1137 Research Article Received: 5 August 2009 Accepted: 2 November 2009 Published online in Wiley Online Library: 26 January 2010 (wileyonlinelibrary.com) DOI 10.1002/jrs.2579 Surface-enhanced Raman spectroscopic analysis of fonofos pesticide adsorbed on silver and gold nanoparticles Jitraporn Vongsvivut, a Evan G. Robertson a,b and Don McNaughton a* Surface-enhanced Raman scattering (SERS) spectra of fonofos, an organophosphorous pesticide (OPP), were recorded using citrate-reduced silver (Ag) and gold (Au) colloidal nanoparticles in the form of dried films. In this study, significant enhancements were achieved with fonofos concentrations down to ∼10 ppm with the Ag colloids, demonstrating a potential for the technique in the analysis of fonofos residues. Successful formation of the SERS-active metal aggregates was indicated by the presence of a plasmon band at longer wavelengths in the UV – visible spectrum. Transmission electron microscopy (TEM) images revealed distinctively different morphologies of the aggregates formed by the two metals, while the observed SERS spectral features of fonofos were also found to be different for molecules adsorbed on Ag and Au colloidal surfaces. This evidence suggests metal-induced changes in adsorption behavior of the fonofos analyte, leading to the different binding structures onto the different metal surfaces. Comparisons between the most prominent SERS-enhanced bands and the precise mode descriptions predicted through density functional theory (DFT) simulations at the B3LYP/6-311+G(d,p) level allowed an in-depth orientation analysis of the adsorbed species on metal surfaces. Nevertheless, the ring vibrations were found to possess a major contribution in the observed SERS enhancements for both metal nanoparticles. Copyright c  2010 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: SERS; organophosphorus pesticide; fonofos; density functional theory; B3LYP/6-311+G(d,p) Introduction Organophosphorus pesticides (OPPs) have become one of the most commonly used groups of pest control chemicals in agriculture throughout the world. The toxicity induced by organophosphates results from inhibiting the enzymes acetylcholinesterases (ChE) in the nervous system of the exposed organisms in either target pests or humans. These enzymes remove acetylcholine (Ach), which carries electrical signals across the synapse, leading to rapid twitching of voluntary muscles and finally paralysis or heart failure. [1] Despite acute poisoning caused by an exposure to OPPs, the use of OPPs has been progressively increasing over the past years, and this has attracted intense public concern worldwide about trace amounts of the residues in agricultural products that might cause long-term non-fatal health effects. A continual improvement in analytical techniques has therefore played a vital role in monitoring chemical residues in foods in order to assure safety for the consumers and to achieve optimal regulation for pesticide uses. According to a review by the US Geological Survey National Water Quality Laboratory (USGS), [2,3] the standard routine analysis for trace pesticide residue is based mainly on gas chromatography (GC) with flame photometric detection offering an excellent limit of detection (LOD) down to sub-parts-per-million. The advantage of using GC arises from its high separation power and, in addition, sensitivity and/or selectivity can be achieved through a selection of detectors such as electron capture detector (ECD), [4] nitrogen–phosphorus detector (NPD), [5] and mass spectrometry (MS). [6,7] Liquid chromatography coupled to MS (LC – MS) has also been demonstrated in recent years to be a potential technique particularly for determination of pesticides of low volatility and thermolability. [8,9] However, sample preparations and extraction process in the chromatographic techniques are time consuming, and require extensive manual handling of toxic pesticide samples as well as large amounts of organic solvents. The sophisticated combinations of instrumentation required for the measurements are also disadvantages of the techniques. We accordingly introduce a highly sensitive surface-enhanced Raman scattering (SERS) spectroscopic technique as an alternative in pesticide analysis. Since its first discovery, [10] the SERS technique has been rapidly developed as a potential trace method widely used in numerous applications for detecting molecules and/or monolayers on a variety of rough metal surfaces, [11] providing an enhancement in the Raman scattering signal by up to 10 6 or even 10 10 . Indeed the exquisite sensitivity of the technique, which has been proved to be sufficient for single-molecule detection, [12,13] is the result of the electromagnetic (EM) field, which is greatly amplified through the excitation of localized plasmon resonance inside the metal nanoparticles. [14] As a consequence, the signal of molecules in a close proximity to the surface of the metal is intensified due to the coupling of the enhanced EM field to the molecular vibrations, leading to a large increase in the ∗ Correspondence to: Don McNaughton, Centre for Biospectroscopy, School of Chemistry, Monash University, Wellington Road, Clayton, Victoria 3800, Australia. E-mail: Don.McNaughton@sci.monash.edu.au a Centre for Biospectroscopy, School of Chemistry, Monash University, Clayton, Victoria 3800, Australia b Department of Chemistry, La Trobe University, Bundoora, Victoria 3086, Australia J. Raman Spectrosc. 2010, 41, 1137–1148 Copyright c  2010 John Wiley & Sons, Ltd.