transport processes in the film, even for films containing only a bilayer of PDDA/Co IIPcTS 42. The peak reduction current increased with the number of layers and showed a tendency to saturation after a deposition of 4 –5 bilayers. © 1999 Academic Press Key Words: cobalt phthalocyanine tetrasulfonate; layered films; electrochemistry; polyions; electrostatic adsorption.
INTRODUCTION
Electrochemical reactions mediated (catalyzed) by cobalt macrocyclic complexes are applicable to a wide variety of synthetic manipulations (1, 2). Recently, we demonstrated advantages of microemulsions (3) in selectivity or stereochemistry for several cyclization reactions using dissolved cobalt complex catalysts. If such reactions could be done using catalytic films attached to electrode surfaces (4), additional benefits including reusability, economy of catalyst, and improvements in efficiency might accrue (5–7). Previous work in our laboratory reported ordered films of water-insoluble dialkylammonium surfactants in which metal phthalocyanine tetrasulfonates are intercalated between cationic surfactant bilayers (8, 9). Association of metal phthalocyanine tetrasulfonates within the film was important for film stability. Cobalt phthalocyanine tetrasulfonate (Scheme 1) is a complex suitable for electrochemical catalysis of important carbon-bond forming reactions (10, 11). We felt that association might also be a stabilizing factor in a layer-by-layer film
EXPERIMENTAL
Materials
1 Permanent address: Department of Chemistry, University of Nairobi, Box 30197, Nairobi, Kenya. 2 To whom correspondence should be addressed.
The tetrasodium salt of cobalt(II) 4,49,40,4--tetrasulfophthalocyanine 2-hydrate (Co IIPcTS 42Na 4) was prepared by a published method (22). Aqueous sodium polystyrene sulfonate (PSS, MW 70000, Aldrich) was used at a concentration of 3 mg/mL. Polydimethyldiallylammonium chloride (PDDA, Aldrich) at a concentration of 2 mg/mL was dissolved in water purified with a Barnstead Nanopure system to a specific resistance, .15 MV cm. Electrolyte solutions were 0.1 M NaCl, 0.01 M acetate buffer, pH 5.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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Voltammetry A BAS-100B/W electrochemical analyzer (Bioanalytical Systems) was used for cyclic voltammetry. The three electrode cell contained the gold-coated electrode as working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). The ohmic drop of the cell was compensated to 96 –98% by the BAS-100B/W system, and uncompensated resistance was typically ,7 V. Prior to voltammetry solutions were purged with purified nitrogen for at least 5 min, and a nitrogen atmosphere was maintained over solutions during experiments. Measurements were performed at ambient temperature of ca. 22–23°C. RESULTS SCHEME 1
Film Assembly The multilayer films were assembled on gold-coated resonator electrodes (0.016 cm 22) of a quartz crystal microbalance (QCM) or on quartz plates, by repeating alternate adsorption of PDDA and Co IIPcTS 42. Fused quartz plates (1-mm thick) were washed with sonication in 59% alcohol 1 40% water 1 1% NaOH for 20 min to induce negative charges on the surface, then rinsed thoroughly. Gold electrode resonators were washed in aqua regia for 20 s (Caution: Aqua regia is corrosive to all body tissues and must be handled with care), then in water and alcohol. A negatively charged surface was prepared by immersing the cleaned gold electrodes in 1 mM ethanolic 3-mercapto-1-propanesulfonic acid (MPS, sodium salt, Aldrich) for 12 h, followed by rinsing in pure ethanol and water (23). To grow multilayers the solid substrates were alternately immersed for 15 min in 1 mg/ml aqueous solutions of Co IIPcTS 42 and solution of polyions with intermediate water washing. The process was periodically interrupted and samples were dried under a stream of nitrogen for UV spectroscopy, QCM, and electrochemical measurements. Monitoring Film Growth We monitored the assembly process by UV absorption spectroscopy (Perkin–Elmer l6 UV-vis spectrometer) for films made on quartz plates. For films made on gold-coated quartz crystal microbalance (QCM, USI System, Japan) resonators, the films were removed from solution and dried in a stream of nitrogen before QCM frequency measurements. Optical studies were done with a Nicon–Methaphot (Japan) microscope in reflection mode. After film preparation, electrochemical studies of the films were done directly on the gold QCM resonators. Steel wires of the resonators were isolated by painting with silicon paint (Toray Dow, PRX 30). No peaks on cyclic voltammograms from such bare gold electrodes were detected in the 20.7 to 10.4 V range.
