Langmuir 1990,6, 1617-1620
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Mediated Electron Transfer by a Surfactant Viologen Bound to Octadecanethiol on Gold Stephen E. Creager, David M. Collard, and Marye Anne Fox* Department of Chemistry, University of Texas, Austin, Texas 78712 Received February 27,1990. I n Final Form: August 13, 1990 The capacity of a self-assembledoctadecanethiol layer on gold to block direct electron transfer to Ru(NH3)s3+in aqueous solution is enhanced by the presence of the surfactants dodecyltrimethylammonium bromide and N-methyl-N'-octadecyl-4,4'-dipyridiniumdibromide (1). Reversible formation of a bilayer structure is postulated. The redox-activesurfactant blocks direct electron transfer while simultaneously providing a pathway for mediated reduction via the viologen 2+/.+ redox couple. Contact angle and X-ray photoelectron spectroscopic (XPS) measurements indicate that the viologen moiety is buried within the interior of the assembly.
Introduction We report here some preliminary electrochemical results on monolayer-modified gold electrodes a t which redox reactions may be either blocked or selectively activated by electron-transfer mediation. The performance of monolayers formed by self-assembly as blocking or mediating layers is enhanced by the presence of suitable surfactants. In contrast to the usual parameters for redox mediation (a thick redox polymer layer interacting with a slow redox couple), our system provides a very thin, organized layer which effectively mediates to a fast couple. This represents a new and powerful approach to electrode modification and provides an attractive alternative t o polymer and Langmuir-Blodgett film techniques. Alkanethiols on gold,' ammonium salts on silver and mercury,2 lipids on glassy ~ a r b o nlipids , ~ and carboxylates on p l a t i n ~ m and , ~ alkylsilanes on tin oxide5 and alumina617all form monolayers which block direct electron transfer to redox species in solution. Several groups have studied the electrochemistry of self-assembled functionalized thiols on gold.8-12 Others have modified electrodes with electroactive adsorbates by ~elf-assembly,'~ adsorption of surfactants on modified hydrophobic electrode~,4*~?~ and Langmuir-Blodgett techniques.14 N-Methyl-N'-octadecyl-4,4'-bipyridinium(1) self-
* Author to whom correspondence should be addressed. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.Am. Chem. SOC.1987,109, 3559. (2) Rusling, J. F.; Couture, E. C. Langmuir 1990, 6, 425. (3) Garcia, 0. J.; Quintela, P. A.; Kaifer, A. E. Anal. Chem. 1989,61,
979. . . (4) Fare, T. L. Langmuir 1990,6, 1172. (5) Okahata, Y.; Yokobori, M.; Ebara, Y.; Ebato, H.; Ariga, K. Langmuir 1990, 6, 1148. (6) Miller. C.: Maida. M. J. Am. Chem. SOC.1986. 108. 3118. (7) (a) Miller,'C. J:; Majda, M. Anal. Chem. 1988,60,1168. (b) Miller, C. J.; Widrig, C. A.; Charych, D. H.; Majda, M. J. Phys. Chem. 1988,92, 1928. (c) Miller, C. J.; Majda, M. J. Electroanal. Chem. 1986, 207, 49. (d) Goss, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988, 92, 1937. (8) (a) Rubinstein, I.; Steinberg, S.;Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426. (b) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987,219, 365. (c) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987,91, 6663. (9) (a) Finklea, H. 0.;Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (b) Finklea, H. 0.; Fedyk, J.; Schwab, J. ACS Symp. Ser 1988, 378, 431. (c) Bunding Lee, K. A. Langmuir 1990, 6, 709. (d) Bunding Lee, K. A.; Mowry, R.; McLennan, G.; Finklea, H. 0.J.Electroanal. Chem. 1988,246,217. (10) Lee, C.; Bard, A. J. J. Electroanal. Chem. 1988,239, 441. (11) (a) Li, T. T.-T.; Liu, H.Y.; Weaver, M. J. J.Am. Chem. SOC.1984, 106,1233. (b) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. SOC.1984,106, 6108. (12) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990,112,4301. (13) Widrig, C. A.; Majda, M. Langmuir 1989,5, 689. (14) Facci, J. S.; Falcigno, P. A.; Gold, J. M. Langmuir 1986, 2, 732.
