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Dependence of Gold Colloid Size on Citrate Concentration in the Synthesis for SERS Applications

Xiomary Cuebas1, William Ortiz-Rivera2, and Samuel P. Hernandez-RIvera2. (1) Chemical Engineering, University of Puerto Rico, Mayaguez, PR 00680, (2) Chemistry, University of Puerto Rico, Mayaguez, PR 00680

Surface-Enhanced Raman Scattering (SERS) is a useful spectroscopic technique which combines extremely high sensitivity, due to enhanced Raman cross-sections comparable or even better than fluorescence emission. With the observation of vibrational spectra of adsorbed species, SERS provides one of the most incisive analytical methods for chemical and biological warfare agents detection and analysis at very low concentrations of analyte. Since nanoparticles have unique optical and chemical properties which depend on their size and the shape, applications to SERS make them even more attractive. Colloid suspensions of metallic zero valence nanoparticles can also work as SERS surface. Colloidal nanoparticle syntheses have been known for a considerable amount of time. These particles may consist of a particular material, be of a particular size or have specialized surface functionality. It has even become possible to have some control over the nanoparticle shape. In this work we study the dependence of the size of the colloid on the concentration of the reducing agent in this case Trisodium citrate. We synthesized gold colloid varying the volume of citrate for each one. The quantities of citrate added to the gold solution were 1 ml, 0.75ml, 0.5ml and 0.3 ml. The diameter of the colloid with was estimate by analysis to TEM images. As a result, the diameter is inversely proportional to the citrate concentration. The greater the concentration of citrate the smaller the particle size (diameter). For the SERS spectra of Adenine, the gold colloid with 0.5 ml of citrate give the most intense band compared with the others. The concentration of this analyte could be detected around 10-5 M at a wavelength of 785 nm. This corresponds to ca. 10-9 g present in the interrogation volume under by a laser beam steered by a 10x objective.

Keywords: SERS, gold colloids, TEM.