Assembly of Co IIPcTS 42/PDDA Monitored by QCM QCM frequency changes were measured on dry films to obtain the weight of each layer. The long-term stability (several hours) of the quartz resonator frequency was 62 Hz. The resonators used are coated by vapor-deposited gold electrodes on both faces and resonance frequency is 9 MHz (AT-cut). Typically, a QCM resonator was immersed into a polyelectrolyte or dye solution for a given period, removed, and dried in a nitrogen stream, and the frequency change was measured. For our resonator electrodes, calibration using SEM cross-sections of dry polyion films (16) independently confirmed the Sauerbrey equation (24), DF 5 21.83 3 10 8 M/A (mass/area) for dry films. Comparative studies of the same films by QCM and SEM, using film density 1.3 g cm 23, and a measured Au surface area 20% larger than a geometric area, showed that layer thickness (d) and QCM frequency shift (DF) are related by d (nm) ' 20.016 DF (Hz). Assembly of films of anionic Co IIPcTS 42 and cationic PDDA (Co IIPcTS 42/PDDA multilayer) was done by 15 min alternate adsorption from their respective solutions at neutral pH. QCM monitoring of dry films showed linear changes of the frequency with increasing number of adsorption steps (Fig. 2). The frequency shift is proportional to the mass of each ad-
FIG. 1. Schematic diagram of multilayer architectures prepared by sequential adsorption of PDDA and Co IIPcTS 42 layers. Additional layers are added as necessary for films of desired thickness.
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because of the superposition of PDDA and Co IIPcTS 42 absorbance maxima at 225 nm. At this wavelength, there is an increase of optical density at both PDDA and Co IIPcTS 42 adsorption steps. At 677 nm there is no absorbance for PDDA, and the film optical density increased only at the Co IIPcTS 42 adsorption steps. Thus, the incremental increase in A is significantly smaller at this longer wavelength. To check the necessity of polycation layers for film assembly, we attempted step-wise Co IIPcTS 42 adsorption without PDDA interlayers. No multilayer growth was observed by QCM with such an approach, and no UV-VIS absorbance
FIG. 2. Influence of number of adsorbed layers on QCM frequency demonstrating linear growth for a dry {(PDDA/PSS) 2 1 (PDDA/Co I42 I PcTS ) 1–9 } multilayer film; (PDDA/PSS) 2 is the precursor.
sorbed layer and indicates a linear increase of film mass on the electrodes with increasing number of deposition cycles. In four independent experiments, we found that the QCM frequency shift for Co IIPcTS 42/PDDA bilayers was 80 Hz (65%): 40 Hz for Co IIPcTS 42 layers and 40 Hz for PDDA layers. Thus, dye monolayer coverage on every other deposition step is 110 ng cm 22. Assuming that the layer is smooth and no microcrystals are formed by dye molecules, we used these frequency shifts to estimate a layer thickness of 0.6 nm for both Co IIPcTS 42 and PDDA. The dye layer thickness is less than the maximum dimension of the Co IIPcTS 42 molecule in the plane of the PcTS ligand of ca. 1.2 nm (25). Thus, it is possible that dye molecule planes are stacked face-to-face and tilted in the layer. Optical Properties Optical microscopy of films on quartz slides revealed lightpurple coverage at a resolution of 0.5 mm, corresponding to the microscope resolution. No color fluctuations in different parts of the film were detected by eye. Dye multilayers of the best optical quality were prepared on slides coated initially by a precursor polycation/polyanion layer of {(PDDA/PSS) 2 1 PDDA}. The thickness of this precursor layer is ca. 6.1 nm, as calculated from corresponding 2DF 5 380 Hz measured for precursor layers assembled on a QCM resonator. The precursor provides a smooth and permanently charged surface, which apparently encourages uniform distribution of the dye layer. Figure 3a shows absorption spectra at different stages of the assembly of the film {(PDDA/PSS) 2 1 (PDDA/Co IIPcTS 42) 1– 8}. A regular increase of absorbance after deposition of every bilayer was found. UV-VIS absorbance at 677 and 225 nm demonstrates a linear increase of the amount of dye in the film with a number of adsorption cycles (Fig. 3b). The increments of these dependencies are different. At 225 nm, A increases by 0.0065 A/step, and for 677 nm 0.0035 A/step. This is partly
FIG. 3. UV monitoring of film assembly: (a) Spectra for individual bilayers of PDDA/Co IIPcTS 42 during {(PDDA/PSS) 2 1 (PDDA/Co IIPcTS 42) 1–8} multilayer assembly on a quartz slide and along with spectrum of aqueous 0.14 mM Co IIPcTS 42; (b) dependence of film absorbance at 225 and 677 nm on a number of layers.
