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Langmuir 1991, 7, 1192-1197
Use of Electroactive Thiols To Study the Formation and Exchange of Alkanethiol Monolayers on Gold David M. Collard and Marye Anne Fox* Department of Chemistry] University of Texas, Austin, Texas 78712 Received October 17, 1990.I n Final Form: December 28, 1990 Treatment of mixed monolayers of hexadecanethiol and 16-(ferrocenylcarbony1oxy)hexadecanethiol on gold with solutions of alkanethiols results in the replacement of approximatelyone-third of the electroactive thiol. This exchange can be monitored by integration of the cyclic voltammetric wave. If this exchange is carried out with a second electroactive thiol, the signal from this species indicates adsorption, and diminution of the original signal represents desorption during exchange. Subsequent treatment of the three-componentmonolayer with a nonelectroactive thiol results in loss of the signal from the second electroactive species without loss of the original species. These results are consistent with exchange of thiols from defect sites, which are immobile over extended periods.
Introduction The structure of well-defined thin-films and monolayers is of great scientific interest and technological importance.' An understanding of monolayer homogeneity and the role of defect structures is required in order to prepare thin films with specific functions. Since the behavior of defects might dominate that of ordered regions of the monolayer, it is important to identify the behavior of these sites and be able to either suppress or control their reactivity. Long chain alkanethiols self-assemble on gold from dilute solutions to form densely packed monolayers.2 Electron3 and helium atom4 diffraction indicate a hexagonal lattice with a S-S distance of 5.0 A which corresponds to the Au-Au (111)face lattice spacing and results in a surface density (I?) of 4.6 X 1014 molecules.cm-2. Infrared spectroscopy indicates that the alkyl chains pack in an all-trans conformation and that they are tilted ca. 30° to the normal so as to achieve a dense, crystalline packing.6 Alkanethiols with different chain lengths or terminal functional groups form mixed The molar is not ratio of the two adsorbates in the monolayer (xBuTf) necessarily identical with that of the same thiols in the contacting solution (xsoh)and cannot be determined by a simple equilibrium expression. The composition of mixed monolayers has previously been studied by contact angles, ellipsometry, and X-ray photoelectron spectrosCOPYTwo-dimensional crystalline monolayers modified at their termini with a variety of functional groups can be prepared. Chidsey recently reported assembly of mixed monolayers of ferrocene-substituted and unsubstituted alkanethiols on gold electrodes in a study of electron ~~
(1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, s.;Israelachvili, J.;McCarthy,T. J.; Murray, R.; Pease,R. F.;Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Ulman, A,; Eilers, J. E.; Tillman, N. Langmuir 1989,5, 1147. (3) Strong, L.; Whitesides, G. M. Langmuir 1988,4, 546. (4) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421. (5) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.Am. Chem. SOC.1987,109,3559. (6) Bain, C . D.; Whitesides, G. M. Science 1988, 240, 62. (7) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111,321. (8)Bain, C. D.; Evall, J.; Whitesides, G. M. J.Am. Chem. SOC.1989, 111, 7155. (9) Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1989,111,7164. (10) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506.
transfer rates." The fraction of ferrocene-substituted thiol was kept low to ensure that the electroactive groups are well separated and that the observed electrochemistry would therefore reflect isolated, identical sites. The unsubstituted thiol formed a densely packed monolayer such that the ferrocene is held a t a known, fixed distance from the metal substrate. The electroactivity of this modified surface is a consequence of electron tunneling between the substrate and ferrocene. When a mixed monolayer consisting of ferrocene-substitutedand unsubstituted thiol was exposed to unsubstituted thiol in ethanol, approximately one-third of the ferrocene label was replaced (determined by diminution of the cyclic voltammetric peak). He postulates that this corresponds to replacement of the thiol at defect sites. The solution phase exchange of surface-bound thiols by a thiol of different surface hydrophobicity has been studied by the change in contact angle. Although the initial adsorption occurs over a period of hours, the displacement of SCllX (X = CH3, OH) by HS(CH2)IiY (Y= OH, CH3) is incomplete after 10 days.* The displacement of short alkanethiols by longer thiols has been described, although complete exchange has not been confirmed.12 Changes in monolayer properties (contact angles, thickness) when longer adsorption times are used might be explained by an initial rapid (kinetically driven) adsorption of a monolayer, followed by slower processes which result in the formation of the thermodynamically favored layer.8 The electrochemistry occurring at alkanethiol monolayer modified gold electrodes has been assumed to occur at defect s i t e ~ , ~ since J ~ J ~pinholes can be passivated by electropolymeri~ation.~~ The kinetics of formation and exchange of these monolayers is poorly understood, although the role of defect sites in the exchange of alkanethiols between the surface and organic solutions has been suggested." Here we elaborate the exchange process by using three families of ferrocene-substituted alkanethiols (1 and 2, 3-5, and 6) with different oxidation potentials. This allows for the quantitative description of both the adsorbed and desorbed surface populations by electrochemical techniques. The sequential exchange of (11) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990,112,4301. (12) Laibinis, P. E.; Fox, M. A.: Folkers, J. P.: Whitesidea, G. M. Submitted for publication. (13) Finklea, H.0.;Avery, S.; Lynch, M.; Furtach, T. Langmuir 1987, .7 Anq
_"I.
