Electrochemical Behavior of LB Films Containing a Mixture of

J. Phys. Chem. B 1998, 102, 6799-6803

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Electrochemical Behavior of LB Films Containing a Mixture of Viologen and a Phospholipid A. J. Ferna´ ndez, M. T. Martı´n, J. J. Ruiz, E. Mun˜ oz,* and L. Camacho The Department of Physical Chemistry and Applied Thermodynamics, Faculty of Sciences, UniVersity of Co´ rdoba, AVd. San Alberto Magno s/n, E-14004 Co´ rdoba, Spain ReceiVed: February 24, 1998; In Final Form: April 28, 1998

The electrochemical behavior of mixed monolayers of N-tetradecyl-N′-methyl viologen (TMV) and L-Rdimyristoylphosphatidic acid (DMPA) has been studied. Mixed monolayers of TMV/DMPA ) 1:2 have been obtained at the air-water interface and transferred by the Langmuir-Blodgett (LB) method at π ) 40 mN m-1 onto In and Sn oxide (ITO) electrodes. These LB films were used as the working electrode in a conventional electrochemical cell and immersed in aqueous solutions with different electrolytes. The influence of the type and the electrolyte concentration on the voltammetric peaks related to the two one-electron processes of TMV has been studied. The formation of an insoluble salt of viologen perchlorate on the surface of the electrode is observed when concentrations of perchlorate higher than 0.3 M are used. Due to this, a reorientation of the polar group of the viologen in the transferred mixed monolayer from a flat to a perpendicular position is postulated.

Introduction The modified electrodes by the immobilized molecular monolayers can be achieved by both the self-assembly or Langmuir-Blodgett (LB) techniques, by which it is possible to immobilize a wide range of compounds.1,2 The electrochemical properties of the LB films have been published elsewhere.3 The bis-alkylated bipyridinium salts, so-called viologens, are a well-known type of electron acceptors.4 These compounds have three states of oxidation and show a highly reversible electrochemical behavior. Viologen derivatives with alkyl substituents of long chains are used as models to modify electrodes due to their easy detection by conventional techniques such as cyclic voltammetry.5 Several groups of researchers have chosen this type of compound to prepare highly organized monolayers on electrode surfaces;5-16 likewise, the electrochemical behavior of viologen LB films has been related to those formed by adsorption from a solution of the viologen amphiphile.5,12-16 In the present work, the stable monolayers at the air-water interface were formed by using the cospreading technique of N-tetradecyl-N′-methyl viologen (TMV) with an amphiphilic anchor, dimyristoylphosphatidic acid (DMPA), negatively charged at neutral pH. The chemical structure and transversal section of both compounds are shown in Scheme 1. The optimum molar ratio for the formation of stable TMV/ DMPA monolayers was found to be 1:2 obtaining an electrically neutral monolayer; that is, the two positive charges of a molecule of TMV are neutralized by the negative charges of two molecules of DMPA. Thus, TMV, despite its aqueous soluble character, is strongly retained at the air-water interface via charge interactions and also hydrophobic interactions between the alkyl chains of both compounds. The DMPA phospholipid has been previously used to prepare well-defined mixed monolayers with water-soluble positively charged molecules, * To whom correspondence should be addressed.

SCHEME 1: Molecular Structure of N-Tetradecyl-N′Methyl Viologen (TMV) and L-r-Dimyristoylphosphatidic Acid (DMPA)

creating Langmuir-Blodgett films.17-20 Our results suggest the head polar groups of TMV in the mixed monolayer, transferred on ITO, occur in a parallel configuration with respect to the electrode surface located between this surface and the DMPA head. The formation of an insoluble salt of viologen at high electrolyte concentrations was observed when the LB film on ITO is immersed in the perchlorate solution. Such an insoluble salt is related to a reorientation of the headgroup of TMV from a flat to perpendicular orientation. The preparation of electrodes modifed with TMV/DMPA films has the purpose of showing new methods of immobilization redox centers on electrodes in a relatively easy manner. The transferred films show a high stability and reversibility.

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6800 J. Phys. Chem. B, Vol. 102, No. 35, 1998

Ferna´ndez et al. SCHEME 2: Organization of the Mixed TMV/DMPA Monolayer at a Molar Ratio 1:2 at the Air-Water Interface (A) and Transferred onto the Surface ITO Electrode (B)

Figure 1. Surface pressure-area (π-A) isotherms of a monolayer of DMPA (curve a) and the cospread TMV/DMPA monolayer at a molar ratio 1:2 (curve b). Both curves are plotted vs area per DMPA molecule; Subphase water, 21 °C.