COBALT PHTHALOCYANINE TETRASULFONATE AND POLYCATIONS
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increase was found. Thus, alternation of oppositely charged dye and polyion layers was necessary to obtain an ordered nanocomposite. It is informative to compare absorbance of Co IIPcTS 42 in the film and in dilute solutions (Fig. 3a). In solution, aggregation of Co IIPcTS 42 is known to occur (22), with the peak near 665 nm assigned to a P–P* transition of the phthalocyanine ligand, characteristic of monomers (26). Peaks or shoulders at about 610 – 630 nm arise from face-to-face aggregates because of extensive coupling between the P-electron systems of adjacent rings. This leads to a blue shift in the P–P* band (27). In our films, the maximum at 665 nm and shoulder at about 620 nm suggest the presence of both monomers and aggregates of Co IIPcTS 42 in the films (Fig. 3a). An additional sharp maximum at 225 nm also appears. These maxima grow linearly with the number of the film layers. Matching of the integrated optical absorbance in the 550 – 750 nm region of dye solutions with those of the multilayer film gives a rough estimate of 15 ng cm 22 for a dye monolayer, much less than measured by QCM at 110 ng/cm 2. The lower value from absorbance is most likely connected with negative deviations from Beer’s law in the condensed films containing aggregated dye molecules. This also suggests Co IIPcTS 42 dimerization or aggregation in the films. Monomer to dimer conversion decreases the monomer concentration, and extinction coefficients for aggregates are roughly 1/3 to 1/4 less compared with the monomer (22). These two combined factors could result in the observed 1/7 decrease of optical absorbance for each layer compared to the measured mass. Voltammetry of Co IIPcTS 42/PDDA Multilayers In all experiments with Co IIPcTS 42/PDDA films, reproducibile pairs of reduction– oxidation peaks were found by cyclic voltammetry. Fig. 4a shows voltammograms for (PDDA/ Co IIPcTS 42) 1–5 films on MPS-treated gold electrodes. A pair of nearly reversible reduction– oxidation peaks was observed centered at about 20.280 V vs SCE. Peak current increased with the number of layers, and peaks all had the unsymmetric shapes characteristic of charge transport diffusion control (4). When an additional polycation–polyanion layer of 1-nm thickness was deposited underneath the first Co IIPcTS 42 layer, resulting in {(PDDA/PSS) 1 1 (PDDA/Co IIPcTS 42) 1–5} films, an increase in reduction peak current was also found with an increasing number of layers, but the peak separation was much larger (Fig. 4b). We measured the layer mass with QCM before and after every electrochemical experiment. About 50% of the initially adsorbed dye-layer was washed out during CV scanning, resulting in a stable coverage at every dye adsorption step of ca. 50 ng cm 22, considerably less than the layer growth step of 110 ng cm 22 found during assembly of the film.
