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Langmuir 2003, 19, 9542-9544
Electrochemical Quartz Crystal Microbalance Study of Self-Assembled Monolayers and Multilayers of Ferrocenylthiol Derivatives on Gold Ana S. Viana,*,† Maher Kalaji,‡ and Luı´sa M. Abrantes†,§ Laborato´ rio de SPM da Faculdade de Cieˆ ncias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal, Department of Chemistry, University of Wales Bangor, Bangor, Gwynedd LL57 2UW, United Kingdom, and Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆ ncias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Received January 27, 2003. In Final Form: April 22, 2003
1. Introduction Self-assembled monolayers (SAMs) on gold electrodes have been extensively studied in the past two decades due to their stability and versatility especially as template platforms for the development of biosensors.1-5 Furthermore, the presence in the SAMs of an electroactive terminal group, such as ferrocene, confers redox properties to the monolayers from which information about the structural and functional behavior of the modified electrodes can be obtained.6-13 The self-assembly method inherently limits the chemical adsorption to one single layer. However, there are a few examples in the literature14,15 where a proper orchestration of the terminal functionality permits the multistep deposition of more layers. Recent studies on SAMs containing substituted ferrocenyl derivatives have shown that it is possible to achieve the controlled deposition of stable monolayers or multilayers onto gold surfaces in one step, depending on the experimental conditions.16 The amount of immobilized compound on the surfaces modified by multilayers, estimated by the cyclic voltammetric response of the † Laborato ´ rio de SPM da Faculdade de Cieˆncias, Universidade de Lisboa. ‡ Department of Chemistry, University of Wales Bangor. § Departamento de Quı´mica e Bioquı´mica, Faculdade de Cie ˆ ncias, Universidade de Lisboa.
(1) Finklea, H. O. Electrochemistry of Organised Monolayer of Thiols and Related Molecules on Electrodes. In Electroanalytical Chemistry, A series of Advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (2) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (3) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mulsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (4) Zhao, Y.-D.; Pang, D.-W.; Hu, S.; Wang, Z.-L.; Cheng, J.-K.; Dai, H.-P. Talanta 1999, 49, 751. (5) Chari, N. K.; Vijayamohanan, K. Biosens. Bioelectron. 2002, 17, 1. (6) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203. (7) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (8) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (9) Ye, S.; Haba, T.; Sato, Y.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653. (10) Popenoe, D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (11) Hocket, L. A.; Creager, S. E. Langmuir 1995, 11, 2318. (12) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (13) Hickman, J. J.; Ofer, D.; Labinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (14) Evans, S. D.; Ulman, A.; Goppert-Berarducci, K. E.; Gerenser, L. J. J. Am. Chem. Soc. 1992, 113, 5866. (15) Kepley, L. J.; Sackett, D. D.; Bell, C. M.; Mallouk, T. E. Thin Solid Films 1992, 208, 132.
ferrocene terminal groups, was found to be much higher than that obtained for the close-packed monolayers. Their presence was also confirmed by ellipsometry, in situ IR spectroscopy, and surface probe microscopy (atomic force microscopy and scanning tunneling microscopy). Electrochemical desorption studies performed for both monolayer and multilayer films revealed the presence of the same amount of chemically bound adsorbates, thus indicating that multilayers grow from a close-packed monolayer. The aim of this contribution is to provide a more complete picture of the formation, organization, and stability of the ferrocenylthiol multilayer arrangements on gold electrodes,16 through the comparison between cyclic voltammetric and electrochemical quartz crystal microbalance (EQCM) data. In this work, the spontaneous adsorption of a ferrocenylthiol from different solution concentrations was monitored by EQCM, and the corresponding electrode coverages were estimated. The reorganization changes occurring in the monolayer or multilayer films induced by the redox behavior of the ferrocene moiety are also addressed. EQCM is one of the numerous techniques that have been utilized to study the spontaneous formation of thiol derivative SAMs on gold substrates, as well as the redox processes of electroactive monolayers.17-23 The frequency changes during the redox switching of ferrocenylthiol monolayers indicated the formation of ion-pairs when the monolayers were studied in perchloric acid. However, the reported mass changes are not always stoichiometric, which may be related with incorporation of water molecules during the electron transfer or with the presence of traces of trapped solvent used during the adsorption.1,17-19 2. Experimental Section The synthesis of the ferrocenylthiol derivative (C5H5)Fe(C5H4)CO(CH2)5SH (FcC6) used for the self-assembly has been reported elsewhere.16 Absolute ethanol and perchloric acid were analytical grade and were used as received. The electrochemical gravimetric experiments were performed in a 420 model CH Instruments electrochemical quartz crystal microbalance. The adsorption was carried out at 23 ( 1 °C onto a 8 MHz AT-cut quartz crystal coated with 1000 Å of gold fitted in a one-compartment Teflon cell (CH Instruments). A platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Although the majority of the data presented in the paper were obtained on new goldcoated quartz crystals, some experiments were performed on cleaned crystals. The electrodes were dipped in piranha solution for a few minutes and thoroughly rinsed with ethanol and water. The surfaces were checked by cyclic voltammetry in 1 M H2SO4 till a characteristic voltammogram was obtained and by ellipsometry. The electrodes were modified inside the EQCM cell using ethanolic solutions of 1, 20, and 40 mM FcC6. After adsorption, (16) Viana, A. S.; Abrantes, L. M.; Jin, G.; Floate, S.; Nichols, R. J.; Kalaji, M. Phys. Chem. Chem. Phys. 2001, 3, 3411. (17) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (18) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 372, 117. (19) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991, 36, 1799. (20) Sato, Y.; Mizutani, F.; Shimazu, K.; Ye, S.; Uosaki, K. J. Electroanal. Chem. 1997, 434, 115. (21) Kim, Y.-T.; McCarley, R.; Bard, A. J. Langmuir 1993, 9, 1941. (22) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620. (23) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391.