0143-1463/90/2406-1617$02.50/0
assembles on gold to form monolayers a t high surface coverage and bilayers a t lower coverage by intercalation of the surfactant alkyl chains.13 Assembly of this surfactant has also been reported on lipid monolayers on glassy carbon3 and alkylsiloxane monolayers on porous alumina: Here we describe the effects of annealing and of added surfactants on the electrochemical behavior of octadecanethiol monolayers on gold. The assembly of 1 on alkanethiol m o n 0 1 a y e r s ~ blocks ~ J ~ direct electron transfer, i.e., inhibits interfacial electron transfer with a concomitant decrease in the current flow at the peak potential, to redox probes in solution while simultaneously promoting mediated electron transfer.
1
Results and Discussion We have prepared monolayer-modified electrodes by selfassembly of octadecyl mercaptan (OM) from chloroform (10 mM) on either sputtered gold or polished gold disk e1e~trodes.l~ Several literature reports have confirmed that layers formed by this procedure consist of close-packed monolayers (approximately 23 8, thick with the alkyl chain canted ca. 30' to the normal to the surface).I6 The non-Faradaic (capacitive) current of sputtered gold electrodes is greatly diminished upon formation of the OM layer (Figure l).lJ7 The layers are completely effective in blocking electron transfer to Fe(CN)64-/3-(not shown) (15) (a) Coating times were generally 12-24 h. (b) Sputtered electrodes (area: 0.20 cm2) consisted of loo0 A of gold on 50 A chromium on glass. They were cleaned sequentially in chromic acid and dilute hydrofluoric acid and were copiously rinsed with water before dried under a flow of nitrogen immediately prior to coating. (c) A Teflon-shrouded gold disk (area: 0.091 cm2) was polished in an aqueous slurry of 0.3-pmalumina, sonicated sequentially in aqueous detergent (Alconox) and distilled water degreased in boiling 2-propanol vapor, and briefly heated in air a t 150 "C. (d) OM-modified gold electrodes were immersed in filtered aqueous solutions of surfactant viologen 1. The reversible voltammetry of the same assembly has been reported previously, along with coadsorption of OM and l,gb but without complete structural characterization or demonstration of mediation. (16) (a) Bain, C. D.; Troughton, E. B.; Tao, Y-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989, 111, 321. (b) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. SOC.1989,111,7155. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111,7164. (d) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989,28, 506. (17) All cyclic voltammetry experiments were conducted on a PAR 173/ 175 potentiostat/universal programmer or a BAS-100 in 0.1 M KCl with
a Ag/AgCl (saturated KCl) reference and platinum wire counter electrode. All potentials are referenced to Ag/AgCl (saturated KCl).
0 1990 American Chemical Society
1618 Langmuir, Vol. 6,No. 10, 1990
Letters
but are somewhat less effective for R u ( N H ~ ) ~ ~(ratio +/~+ of current a t Eo a t the modified electrode (i) to the peak current at the bare electrode (io): i/io = 0.02). The cyclic voltammograms shown in Figure 1 indicate that the residual current observed after formation of the monolayer increases slightly with increasing overpotential. This behavior can be attributed to the substantial absence of pinholes, with electron transfer occurring a t other sites.18J9 The residual currents obtained here are smaller than those obtained for similar layers on polished or evaporated surfaces formed over shorter soaking periods and similar to those obtained after passivation of pinholes by phenol polymerization.'* Blocking is improved by heating the films in an oven at 150 "C for 5 min in air (Figure 1;i/io = 0.01), but longer annealing times damage the layers, resulting in larger reversible voltammetric waves. Initial adsorption of alkanethiols on gold is rapid, to afford an incomplete, disordered layer.'6 Additional adsorption, consolidation, displacement of contaminants, expulsion of solvent, and reduction of defects t o enhance packing are slower processes. We postulate that these slower processes are not complete after 48 h and that annealing results in the consolidation of two-dimensional crystalline domains of alkanethiol to reduce the defect density. Such defects might include two-dimensional crystal boundaries with conformational disorder in the alkanethiol, rather than pinholes, which would give a voltammetric response with a reversible wave a t EO. Upon addition of a cationic surfactant, dodecyltrimethylammonium bromide (2)(1mM), to the electrolyte solution (0.1 M KCl), a further decrease in the residual current for electron transfer to Ru(NH&~+is observed; i/io < 0.005, presumably by decreasing the rate of heterogeneous electron transfer at defect sites by a primarily electrostatic mechanism.