(14) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987,91, 6663. (15) Finklea, H.0.;Snider, D. A.; Fedyk, J. Langmuir 1990,6, 371.
0743-7463/91/2407-1192$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 6,1991 1193
Alkanethiol Monolayers on Gold monolayers with the electroactive (1-6) and inactive (7-9) alkanethiols allows us to monitor processes which reexchange thiols initially adsorbed into defect sites. We also studied the effect of alkanethiol chain length, solvent, and concentration on the exchange rate. The monolayer formed by exchange differs from that formed by coadsorption of the constituent thiols. The exchange process allows for the selective functionalization of defect sites and affords some control over the structure of the monolayer over the two-dimensional surface.
10
a E
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,a6 q
, D 4 . i
2
5
0
15
10
time/ d 1,n=16
2.n-11
6,n = 4,R = octyl
3,n=16 4,n-11 5,1714
Figure 1. Plots of surfacecoverage(r)of 1and 2 versus exchange time: exchange of Au/S(CH~)l&COFc(l), S(CH2)laCHs (7) mixed monolayer with HS(CH2)&Hs (7); exchange of Au S(CH2)210COFc (21, S(CHt)&H3 (8) mixed monolayer wit HS(CH2)&H3(8). Monolayermodified electrodes were formed by coadsorption of 1 and 7, or 2 and 8, from ethanol (1mM total thiol concentration, xmh(Fc)= 0.2); exchange solutions were 1 mM thiol (7 and 8, respectively) in ethanol.
6
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8,n=9 9,n=3
Results General Electrochemical Features of MonolayerConfined Ferrocene. Treatment of a gold surface with a dilute solution of 16-(ferrocenylcarbonyloxy)hexadecanethiol(1) and hexadecanethiol(7) (total thiol concentration 1 mM, xsoh(Fc) = 0.1-0.3) in ethanol affords a mixed monolayer which gives Nernstian electrochemical behavior with reversible, symmetrical, sharp (M,p== 90 mV) cyclic voltammetric peaks at Eo == +500 mV versus Ag/AgCl (saturated KC1) in 1M HClOr. The molar ratio of adsorbants on the surface (xsurf)is similar to the solution ratio (xs0h) at low coverage of ferrocene.l1 The peak current (i,) is proportional to the potential scan rate ( u ) , indicating that the wave is due to a surface-bound species. The surface coverage (I', in molecules.cm-2) of ferrocenesubstituted alkanethiol in the monolayer can be determined by integration of the charge beneath the voltammetric wave. The electrochemicalbehavior is unchanged upon rinsing in solvents or electrochemicalcycling between -100 and +800 mV. The charging current at potentials positive of the ferrocene wave is greater than that at negative potentials, indicating a change in the structure (polarity)of the monolayer upon oxidation to ferrocinium ion. Monolayer Exchange. Exposure of the mixed monolayer (Au/ 1,7)to 1mM hexadecanethiol in ethanol results in a decrease in the signal corresponding to a replacement of approximately one-third of the ferrocene in 48 h. Further exposure results in slow loss of ferrocenesubstituted thiol, Figure 1. If the exchange is carried out with 16-(ferrocenyl)hexadecanethiol (3), a new reversible wave is observed at ca. +200 mV corresponding to surface-bound alkylferrocene 3, Figure 2. Integration of this wave specifies the amount of exchanged thiol adsorbed. The ratio of acylferrocene desorbed (diminution of the original wave) to alkylferrocene adsorbed, which indicates the total amount of thiol desorbed (acylferrocene substituted and unsubstituted) in a 1:l exchange, is in accord with the original ratio of adsorbates, x S d F c ) ,indicating that there is no preferential desorption of substituted or unsubstituted thiol. For
A I
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I
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I
I
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I
I
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I
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Figure 2. Cyclic voltammograms (10 mV s-l in 1 M aqueous HC104,Ag/AgCI (saturated KCl) reference,platinum wire counter electrode) of Au/S(CHZ)I~OCOFC (11,S(CH2)16CH3(7) modified electrodes (0.2 cm2): A, after formation from mixture of thiols in ethanol (1 mM total thiol concentration, xmh(l)= 0.2); B, after exchange with HS(CH2)leFc (2) (1 mM in ethanol, 24 h); C, after subsequent exchange in HS(CH2)1&Hs (7) (1 mM in ethanol, 24 h).
