Determination of Thickness, Dielectric Constant of Thiol Films, and Kinetics of Adsorption Using Surface Plasmon Resonance Flavio S. Damos, Rita C. S. Luz, and Lauro T. Kubota* Institute of Chemistry, UNICAMP, P.O. Box 6154, 13084-971, Campinas, SP, Brazil Received May 26, 2004. In Final Form: August 26, 2004 This paper describes the formation of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) under different concentrations on a gold sensor disk, monitoring in situ and in real time using surface plasmon resonance spectroscopy (SPR). The film thickness and dielectric constant were determined for a fully formed monolayer using one-color approach SPR. The kinetic studies of the film formation in ethanol solution indicated that the self-assembled monolayer is formed in a two-step adsorption process. In this sense, this unpublished route was applied on the basis of a model where many molecules are adsorbed at an initial step and then can be desorbed and/or rearranged to form a perfect monolayer. 1. Introduction Self-assembled monolayers (SAMs) and multilayers have promoted the control and selection of the physical and chemical properties of surfaces. As a result, the use of self-assembled layers has attracted considerable interest nowadays, owing to their potential technological applica- tions. In addition, structurally well-defined layers on solid surfaces allow researchers to simply model a large variety of interfacial phenomena that are often difficult to study at bare surfaces. 1 In this sense, alkanethiol carboxy- and amine-termi- nated SAMs have drawn special attention because these films are easy to prepare with an organization comparable to that ofLangmuir-Blodgett films. As shown by a variety of analytical techniques, such as atomic force microscopy (AFM), infrared spectroscopy (IR), 2 sum frequency gen- eration (SFG), 3 scanning tunneling microscopy (STM), 4 and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), 5 thiol SAMs are densely packed films. As a result of the application of these techniques, the molecular orientation of the alkyl chains in SAMs is well- known without any controversy. It has also been reported that these films form spontaneously upon immersion of gold substrates into thiol-containing solution, generating densely packed arrays of Au thiolates that have an average tilt angle of 27-48° from the normal of the surface. 1,6,7 On the other hand, there is a variety of contradictory publications concerning the kinetics of formation and adsorption mechanism. These controversial results have been attributed to several factors such as the presence of preadsorbed contaminants on the gold surface, differences between in situ and ex situ methods, and solvent or thiol concentration effects. 8 In one of the earliest works on the kinetics of thiol adsorption on gold surfaces, 9 the self-assembled mono- layers of long alkanethiol chains were followed ex situ using ellipsometry for film thickness and contact angle for wettability measurements. As reported, a two-stage adsorption process was observed. In the first step, the initial rapid formation of the monolayer, in a few minutes, the thickness reached 80-90% of its maximum. This rapid initial adsorption was followed by a slower period of several hours until the thickness approached its final value. In a similar way, Hu and Bard 10 examined the adsorp- tion of carboxy-terminated thiol on a gold surface formed in an alkaline solution of mercaptoundecanoic acid using AFM. They also observed the adsorption process in two steps, where the surface coverage reaches 60% of full monolayer coverage in the first step, occurring within 15 min, followed by a slower process of up to 2-3 h to reach the maximum coverage. Several other researchers have verified the two-step adsorption process, 8,11 and as a result, they in general agree that the adsorption takes place in two steps. On the other hand, there are conflicting reports on the models of the dynamic adsorption of thiol on the gold surface. Pan 8 et al. using a quartz crystal microbalance have shown that the dynamics of the adsorption was consistent with a simple Langmuir rate law in agreement with those observed by Karpovich and Blanchard. 12 However, Subramanian and Lakshminarayanan 13 have shown that the adsorption follows a diffusion-controlled Langmuir model at lower concentrations (<5 µmol L -1 ) and Langmuir kinetics at higher concentrations for several alkanethiols. Peterlinz and Georgiadis 14 also observed a * To whom correspondence should be addressed. E-mail: kubota@iqm.unicamp.br. Phone: 55 19 3788 3127. Fax: 55 19 3788 3023. (1) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (3) Capaz, R. B.; Cho, K.; Joannopoulos, J. D. Phys. Rev. Lett. 1995, 75, 1811-1815. (4) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guen- therodt, H.-J.; Wolf, H.; Ringsdirf, H. Langmuir 1994, 10, 2869-2871. (5) Hahner, G.; Woll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955-1958. (6) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Jager, H. B.; Buck, M.; Woll, Ch. Thin Solid Films 1997, 307, 183-191. (7) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Nelson, A. J.; Terminello, L. J.; Fadley, C. S. Langmuir 2004, 20, 2746-2752. (8) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469- 4473. (9) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (10) Hu, K.; Bard, A. J. Langmuir 1998, 14, 4790-4794. (11) DeBonno, R. F.; Loucks, G. D.; Manna, D.; D.; Krull, U. Can. J. Chem. 1995, 74, 677-688. (12) Karpovich, D. S.; Blanchardt, G. J. Langmuir 1994, 10, 3315- 3322. (13) Subramanian, R.; Lakshminarayanan, V. Electrochim. Acta 2000, 45, 4501-4509. (14) Peterlins, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740. 602 Langmuir 2005, 21, 602-609 10.1021/la0487038 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2004