2794
Langmuir 1991, 7, 2794-2802
Monomolecular Langmuir-Blodgett Films at Electrodes. Formation of Passivating Monolayers and Incorporation of Electroactive Reagents Renata Bilewiczt and Marcin Majda' Department of Chemistry, University of California a t Berkeley, Berkeley, California 94720 Received March 25,1991. I n Final Form: June 14, 1991 Langmuir-Blodgett (L-B) transfer of mixed octadecanethiol (C1BSH)-octadecanol(ClBOH) monlayers onto vapor-deposited gold films on glass slides leads to the formation of stable, densely packed monomolecular films when the composition of the mixed monolayers involves 60-80 mol % octadecanethiol. This composition of the mixed L-B films is a result of a trade-off between the mole fraction of C180H, whose increasing magnitude allows one to increase surface pressure of a mixed monolayer during an L-B transfer, and the mole fraction of CleSH, whose increasing magnitude enhances stability of the mixed monolayer on the gold surface following the L-B transfer. The passivating properties of ClsSH/ClsOH L-B monolayers were investigated electrochemically. Determination of interfacial capacitance and measurements of the extent of permeabilityof the surface monolayers by cyclic voltammetry demonstrated that the passivating characteristics of the Cl&H/Cl&H L-B monlayers are essentially identical to the best self-assembled octadecanethiolmonolayers described in the literature. L-B transfer of monolayers containing small quantities of an electroactive reagent can be used as a general technique of reagent immobilizationat the electrode surface. Unlike self-assembly methods,reagent incorporation in Langmuir Blodgett monolayers offers precise control of their composition in a broad range of concentrations. Comparison of the surface concentrationsof ubiquinone incorporated in C&H/C~BOHmonolayers at the air/water interface before L-B transfer and at the electrode surface following L-B transfer gave 1:l correlations in the concentration range 1.0 X 10-12-1.0 X 10-10 mol/cm2.
Introduction It is now well established that self-assembly of n-alkanethiolsl and n-alkylsilanes2 (with 10 or more carbon atoms in a chain) and their derivatives, such as w-hydroxyalkanethiol monolayers,3can lead to the formation of wellordered structures on gold surfaces. A substantial volume of recently published reports concerned with self-assembly processes and with the structural properties of selfassembled monolayers has been motivated by a general goal of controlling physical and chemical properties of + Permanent address: Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland. (1) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559. (b) Finklea, H.0.;Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987,3,409. (c)Troughton, E. B.; Bain, C. D.; Whiteaides, G.M.; Nuzzo, R. G.;Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (d) Strong, L.; Whitesides, G. M. Langmuir 1988,4,546. (e) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989, 111, 321. (0 Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989,111,7155. (g) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111, 7164. (h) Bain, C. D.; Biebuyck, H.A,; Whitesides, G . M. Langmuir 1989,5,723. (i) Bain, C. D.; Whitesides, G.M. Langmuir 1989,5,1370. (j) Bain, C. D.; Whitesides, G.M. Angew. Chem., Int. Ed. Engl. 1989,28,506. (k) Chidsey, C. E. D.; Lin, G.-Y.; Rountree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421. (1) Nuzzo, R. G.;Dubois, L. H.;Allara, D. L. J.Am. Chem. SOC.1990,112, 558. (m) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. J. J. Am. Chem. Soe. 1990,112, 4301. (n) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6,87. ( 0 ) Chidsey, C. E. D.; Loiacono,D. N . Langmuir 1990,6,682. (p) Finklea, H.0.; Snider, D. A.; Fedyk, J. Langmuir 1990,6,371. (9)Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem., in press. (r) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. SOC.1991,113,2370. ( 8 ) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC.1991, 113, 2805. (2) (a) Sagiv, J. J. Am. Chem. SOC.1980,102,92. (b) Sagiv, J. Isr. J. Chem. 1979, 18, 346. (c) Netzer, L.; Sagiv, J. J. Am. Chem. SOC.1983, 105,647. (d) Netzer, L.; Iscovici, R.; Sagiv,J. Thin Solid Films 1983,99, 235; 1989,100,67. (e) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100,465. (0 Gun, J.; Iscovici, R.; Sagiv, J. J. ColloidInterface Sei. 1984, 101, 201. (g) Maoz, R.; Sagiv, J. Thin Solid Films 1986, 132, 135. (h) Pomerantz, M.; Segmuller, A,; Netzer, L.; Sagiv, J. Thin Solid Films 1986,132, 153. (i) Gun, J.; Sagiv, J. J. Collid Interface Sci. 1986, 112, 457. (j) Cohen, S. R.; Naaman,R.;Sagiv, J. J.Phys. Chem. 1986,90,3054. (3) (a) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991,95, 877. (b) Miller, C.; Gratzel, M. J. Phys. Chem., in press.
interfaces, with long-term applications in such diverse areas as catalysis,corrosion,friction/lubrication, adhesion, and biomimetic research.lj1" In electrochemistry, formation of self-assemblingmonomolecular films on electrodes has been used in the measurements of the rate constants of rapid heterogeneous electron transfe# and in the studies of long-range electrontransfer Within this group of applications, Sabatani and Rubinstein used a mixed monolayer of octadecyltrichlorosilane and octadecanethiol molecules as a barrier restricting access of redox species in solution to the electrode surface.4b Chidsey demonstrated a selfassembly scheme in which ferrocene centers were spaced away from the electrode surface by a fixed distance corresponding to the thickness of the hydrocarbon layer of the self-assembled mole~ules.~This system served to investigate long-range electron-transfer kinetics of the bound ferrocene centers. Most recently, studies of the distance dependence of the electron-transfer kinetics involving blocking monolayer assemblies of w-hydroxyalkanethiols of variable thickness were reported by Miller et al.3 Treating molecular self-assembly as a method of immobilization of specific reagents at the electrode surface considerably broadens the range of electrochemical applications of self-assembly techniques.6Jm Most notable (4) (a) Sabatini, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J.Electroanal. Chem. 1987, 219, 365. (b) Sabatini, E.; Rubinstein, I. J. Phys. Chem.
1987,91,6663. (5) Chidsey, C. E. D. Science 1991,251,919. (6) (a) Facci, J. 5.Langmuir 1987,3,525. (b) Diu, A.; Kaifer, A. E. J. Electroanul. Chem. 1988,249,333. (c) Lee, H.;Kepley, J. K.; How, H. G.;Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2697. (d) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sa&, J. Nature 1988, 332,426. (e) Donohue, J. J.; Buttry, D. A. Langmuir 1989,5,671. (0Van Galen, D. A.; Majda, M. A d . Chem. 1988,60,1549. (g) Widrig, C. A.; Majda, M. Langmuir 1989,5, 689. (h) Bunding-Lee, K. A. Langmuir 1990,6,709. (i) DeLong, H.C.; Buttry, D. A. Langmuir 1990,6,1319. 6)Creager, S. E.; Collard, D. M.; Fox, M. A. Langmuir 1990,6,1617. (k)
Kunitake, M.; Akiyoshi, K.; Kawatana, K.; Nakashima, N.; Manabe, 0. J. Electroanal. Chem. 1990,292, 277.