CH3
Bi CH3
W B
00 E(v0ltS)
Figure 1. Cyclic voltammetry of 1mM Ru(NH3)s3+, 100 mV s-1, electrode area = 0.20 cm2: (A) on bare Au in 1 M KC1 (note different scale); (B) Au/OM after rinsing; (C) Au/OM after annealing at 150 "C for 5 min; (D)Au/OM in presence of 1mmol C12H26N(CH&+Br-. (surfactant viologen I), using OM-treated gold (Au/ OM) as the working electrode, results in a peak a t about 4 . 4 5 V corresponding to reversible viologen reduction (V+ to Vet) which becomes prominent on extended exposure (30 min). The increase in surface coverage to reach a maximum indicates adsorption of the electroactive surfactant on the modified electrode from the dilute aqueous solution. No voltammetric wave is observed with a bare gold working electrode in the same solution. The octadecanethiol monolayer partially blocks the reduction of dimethylviologen 3, a hydrophilic analogue of the surfactant 1 (i/io = 0.2; a t a concentration of 2 mM in 1M aqueous KCl.) No surface wave is observed a t lower concentration (S10-5M in 1M KCl), indicating that there is no preconcentration of 3 a t the modified electrode surface as was observed for 1. Although methylviologen does permeate the octadecanethiol monolayer, it does not assemble at the surface. The assembly of the surfactant viologen 1 is thus a consequence of its long hydrophobic tail.
2 3
Control experiments with gold electrodes show that the added surfactant 2 has no effect on the otherwise reversible Ru(NH3)s3+voltammetry, unless the electrode has first been coated with the alkanethiol monolayer. This improvement in blocking is completely reversible: rinsing in water and transfer to surfactant-free solution regenerates the partially blocked voltammetric response. We postulate that this increase in blocking behavior can be attributed to the formation of a bilayer-like (or multilayer) structure on the electrode. The surfactant assembles on the alkanethiol monolayer by hydrophobic interaction between the hydrocarbon tails with the polar ammonium head group facing the solution. Thus, an aqueous surfactant solution wets the alkanethiol monolayer-modified surface, whereas surfactant-free water does not. The use of a surfactant substituted with a redoxactive moiety can impart a new function to the selfassembled monolayer, i.e., as an electron-transfer mediator. Cyclic voltammetry of dilute (10-5 M) aqueous surfactant N-methyl-N'-octadecyl-4,4'-bipyridiniumdibromideevm
The octadecanethiol-modified electrodes emerge dry upon removal from surfactant viologen solution, and the redox wave persists after the dry electrode is transferred to viologen-free electrolyte (Figure 2). Integration of the charge below the voltammetric peak gives a surface coverage (I') of (1.5 f 0.1) X 1014molecules/cm2 (Le., (2.3 f 0.2) X 10-lomol/cm2, 66 f 4 A2/molecule). Upon a brief rinsing (agitation in water), the surface coverage decreases to (0.2 f 0.1) X 1014 molecules/cm2. Removal of the remaining viologen is incomplete even after standing in stirred aqueous solutions for 48 h. The voltammetric waves are sharp (aE,p = 110 mV), reversible, and symmetrical with a small peak separation (hE, < 10 mV), and the peak current varies linearly with scan rate (Figure 2). This indicates that 1 is surface-bound to the OM layer and is not subject to diffusion to the electrode. The potential shifts more negative as surface coverage decreases (in the range -410 to -490 mV for r = (1.6-0.1) X 1014molecules/ cmz).