example, in the first step of the exchange (Figures 2 and 3) with an initial coverage of xs,,&) = 0.17, the amount of adsorbed 3 is ca. 5 times the amount of desorbed 1. When the electrode subjected to exchange with 3 is treated with hexadecanethiol, there is a rapid loss of alkylferrocene 3 with only slow loss of acylferrocene l (Figures 2 and 3). This indicates that there are two kinetically independent populationsof surface-boundthiol (differing with respect to rate of exchange from the monolayer) which are not in rapid equilibrium. Similarly,when the mixed monolayer modified electrode (Au/ 1,7) initially exchanged with hexadecanethiol (7) is treated with 3, there is a rapid growth of a signal for alkylferrocene 3 with little further loss of acylferrocene l. This represents replacement of exchangeablethiol which, by virtue of the first exchange, consists entirely of nonelectroactive alkanethiol. Subsequent treatment with hexadecanethiol then exchanges rapidly with 3, with only slow further loss of 1. Despite the rapid rate of reexchange of thiols incorporated in the first exchange (i.e., day 2 to day 4 in Figure 3), the process is incomplete after standing the electrode in alkanethiol solution for more than 2 weeks (Le., in Figure
Collard and Fox
1194 Langmuir, Vol. 7, No. 6,1991
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3, the reexchange of 3, which was incorporated in the first exchange, is incomplete a t day 14). The electrodes can be repeatedly recycled through this alkanethiol/ferrocenylalkanethiol sequence with reexchange of only the exchanged thiol. Treatment of mixed monolayers of 3 and 7 with 1 gives similar results. A new reversible wave corresponding to the acylferrocene 1 is observed with a diminution of the original alkylferrocene, 3, signal. Subsequent treatment with hexadecanethiol, 7, results in the rapid reexchange of the acylferrocene 1 without further loss of alkylferrocene 2. Treatment of a monolayer of hexadecanethiol on gold with 1 (1mM in ethanol, 24 h) results in a surface coverage of the electroactive thiol of 1.1X 1014molecules.cm-2 (Le. 24% exchange). Subsequent treatment with 2 (1mM in ethanol, 24 h) results in diminution of the acylferrocene signal to ca. 0.3 X 10" molecules*cm-2and the incorporation of ca. 1.2 X 1014moleculesecm-2 of the alkylferrocene. This corresponds to the fast exchange of most (73%) of the acylferrocene adsorbed in the first exchange and further ' ) of hexadecanethiol, which was incomplete exchange (9% in the first 24 h exchange. Further exposure to 2 results in no change. Exchange of mixed monolayers of hexadecane and 1 or 2 with mixtures of ferrocene-substituted and unsubstituted alkanethiols also results in the rapid exchange of ca. one-third of the monolayer. Effect of Chain Length on Exchange Kinetics. We have studied the effect of chain length by the use of electrochemical thiols, Mixed monolayers of ll-(ferrocenylcarbony1oxy)undecanethiol (2) or ll-ferrocenylundecanethiol (4) and decanethiol (8)were treated in a similar fashion as the hexadecyl derivatives 1 and 3. The displacement of 2 by decanethiol is faster than for the longer homologue (Figure 1). Displacement of 4 by decanethiol is particularly fast (at least two-thirds of the ferrocene-substituted thiol is displaced in 24 h). When a mixed monolayer of 4 and 8 (I'(4) = 0.34 X 1014 molecules.cm-2) is immersed in a solution of 2 in ethanol, a rapid growth of a signal for the acylferrocene (2) accompanies a decrease in the intensity of the alkylferrocene 4 signal. The acylferrocene wave then slowly approaches a limit of r(3) = 2.8 X 1014 moleculewcm-2 (Figure 4). This coverage corresponds to 36 A2 per ferrocene, which is in good agreement with the size offer-
6
5
time/ d
time/ d
Figure 3. Plot of surface coverages (I') of 1 and 3 versus time for treatment of Au/S(CH&eOCOFc (11, S(CHZ)I~CHS (7) modified electrode (l'(l)t,o = 0.8 X 10" molecules.cm-*) with HS(CH2)leFc(3) (1mM in ethanol, 2 days), and HS(CHg)$H3 (7)(1mM in ethanol, 12 days). Surface coverage is determined by integration of cyclic voltammograms shown in Figure 2.