0743-7463/91/2407-2794$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No.11, 1991 2795
Monomolecular Langmuir-Blodgett Films at Electrodes are attempts to induce selective and/or catalytic response of the electrode monolayer assembly. Here, a general strategy calls for immobilization of molecular active sites chosen to induce catalytic or selective response in an impermeable, passivating monolayer of long-chain molecules, so that access to the electrode surface is allowed only at the active sites. One example illustrating this strategy was described by Rubinstein and co-workers,who relied on co-self-assemblyof octadecanethioland 2,2'-thiobis(ethy1acetoacetate) to form a mixed monolayer on gold electrodes. The selective binding of copper ions by the acetoacetate sites in the presence of ferric ions led to a selective response toward copper ions.Bd A similar selfassembly approach was used recently by Kunitake and co-workers to immobilize a quinone derivative in a monolayer of octadecanethiol, in order to carry out electrocatalytic oxidation of NADH.6k The main limitation of the self-assembly techniques used in preparation of such twocomponent monolayer assemblies stems from the fact that choice of molecular active sites is limited to compounds capable of binding to the electrode surface, such as thiols. Also, self-assembly techniques do not allow adequate control of the concentration of the active sites in a surface assembly. An alternative technique, Langmuir-Blodgett (L-B) transfer, is free of both of these disadvantages.' Examples of electrode modification with L-B films have been described in the literature.8 However, the structural stability of monomolecular L-B films, which is required to maintain their passivating character, leaves much to be desired. For example, despite the fact that octadecyl alcohol and many other long-chain surfactants can be compressed at the air/water interface to form well-ordered Langmuir monolayers, their transfer onto hydrophilic solid supports such as gold surfaces and immersion in an aqueous electrolyte leads inevitably to the destruction of the monolayer and to complete loss of its expected impermeability. It is clear that chemical bonding of octadecanethiol to gold surfaces in self-assembly is a key element responsible for the stability and the passivating character of the selfassembled alkanethiol monolayers. Unfortunately, literature reports indicate that octadecanethiol monolayers cannot be compressed on the air/water interface and collapse beyond ca. 15 mN/m.g Most recently, Itaya and co-workers showed that compression of octadecanethiol on a CdClz subphase resulted in the formation of what appears to be a bilayer on the water surface. A limiting area per molecule of 9 A2 was reported under these conditions.% An L-B transfer of octadecanethiol at accessible pressures for this system (ca. 12 mN/m) produces surface monolayers which are not sufficiently compact to form passivating films on gold surfaces (see Results and Discussion). Thus, to our best knowledge, Langmuir-Blodgett techniques have never been used (7) Roberta, G.,Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (8) (a) Fromherz, P.; Arden, W. J. Am. Chem. Soe. 1980,102,6211. (b) Daifuku, H.; Aoki, K.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. 1986,183,1. (c) Daifuku, H.; Ywhimura, I.; Hirata, I.; Aoki, K.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. 1986, 199, 47. (d) Facci, J. S.; Falcigno, P. A.; Gold, J. M. Langmuir 1986, 2, 732. (e) Fujihira, M.; Poosittisak, S. J.Electroanal. Chem. 1986,199,481. (f) Lee, C. W.; Bard, A. J. J.Electroanal. Chem. 1988,239,441. (9) Zhang, X.;Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (h) Ueyama, S.; Isoda, S.; Maeda, M. J. Electroanal. Chem. 1989, 264, 149. (i) Nagase, S.; Kataoka, M.; Naganawa, R.; Komatau, R.; Odashima, K.; Umezawa, Y. Anal. Chem. 1990, 62, 1252. (9) (a)Sobotka, H.; Rosenberg, S. InMonomolecuZarLayers; Sobotka, H., Ed.; American Association for the Advancement of Science: Washington DC, 1954;p 175. (b) Livingstone, H. K.; Swingley,C. S.J. Colloid Interface Sci. 1972, 38,643. (c) Itaya, A.; Van der Fluweraer, M.; De Schryver, F. C. Langmuir 1989,5, 1123.
successfully to produce passivating monomolecular films on electrode surfaces. The goal of the research described below was to improve the stability of monolayer L-B films on electrodes and to explore the usefulness of L-B transfer as a general technique of reagent immobilization on electrode surfaces. We present experimental conditions under which stable passivating L-B monolayers can be formed on gold electrodes. Their passivating properties are characterized by inhibition of the gold electrooxidation and Ru(NH3)e3+ reduction processes, and also by a decrease of the electrode interfacial capacitance. In view of these measurements, the passivating characteristicof our L-B films is essentially the same as that reported in the literature for the wellordered self-assembled alkanethiol monolayers. We also show that other molecules, not necessarily amphiphilic, can be incorporated in Langmuir monolayers a t the air/ water interface and then quantitatively transferred to the electrode surface using L-B techniques.
