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Langrnuir 1991, 7, 1510-1514
Electrochemical Characteristics of a Gold Electrode Modified with a Self -Assembled Monolayer of Ferrocenylalkanethiols Kohei Uosaki,' Yukari Sato, and Hideaki Kita Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received August 6,1990. I n Final Form: December 21, 1990 A gold surface was modified with ferrocenylalkanethiolsby the self-assemblingmethod. Symmetrical reversible redox peaks due to immobilized ferrocene groups were observed in HC104solution. Effects of modification conditions on electrochemical characteristics of the modified electrodes were investigated. It was shown that although the adsorption of the thiols was complete within 1 h, the ordering of the adsorbed monolayer required a much longer time. Shape and position of the redox peaks were strongly affected by the anion in solution. When a self-assembledmonolayer was formed in relatively concentrated thiol solution at higher temperature for longer immersion time, very sharp current spikes which seem to reflect the structure change of the self-assembled monolayer were observed.
Introduction Studies of chemical modification of metal and semiconductor surfaces have been carried out very extensively. There are many methods to form functional thin layers on solid surfaces. Polymers have been very popular modifiers and many papers on polymer modifications are available.'+ Unfortunately, however, since polymers have molecular weight distribution and the structures are heterogeneous, it is difficult to control the functions of the modified layer at the molecular level. Furthermore, it is impossible to discuss the mechanism of electron transfer and functions quantitatively. Higher order structure can be controlled better if surface is modified by sequential deposition of monolayer of functional molecules. In this case, it is much easier to discuss the relation between structure and function of the modified surface at the molecular level. The Langmuir-Blodgett (LB) method has been the most popular technique to form mono- and multilayers of assembled molecules on solid The LB method has been applied to form many kinds of devices such as electric insulators and semiconducting thin layers and also has received strong attention as one of the techniques to construct organized molecule structures for molecular and biomolecular devices. Unfortunately, however, monolayers formed by the LB method adsorb on solid substrates only physically and, therefore, are usually not stable. Recently, new approach for the formation of oriented stable monolayers, namely the self-assembling method, has been employed by many groups.12-18 In the case of the self-assembling method, (1)Merz, A,; Bard, A. J. J. Am. Chem. SOC.1978,100, 3222. (2)Peerce, P. J.; Bard, A. J. J.Electroanal. Chem. Interfacial Electrochem. 1980,112,97. ( 3 ) Inzelt, G. Electrochim. Acta 1989,34,83. (4)Peerce, P.J.; Bard, A. J. J.Electroanal. Chem. Interfacial Electrochem. 1980,114,89. (5)Inzelt, G.; Szabo, L. Electrochim. Acta 1986,31,1381. (6)Linford, R. G., Ed. Electrochemical Science and Technology of Polymers; Elsevier Applied Science: London, 1987;Vol 1. (7) Sakaguchi, H.;Nakamura, H.; Nagamura, T.; Ogawa, T.; Matauo, T. Chem. Lett. 1989,1715. (8)Takehara, K.; Niino, H.; Oozono,Y.; Isomura, K.; Taniguchi, H. Chem. Lett. 1989,2091. (9)Okahata, Y.;Tsuruta, T.; Ijiro, K.; Ariga, K. Langmuir 1988, 4, 1373. (10)Nishiyama, K.; Kurihara, M.; Fujihira, M. Thin Solid F i l m 1989, 179,477. (11)Ogawa, K.; Tamura, H.; Hatada, M.; Iahihara, T. Langmuir 1988, 4,195. (12)Sagiv, J. J. Am. Chem. SOC.1980,102,92.