Experimental Section N-Tetradecyl-N′-methyl viologen (TMV) and L-R-dimyristoylphosphatidic acid (DMPA) were supplied by Fluka and Sigma, respectively. Perchlorate salts of potassium and lithium were obtained from Fluka. Sodium perchlorate, sodium chloride, and sodium sulfate were obtained from Merck. A mixture of trichloromethane and methanol, ratio 3:1 (v/v), was used as a spreading solvent. The pure solvents were obtained without further purification from Panreac. Ultrapure water from a Millipore Milli-Q system was used throughout. All reagents were used as received and without further purification. The ITO-coated glass substrate was purchased from Balzers Ltd. and used with a previous cleaning treatment, as described previously.19 The monolayers of DMPA and TMV/DMPA ) 1:2 were prepared by spreading on a Lauda Filmwaage FW2, and the temperature was kept constant at 21 °C. Details on the method used for pressure (π)-area (A) measurement and transfer process were described elsewhere.18 In all cases, the ratio transfer was close to 1. Voltammetric recordings were obtained by using Quiceltron electrochemical multifunctional equipment. A wire of Pt and a saturated calomel electrode (SCE) were used as the auxiliary and reference electrodes, respectively. ITO supports, coated with a transferred LB monolayer, were used as the working electrode. The working surface area equal to 0.48 ( 0.03 cm2 was calculated by comparison with the cyclic voltammograms of 5 × 10-5 M [Fe(CN)6]3- in aqueous 0.1 M KCl of the known diffusion coefficient D ) 7.6 × 10-6 cm2 s-1.21 In most electrochemical experiments, an initial potential of -0.25 V is applied for 3 s, and subsequently several cyclic scans to a final potential of -1.15 V (different values will be indicated) were obtained. The IR ohmic drop was uncompensated. All voltammetric measurements were made under nitrogen atmosphere at 21 °C, and all potentials are given with respect to SCE. Results and Discussion Behavior of the Mixed TMV/DMPA Monolayer in a Molar Ratio 1:2 at the Air-Water Interface. The surface pressure versus area (π-A) isotherms of Langmuir monolayers of DMPA (curve a) and TMV/DMPA ) 1:2 (curve b) at the air-water interface are recorded in Figure 1. The π-A isotherm

of the mixed monolayer shows an expansion with respect to the reference DMPA isotherm. This is clear evidence for the presence of TMV at the interface. The stability of the TMV/DMPA monolayer at the air-water interface was observed following the relaxation of the film compressed at 40 mN m-1 and kept constant by a feedback system for 1 h at least. No change in the value area per molecule after that time was noted. The area per molecule of DMPA at π ) 40 mN m-1 was 0.40 and 0.51 nm2 for curve a and b (Figure 1), respectively. The area per pure DMPA is 0.40 nm2,20 and considering the molar ratio 1:2 for the TMV/DMPA monolayer, the area per viologen molecule in the mixed monolayer is equal to 0.22 nm2. This value is smaller than those found from the isotherms for pure monolayers of alkylmethyl viologens (0.45 and 0.50 nm2 for octadecylmethyl viologen12 and hexadecylmethyl viologen,14 respectively) with a perpendicular orientation for the viologen derivative. The area per molecule equal to 0.22 nm2 agrees with the cross section of a hydrocarbon chain.22 Thus, we can suggest that the expansion observed in the isotherm for the mixed monolayer (cuve b in Figure 1) at high surface pressures is only due to the alkyl chain from the TMV molecule, which is retained between the DMPA molecules without the contribution of the bipyridyl group. Therefore, the headgroup may be located underneath the headgroups of the phospholipid (see Scheme 2A). This type of structure has been proposed elsewhere for TMV binding in vesicles of dihexadecyl phosphate.23 According to Scheme 2A, the mixed monolayer has the positive charges completely neutralized with the negative density from DMPA. Then, assuming no loss of TMV to the aqueous subphase and taking the data from the isotherms, the area per TMV molecule at the air-water interface under 40 mN/m is 2 × 0.51 nm2 ) 1.02 nm2, which corresponds to 1.63 × 10-10 mol cm-2 in terms of viologen surface coverage, Γ. Electrochemical Behavior of Transferred TMV/DMPA ) 1:2 Films on ITO. An organized layer was transferred from the air-water interface to a hydrophilic ITO electrode keeping the surface pressure constant at 40 mN m-1. If the molecular organization suggested in Scheme 2A at the air-water interface is preserved, the headgroups of TMV in the transferred layer will be intercalated between those of the DMPA molecules and the surface of the ITO electrode (see Scheme 2B) with a flat orientation with respect to the surface of the electrode and an area per molecule equal to 1.02 nm2 (determined from curve b, Figure 1). The published values of the area per viologen