Experimental Section Materials. l-Octadecanethiol(Aldrich, 98 % ) and 1-octadecan01 (Aldrich, 99%) were recrystallized from 100% ethanol. N-Butylferrocene (Strem Chemicals), vitamin Kl (Sigma),and hexaa"ineruthenium(II1) chloride (Aldrich)were used without further purification. Os(II)-tris(2,2'-bipyridine chloride)and Os(II)-tris(4,7-diphenyl-l,l0-phenanthroliie) perchlorate were synthesized according to the literature procedures.10 Chloroform (Fisher,ACS certified, spectranalyzed)was used as the spreading solvent. Reagent grade 70% perchloric acid (Aldrich) and potassium chlmide (Fisher,ACS certified) were used to prepare the supportingelectrolyte solutionswithout further purifications. House distilled water was passed through a four-cartridge Barnstead Nanopure 11 purification train consisting of Macropure pretreatment (OrganicsFree), two ion exchangers, and a 0.2-pm hollow-fiber final filter for removing particles. Water resistivity was in the range 17.9-18.3 MQ cm. Langmuir-Blodgett Monolayer Techniques. The monolayer spreading solutions were prepared daily by dissolving appropriate amounts of compounds in CHCL Aliquots of a spreadingsolutionswere delivered to the water surface in several locations with a Hamilton 50-pL gas-tight syringe. The purity of the subphase and chloroform was checked by sweeping the barrier and measuring the surface pressure. If any pressure rise was detected, the water surface was aspirated following its compression to a small area using disposable pipets. Subeequently, the barrier was moved back and swept across the water surface again to repeat the entire cycle. Pure solventa gave typically no detectable rise in the surface pressure. The initial trough surfacearea was 375cm2. Langmuirfilmswere compressed at a rate of 10 A* molecule-1min-*. Langmuir-Blodgett transfer of compressed monolayers onto gold-coated substrates (electrodes) was done by mechanicallylifting a substratethrough the monolayer at the air/water interface at a rate of 10 mm/min. The hydmphilicsubstrates were immersed into a subphasebefore monolayer spreading. After L-B transfer, the coated electrode substrates were left in the nitrogen atmospherefor severalminutea beforethey were transferred into an electrochemicalcell or stored in a sealed box for later measurements. Instrumentation. Surface pressure vs area per molecule isothermswere recorded using a standard Langmuirtrough (KSV 2200 System) equipped with a Wilhelmy plate type microbalance. The instrument and barrier motion were externally controlled by a PC AT clone computer and KSV Dynamic film control system software Version 2.0. The Langmuir trough and all the accessories except external electronics and computer control units were completelyenclosed in a Plexiglassbox,which was continously purged with filtered (0.01-pm Matheson membrane gas filter) nitrogen. During most operations, relative (10) (a) Palmer, R. A.; Piper, T. 5.Inorg. Chem. 1966, 6, 864. (b) Creutz, C.; Chou, M.; Netze1,T. L.; Okumura, M.; Sutin, M. J. Am. Chem. SOC.1980,102, 1309.
Bilewicz and Majda
2796 Langmuir, Vol. 7, No. 11, 2991
25
r--
60
40
d
20
0
10
0 20
30
A, A2/molecule Figure 1. P A isotherm of Cl&H on a pure aqueous subphase at 21 OC. Averagelimitingareaatu =Ois21.5f0.5~2/molecule.
humidity was maintained below 50%. Routine cleaning procedures are described elsewhere." Electrochemical experiments were carried out in a threeelectrode cell with a saturated calomel electrode (SCE) or a saturated mercury sulfate electrode as reference electrodes and a platinum counterelectrode. All electrode potentials were referenced with respect to SCE. Working I electrodes were fabricated by vapor deposition of ca. 70-100-nm-thick gold films (99.999% Lawrence Berkeley Laboratory) onto thoroughly precleaned glass slides as described previously.12 A 5-nm-thick underlayer of Cr, customarily used to improve gold adhesion to the substrate, was replaced by a thin film of an organosilane coupling agent, (3-mercaptopropyl)trichlorosilane(MPS),according to a procedure described elsewhere.12 The use of MPS instead of Cr to enhance gold adhesion also improved the passivatingproperties of the Langmuir-Blodgett monolayer films. Immediately before L-B transfer experiments, gold-coated substrates were subjected to plasma treatment for ca. 7 min in a Harrick Model PDC23G plasma cleaner (100 mTorr Ar plasma). This step significantly improves wettability of the gold substrates, which is a necessary characteristic in the L-B transfer step. The electrode surface area was 0.40 cm2. All electrochemical experiments were carried out with a BAS-100A electrochemical analyzer (Bioanalytical Systems Inc.). Uncompensated resistance between the working and reference electrodes was minimized electronically by a routing executed internally by BAS 100A. Advancing contact angles were measured with a Rame-Hart Model 100contact angle goniometer.
Results and Discussion Langmuir Experimentsat the Air/ Water Interface. Behavior of octadecanethiol (Cl8SH) at the air/water interface is illustrated in Figure 1. On a pure aqueous subphase, the C18SH monolayer becomes unstable above ca. 14 mN/m and collapsesabove 19mN/m. The limiting area per molecule obtained by extrapolation of the initial raising portion of the isotherm to ?r = 0 is 21.5 f 0.5 A2/ molecule, which is consistent with the cross-sectionalarea of a saturated hydrocarbon chain. The second pressure rise is reproducible and gives approximately 8 A2 as a limiting area, suggesting formation of a multilayer phase at the water surface, Octadecanol (ClsOH) forms very stable monolayers at the air/water interface, which can be compressed and which remain stable at surface pressures above 50 mN/m (lOCharych, D. H.; Landau, E. M.; Majda, M. J . Am, Chem. SOC.
1991, 113, 3340. (12)GOSS,C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991,63,85.
A, A2/molecule F i g u r e 2. PA isotherms of Cl&H/CleOH mixed monolayers on pure water at 21 "C. Mole fraction of C&H: (A) 1.0; (B) 0.72; (C) 0.67; (D) 0.59; (E) 0.0. (See also Table I.)
0 0
10
20
30
A, A2/molecule
Figure 3. P A isotherms of C&3H on 0.01 M NaOH subphase. Prior to compressions, the monolayers were incubated at 60 Az/ molecule for (A) 0 and (B) 10 min. (See also Table I.)
(see Figure 2E). Thus, the fact that the C&H monolayer cannot be compressed beyond ca. 20 mN/m on an aqueous subphase is due to the insufficient polarity of the thiol group (dipole moments of CH30H and CH3SH are 1.71 and 1.26 D, respectively). Indeed, the stability of the CISSH monolayer on 0.01 M NaOH subphases (pK, of CleSH can been estimated to be between 9 and 12)'qJ3 is substantially higher, allowing compression to above 35 mN/m as shown in Figure 3A. Under these conditions, however, chemical decomposition or oxidation of thiolate to disulfide has a destabilizing effect on the surface monolayer. We found, for example, that when a Cl&H monolayer following ita compression is held at constant surface area in the condensed-phase region of the isotherm, the surface pressure decreases rapidly to zero (a 40-50% decrease in ca. 10 min was typically observed) regardless of the subphase NaOH concentration in the range 0.0050.1 M. A chemical decomposition is also evident when one considersthe result in Figure 3B. Here a C&H monolayer was maintained initially at A = 0 (60 A2/molecule) for 10 min before the isotherm was recorded. The shift in the isotherm can be unambiguously related to a chemical
-
(13) Ionization Constants of Organic Compounds in Aqueous Solutions; IUPAC Chemical Data Series 23; Pergammon Press: New
York, 1983.