molecules that have long hydrocarbon chains chemisorb on a solid surface by making covalent bonds with atoms on the solid surface and self-assembling with high structural order. For example, it was reported that molecules with terminal trichlorosilyl12or trimeth~xysilyll~ groups form self-assembled monolayers on silicon surface by reacting with hydroxyl groups on a silicon surface. Similarly, there are many reports of self-assembled monolayer formation of thiols on gold.14-18 Some attempts are also made to form multilayer films by sequential depos i t i ~ n lof~self-assembled *~~ monolayers. Although many efforts have been taken to form and characterize the selfassembled monolayers of unsubstituted alkanethiols on gold, only a few reports are available on the attempt to make functional monolayers on the surface. Rubinstein et al. have constructed the self-assembled monolayer of 2,2'-thiobis(ethy1 acetoacetate) on gold and found that the monolayer showed the ionic recognition and ion selective responsea2' Recently, Chidsey et al., who have shown that stable monolayers of ferrocene-terminated alkanethiols can be formed by coadsorption of ferrocenylalkanethiol and unsubstituted alkanethiol, studied the electrochemical behavior of the monolayers and proposed a model to explain the adsorption and electrochemical behavior of the mixed monolayers.22 In both reports, ester type compounds were mainly used to form the monolayers, although Chidsey et al. reported briefly the properties of the self-assembled monolayer in which alkanethiols linked directly to the nonpolar ferrocene group were employed as well. In this work, we have tried to understand how a selfassembled monolayer is formed and how the functional (13)Simon, R.A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. SOC. 1982,104,2031.
(14)Porter,M.D.;Bright,T.B.;Allara,D.L.;Chidsay,C.E.D.J.Am. Chem. SOC.1987,109,3559. (15)Finklea, H.0.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3,409. Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6,87. (16) (17)Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (18)Bain, C. D.;Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G.M.; Nuzzo, R. G. J.Am. Chem. SOC.1989,111,321. (19)Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid F i l m 1983,99,235. (20)Lee, H.;Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988,92,2597. (21)Rubinstein, I.;Steinberg, S.;Tor, Y.; Shanzer, A.; Sagiv,J. Nature 1988,332,426. (22)Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990,112,4301.
0743-7463/91/2407-1510$02.50/0 0 1991 American Chemical Society
Uosaki et al.
1512 Langmuir, Vol. 7,No. 7, 1991 (a)
0
Y
c
,. 80
I
0
1
2
3
4
5
6
Time/logct/sec) Figure 3. Relations between ch e of anodic eak and soaking and CllFY.) rnodifiefgold electrodes. time for both C 8 , (0) Concentration of the adsorbates in hexane was 50 fiM and modification temperature was 0 O C . and the anodic (shoulder) peak of 280 mV continued to grow even after 1h (Figure 2). Naturally, the redox peaks that appear at more positive potential should correspond to the ferrocene group which is more difficult to oxidize. Since only one type of ferrocene group is under consideration, we speculate that the redox potential reflects the order of the self-assembled monolayer and the ferrocene group which is oxidized at more positive potential is the one in the well-ordered structure. This means that the relative stability of the reduced form to the oxidized form increases when the order of the monolayer increases. This is reasonable because of the following reason. While the reduced form (ferrocene) is neutral, the oxidized (ferrocenium ion) is charged and replusion interaction between the latter groups should be larger in the more well ordered monolayer than in the less ordered monolayer. Thus, the experimental results suggest that although the adsorption of the thiols completes within 1h, the ordering of the film structure needs much longer time. We are currently performing the impedance measurements to obtain the information on the compactness of the monolayer. Figure 3 also shows that the number of CnF, molecules adsorbed on gold is larger than that of C$, molecules. Since the size of the ferrocene group is large, the presence of the ferrocene group is unfavorable for the formation of the ordered monolayer. Since the ordering of the monolayer is achieved due to the cohesion interaction between the neighboring alkyl chains, a better ordered structure is expected for a monolayer of long alkyl chain molecules than for that of short chain ones. Thus, because CllF, is a longer hydrocarbon chain than CsF,, the former is expected to make an ordered monolayer more easily and, therefore, to have a larger amount of adsorption. Similar results were obtained when the soaking temperature was 25 "C, although the charge for the redox reaction increased little. Figure 4 shows the relation between the concentration of CsF, in hexane solution and the anodic charge. The experiments were carried out for the modified gold electrodes prepared by immersing in hexane solution a t 0 "C for 18 h. The saturation adsorption was reached a t a concentration as low as 10 p M . These results suggest the very strong interaction between the adsorbate and gold substrate. Effect of Anion on Electrochemical Response. The ferrocene group of ferrocenylalkanethiol, which is immobilized on a gold surface, becomes ferrocenium cation (F,+) upon oxidation. In this process, anions are required to compensate for the electric charge on the gold surface. It is interesting to see whether an ion pair is formed between N
L
*
-
.