FIG. 4. Cyclic voltammograms at 1.0 V s 21 of Co IIPcTS 42 films on MPS-gold electrode in a pH 5 buffer: (a) (PDDA/Co IIPcTS 42) 1,3,5; (b) {PDDA/ PSS 1 (PDDA/Co IIPcTS 42) 1–5}.
DISCUSSION
Layer-by-layer assembly of films of Co IIPcTS 42 and oppositely charged polycations provided films with multiple layers of the electroactive dye. QCM monitoring (Fig. 2) demonstrated a linear increase of the film mass with the number of adsorption steps, which corresponds to monolayer formation at every adsorption step for polyion and Co IIPcTS 42. UV-visible absorbance (Fig. 3) was also consistent with a regular layerby-layer film growth and suggested that Co IIPcTS 42 is partly aggregated in the films. Similar aggregation was found in ordered cast films of M IIPcTS 42 complexes and DDAB (8, 9). The layered Co IIPcTS 42/PDDA films gave reduction– oxidation peaks that increase in height with the number of Co IIPcTS 42 layers. For (PDDA/Co IIPcTS 42) 1–5 films, nearly reversible pairs of reduction– oxidation peaks were observed (Fig. 4a) with average midpoint potential of 20.28 V vs SCE at pH 5. These peaks appear at similar potentials to those of the
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FIG. 5. Influence on scan rate on peak current for {PDDA/PSS 1 (PDDA/ Co IIPcTS 42) 1,2,4} films in a pH 5 buffer.
Co(II)/Co(I) redox couple of Co IIPcTS 42 dissolved in dimethylformamide (E 0 5 20.32 V) and in a microemulsion (20.25 V) (10, 11). Thus, the CV results suggest that the catalytically active Co(I) form of the complex (10) can be produced reversibly in these films. With an additional 1-nm polymer barrier between the gold surface and the dye layer in {(PDDA/PSS) 1 1 (PDDA/Co I42 I PcTS ) 1–5 } films, the peak separation increases and the voltammetry is less reversible (Fig. 4b). Nevertheless, integrations of the reduction peaks suggest about the same amount of charge is passed, i.e., 6 mC/cm 2 (1 V s 21) for 5 bilayers of both types of films. This suggests that roughly the same amount of electroactive material is being addressed in both types of films. Plots of reduction peak current (I p) vs scan rate (n) gave nonlinear dependencies which fit I p 5 n 0.65 for films with Co IIPcTS 42 monolayer and I p 5 n 0.5 for four-layer films, with linear regression correlation coefficients 0.99. (Fig. 5). The latter square root dependence of I p on scan rate suggests a diffusional limitation of charge transport (4) in the Co IIPcTS 42/ PDDA multilayers. This view is also consistent with the characteristic unsymmetric diffusional CV peak shape. Thus, as additional Co IIPcTS 42/PDDA layers are added above the first PDDA/Co IIPcTS 42 bilayer, peak current shifts from mixed surface– diffusion control to full diffusion control at four layers. While we have not investigated this point in detail, the I p 5 n 0.65 dependence for films with a single monolayer of electroactive dye (i.e., Au-MPS-PDDA/Co IIPcTS 42) consistent with partial diffusion controlled charge transport probably does not reflect actual physical diffusion of the dye molecules within the films. This suggestion is further supported by the larger peak separation and decreased reversibility of the CVs for {(PDDA/ PSS) 1 1 (PDDA/Co IIPcTS 42) 1–5} films. Here, with a 1-nm layer of polymer between the electrode surface and the first dye
layer, and all other properties assumed the same, electron transfer would seem to be slower than in the (PDDA/ Co IIPcTS 42) 1–5 films, which have only one PDDA layer separating the electrode and the first dye layer. If the dye could diffuse readily through the polymer layers to the electrode, we would expect the steady state CV reversibility and charge transport after multiple scans to be similar for both types of films, but this was not the case. The larger peak separations in the films with the 1 nm “polymer barrier” between dye and electrode seem to give slower electron transfer. Figure 6 shows the dependence of the CV reduction peak current on the number of dye layers. The increase in peak current is linear for the first three layers, then begins to bend off at the 4th and 5th layers of Co IIPcTS 42. This coincides with the loss of surface control of current as the number of film layers increases. Schlenoff et al. (19) found for polybutanyl viologen/PSS