Langmuir, Vol. 7, No. 11, 1991 2797
Monomolecular Langmuir-Blodgett Films at Electrodes
reaction of the thiol moieties on the 0.01 M NaOH subphase, resulting in a drastic decrease of the amphiphilic character of the surfactant. Considering desirable surface stability and compressibility of Cl8OH monolayers and structural similarities of octadecanol and octadecanethiol, we investigated the surface properties of mixed C&H/ClsOH monolayers. The results of these Langmuir experiments are shown in Figure 2 for several mixtures of the two amphiphiles. It is apparent that increasing the &OH content in mixed monolayers results in an increase of their stability in the high-pressure region. The limiting area per molecule extrapolated from the region of high surface pressure is independent of the monolayer composition since the crosssectional areas of both componentsare similar. The values of the collapse pressure for several C18SH/C@H mixtures are listed in Table I. The proportionality of the collapse pressure to the mole fraction of &OH suggests ideal mixing of both monolayer components.14 Essentially identical changes in the T-A isotherms of C&H were also observed upon addition of octadecylamine (C18NHz). These results are shown in Figure 4 and Table I. Langmuir-Blodgett Transfer of Mixed Monolayers on Gold-CoatedSubstrates. L-B transfer of a mono-
layer from the air/water interface to a solid substrate requires a certain minimal stability of the monolayer on the water ~urface.~ Thus, in all cases, the optimal pressure for L-B transfer listed in Table I is somewhat lower than the collapse pressure for a given monolayer composition. The L-B transfer ratio (defined as Al/A,, where AI is the decrease in the area occupied by the monolayer on the water surface during a transfer, and A, is the surface area of the coated substrate) varied from 0.95 to 1.05 in all cases, except when the mole fraction of C&H in a monolayer was in excess of 0.85. In other terms, a successful L-B transfer required stability of a surface monolayer at a pressure higher than ca. 17 mN/m. In addition, L-B transfer ratios can be affected by the characteristics of the solid substrates. Of particular importance here is surface hydrophilicity. The best results required the use of substrates immediately following plasma treatment (see Experimental Section). Consistent with the known instability of C180H L-B monolayers in experiments involving contact of coated substrates with water or their immersion into aqueous solutions (seealso electrochemicalresults below), attempts to characterize these types of films by contact angle measurements resulted in rather irreproducible results ) the C&H/ unless the C1&H mole fraction ( X C ~ ~ S Hin C180Hand C18SH/C18NH2L-B monolayers was in excess of ca. 0.5. The average advancing contact angle for (218SH/ClsOH ( X C ~ ~ S = H 0.7) was 111' f 2, (based on six different substrates). This value is similar to the advancing contact angles reported for self-assembled CleSH monolayers on goldsurfaces,which range from 111' to 113O.ljaJ2 (As will be shown below, mixed monolayers of this particular composition exhibited the best passivating characteristics.) Electrochemical Characterization of L-B Monolayers on Au Electrodes. The passivating character of the Cl&H and C&H/C18OH L-B monolayers on gold electrodes was investigated by cyclic voltammetry in 1.0 mM Ru(NH&~+,0.5 M KC1 solution. A series of cyclic voltammograms recorded on monolayer-coated electrodes produced under different conditions is shown in Figure 5. In general, as the extent of electrode passivation increases, the cyclic voltammograms become initially smaller and develop larger peak-to-peak separation than usually observed on uncoated electrodes (Figure 5B). This suggests that electrode reactions take place at a large number of sites still free of the octadecyl molecules. Linear diffusion conditions continue to prevail. However, partial blocking of the electrode surface has an effect on the shape of cyclic voltammograms similar to that observed in cases of quasi-reversible electron-transfer kinetics.ls When the extent of passivation increases, current decreases further and cyclic voltammograms become sigmoidal in shape (Figure 5C,D). At this stage the L-B surface monolayer contains a number of pinhole defects, which are located sufficiently far apart to act as an array of microelectrodes.16 Finally, under the best passivating conditions (Figure 5E), the cathodic current is further reduced and loses its sigmoidal character. Under these conditions, the current contains a significant component due to electron tunneling across the CleSH/CleOH m o n ~ l a y e r .It~ is ~ ~worth ~~~~ pointing out that the R u ( N H ~ ) ~ ~redox + / ~ couple + used in these experiments is known to have a very fast heterogeneous electron-transfer rate constant (recent measure-
(14) Hann,R.A. InLangmuir-BZodgettFilms;Roberts,G.,Ed.; Plenum Press: New York, 1990; Chapter 2, p 17.
1983,147, 39.
Table I. Properties of CleSH/CieOH and CieSH/CieNHz Monolayers at the Air/Water Interface. optimal L-B transfer Limiting collapse pressure, area/molecule,b pressure: mN/m X C , ~ H ZC,PH X C , ~ H ~ A*/molecule mN/m 0 1.00 21.9 56 35-40 0.49 0.51 21.6 36 20-25 0.59 0.41 21.6 32 20-25 0.67 0.33 21.4 29 20-22 0.78 0.22 21.4 24 18-20 1.00 0 21.3 16 10-12 0 1 20.5 63 40-45 0.36 0.64 20.8 46 38-40 0.62 0.38 20.9 34 28-30 0.77 0.23 21.1 21 20-22 1.W 21.5 30 10-12 With the exception of the last entry, all the data were obtained on pure aqueous subphase. Average precision of the limiting area data is 10.6 A/molecule. Average precision of the collapse pressure measurements is f1.5 mN/m. Obtained on 0.01 M NaOH subphase.
6ol i' -I '
5
2
'
40
E e-
20
n
15
20 A,
25 30 A*/moiecuie
35
Figure 4. n-A isotherms of C ~ & ~ H / C I ~mixed N H ~monolayers on pure water at 21 OC. Mole fraction of C&H: (A) 1.0; (B) 0.77; (C) 0.62; (D)0.36; (E) 0.0. (See also Table I.)
(15) Amatore, C.;Savbant, J.-M.; Tessier, D. J. Electroanal. Chem.
Bilewicz and Majda
2798 Langmuir, Vol. 7, No. 11,1991
I
5 '
c
I I
I
I
0.3
0
-0.3
E, V v s S C E Figure 5. Cyclic voltammogramsof Ru(NH3)e3+(1.0 mM in 0.5 M KCI) at a bare gold electrode (A) and at the Au electrodes
coated with C&H/ClBOH L-B monolayers (B-E). Monolayer n dtransfer pressure compositionsexpressedin terms of xc e ~ ~ athe were the following: (B)1.0,12 mN m; (C) 0.40, 20 mN/m; (D) 0.40,30mN m; (E)0.70,20mN/m;t edashedlineinErepresents backgroun current recorded at the same electrode in the absence of the ruthenium complex. A = 0.40 cm2;u = 50 mV/s. (See also data in Table 11.)
d
b
1.5
I
I
1.0 0.5 E, V v s SCE
I
0
Figure6. Cyclicvoltammo ams of Os(bpy)g2+(1.OmM chloride salt in 0.1 M H2SO4) at arare gold electrode (A) and at the electrodescoated with C1&3H/ClBOHL-B monolayerscontaining 70 mol % of Cl&H transferred at (B) 10 and (C) 22 mN/m. A = 0.40 cm2; u = 50 mV/s. A set of small voltammetric peaks at 1.1 V in A is due to chloride adsorption on clean gold surface.
