Vibrational analysis of herbicide diquat: A normal Raman and SERS study on Ag nanoparticles M.R. Lopez-Ramirez a , Luca Guerrini b , Jose V. Garcia-Ramos b , Santiago Sanchez-Cortes b, * a Department of Physical Chemistry, University of Ma ´laga, E-29071 Ma ´laga, Spain b Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain Received 24 July 2007; received in revised form 30 November 2007; accepted 3 December 2007 Available online 23 December 2007 Abstract A surface-enhanced Raman scattering (SERS) study of diquat (DQ) is reported in this work for the first time. Previously, we have accomplished the assignment of the Raman spectra of DQ and its radical cation DQ  on the basis of theoretical DFT calculations. SERS spectra on Ag colloid were obtained at different conditions: varying the added anion, the surface coverage and the excitation wavelength. Structural marker bands related to the molecular planarity and the environment of N atoms were identified on the basis of the calculated frequencies and the SERS spectra. The analysis of SERS spectra at different conditions revealed that two DQ species are present on the metal surface: DQ attached to the surface through the formation of charge-transfer (CT) complexes with adsorbed anion, bearing a structure similar to that of DQ  , and other DQ species which are adsorbed on the metal forming multilayers. # 2008 Elsevier B.V. All rights reserved. Keywords: Diquat; Raman; Density functional theory calculation; SERS; Metal nanoparticles 1. Introduction Diquat (N,N 0 -ethylene-2,2 0 -bipyridynium, DQ, Fig. 1a) dibromide is a non-selective herbicide and plant growth regulator, structurally related to paraquat, normally used in agricultural practices, which may cause moderate hepatic and renal toxicity [1]. The quantitative analysis of these compounds is of remarkable interest because of their acute toxicity to mammals and men [2,3]. DQ belongs to the chemical family of viologens, which are dication species with interesting proper- ties: their redox chemistry, the electron-poor nature and charge- transfer (CT) complexes, the way electrons are delocalized within the radical cation and the ability to form charge-transfer complexes with electron donors [4]. Despite its obvious interest in the fields indicated above, it is surprising the fact that there is none complete vibrational assignment for this molecule in the scientific literature. The only vibrational spectroscopic studies are limited to a brief report of the infrared spectrum of DQ in the dibromide form [5] and a wider study of the resonance Raman spectrum of DQ and its radical cation form [6]. Viologens in general are able to interact with metal surfaces [7] combining the electromagnetic properties of metals and the electrochemistry properties of viologens. The adsorption of viologens and other similar cationic species onto metal surfaces seems to take place through anions previously adsorbed on the metal as suggested by us in a previous work [8]. The application of surface-enhanced vibrational techniques in the characteriza- tion of DQ adsorbed on metallic surfaces could be then of interest in order to detect small amounts of the herbicide in the environment and in the characterization of its adsorption on nanostructured metals in a possible functionalization of metals used as sensing substrates. Surface-enhanced Raman spectroscopy (SERS) is a very useful technique in the analysis of the adsorption of molecules on metal surfaces. The enhancement of the Raman signal induced by metal nanoparticles (NPs), which is the basis of SERS spectroscopy, can be understood as a contribution from two different mechanisms: electromagnetic and charge-transfer [9–12] mechanisms. In the electromagnetic mechanism the intensity of the surface plasmon resonance of the metal depends www.elsevier.com/locate/vibspec Available online at www.sciencedirect.com Vibrational Spectroscopy 48 (2008) 58–64 * Corresponding author. Tel.: +34 91 561 68 00; fax: +34 91 564 55 57. E-mail address: imts158@iem.cfmac.csic.es (S. Sanchez-Cortes). 0924-2031/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2007.12.003