'
I
-200 0 200 400 600 800 PotentiaVrnV vs. SSCE
Figure 2. Cyclicvoltammograms of the gold electrodes modified by immersing in 50 WMCllF, hexane solution at 0 O C for various time durations. The soaking times were (a) 10 8, (b) 30 s, (c) 2 min, (d) 5 min, (e) 10 min, (f) 45 min, (8) 2 h, (h) 4 h, and (i) 17 h. Solution was 1 M HClO,. Sweep rate was 100 mV.8-1.
that a linear relation exists between the sweep rate and the anodic peak current. This fact also supports that CsF, was immobilized on a gold surface. Charge of the anodic peak in this case is 53.0 pC/cm2, which is equivalent to 3.3 X 10" adsorbed molecules/cm2. Chidsey et al. reported that the maximum coverage of substituted alkanethiol should be 2.7 X 1014molecules/cm2based on the calculation assuming 6.6 A for the diameter of ferrocene group.22If one takes into account the roughness factor of the electrode (-1.6) and the reproducibility of the experiments, the present result is in good agreement with their estimation. Figure 2 shows the effect of immersing time on the shape of cyclic voltammograms of gold electrodes which were prepared in 50 pM C1lFc-hexane solution at 0 "C. As mentioned before, redox peaks were observed around +200 mV with a shoulder a t +280 mV. Generally, speaking, as immersing time increased, the shoulder grew and became clear peaks, although scattering of the data made the trend less clear. Adsorption Behavior. Figure 3 shows the relations between charge of anodic peak and immersing time for both the CsF,- and C1lFc-modifiedgold electrodes. Modification temperature was 0 "C. The number of the adsorbed molecules can be calculated from the anodic charge. In both cases the amount of adsorbed thiols increased rather quickly a t the beginning and then slowly with time and reached saturation after.about 1h, although the exact time when saturation was reached is difficult to say since the data are scattered very much. One reason for the scattering of the data may be the variation of surface condition of gold substrate. Although the gold substrates were prepared very carefully and the preparation conditions were kept as constant as possible, there may be a possibility that the surface conditions, e.g., orientation of the surface, roughness factor, and cleanliness, differ from sample to sample. While the number of adsorbed molecules reached saturation value after about l h, shape of the CV continued to change as mentioned before (Figure 2). The redox peak observed at -200 mV became smaller
Langmuir, Vol. 7, No. 7, 1991 1513
Gold Surface Modified with Ferrocenylalkanethiols
Table I. Anodic and Cathodic Peak Potentials of C$,-Modified Gold Electrode in Various Solutions' solution 0.2MKNOs 0.2MNafi04 0.2M NaClOd
Em, mV +430
+380
+400
+350
+200
E,, mV
solution 0.2MNaCl
Ep.9
mV
E,, mV
+260
+230
+460
+400
+150
a. Sweep rate 100mV-8-1. Modification was carried out in 60 pM C$F, hexane solution at 0 O C .
Concentration o f C G ~ /rnM c Figure 4. Relation between the concentrationof Cfl, in hexane solution and anodic charge. The soaking time was 18 h and modification temperature was 0 O C .
,
60.'
'E
40-
V
t
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,
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400 600 800 1000
P o t e n t i a l / m V vs. SSCE +
I
60tb
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5 -40 "-601
,
-200
,
,
0
200
,
,
.