LB Films Containing Viologen and a Phospholipid

Figure 2. Consecutive voltammetric cycles (1st, 2nd, and 10th cycles) of a monolayer of TMV/DMPA ) 1:2 transferred at 40 mN m-1 from the water-air interface onto ITO. Scan rate V ) 0.1 V/s in 0.1 M KClO4. T ) 21 °C.

headgroup range between 0.905 and 1.20 nm2.16 The value measured is a median value between those mentioned above and agrees with that obtained for a rectangular area of the molecule with a flat orientation using the dimensions given elsewhere,5 i.e., 0.63 nm × 1.63 nm ) 1.03 nm2. This fact indicates that the electrode is totally coated by a dense monolayer of bipyridine groups of TMV (see Scheme 2B). The ITO electrode, coated with a TMV/DMPA ) 1:2 monolayer, was immersed in an aqueous solution with different electrolytes and used as the working electrode in a conventional electrochemical cell. In Figure 2, several voltammetric cycles for this system obtained with 0.1 M KClO4 and at a scan rate V ) 0.1 V/s are shown. The cyclic voltammogram shows two well-defined as well as reversible peaks. These correspond to the two consecutive monoelectronic processes (peaks A/A′ and B/B′) by the dication viologen.4 Likewise a difference equal to 20-30 mV between peak potentials of A and A′ and B and B′ has been found for both processes, and a peak half-width higher than 90 mV detects the presence of repulsive interactions between the electroactive species.24,25 Furthermore, the peak currents for A and A′ are linearly dependent on V in the case of the first voltammetry cycle. The electrochemical behavior observed for the mixed TMV/ DMPA ) 1:2 films in different perchlorate media of Li, Na, or K is identical. Once the background current has been subtracted, the integration of the voltammetric peaks was performed obtaining the average value of the exchange charge, Q. For those salt media during the first voltammetric scan, the values of Q were 17.7 ( 1.3 and 15.4 ( 0.4 µC cm-2 for A and A′, respectively, and this involves their corresponding average Γ values of (1.83 ( 0.13) × 10-10 and (1.60 ( 0.04) × 10-10 mol cm-2. These values stay constant independently of the concentration of perchlorate solutions. The surface concentration obtained by the integration of peak A′ agrees with that determined from the isotherm during the transfer process (1.63 × 10-10 mol cm-2). This is clear evidence that all the molecules of TMV at the air-water interface were transferred onto the ITO substrate. However, the value of Γ obtained from the integration of peak A is always higher, ∼10%, than those determined from the isotherm. Bard16 relates this phenomenon to the catalytic action of the viologen on oxygen dissolved in the electrolyte medium. Although the voltammograms were

J. Phys. Chem. B, Vol. 102, No. 35, 1998 6801

Figure 3. Cyclic voltammograms for TMV/DMPA ) 1:2 monolayers at V ) 0.1 V/s obtained in different concentrations of NaClO4: 0.5 M (solid line), 0.35 M (dotted line), 0.1 M (dashed line).