(16)Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. s. Science
layer. It becomes clear that while CleOH enhances monolayer stability on the water surface, Cl&H is responsible for the monolayer integrity on gold surfaces. Thus, the optimal conditions involve a trade-off between the L-B transfer surface pressure and the Cl8SH mole fraction. The best results require XC,&H to be in the range 0.6-0.8. Under these conditions, the L-B transfer is carried out at a surface pressure between 20 and 22 mN/m. Following L-B transfer, the coated electrodes can be stored in clean boxes for several days without any deterioration noticeable in subsequent electrochemical experiments. Also, we have not observed any decrease of the passivating character of an electrode monolayer in the course of a 3-h-long immersion in an electrolyte solution. Stability in aqueous solution for longer periods of time was not investigated. Another test of the passivating character of the CISSH/ClaOH L-B monolayers is inhibition of gold electrooxidation in 0.1 M HC104.1b*0*4J8 Figure 7 illustrates this effect. Under the optimal conditions of L-B transfer mentioned above, gold redox processes, and specifically the oxide reduction peak a t 0.8 V, are completely eliminated. In comparison, based on the experiments in our laboratory,lz self-assembly of CleSH monolayers on the same type of gold electrode results in the decrease of the gold oxide reduction charge to 0.12 % . Thus,in our hands, the self-assembled Cl&H monolayers appear less passivating, in view of this criterion, than the CleSH/CleOH ( X C , ~ H= 0.7) L-B monolayer films. Similar to the selfassembled mono1ayers,l0J2potential scan excursions beyond +1.4 V vs SCE lead to a progressive deterioration of the passivating character of the L-B monolayer films. Perhaps the best and most quantitative test of the structural integrity of alkyl monolayer films and their passivating character is the magnitude of the interfacial capacitance of the coated electrodes. ChidseylO, Miller,&
(17)Sutin, N.; Brunschwig, B. S.;Creutz, C.; Winkler,J. R. Atre Appl. Chem. 1988,60, 1817.
(18)Angeratein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J . Electroanal. Chem. 1987,228, 429.
ments put this value at 80 cm/s),16 and thus it is a particularly demanding probe of film passivating properties.'O Besides the kinetic facility of a redox probe, its hydrophobicity also appears to be an important characteristic. Consider the cyclic voltammograms in Figure 6, where passivating properties of the Cl&H/CleOH L-B monolayers were probed with O s ( b p y ) ~(bpy, ~ + 2,2'-bipyridyl). These experiments parallel those in Figure 5 as far as the type and quality of the L-B films are concerned. Yet, it is apparent that the extent of passivation deduced from the O~(bpy)3~+ experiments isless than that apparent in the Ru(NH)a3+series. This difference cannot be related to the differences in the size of the electroactive cations since the osmium complex is larger than the ruthenium hexamine cation (the radii of the osmium and ruthenium complexes are 6.8 and 3.4 A, re~pectively).~'Since the sigmoidal shape of the Os(bpy)32+voltammograms (which indicates mass-transport limitation) persists under conditions where the Ru(NH3)s3+ current voltage curve is dominated by electron tunneling (compare voltammograms in Figures 5E and 6C),the differences between these two electroactive cations should be understood in terms of their hydrophobicity. Hence it seems likely that a more lipophilic Os(bpy)a2+cation can accessthe electrode surface more readily than Ru(NH3)s3+by partitioning into the C1&3H/CIeOH monolayer. From the experimental point of view, the increasing extent of passivation observed in Figures 5 and 6 is related to two parameters, the surface pressure during and L-B transfer and the composition of the CleSH/ClsOH mono1990,250, 1118.
~~
~
~
Langmuir, Vol. 7, No. 11, 1991 2799
Monomolecular Langmuir-Blodgett Films at Electrodes I
I '
' I
I *>
Table 11. Electrochemical Characterization of Monomolecular L-B Films on Gold Electrode@ Ru(NHa)#+ gold oxide L-B reductn reductn monolayer transfer current current interfacial compoetn, XClbSH
Y
1.5
1.o
0.5
0
E, VvsSCE
Figure 7. Cyclic voltammograms recorded in 0.1 M HClOd at a clean gold electrode (A) and at the gold electrodes coated with (B)C&H L-B monolayer transferred at 10mN/m and (C) Cl8SH/C&H ( I C C , ~ H= 0.60) L-B monolayer transferred at 20 mN/ m. A = 0.40 em2, u = 50 mV/s.