I
400 600 800 1000
Figure 5. (a) Cyclic voltammograms in 1M HC104and in 1M HzSO4 (dotted line) of the gold electrode modified by dipping treatment in 50 pM C g , hexane solution for 2 days. Sweep rate was 100 m V 4 . (b) Cyclic voltammogram in 1 M HCl of the same gold electrode used to obtain Figure 5a. Sweep rate was 100 mV.s-1.
ferroceniumion and anion. The redox process of ferrocene in this case can be expressed by
-
(F;X-),
+ e-
K+ E = ERT o ln -F
In [F,+x-I,
+c
(3)
where C is a constant. If it is assumed that [Fc+X-] is constant regardless of X-, eq 3 can be written as
P o t e n t i a l / m V vs. SSCE
(Fc)M+ X-
1M HzSO4 solution (dotted line). In the latter case, redox peaks shifted positively by about 100 mV. The CV was also measured in 1M HC1 solution and is shown in Figure 5b. In this case anodic peaks were observed at +260 and +480 mV and broad cathodic peak was observed around +300 mV and the electrochemical response was irreversible. Effects of anions on the electrochemical response were also investigated in solutions of neutral pH, and similar results were obtained as the redox potentials were listed in Table I. In NaClO4 solution, relatively reversible CV was observed as in HClOd solution and the redox peaks appeared also a t similar potentials. In NaCl solution, the response was rather irreversible and the redox peaks shifted positively in NazSO4 and KNOs. Effects of anion on electrochemical response of ferrocene groups have been studied at poly(vinylferr0cene) (PVF) modified electrodes.6 The positions of the redox peaks varied with anion species. The anodic peak appears at more positive potential than the cathodic peak at PVFmodified electrode while two peaks appear at the same potential in the present system. The peak potential for different anions in the PVF-modified electrode system is C104- < NOa- < Sod2-, which is in agreement with that observed in the present study. These results strongly suggest the ion pair formation and can be discussed based on eq 2. Since [X-] and [F,] are constant, eq 2 can be written as
(1)
where ( F c )and ~ (FOX-), are F, and Fc+C-immobilized on the surface, respectively. Therefore, the redox potential of this process can be written ass
where K is the formation constant of the ion pairs, [F,+~-]M.Thus, the electrochemical behavior of the modified electrode should be strongly affected by the nature of these anions, particularly the formation constant of the ion pairs. Figure 5a shows the cyclic voltammogram of the modified gold electrode, which was prepared by immersion in 50 p M CsF, hexane solution for about 2 days, measured in 1M HC104 solution (solid line) and in
RT E = Eo --ln
F
K
+ C'
(4)
where C' is a constant. Thus, the effect of the anion is governed only by K. The larger the value of K is, the more negative the redox potential becomes. The tendency of the peak shift suggests that K is in the order of C104- > NO3- > S042-. Ion pair formation is also confirmed by the effect of anion concentration on the electrochemical response of the modified electrode. Figure 6 shows the cyclic voltammograms of the gold electrode, which was surface modified by immersion in 1mM C19, hexane solution for 27 h, measured in HClO4 solution of various concentrations. As clearly seen in the figure, the redox peaks shifted positively as the concentration of HClO4 became low. A linear relation was observed between the peak potential and the concentration of HC104with the slope of 60 mV/ decade (Figure 71, which agrees with the value expected from eq 2. In the studies of electrochemical behavior of polymermodified, electrodes the importance of the understanding of the movement of counterionduring oxidation/reduction process of electroactive groups which are bound to polymer
1514 Langmuir, Vol. 7,No. 7, 1991
Uosaki et al.
1 1OpA 2001 N
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100-
Q
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\
t
c
0 -
a
L
5
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.