multilayer films that the CV peak heights increased with the number of layers up to 10 bilayers. For films of the iron heme protein myoglobin and polystyrene sulfonate, only two layers of the electroactive protein were electrochemically addressable (23). In the present case, the CV peak heights at the 4th layer begin to show a tendency to saturation, but the data suggest that all five layers in the thickest films studied communicate with the electrode. Charge transport in polyion films may be limited by electron diffusion and/or transport of electroinactive counterions into and out of the films (4). Transport of charge involving the outermost phthalocyanine layers in our films may become difficult because the rate of electron transfer decreases exponentially with distance from the electrode and distance between redox partners (28) and/or because of decreasing rates of counterion transport in the increasingly thicker films. Electron self-exchange rates between electroactive layers could also be an important feature of charge transport in these films. The I p 5 n 0.65 dependence (Fig. 5) for films with one monolayer of electroactive dye shows that even for these
FIG. 6. Dependence of peak current at 1.0 V s 21 in a pH 5 buffer on a number of dye layers in PDDA/Co IIPcTS 42 films.
COBALT PHTHALOCYANINE TETRASULFONATE AND POLYCATIONS
systems there is significant charge diffusion control operating in the system. Note that the electroactive layer in these particular films lies on a bed of polymer, which could slow charge diffusion. For films with four and five layers, the data clearly show a square root dependence of peak current on scan rate, suggesting nearly complete charge diffusion control. Using the weights of materials estimated by QCM to estimate film volume and the concentration of Co IIPcTS 42 in the films (assuming full electroactivity), the slope of the I p vs. n 0.5 plots, and the Randles Sevcik equation (4), we estimated a lower limit of 5 3 10 213 cm 2 s 21 for the charge transport diffusion coefficient (D ct) in films with five layers of Co IIPcTS 42. Actual D-values of 10 215 cm 2 s 21 were found for watersoluble rhodamine and 10 213 cm 2 s 21 for the smaller tetramethyl-4-piperedinol through layered polyion films (29). These values are slightly smaller than the lower limit D ct in our films. D-values in the polyion films are significantly smaller than those of films of Co IIPcTS 42 and the cationic surfactant didodecyldimethylammonium bromide (DDAB). D ct values were 3 3 10 28 cm 2 s 21 for a liquid crystal DDAB film into which CuPcTS 42 had been ion exchanged and 5 3 10 29 cm 2 s 21 for a solid film cast from a CuPcTS 42/DDA salt in pH 12 phosphate buffer (30). Spectroscopic data for Co IIPcTS 42/polycation films suggest that face-to-face dimers are present in the films. QCM mass measurements are consistent with a face-to-face, tilted orientation in the layer. However, neutron reflectivity of polyion films made by the layer-by-layer method showed considerable mixing between adjacent oppositely charge layers, but no mixing between layers separated by two intervening layers (31). It is possible that partial mixing of adjacent layers also occurs in the Co IIPcTS 42/polycation films, and this might indeed facilitate charge transport by bringing the redox centers in adjacent bilayers closer together. In summary, electroactive films of Co IIPcTS 42 and polycations were prepared by a layer-by-layer growth method. Reversible steady state cyclic voltammograms of the Co(II)/Co(I) redox couple were obtained and were controlled by charge transport diffusion at 0.1–1 V s 21 even for films containing only a monolayer of electroactive dye. While charge transport diffusion coefficients were smaller than for films of metal phthalocyanine tetrasulfonates and cationic surfactants, the layer-by-layer method provides advantages of precise control of film thickness and the possibility of higher concentrations of electroactive species in the films.
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ACKNOWLEDGMENT This work was supported by Grant CTS-9632391 from the National Science Foundation. G.N.K. thanks UNESCO Paris/Nairobi for a grant to travel to the University of Connecticut.
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