Porter,lB,Widrig,lq and co-workershave reported detailed measurements of interfacial capacitance of carefully prepared self-assembled alkanethiol and w-hydroxyalkanethiol monolayers on polycrystalline gold surfaces. In all cases,the values of interfacialcapacitancewere inversely proportional to the number of methylene units in the alkyl chains and could be interpreted in terms of parallel-plate capacitor with the dielectric constant similar to the value for bulk polyethylene. For octadecyl-type films, the reported interfacial capacitance values ranged in these reports from 1.0to 1.5 pF/cm2.1a~o~q~~ These are the lowest capacitance values for self-assembled C~a-typefilms reported in the literature to date. Since the presence of defects in surface monolayers would result in higher interfacial capacitance, these values could be taken as bench mark values for well-executed self-assembly. Our best Cl&H/Cl&H L-B monolayer coated electrodes exhibit interfacial capacitance of 1.1pF/cm2 measured in 0.2 M HClOI electrolyte at + 0.2 V vs SCE at scan rates in the range of 50-1000 mV/s (see Figure 7). Table I1 provides a comparison of the electrochemical results obtained in the studies of several L-B monolayer films and thier passivating properties. Besides the factors related to the structure and composition of the L-B films discussed above, the smoothness and cleanliness of gold substrates become important in achieving structurally perfect monolayer assemblies. The latter group of factors broadens the range of typically observed values of interfacial capacitance to 1.1-1.8 pF/cm2. Direct comparison of interfacial capacitance values of our films with those of self-assembled alkanethiols of the same chain length involves an assumption of structural similarityof these two types of surface monolayers. Specifically, the packing density and thickness of the selfassembled films are affected by the hexagonally close packed structure of C&H monolayerscommensuratewith gold (111) surface structure.'a.dJIOl* (Gold (111) is the predominant atomic surface structure of vapor-deposited gold films.)Ia Such structure involves an approximate 3035' tilt of the all-trans alkyl chains with respect to surface normal. It is reasonable to hypothesize that the L-B monolayers, which are transferred from the water surface in vertical orientation at ca. 18A2/molecule(9.2 X 10-lOmol/
dens,
pA/cm2
capacitance,'
pF/cm*
129
281
38
0.2 0.2 0.4 0.4 0.4 0.6 0.7 0.7 0.8 1.0
20 40 10 20 30 25 15 22 20 12
896 5gb 70b 4.3' 1.6c
98b
12 10 13 1.1 1.1 0 1.4 0 0 12
6.9 4.2 6.9 3.2 2.1 1.2 3.8 1.1 1.7 13.2
1.5'
0.3
1.5
0.7
I
dens,
pA/cm2
-
CisSH/Ci$JHz I
mN/m
bare gold
CieSH/CisOH
I
preesure,
22
0.8c
4.1'
0.9 1.1c
Measured in 0.1 M HClOd at +0.2 V; roughness factor of 1.2 was used in the measurements of the electrode surface area. b Cathodic peak current measured at ca. -0.23 V. Plateau current measured at ca. -0.30 V. a
cm2), subsequently relax and adopt a commensurate structure with the gold surface. This respreading is necessary to correct for the microscopic roughness of ca. 20% of the gold The resulting surface coverage would then be consistent with the coverage of self-assembledCl&H monolayers on polycrystalline gold surfaces reported to be 7.6 X 10-lo mo1/cm2.1aJ*a The same electrochemicalexperiments described above were also carried out to probe passivating properties of Cl&H/C18NH2 L-B monolayers. In general, we find that both types of monlayers have comparable characteristics for comparable Cl&H mole fraction and surface pressure of L-B transfer (see results in Table 11). It is worth noting that the best results involving C&H/C&JH2 films were obtained for the same ratio of the two components as in the case of the ClSSH/ClBOH L-B monolayers. While this research was in progress, we obtained the results of Sawaguchi and co-workers, who investigated the passivating characteristics of mixed L-B monolayers of l-hexadecanethiol and stearic acid on vapor-deposited gold electrodes.19 They obtained the most stable L-B monolayers for mixed monolayers containing 10mol % of hexadecanethiol transferred at 40 mN/m. Judging from the sigmoidal character of the voltammetric waves of ferrocyanide oxidation (a redox probe of considerably lower kinetic facility), these monolayers were not as passivating as our best C~&H/CI@HL-B films. We have, subsequently, carried out experimentswith mixed Cl&H/stearic acid monolayers of different compositions and found that this system yields consistently less favorable results compared to the other two described above. We postulate that larger structural differences between thiol and carboxyl groups compared to thiol and hydroxyl (and SH and NH2) groups could account for this effect. Incorporation of Electroactive Reagents into ClsSH/Cl8OH L-B Monolayers. Having establishedthe conditions for the formation of stable and passivating monolayers on gold electrodes using L-B techniques, we now focus on the possibility of reagent incorporation into these monolayer assemblies. We seek to develop a general scheme of reagent immobilization on the electrode surface in an impermeable monolayer matrix consisting of a c18SH/ClsOH L-B film. In such a scheme,a water-insoluble (19) Sawaguchi,T.;Nishizewa, M.; Mataue, T.; Uchida,I. Chem. Lett. 1990, 1437.
Bilewicz and Majda
2800 Langmuir, Vol. 7, No. 11, 1991 -
I
1
I
I
A
Table 111. Effect of Os(DPP)s(ClO& on the Limiting Area per Molecule in Mixed C&H/ClaOH Monolayers at the Air/Water Interface limiting area, A*/molecule
-m2 0 0.06 0.09 0.11 0.18 0.31
obs 20.5 26.4 29.6 31.4 38.5 51.5
a x b , mole fraction of Os(DPP)&lO&. = .W,BSHAC,BSH + XC~~OHAC~PH +x d b .
I
1.o
I
I
0.8 0.6 E, V v s S C E
I
0.2
Figure 8. Cyclic voltammograms recorded in 0.1 M HCIOI at gold electrodes coated with (A) ClsSH/ClsOH ( X C ~ ~=H 0.65) L-B monolayer transferred at 20 mN/m. (B) The L-B film contained 20 mol % butylferrocene. A = 0.40 cmz, u = 50 mV/s. reagent to be immobilized is spread at a controlled mole fraction together with the C18SH and Cl8OH on the water surface, compressed, and then transferred onto a goldcoated glass substrate. In the experiments presented below, the molar ratio of ClaSH/ClaOH was kept constant at ca. 1.8. This corresponds to the composition of the best stability and to the highest degree of passivation described above. In comparison to single-component L-B films, mixed L-B monolayers have not been investigated extensively. Nevertheless, there exist a number of literature reports, reviewed recently,14 concerning properties of two-component systems where one of the components is nonamphiphilic. Butylferrocene (BF) is not an amphiphilic molecule and cannot be spread on the water surface. However, it can be dispersed in a ClaSH/ClaOH monolayer and forms stable Langmuir films when its mole fraction does not exceed 25 % . Under these conditions, compression isotherms similar to those in Figure 2 are obtained, except the limiting area at a = 0 reflects the presence of BF according to the following expression:14 Ai = XC,,SHAC,,SH + XC,,OHAC,,OH + XBFABF (1)
Based on several measurements of the limiting area per molecule, AI, we found the limiting area of butylferrocene, ABF,to be 45 f 4 A2/molecule. This value is consistent with the cross-sectional area of ferrocene. When the concentration of BF in the surface monolayer is increased beyond 30 mol ?4 , A1 decreases significantly, suggesting that some BF is expelled from the monolayer during compression. Mixed monolayers with XBF < 0.25 can be transferred onto gold-coated substrates with a 0.9 f 0.1 L-B transfer ratio at a surface pressure of 20 mN/m. The electrochemicalresults shown in Figure 8 provide a strong indication that butylferrocene remains immobilized and electroactive in the surface monolayer when the coated electrode is immersed in 0.1 M HC104 electrolyte. Integration of the anodic peak current at 0.5 V gives BF surface converage of 1.6 X 10-lo mol/cm2. This is in good agreement with the expected value of 1.5 X 10-lomol/cm2, which is based on the known initial composition of the monolayer on water surface. A relatively high level of charging currents in this experiment reflects the apparent increase of the average dielectric constant of the surface monolayer brought about by BF incorporation end a resulting disruption of the monolayer continuous structure.