recorded in well-degassed solutions, a possible explanation of this phenomenon could be the presence of small amounts of oxygen strongly associated to the LB film. Figure 2 shows the electrochemical signal decreasing with consecutive voltammetric cycles. However, this phenomenon strongly depends on the perchlorate concentration; for example, after 10 consecutive voltammetric cycles in 0.5 M perchlorate and for V ) 0.1 V/s, Q is reduced to less than 1% for peak A′. By using different anions (SO42- or Cl-), the electrochemical signal decreases more strongly than that obtained in perchlorate medium. This phenomenon has to be related to an increase of the viologen solubility in SO42- and Cl- solutions.14 The cyclic voltammograms for several TMV/DMPA ) 1:2 monolayers using different concentrations of sodium perchlorate as the supporting electrolyte are shown in Figure 3. The voltammogram in 0.5 M NaClO4 (solid line in Figure 3) shows peaks A and A′ higher and sharper than those at a low concentration, shifting the peak potentials to positive values as well. On the other hand, peaks B and B′ clearly appear split into three peaks. The central peak (B) appears at potentials close to those observed for the same peak in 0.1 M NaClO4 (dashed line in Figure 3). With respect to the others, the peak that appears at more positive potentials (peak C) has a sharp, narrow shape, while the one at more negative potentials (peak D) is broader and slightly overlaps the central peak. The explanation of these phenomena has required additional experiments as described below. First, a cyclic voltammogram was recorded for a TMV/ DMPA ) 1:2 monolayer in 0.5 M perchlorate as the supporting electrolyte (see Figure 3, solid line). After the recording, the solution was diluted to 0.1 M in ClO4- with pure water without taking out the electrode, and a new cyclic voltammogram was recorded. This recording had the same morphology as that obtained by directly using 0.1 M aqueous solutions (see Figure 3, dashed line). A second experiment has been performed adding TMV to the aqueous media. Figure 4 (solid line) shows the voltammogram corresponding to a mixed TMV/DMPA ) 1:2 monolayer in 0.1 M NaClO4. A voltammogram (dotted line) is also recorded on the same monolayer under the same experimental conditions as before and adding 3 × 10-6 M TMV to the solution. The morphology of peaks in the TMV solution is identical to that obtained when the concentration of perchlorate without

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Ferna´ndez et al. SCHEME 3 Hypothetical Reorientation of the Viologen Head Groups in the Mixed TMV/DMPA Monolayer, in a Molar Ratio 1:2, at the Solution-ITO Electrode Interface When the High Concentration of the Supporting Electrolyte Is Used. Transversal Section and Top View for the LB Film-Electrode Are Drawn. The Top Performance Does Not Show the Na+ Ions Because They Are underneath the Head Group of DMPA

Figure 4. Cyclic voltammograms of a monolayer of TMV/DMPA ) 1:2 at V ) 0.1 V/s obtained in 0.1 M NaClO4 (solid line) and after adding a concentration of 3 × 10-6 M TMV (dotted line).

TMV is increased (see Figure 3). The charge values for peaks A and A′ in the presence of 3 × 10-6 M TMV in solution were equal to 21.2 and 20.9 µC cm-2, respectively. No morphological changes of the peaks and the charge values mentioned previously were observed after subsequent additions of TMV in the solution to reach 10-5 M. It is necessary to note that if the ITO electrode coated with the mixed monolayer is taken from the solution with 3 × 10-6 M TMV and then put into the solution of pure electrolyte 0.1 M NaClO4, the voltammogram recorded recovers the characteristic morphology in this medium. Finally, it has been proven that by using 1 M NaCl as the supporting electrolyte, only a slight shift of peaks to more positive potentials is observed, in contrast to the morphology changes shown previously in the presence of perchlorate. All these results suggest that adding TMV to the solution with a constant perchlorate concentration or increasing the perchlorate concentration in the absence of TMV in solution has the same effect on the shape of the voltammograms. The fact that the presence of Cl- instead of perchlorate ions produces no change in the morphology of the voltammograms suggests the formation of an insoluble salt of viologen perchlorate ((ClO4)2TMV). The formation of this insoluble salt involves a massive penetration of ions in the monolayer at high concentrations of perchlorate before the reduction process occurs. Under these conditions, the charge complex formed between the TMV2+ molecules and DMPA- is broken, and the subsequent formation both of new ionic pars, DMPA- and Na+, and of the (ClO4)2TMV is caused. The presence of TMV2+ in solution produces a value of charge equal to 20.9 µC/cm2 for peak A′, i.e., approximately 35% higher than that obtained in the absence of TMV in solution (15.4 µC/cm2), as commented above. This fact is indicative of the flexibility of the film organized in a denser structure incorporating an excess of TMV2+ molecules. However, this is only possible if the TMV2+ molecules penetrate with their counterion keeping the film electrically neutral. Thus, the TMV molecules incorporated to the monolayer will be as perchlorateTMV2+. On the other hand, the viologen dication from the solution has to penetrate with an orientation with respect to the electrode very different from those in the initial mixed monolayer, because the surface of the electrode is totally coated with the bipyridine groups for the TMV/DMPA ) 1:2 monolayer in flat orientation, as commented previously (see Scheme 2B). We propose that the TMV molecules penetrate from the solution to the surface electrode with the head polar group in perpendicular