(18) Finklea, H. 0.; Snider, D. A.; Fedyk, J. Langmuir 1990, 6, 371. (19) Chidsey, C. E. D.; Loiacono, D.N. Langmuir 1990,6,682.
(20) Krieg, M.; Pileni, M.-P.; Braun, A. M.; Grltzel, M. J. Colloid Interface Sci. 1981, 83, 209.
Langmuir, Vol. 6, No. 10, 1990 1619
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Figure 2. Cyclic voltammograms of Au/OM/l (r = 0.45 X 10" molecules/cm2),area = 0.20 cm*in 0.1 KC. Insert: plot of peak potential vs scan rate.
, _-
I I
-y6'
I
-
- _ -- _ -
I
-0.700
E(volts)
I
.
I
, '\
\
I
Figure 3. Cyclic voltammograms on Au (0.20 cm2) in 0.1 M KC1: (A) Au/OM/l (r = 0.6 X 1014molecules/cm2) with no additional electroactive species in electrolyte; (B) same electrode in electrolyte containing 0.5 mM Ru(NH&~+;(C) bare Au in electrolyte containing 0.5 mM Ru(NH&3+.
Figure 3 clearly shows that 1 enhances the blocking by OM for the direct Ru(NH3)s3+reduction (Eo= -0.14 V vs Ag/AgCl, Scheme I, eq l),while simultaneously promoting the mediated reduction through immobilized 1 (2+/*+, Eo = -0.45 V, eqs 2 and 3).
Scheme I RU(NHJ;+
+ e-
-0.14 V +
Ru(NH3)?+
v*++ R ~ ( N H ~ ) v2+ ~ ~+ + RU(NHJ;+
(1)
(3)
The surface-bound viologen also mediates electron transfer to oxygen. In the presence of dissolved oxygen, the cathodic current is enhanced relative to the anodic current. The symmetrical wave shape can be restored by purging the electrolyte solution with nitrogen. The observed electroactivity of 1 is interesting, given that the viologen head group is likely to be situated relatively far (>20 A) from the gold surface. The solubilization of dialkylviologensby micelles2112 and vesic1esB.H and their assembly on hydrophobic monolayer-modified electrodes3v6indicate their amphiphilic character. These studies show that the reduced form of surfactant violo(21) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985,89,4876. (22) Hoshino, K.; Sasaki, H.; Suga, K.; Saji, T. Bull. Chem. SOC. Jpn. 1987,60,1521. (23) Kaifer, A. E. J. Am. Chem. SOC.1986, 108, 6837. (24) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J. Electroanal. Chem. 1988,246, 337.
gens (V*+)are more hydrophobic than the corresponding oxidized form and that v'+partitions between the polar (aqueous phase) and hydrophobic (alkyl chain) regions in these a ~ s e m b l i e s . ~The ~ - ~electroactivity ~ of 3 a t Au/ OM electrodes indicates that the head group can penetrate the hydrophobic monolayer. The reduction of all of the adsorbed monolayer probably relies on mediation via diffusion of some polar head groups to the electrode surface, followed by lateral electron transfer.6*26 X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition of the treated surfaces.*' The large C(ls)/S(Bp) ratio ( ~ 2 8 found ) for the Au/OM surface indicates attenuation of the sulfur signal by inelastic collision of the photoejected electron with overlaying methylene units.28 After treatment with 1, a N(1s) signal is observed and a larger-than-stoichiometric C / N ratio is observed (61 as opposed to 44 calculated for a homogeneous layer with I'(OM) = 5 X 1014 and r(l)= 1.5 X 1014molecules/cm2). This indicates that the N signal is also attenuated and that the viologen head group does not reside at the exposed surface. The sulfur signal is further attenuated by this thicker layer. After the surface is rinsed to remove most of the viologen (I' = 0.15 X 1014molecules.cm-2 by integration of the voltammetric wave), the nitrogen signal could not be observed. EllipsometryZ9indicates that the thickness of the OM film on gold is 22 A and that after soaking in surfactant viologen this value increases to 42 A, which implies that 1 forms the second half of a bilayer. Rinsing results in a decrease in thickness to 23 A. Surprisingly, the contact angle changes very little in this sequence.30 For the Au/ OM surface, we obtained contact angles for water