calcb
26.5 29.5 31.5 38.4 51.4
* Calculated from A",
We have also observed considerable stability of the perchlorate salt of O S ( D P P ) ~complex ~+ (DPP = 4,7-diphenyl1,lO-phenanthroline)in Langmuir monolayers of ClSSH/ ClaOH surfactants. The lipophilic character of diphenylphenanthroline accounts for the insolubility of this compound in water and its stability in surface films at high pressure. In comparison, a less lipophilic Os(bpy)3(C104)2 (bpy = 2,2'-bipyridine) is too soluble in water and cannot be entrapped in the surfactant monolayer at the water surface. Incorporation of the perchlorate salt of the Os(DPP)32+complex in a Cl8SH/ClaOH monolayer led to a positive shift of the limiting area per molecule in a broad range of osmium complex concentrations. The values of the limiting area were consistent with the sum of the molecular area of the three components weighted by their respective mole fractions (consult eq 1 and Table 111). Based on these measurements, an average molecular area of Os(DPP)3(C104)2was 120.2 f 0.9 A2/molecule, which is in reasonable agreement with the size of the osmium complex,20particularly when one considers a possibility of intermixing of the ligands' phenyl rings with the surrounding octadecyl chains of the monolayer surfactants. These results differ from a recent report of Murakata and co-workers, who investigated mixed monolayers of a close analogue of our osmium complex, Ru(DPP)s2+ and reported that it could not be dispersed in octadecanol films.20c Stable mixed monolayers of the ruthenium complex were obtained with stearic acid under conditions where it forms ion pairs with R u ( D P P ) ~ ~ + . ~ ~ The L-B transfer ratios of mixed monolayers containing osmium species onto gold electrodes were 1.0 f 0.1 a t 22 mN/m. Figure 9 shows a series of cyclic voltammograms of Os(DPP)3(C104)2 incorporated in L-B monolayer assemblies at different surface concentrations. The peakto-peak separation of approximately 100 mV is most probably due to kinetic effects of perchlorate counterion insertion and expulsion processes. Integration of the voltammetric current gave the surface concentrations of the osmium complex, which were in very good agreement with the initial surface concentrations on the water surface as listed in Table IV. This finding reflects the excellent stability of the system on the water surface, both during L-B transfer and in the 0.1 M HC104electrolyte solution. It is worth noting that stability of this system persists over a period of several days following an L-B transfer. Incubation of the coated electrode in the electrolyte solution overnight or continuous voltpnmetric cycling does not result in any appreciable loss of Os(DPP)3(C104)2electroactivity . Another class of compounds which can be easily incorporated in Langmuir monolayers and transferred onto (20)(a) Goldstein, B. M.; Barton, J. K.; Berman, H. M. Inorg. Chem.
1986,25, 842. (b) Zalkin, A.; Templeton, D. H.; Ueki, T. Inorg. Chem. 1973, 12, 1641. (c) Murakata, T.; Miyashita, T.; Matsuda, M. J. Phys. Chem. 1988,92,6040.
Langmuir, Vol. 7, No. 11, 1991 2801
Monomolecular Langmuir-Blodgett Films at Electrodes
6 __-----__
-- - - - - - - - -
I
I
1.0
I
I
I
0.8 0.6 0.4 E, V v s S C E
I
1
0.2 10
20
30
50
40 A,
60
70
80
molecule
90
100
Figure 9. Cyclic voltammogramsof Os(DPP)S(C104)2transferred onto the electrode surface in Cl&H/ClSOH L-B monolayers (XC,&H:XC,~H = 2.3, 7 = 22 mN/m). Mole fraction of the Os complexin the film: (dashed line) 0.0; (A) 0.10;(B)0.18;(C) 0.28. 0.1 M HClOd. A = 0.4 cm2/s, u = 100 mV/s.
Figure 10. P A isotherms of Cl&H/C&H (mole ratio 2.3) containing variable mole fractions of Qm at a pure aqueous subphase: (A) 1.0; (B)0.21; (C) 0.14;(D)0.08; (E)0.04;(F)0.0.T = 21 O C . (See also data in Table V.)
Table IV. Comparison of Oe(DPP),(C104), Surface Concentrationsin Cl8SH/Cl8OH Monolayers at the Air/ Water Interface and in the L-B Films on Gold Electrodes area per molecule,o iolOr.la: 1010rA”: xg. A2/molecule mol/cm2 mol/cm2
Table V. Effect of QWon the Limiting Area per Molecule in Mixed ClsSH/ClaOH Monolayers at the Air/Water Interface. Comparison of the Observed and Calculated Values area, A2/molecde
0.11 0.19 0.31
26.9 33.0 42.1
0.68 0.96 1.22
0.66 0.94 1.15
OAverage area per molecule in mixed monolayers at the L-B transfer pressure of 22 mN/m. b Surface concentration at the air/ water interface at 22 mN/m. c Surface concentration obtained by integration of cyclic voltammetric current.
solid surfaces is represented by the long-chain quinone derivatives, ubiquinone (Qx)(also known as coenzyme Q) and vitamin K1 (K1). 0
0
Due to the amphiphilic character of these compounds, they can be spread and compressed on the water surface alone (see Figure 10, curve A). The limiting area per molecule and the collapse pressure in the case of Qm are 85 A2/molecule and 12.3 mN/m, while the same parameters for K1 are 66 A2/molecule and 13.0 mN/m. When QW is dispersed in the CleSH/ClaOH matrix on the water surface, a series of isotherms shown in Figure 10 can be obtained (K1-C&3H/ClsOH system exhibits essentially identical behavior). The limiting area, AI, obtained by the extrapolation of the first rising portion of each isotherm below 12 mN/m increases with the mole fraction of QSO in the monolayer. AI can be calculated by summing the molecular areas of Qx, C&H, and CleOH weighted by their mole fractions. The calculated values of A1 are in good agreement with those found experimentally, as shown in Table V. Compression of the mixed monolayers beyond approximately 12 mN/m results in their collapse followed
0.02 0.04 0.05 0.06 0.07 0.09 0.12 0.13 0.14 0.17 0.19 0.21
0.98 0.96 0.95 0.94 0.93 0.91 0.88 0.87 0.86 0.83 0.81 0.79
22.8
22.7
25.0
24.6
27.2
27.1
30.2
29.6
32.3 33.3 34.3
32.1 33.4 34.6
22.8 19.7
19.8 19.4
19.4 20.6 19.3 19.2
19.0 18.8 18.4 17.8
18.9
17.4
15.7 17.7
16.4 16.0
a Limiting area obtained by extrapolation of the initial rising portion (below r = 12 mN/m) of the isotherms to r = 0 (see Figure 10).b Based on AlQLC= ( X C ~ ~+HX C ~ H ) A ’ CC ~~&HH+ with A‘cIaH/c,&H= 21.4 A2/molecule, A& = 84.6 hjmolecule. e Limiting area obtained by extrapolationof the second rising portion (above r = 15 mN/m) of the isotherms to r = 0 (see Figure 10). Based on = XC&H xC,&H)A”ClgH/C,pH; A”Cl&H/Cl&H 20.2A2/mOlede.