10.1021/la0300333 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003
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Langmuir, Vol. 19, No. 22, 2003 9543
Figure 1. Frequency change during the self-assembling of 1 mM (a) and 40 mM (b) FcC6 on gold in ethanol. The inset of (b) shows the initial frequency change using a similar scale as in (a). The solid arrow means the injection of FcC6. the electrodes were rinsed with copious amounts of water and ethanol.
3. Results and Discussion Figure 1 shows the frequency changes monitored by EQCM during the spontaneous adsorption of FcC6 onto gold from dilute (Figure 1a) and concentrated (Figure 1b) solutions. The addition of FcC6 was performed only after a constant frequency value was obtained in pure ethanol. Immediately after the addition of adsorbate, the frequency decreases steeply reaching a constant value after some seconds. This behavior is related with the fast formation of SAMs. The change in frequency was converted into the mass added to the electrode surface using the Sauerbrey equation.24-26 Using a crystal with a fundamental frequency of 7.995 MHz, a decrease of 1 Hz in the oscillation frequency corresponds to 1.38 ng of material deposited onto the gold surface for an electrode area of 0.2 cm2. A frequency variation of 36 Hz (Figure 1a) corresponds to a surface concentration of 7.5 × 10-10 mol cm-2. This value is higher than that predicted for a close-packed monolayer of ferrocene derivatives (∼5 × 10-10 mol cm-2 3), which after accounting for a roughness factor of 1.2, estimated by the oxidative stripping of chemisorbed iodine27 (a common value for evaporated gold films28,29), gives a surface concentration of 6.2 × 10-10 mol cm-2. This value is still slightly higher than that expected for a fully packed (24) Sauerbrey, G. Z. Z. Phys. 1959, 155, 206. (25) Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295. (26) Deakin, M. R.; Buttry, D. A. Anal. Chem. 1989, 61, 1147A. (27) Rodriguez, F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283.
monolayer. However, after the sample was thoroughly rinsed with water and ethanol, the charge estimated from the voltammetric peaks of the ferrocene, 4.6 × 10-10 mol cm-2, corresponds to a monolayer coverage. Despite the limitations inherent to the application of the Sauerbrey equation in liquids, such as film uniformity and rigidity and maintenance of the viscoelastic properties,24-26 the good agreement between the estimated coverages using the EQCM data and the voltammetric charges is strong evidence that this is a sufficiently good approximation to be used in the data analysis. The frequency evolution with time for a multilayer film (Figure 1b) shows three distinct regions. During the first 120 s, the frequency decreases (30 Hz) and reaches a plateau, similar to that observed for the frequency change in the monolayer film, giving a surface coverage of 6.0 × 10-10 mol cm-2. After this period, the frequency decreases again until a stable value is achieved at about 1200 s. The frequency decay with time is however nonreproducible, since the plateau can be reached before the 1200 s. However, several independent experiments have shown that after the first decrease (30 ( 5 Hz), which is always consistent with a monolayer coverage (5-7 × 10-10 mol cm-2), the frequency decreases in a nonreproducible way, till a total change of (100 ( 10 Hz) is obtained. These variations correspond to surface concentrations of 18-22 × 10-10 mol cm-2, which are much larger than those expected for a monomolecular coverage and consistent with previous voltammetric studies.16 The reason for the nonreproducible behavior is probably related with the solution injection procedure, and therefore with the diffusion of the compound in the ethanolic media till it reaches the surface. Besides confirming the deposition of a larger amount of molecules on gold than that expected for a monolayer, the EQCM data clearly indicate that the first step of multilayer formation starts with the adsorption of a monolayer, to which other molecules will be attached. Deposition of subsequent layers, where the frequency would decrease in steps, could not be observed during EQCM investigations. This behavior is in agreement with our previously reported ellipsometric data, which show that the estimated thickness for multilayer films could not be directly correlated with the subsequent adsorption of discrete close-packed layers. In fact, thickness values larger than that expected for a fully packed multilayer system were obtained, pointing to the formation of an open overlayer structure developing from a chemisorbed monolayer.16,30 The cyclic voltammograms of the gold electrodes modified in 1, 20, and 40 mM ethanolic solutions of the ferrocenylthiol are shown in Figure 2. The electrochemical response of the immobilized ferrocene groups is clearly evident in all the presented voltammograms; however, the charge under the oxidation and reduction peaks increases with the concentration of the deposition solution. The surface coverage of each electrode is shown in Table 1. For the case of the electrodes modified in 1 and 40 mM FcCn, the values are consistent with those obtained from the EQCM measurements during adsorption, confirming the presence of a monolayer and an overlayer on the top of a monolayer, respectively. (28) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9. (29) Walzack, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 114. (30) Viana, A. S. Ph.D. Thesis, University of Lisbon, Lisbon, Portugal, 2002.