n
I
0
Figure 4. Plot of ferrocenesurfacecoverage (r)versus exchange time for treatment of Au/S(CH2)11Fc (41, S(CH2)eCHa (8) modified electrode (formed by coadsorption of 4 and 8 from ethanol, 1 mM total thiol concentration, x.0ln(4) = 0.2) with HS(CH2)110COFc(2) (1 mM in ethanol, 3 days) followed by treatment with HS(CH&CHs (8) (1 mM in ethanol, 2 days). A
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Figure 5. Cyclic voltammograms (50 mV 8-l in 1 M aqueous HCI04,Ag/AgC1(saturated KC1) reference,platinumwirecounter electrode) of Au/S(CH&Fc (S), S(CH2)&Hs (9) modified electrodes (0.2 cm2)upon treatment with: A, HS(CH&CHs (9) (1 mM in ethanol);B, HS(CHZ)&Hs (7) (1mM in ethanol), after time, t . rocene previously determined.I6 Therefore, this coverage is similar to that expected for an ordered monolayer of alkanethiol with a close-packed monolayer of ferrocene at the monolayer-solution interface. Treatment of this mixed monolayer with decanethiol results in the rapid loss of the acylferrocene signal similar to that seen with fresh Bldecanethiol mixed monolayers on treatment with decanethiol. The surface wave for a monolayer of 4-ferrocenylbutanethiol (5) and butanethiol (9) is reversible but broad, Figure 5. This indicates that the ferrocene units do not reside in the same environment (Le., at a single distance from the electrode). It is known that short-chain thiol monolayers are substantially disordered. The observed electrochemistry, in this case, consists of contributions from ferrocene in an aqueous environment at the monolayer-solution interface and in an alkanelike environment (16)Seiler,P.;Dunitz, J.D. Acta Crystallogr., Sect. B 1979,35&1068.
Alkanethiol Monolayers on Gold
Langmuir, Vol. 7, No. 6,1991 1195
within the monolayer. The exchange of this mixed monolayer (r(5)= 1.1X 1014moleculescm-2) with butanethiol (9) (1 mM in ethanol) is complete in 3 h, Figure 5A. During the exchange with butanethiol, there is no change in the peak potential or width. The charging current a t a potential negative of the surface wave remains unchanged, whereas that to positive potentials decreases as ferrocene is desorbed. In contrast, exchange of the same monolayer with hexadecanethiol(7) is incomplete after 72 h, Figure 5B. There is an initial rapid desorption of electroactive thiol along with a shift in the oxidation to positive p ~ t e n t i a l .The ~ ~ capacitive current decreases as a thicker insulating monolayer replaces the thinner monolayer. The positive shift of the peak potential is consistent with a decrease in the polarity of the surrounding medium. In the initial monolayer, the ferrocene is at the monolayeraqueous phase interface or buried in the thin monolayer. As the exchange proceeds, it becomes surrounded by long alkyl chains, although pockets of aqueous solution and monolayer permeability are important in determining the composition of the local environment. The shift to positive potential for ferrocene oxidation in less polar solvents has been described.18 In the homologous series of n-alcohols (in 0.1 M LiClOr), there is a shift of +75 mV on proceeding from methanol to butanol. Greater shifts in other solvents correspond to changes in polarity. We have also noted a shift to positive potential upon treatment of w-ferrocenylalkanethiol/alkanethiol monolayers with dodecyltrimethylammonium bromide. In this case, assembly of surfactant molecules on the monolayer shields the ferrocene from electrolyte solution. In addition, we have prepared monolayers of 144-mercaptobuty1)-1'-octylferrocene (6) on gold. Although the oxidation of ferrocene is thermodynamicallyfavored upon substitution with electron-donating alkyl substituent.s,1g~~ the oxidation potential of ferrocene in this monolayer is +310 mV. The effect of the second alkyl substituent is to shift the oxidation potential positive. This is contrathermodynamic and may be explained by burying the ferrocene in the hydrophobic monolayer where it is shielded from electrolyte, so that oxidation to ferrocinium becomes disfavored. Presumably, this monolayer is substantially disordered owing to the bulky ferrocene substituent in the middle of the chain. As a consequence, the voltammetric peak is broadened. Effect of Ferrocene Surface Density. At low ferrocene surface coverage ( x 8 d < 0.5, i.e. I'
