pubs.acs.org/Langmuir Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on Nanostructured Gold Substrates Emiliano Cort es, † Aldo A. Rubert, † Guillermo Benitez, † Pilar Carro, ‡ Maria E. Vela, † and Roberto C. Salvarezza* ,† † Instituto de Investigaciones Fisicoqu ımicas, Te  oricas y Aplicadas, Universidad Nacional de La Plata, CONICET, Sucursal 4 Casilla de Correo 16, La Plata 1900, Argentina, and ‡ Departamento de Qu ımica F ısica, Universidad de la Laguna, Avda. Astrof ısico Francisco S  anchez S/N, La Laguna 38071, Tenerife, Spain Received December 24, 2008. Revised Manuscript Received March 2, 2009 Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environments seriously limits the fabrication of thiol-based devices. Here, we demonstrate that nanostructured gold exhibits higher resistance to SAM degradation and increased electrochemical stability against thiolate desorption in relation to polycrystalline preferred oriented Au(111). The increased stability can be related to the presence of a large number of defects, such as adatoms, vacancies, and steps where the thiolate binding energy is stronger than at terraces. The nanostructured Au is an interesting platform because it can be easily prepared, has surface enhanced Raman spectroscopy (SERS) activity, and exhibits a high signal/noise ratio for amperometric detection because of its large real surface area. 1. Introduction Self-assembled monolayers (SAMs) on solid surfaces are key elements for many promising applications in the wide field of nanoscience and nanotechnology. 1 SAMs of thiols on metal surfaces are the most popular because they can be easily prepared in gas or liquid phases, forming high-quality close-packed organic layers of controlled thickness and functionality. 2 In fact, the sulfur head links the hydrocarbon chain of variable length to the metal surface through a strong covalent bond. The van der Waals forces between neighboring molecules stabilize the structure. 3-5 The alkyl chain ends in a terminal group that confers the desired chemical and physical properties to the layer. 1-5 Thus, -CH 3 and -CF 3 groups turn the SAM surface hydrophobic and highly anti- adherent, while -COOH, -NH 2 , or -OH groups yield hydro- philic surfaces with good metal ion and protein binding proper- ties. Also, -SH-terminated thiols efficiently bind metallic ions and nanoparticles to the SAMs. Therefore, thiols are essential in many of the so-called “bottom-up” methods proposed to build a wide variety of devices and materials. Thiols on Au are particu- larly attractive because they represent an easy path to link inorganic, organic, and biological materials to a stable and chemically inert surface. 1,2 Thiols on Au surfaces have been proposed as basic units in molecular electronics, as building blocks in sensing and biore- cognition devices, in actuators, molecular motors, and biomi- metic phospholipid membranes, among others. 1-5 In all cases, thiols acts as molecular alligators able to anchor the active element of the device to the gold surface. However, there are some problems that limit the technological applications of thiols on Au surfaces. The main limitation arises from the SAM degradation when the thiolate-Au bonds are exposed to the ambient conditions or liquid media. 6,7 The principal reactions involved in SAM degradation can be written as follows: 2ðRSÞ ad AufRS-SR þ 2Au ð1Þ ðRSÞ ad Au þ H 2 O þ O 3 fRSO 3 H þðOHÞ ad Au ð2Þ The organic S species resulting from these reactions are not chemisorbed and may easily be removed from the Au surface. 6 Thus, these processes lead to a rapid deterioration of the thiol SAM-based devices or SAM-modified surfaces. 8-10 Some work has been performed to understand the physical chemistry in- volved in these reactions, in particular, the role of oxygen, 10 ozone, UV radiation, 11,12 hydrocarbon chain length and end- group properties, 13-15 substrate structure, 16 and the environment where the reaction takes place (air, water, and ethanol). 8,9 There is evidence that the oxidation rate decreases as the chain length increases, while it increases when the grain size of the Au substrate becomes smaller. 16,17 Oxidation in the absence of light has also been observed, 10 while ethanol seems to be the most aggressive environment for the thiolate headgroup. 7,9,18 Disulfides have been *To whom correspondence should be addressed. Fax: (+54) 0221425-4642. E-mail: robsalva@inifta.unlp.edu.ar. Website: http//nano.quimica.unlp.edu. ar. (1) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (2) Love, C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (3) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (4) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258–3268. (5) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (6) Vericat, C.; Benitez, G. A.; Grumelli, D. E.; Vela, M. E.; Salvarezza, R. C. J. Phys.: Condens. Matter 2008, 20, 184004. (7) Yang, G.; Amro, N. A.; Starkewolfe, Z. B.; Liu, G. Langmuir 2004, 20, 3995– 4003. (8) Flynn, N. T.; Tran, T. N. T.; Cima, M. J.; Langer, R. Langmuir 2003, 19, 10909–10915. (9) Jans, K.; Bonroy, K.; De Palmas, R.; Reekmans, G.; Jans, H.; Laureyn, W.; Smet, M.; Borghs, G.; Maes, G. Langmuir 2008, 24, 3949–3954. (10) Willey, T. M.; Vance, A. L.; Van Buuren, T.; Bostedt, C.; Terminello, L. J.; Fadley, C. S. Surf. Sci. 2005, 576, 188–196. (11) Lewis, M.; Tarlov, M.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574–9575. (12) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 2191–2198. (13) Garg, N.; Carrasquillo-Molina, E.; Lee, T. R. Langmuir 2002, 18, 2717–2726. (14) Ito, E.; Noh, J.; Hara, M. Chem. Phys. Lett. 2008, 462, 209–212. (15) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657–6662. (16) Lee, M. T.; Hsueh, C. C.; Freund, M. S.; Ferguson, G. S. Langmuir 1998, 14, 6419–6423. (17) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502–5413. (18) Noh, J.; Hara, M. Langmuir 2001, 17, 7280–7285. Published on Web 4/6/2009 © 2009 American Chemical Society DOI: 10.1021/la804251a Langmuir 2009, 25(10), 5661–5666 5661