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increase of mass due to the approach of the perchlorate anions toward the gold surface. It can be seen from Figure 2a that frequency changes, for a monolayer coverage, are almost reversible, indicating a fast ion-pair formation accompanying the electron transfer. However, in the case of the modified electrodes containing larger amounts of adsorbates (Figure 2b,c) the hysteresis between the forward and reverse sweeps increases, which could be related to reorganization changes occurring in the disorganized overmonolayer structure induced by the change of the oxidation state of the terminal ferrocene groups, and consequent ion-pair formations. In the case of the film with higher surface charge (Figure 2c), the hysteresis is significantly larger and the final value of frequency is slightly higher than that at the beginning of the cycle. This fact might be due to the release of water molecules trapped within the multilayer arrangement as a result of the reorganization of the molecules in the overmonolayer toward a more compact structure. Furthermore, it was observed that after two redox cycles the hysteresis in the frequency response is substantially reduced, supporting the above speculation that the overlayer structure achieves a more compact and stable configuration with potential cycling. The estimated mass added to the modified gold surface during the redox cycling was converted into surface concentration of perchlorate anions and is displayed in Table 1. For the electrodes modified with 1 and 20 mM FcC6, there is a close agreement between the surface coverage obtained by cyclic voltammetry and the estimated [ClO4-] values by EQCM, revealing that all the ferricenium cations will form stable ionic pairs with the anions of the electrolyte. In the case of the electrode with a high surface concentration, [ClO4-] is slightly lower than expected but still consistent with more than one monolayer coverage. This fact may arise from the following: (i) steric hindrance of the redox couples due to the presence of a disordered overlayer, and the consequent poor accessibility of the ferricinium cations to the perchlorate anions; (ii) the overlayer may be viscoelastic in nature, thus damping the oscillation of the quartz crystal. The binding of perchlorate ions may induce a conformational (or phase) change in the layer, altering its viscoelasticity and the subsequent frequency response. 4. Conclusions Figure 2. Cyclic voltammograms and correspondent frequency change during the redox process of the SAMs prepared in 1 mM (a), 20 mM (b), and 40 mM (c) solutions. 1 M HClO4; 100 mV s-1. Table 1. Surface Coverage and Surface Concentration of Perchlorate Anions Estimated from the Voltammograms and Frequency Variations Presented in Figure 2 cyclic voltammetry EQCM Γ[FcC6]/mol cm-2 ∆f /Hz [ClO4-]/mol cm-2 monolayer
1 mM 20 mM multilayer 40 mM
4.6 × 10-10 10 × 10-10 21 × 10-10
6 17 26
4.2 × 10-10 11 × 10-10 18 × 10-10
With the purpose of investigating the ionic-pair formation during the ferrocene redox switching in both monolayer and multilayer films, the change in frequency was also recorded as a function of the applied potential to the working electrode. It has been reported that ferricenium cations can form very stable ion-pairs with perchlorate anions from solution.17,18,31 Therefore, the frequency variations recorded simultaneously with the cyclic voltammograms (Figure 2) should be associated with the
EQCM investigations have confirmed the formation of monolayers and multilayers of ferrocenylthiol derivative; the surface coverage was dependent on the concentration of the deposition solution. The self-assembly of multilayers involves the formation of a close-packed monolayer before the attachment of other molecules, pointing to the formation of a disordered overlayer on the top of a more organized monolayer. EQCM data indicate that multilayers are not formed via layer by layer deposition of ferrocenylthiols. Frequency variation during the redox cycle of monomolecular films is almost reversible and consistent with fast electron transfer and ion-pair formation between ferrocenium cations and perchlorate anions from the electrolyte solution. A decrease in the reversibility is observed as the amount of adsorbates on gold increases, thus pointing to steric hindrance of the redox couples in the disordered overlayer. However, the potential cycling leads to the reorganization of the overlayer toward a more stable and compact structure. LA0300333 (31) Ju, H.; Leech, D. Phys. Chem. Chem. Phys. 1999, 1, 1549.