-200
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200
400
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P o t e n t i a l / m V Vs. SSCE
chain is pointed 0 ~ t . ~ 7 ~ 2The 8 present study clearly demonstrates that the effect of anion on the electrochemical response of electroactive group can be investigated more quantitatively and clearly in monolayer systems than in polymer systems by avoiding the effect of diffusion of counterions within the polymer layer. Spikes and Surface Structure. Under certain conditions, current spikes were observed. Figure 8 shows a cyclic voltammogram of the gold electrode modified with a monolayer of Cat, which was formed by dipping in 1 mM CsF, hexane solution at 25 OC for 2 h, in 1 M HClOd. In addition to the redox peaks that were seen in Figure 1, very sharp spikes were observed. The chance for observing these spikes is higher when the immersion temperature was higher, the concentration of thiol in hexane was higher, and immersing time was longer. The shape of these spikes also varied depending on the preparation
Figure 8. Cyclic voltammogram of the gold electrodemodified with C a , by dipping treatment in 1mM C a , hexane solution at 25 O C for 2 h solution, 1 M HClO,; sweep rate, 100 m V d . conditions. In the particular case shown in Figure 8, the full width a t half maximum (fwhm) was 21 mV (scan rate; 50 m V d ) , which is much narrower than the value expected for asimple Faradaic process.29 Spikes were not observed if gold substrate was not really clean. For example, spikes were never observed if gold substrate prepared more than 3 days before the adsorption process was used. The observations of similar spikes were reported for aminophenylferrocene (fwhm = 28 mV)30and poly(viny1ferrocene)' modified Pt electrodes. Willman et al. suggested that the phase change leads to the current spikes.30 Non-Faradaic reorganization peaks were also reported even for the cases where no redox reactions are involved by many groups. For example Rusting and Conture found current spikes at Hg and Ag electrodes in an aqueous micellar system containing hexadecyltrimethylammonium bromide (CTAB) and straight chain alcohols of two to five carb0ns.3~They attributed the peak to rapid structural reorganization of a mixed alcohol/CTAB layer on the electrode surface. In the present system, when potential becomes positive, ferrocene is changed to ferrocenium ion according to eq 1, and the incorporation of anion occurs. Thus, the structure change of the self-assembled monolayer is expected upon oxidation/reduction of ferrocene groups. This structure change would lead to the current spikes. The another possibility for the cause of the spikes is narrowing of the redox peaks due to strong interaction between electroactive groups. In both cases, the quality of the monolayer may be the important factor whether the spikes are observed or not. To clarify the origin of the current spikes, the detailed spectroscopic and electrochemical studies are under way. Acknowledgment. This work was partially supported by Grant-in-Aid for Priority Area Research from the Ministry of Education, Science and Culture (02205003). We are grateful to H. Itoigawa and M. Yanagida for experimental assistance. NMR and mass spectrum measurements were carried out at the Instrumental Analysis Center of Hokkaido University. Dr. C. E. D. Chidsey is acknowledged for providing us with a preprint of ref 22 prior to publication.
(27) Hillman, A. R. In Electrochemical Science and Technology of Polymers; Linford, R. G., Ede.; Elaevier Applied Science: London, 1987; Vol. 1, Chapter 5. (28) Anaon, F. C.; Saveant, J. M.; Shigehara, K. J. Am. Chem. SOC. 1983,105, 1096.
(29) Bard, A. J.; Faulkner, L. R. ElectrochemicalMethods; John Wiley & Sons: New York, 1980; Chapter 12. (30) Willman,K. W.; Rocklin, R. D.; Nowak, R.; Kuo,K.-N.; Schultz, F. A.; Murray, R. W. J. Am. Chem. SOC.1980,102,7629. (31) Rusling, J. F.; Couture, E. C. Langmuir 1990,6, 425.
-200
800
Potential/mV vs. SSCE Figure 6. Cyclic voltammogram of the gold electrode modified with CllF, by dipping treatment in 1mM C a , hexane solution at 25 "C for 27 h: (a) 1 M, (b) 0.1 M, (c) 0.01 M, and (d) 0.001 M HClO,. Sweep rate was 100 mV-5-1.
Concentration/ (og(C/M)
Figure 7. Relation between the peak potential and the concentration of HClOd. Other conditions are given in Figure 6.