by the second region of low compressibility. The second pressure rise can be approximated by a weighted sum of ClaSH and ClaOH molecular areas (Table V). These observations suggestthat the two componentsof the monolayer 650 and C&H/C1SOH are not mi~cib1e.l~ It is also apparent that a t 12 mN/m ubiquinone is quantitatively squeezed out of the monolayer. This holds for allmixtures with a QSOmole fraction greater than 2%. The higher collapse pressure with the most dilute mixed monolayers (see Figure 10E) suggests improved mixing at that corlcentration level. It is reasonable to postulate that Q ~ J is retained in surface monolayers when present at low concentrations. Mixed monolayers containing either 660or K1 can be transferred in a broad range of mole fractions onto goldcoated substrates a t 10 mN/m with a L-B transfer ratio of 0.9-1.0. The coated electrodes exhibited stability and durability similar to those carrying the Os(DPP)s(C104)2C&H/CuOH L-B monolayers described above. The electrochemical behavior of QW and K1 in L-B films was investigated in 0.1 M NaOH electrolyte, where these compounds are known to show reversible two-electron re-
2802 Langmuir, Vol. 7, No.11, 1991 I
V
0 0
I
Bilewicz and Majda I
I
I
1
4
I
1
I
0
Figure 11. Electrochemistry of Qm immobilized at the gold electrodes in Cl&H/ClsOH L-B films (XC~~H:XC,PH = 2.3, ?r = 10mN/m). (A) Cyclicvoltammogramsrecorded m0.1 M NaOH; the inner and outer curves correspond to the experiments in which surfacefilms contained 1.2 and 2.5 mol % Qw, respectively. A = 0.40 cma, u = 50 mV 8. (B)A correlation of the &w surface concentrations on gold e ectrodes (measured electrochemically) and on the air/water interface (calculated for ?r = 10 mN/m from the average area per molecule and the composition of the Langmuir monolayers) prior to L-B transfer.
I
dox processes.21 Representative cyclic voltammograms of QM a t two different surface concentrations are shown in Figure 11A (under these conditions, the formal redox potential of K1 is -0.524 V vs SCE). The surface concentration of Qw obtained by the integration of the voltammetric current due to reduction or oxidation of ubiquinone was compared with ita initial concentration on the water surface a t 10 mN/m prior to L-B transfer. The excellent 1:l correlation shown in Figure 11B emphasizes the reliability of the L-B transfer techniques and the stability of this system a t all stages of this experiment. Our ability to electrochemically assay low quantities (>1 mol 9% ) of Qw at the electrode surface is due, in part, to the low capacitive background of the monolayer-coated electrodes. This reflects again the densely packed structure of the CleSH/C18OH and of Qw domains in the L-B film. Detailed mechanistic studies of the redox properties of QM immobilized in L-B monolayer assemblies as well as gating properties of Qm sites a t electrodes will be presented in a separate report.
Conclusions We have developed an experimental protocol allowing formation of stable monomolecular Langmuir-Blodgett films of octadecanethiol and octadecanol on gold surfaces. We showed that the stability and passivating properties of these types of monolayers depend on the trade-off between the surface pressure maintained during L-B transfer and the mole fraction of C&H in the Langmuir monolayer. The first of these parameters enhances the (21) (a) Ksenzhek, 0. 5.; Petrova, S. A.; Kolodyazhny, M. V. Bioelectrochem. Bioener. 1982,9,167.(b)Petrova, 5.A,; Kolodyazhny, M. V.;h n z h e k , 0. S.J. Electroanal. Chem. ISSO,277,189. (c) Schrebler, R.S.;Arratia, A.; Sanchez,S.;Ham, M.;Duran, N.Bioelectrochem. Bioenerg. 1990,23,81.
packing density of the surfactant molecules on the gold surface and increases proportionately with the fraction of CleOH in the monolayer assembly. However, stability of the L-B film on gold surfaces is a result of C&H binding to gold and thus increases with the mole fraction of CleSH. The optimal composition for the formation of stable and passivating monolayer L-B films was determined to include 60-80 mol 76 C&H. Langmuir monolayers of this composition can be transferred at a surface pressure of 26.22 mN/m. Mixed monolayer assemblies of ClaSH and ClsNHz were found to have essentially identical characteristics at the analogous fractional composition. The passivating properties of a CleSH/CleOH L-B monolayer were investigated electrochemically. Interfacial capacitance measurements, measurements of the extent of permeability of the passivating monolayers by cyclic voltammetry of Ru(NH3)$+ and of O~(bpy)3~+, and electrooxidation of the monolayer-coated gold electrodes demonstrated that the extent of passivation accomplished by the Cl&H/CleOH L-B monolayers is essentially identical to the best cases of self-assembled octadecanethio1 and related monolayers.la-o~qJa We also demonstrated that L-B transfer of single monolayers can be used as a general technique of reagent immobilization on the electrode surface. There are two key advantageswhich make this L-B approach a technique superior to self-assembly. The first stems from the fact that the surface immobilization involves physical entrapment in an octadecyl monolayer matrix. Thus, molecules to be immobilized do not have to be derivatized with a particular functional groupsuch as thiol, which would allow their binding to the electrode surface. It is difficult to draw general quantitative conclusions based on the data presented above. Nevertheless, we can conclude that water insolubility is apparently the only essential requirement for the immobilization/entrapment in an L-B monolayer. There appear to be additional factors related to the shape and amphiphilic character of the species to be immobilized which also matter. However, these factors influence only the range of concentrations in which a mixed-monolayer system is stable and amenable to the L-B transfer. The second significantadvantage of the L-B entrapment technique over self-assembly is a precise control of the quantity of a reagent to be incorporated and immobilized in a surface monolayer. This feature opens new possibilities in designing surface monolayers assemblies capable of controlling acce8s to the electrode surface at a single molecule and for inducing selectivity in the heterogeneous electron-transfer reactions.
Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We gratefully acknowledge and thank the National Science Foundation for supportingthis research under Grant CHE8807846. We also thank Drs. C. A. Widrig, C. Miller, and T. Sawaguchi for sendingus preprints of their publications. (22) Bilewicz, R.; Majda, M. J. Am. Chem. SOC.1991,113,5464.