and hexadecane of 108' and 47', respectively. After withdrawal from the surfactant viologen solution, these values are 105' and 47', and after the surface is rinsed to remove most of the viologen the water contact angle decreases to 95'. When a drop of water is left on the viologen-modified surface, it slowly (5-30 min) spreads (B(Hz0)decreases to ca 30°) as 1 is dissolved, whereas a water droplet on the Au/OM surface is stable over this time scale. A contact angle of 107' is initially obtained for an aqueous solution of 1 on Au/OM. This drop spreads as 1 is absorbed onto the surface. These surface analytical data indicate that the viologen is not bound to the surface by hydrophobic interactions between the OM and surfactant tails (i.e., tail-to-tail, Figure 4C) with the polar head groups exposed but, rather, that the viologen head group is buried within the interior of the assembly with alkyl chains exposed at the assemblyair (or solution) interface. We propose that some of the surfactant viologen intercalates with the alkanethiol monolayer (Figure 4A) and t h a t the remaining viologen assembles head down (Figure 4B) to afford a hydrophobic surface. The facile removal of most of the viologen corresponds to removal of the outer layer; the intercalated surfactant is more difficult to remove. This causes a (25) Tricot, Y.-M.; Manassen, J. J. Phys. Chem. 1988, 92, 5239. (26) Feldberg, S. W. J. Electroanal. Chem. 1986, 198, 1. (27) XPS measurements were performed on a B. G. Scientific Ltd. ESCALAB Mark 1 spectrometer with A1 Kor radiation analyzed normal to the surface. Peak areas were corrected for atomic sensitivity by Scofield factors. (28) Bain, C. D.; Whitesides, C . M. J. Phys. Chem. 1989, 93, 1670. (29) Ellipsometry was conducted on a Rudolph Instruments Model 423 null oint instrument equipped with a 6328-A laser. These measurements were made assuming that the refractive index remains constant (f2 (nf = 1.45). (30)Contact angles were measured on a Rame Hart, Inc. Model 10000 goniometer by a sessile drop method at ambient humidity, *3O. Literature values for Au/OM are B(H2O) = 112' (Hz0, saturated atmosphere) and B(hexadecane) = 47°.16a*d
1)
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1620 Langmuir, Vol. 6, No. 10, 1990
must either he accompanied by a change in the tilt angle or induction of disorder to accommodate the surfactant alkyl chains or must take place exclusively a t defect sites.31 Further studies me in progress to further characterize these intriguing assemblies. H : octadecylmsrcaplan
y
Figure 4. Schematic representation of surface binding of surfactant viologen I to Au/OM (A). The polar viologen head group is on the interior of the assembly (B) as opposed to the exposed surface (C). Rinsing rapidly removes the outer layer of I to give (D).This study does not establish structural details associated with intercalation (i.e., tilt angle, disorder, e t 4
Acknowledgment. We gratefully acknowledge the financial support of the Office of Basic Research, U.S. Department of Energy, for support of our program on chemically modified electrode surfaces and of the National Institutes of Health for a National Research Service Award to S.E.C. We thank Jeff Cook for the preparing the sputtered electrodes and for assistance in conducting the XPS measurements.
decrease in layer thickness from 42 to 23 A and exposure of a more hydrophilic surface (Figure 4D). If the octadecanethiol monolayer is closely packed with the alkyl chains tilted to maximize packing efficiency, intercalation
(31) The question of changes in the structure of the layer upon interahtion can be a d d d by grazing angle infrared experimenta. This general point of interest (see Stale, S. M.; Porter, M.D.Langmuir 1990. 6,1199)isbeyond thescopeofthiseleehaehemiealstudybutwiubetreated elsewhere.
: ~ulfactantviologen. 1
