Adsorption of 2-aminobenzothiazole on nano-colloidal silver surface: A concentration and time dependent SERS study aided by density functional theory Joydeep Chowdhury * Department of Physics, Sammilani Mahavidyalaya, Baghajatin Station, E.M. Bypass, Kolkata 700 075, West Bengal, India 1. Introduction Surface enhanced Raman scattering (SERS) has opened up wide research fields in Raman spectroscopy and in physics and chemistry of interfaces [1]. It is a useful tool in surface chemistry because of its high sensitivity and potential in providing useful information regarding metal-adsorbate interactions [2]. The origin of SERS still remains a matter of controversy, though it is now widely accepted that there are two main contributions of the overall effect. One is the long range electromagnetic (EM) effect and the other is the short-range chemical (CHEM) effect, which is now thought to be simultaneously operative. The EM mechanism is based on the amplification of the electromagnetic field generated due to coupling of the radiation field with the surface plasmons of the metal nano-particles [3]. The CHEM mechanism is associated with the charge transfer (CT) interaction between the metal surface and the adsorbed molecule, resulting in the change of molecular polarizability [4]. Recently quantum chemical calcula- tions are successfully utilized to model the experimentally observed SERS spectra and their results help one to understand the CHEM mechanism of SERS in particular [5–8]. 2-Aminobenzothiazole (2-ABT) molecule is known for its local anesthetic action and has numerous applications in human and veterinary medicine [9]. The optimized structure of 2-ABT molecule is shown in Fig. 1. 2-ABT is a metabolite of methabenzthiazuron and is reported to form the main fraction of soil bound residues [10]. Considering the enormous biological importance, we present here a concentration and time dependent SERS study of 2-ABT molecule adsorbed on nano-colloidal silver surface. The concentration dependent SERS spectra of the molecule enable us to understand the adsorptive behavior of the probe molecule on the nano-colloidal silver surface at different adsorbate concentrations, close to that encountered under physiological conditions in living systems. The time dependent SERS spectra help us to comprehend the charge transfer contribution to SERS. 2. Materials and methods 2.1. Chemicals and procedure 2-ABT was purchased from Aldrich Chemical Co. and used without further purification. The molecule is readily soluble in acetonitrile solution. Stable silver sol was prepared by the process of Creighton et al. [11]. Mixing a specific volume of stock solution in acetonitrile with an appropriate volume of silver nano-colloid, the desired concentration of 2-ABT on the nano-colloidal surface has been attained. 2.2. Instrumentation Raman spectra were recorded by a Spex double monochro- mator (Model 1403) fitted with a holographic grating of 1800 Vibrational Spectroscopy 52 (2010) 85–92 ARTICLE INFO Article history: Received 12 May 2009 Received in revised form 10 August 2009 Accepted 9 November 2009 Available online 18 November 2009 Keywords: 2-Aminobenzothiazole Surface-enhanced Raman scattering (SERS) Silver nano-colloids Adsorption behavior Density functional theory 2D correlation spectroscopy ABSTRACT Concentration and time dependent SERS spectra of 2-aminobenzothiazole molecule adsorbed on nano- colloidal silver surface have been investigated. The experimental observations are aided by density functional theory. The general interpretation of the concentration dependent SERS spectra corroborates the predominant existence of the ‘‘N’’ specie of the molecule at 1.0  10 6 M adsorbate concentration which ensures the edge on adsorption of the molecule with its short axis (y-axis) almost normal to the nano-colloidal silver surface. The time dependent SERS spectra of the molecule are characterized by the increase in intensity of bands primarily representing the vibrational signatures emanating from the benzene ring moiety of the molecule. The possible rationale of the selective enhancement of the vibrational signatures emanating from the benzene ring moiety of the molecule with time has been envisaged. ß 2009 Elsevier B.V. All rights reserved. * Tel.: +91 33 24626869; fax: +91 33 24626869. E-mail address: joydeep72_c@rediffmail.com. Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec 0924-2031/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2009.11.002