orientation with respect to the surface of the electrode with the aim of reducing the surface area. At high concentrations of perchlorate and in the absence of TMV in solution, there are some changes of the morphology for the voltammetric peaks identical to those observed in the presence of TMV in solution (see Figures 3 and 4). This phenomenon suggests that the TMV molecules in the mixed monolayer take the same configuration as those molecules from the solution. This involves a reorientation from a flat to perpendicular configuration for the molecules of TMV in the mixed monolayer, which is produced by the breaking of the charge complex between DMPA- and TMV2+ and the subsequent formation of the insoluble salt. In Scheme 3, both a transversal section and top view for that perpendicular configuration are drawn. Finally, at high concentrations of perchlorate or in the presence of TMV in solution, the splitting of peak B is clearly observed. A similar splitting has been observed elsewhere26 for the second one-electron transfer of 4,4′-bipyridine in an acid medium and related to the dimer formation of radical cation. Thus, after the formation of radical cation the bipyridine groups located perpendicular to the electrode surface and between the chains of DMPA (see Scheme 3) could form a dimer structure, which is responsible for peaks C and D.26 The formation of the dimer is possible due to the coplanar position of the bipyridine groups. However, at low concentrations of the supporting electrolyte the dimerization is impossible since the groups are lying flat with respect to the electrode and under the head polar groups of DMPA so it is not possible that a coplanar distribution forms the dimer. Acknowledgment. The authors express their gratitude to the Spanish DGICyT (Project PB94-0446 and PB94-0448) for financial support awarded for the realization of this work. References and Notes (1) Murray, R. W. Molecular Design of Electrode Surfaces. In Techniques of Chemistry; Murray, R. W., Ed.; Jonh Wiley & Sons: New York, 1992; Vol. 22, Chapter 1. (2) Facci, J. S. Ibid., Chapter 3. (3) Goldenberg, L. M. J. Electroanal. Chem. 1994, 379, 3. (4) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49. (5) Cotton, T. M.; Kim, J. H.; Uphaus, R. A. Microchem. J. 1990, 42, 44.

LB Films Containing Viologen and a Phospholipid (6) Kim, J. H.; Bunding Lee, K. A.; Uphaus, R. A.; Cotton, T. M. Thin Solid Films 1992, 210/211, 825. (7) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (8) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392. (9) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (10) Diaz, A.; Kaifer, A. E. J. Electroanal. Chem. 1988, 249, 333. (11) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (12) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689. (13) Ye, S.; Kim, J. H.; Uphaus, R. A.; Cotton, T. M.; Lu, T.; Dong, S. Thin Solid Films 1992, 210/211, 822. (14) Lee, C. W.; Bard, A. J. J. Electroanal. Chem. 1988, 239, 441. (15) Lee, C. W.; Bard, A. J. Chem. Phys. Lett. 1990, 170, 57. (16) Obeng, Y. S.; Founta, A.; Bard, A. J. New J. Chem. 1992, 16, 121. (17) Ahuja, R. C.; Caruso, P. L.; Mo¨bius, D.; Wildburg, G.; Ringsdorf, H.; Philp, D.; Preece, J. A.; Stoddart, J. F. Langmuir 1993, 9, 1534. (18) Martı´n, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554.

J. Phys. Chem. B, Vol. 102, No. 35, 1998 6803 (19) Prieto, I.; Ferna´ndez, A. J.; Mun˜oz, E.; Martı´n, M. T.; Camacho, L. Thin Solid Films 1996, 284/285, 162. (20) Prieto, I.; Martı´n, M. T.; Mo¨bius, D.; Camacho, L. J. Phys. Chem. B 1998, 102, 2523. (21) Sawyers, D. T.; Roberts, J. L., Jr. In Experimental Electrochemistry for Chemists; Wiley: New York, 1977; p 77. (22) Gaines, G. L., Jr. In Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (23) Thomson, D. H. P.; Barrette, W. C., Jr.; Hurst, J. K. J. Am. Chem. Soc. 1987, 109, 2003. (24) Bard, A. J.; Flaulkner, L. R. In Electrochemical Methods: Fundamentals and Applications; Jonh Wiley & Sons: New York, 1980. (25) Murray, R. W. In Electroanalytical Chemistry: A Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1983. (26) Sa´nchez Maestre, M.; Rodrı´guez Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. Electrochim. Acta 1996, 41, 819.