Published: February 22, 2011 r2011 American Chemical Society 4184 dx.doi.org/10.1021/jp112021j | J. Phys. Chem. C 2011, 115, 4184–4190 ARTICLE pubs.acs.org/JPCC Understanding the Effect of Adsorption Geometry over Substrate Selectivity in the Surface-Enhanced Raman Scattering Spectra of Simazine and Atrazine Jean C. S. Costa, R^ omulo A. Ando, Pedro H. C. Camargo,* and Paola Corio* Departamento de Química Fundamental, Instituto de Química, Universidade de S ~ ao Paulo, CP 26.077, 05513-970, S ~ ao Paulo-SP, Brazil b S Supporting Information 1. INTRODUCTION The surface-enhanced Raman scattering (SERS) phenomenon consists of the amplification, by several orders of magnitude, of the intensities of Raman signals when probe molecules are located in the proximity of the surface of metallic nanomaterials, such as gold (Au) and silver (Ag). 1 Thus, SERS spectroscopy employing Au and Ag nanostructures as substrates has evolved into a robust technique for detection and structural characterization at low concentration levels, including applications at the single-molecule limit. 2 Recently, SERS has emerged as a potential candidate to address the demand for the development of selective and sensitive analytical techniques to monitor environmental pollutants. 3 Nevertheless, the application of SERS to unknown and/or complex mixtures has been limited. One of the reasons for this is the fact that the SERS spectrum often displays a strong dependence upon the site of interaction/adsorption of a probe molecule with the metal sub- strate as well as the relative orientation of the analyte with respect to the metal surface. 4,5 In addition, as distinct atoms and/or functional groups in a probe molecule display different affinities for Au or Ag, the SERS spectra may present a strong dependence upon the nature of the metal employed as substrate. 6 Therefore, the inter- pretation of observed SERS spectra is not always straightforward. In this context, it is clear that the understanding of the interaction between target analytes and the surface of metallic nanostructures is essential. This can enable, for instance, a better choice of substrate (Au versus Ag) that displays the best interac- tion in terms of adsorption and orientation in order to optimize SERS sensitivity and selectivity. This is particularly important and required when the analysis of samples containing probe molecules that display similar molecular structures is desired. In this paper, we describe a study on the SERS selectivity of Au nanocubes/ nanorods and Ag nanowires as substrates for the SERS detection of the herbicides simazine (6-chloro-N,N 0 -diethyl-1,3,5-triazine- 2,4-diamine) and atrazine (6-chloro-N-ethyl-N 0 -isopropyl-1,3, 5-triazine-2,4-diamine). Simazine and atrazine were chosen as probe molecules because they represent suitable molecules for the investigation of the SERS selectivity of Au and Ag substrates to analytes presenting similar molecular structures. Furthermore, although they are commonly employed worldwide, they have attracted a great deal of environmental concern due to their association with the contamination of groundwater, endocrine disruptor effects, possible carcinogenic effects, and epidemiologi- cal connection with the inhibition of testosterone production. 7-9 Au nanocubes/nanorods and Ag nanowires were chosen as substrates because it is well established that nanostructures displaying sharp corners and/or tips can provide stronger Raman scattering enhancements relative to their rounded counterparts, due to the lightning rod effect. 10-13 Density functional theory (DFT) calculations for ordinary Raman and SERS spectra were performed to obtain a reliable analysis of specific molecule- surface interactions and assignment of vibrational modes. 2. EXPERIMENTAL SECTION 2.1. Materials and Instrumentation. Analytical grade chemi- cals HAuCl 4 3 3H 2 O (hydrogen tetrachloroaurate trihydrate), Received: December 17, 2010 Revised: January 28, 2011 ABSTRACT: This paper studies the selectivity of well-defined Au and Ag nanostructures as substrates for the SERS (surface-enhanced Raman scattering) detection of simazine (6-chloro-N,N 0 -diethyl- 1,3,5-triazine-2,4-diamine) and atrazine (6-chloro-N-ethyl-N 0 -isopro- pyl-1,3,5-triazine-2,4-diamine). Our data showed that simazine and atrazine displayed similar SERS spectra when the Au was employed as substrate. Conversely, distinct SERS signatures were obtained upon the utilization of Ag substrates. Density functional theory (DFT) calculations and vibrational assignments suggested that, while sima- zine and atrazine adsorbed on Au via the N3 position of the triazine ring, simazine adsorbed on Ag via N3 and atrazine via N5. The results presented herein demonstrated that the adsorption geometry of analyte molecules can play a central role over substrate selectivity in SERS, which is particularly important in applications involving ultrasensitive analysis of mixtures containing structurally similar molecules.