IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS J. Opt. A: Pure Appl. Opt. 10 (2008) 104024 (4pp) doi:10.1088/1464-4258/10/10/104024 Thermo-optical properties of gold nanoparticles in colloidal systems M A Ortega 1 , L Rodriguez 2 , J Castillo 3 , V Piscitelli 3 , A Fernandez 3 and L Echevarria 1,4 1 Departamento de Qu´ ımica, Universidad Sim´ on Bol´ ıvar, Caracas 1080-A, AP 89000, Venezuela 2 Departamento de F´ ısica, Universidad Sim´ on Bol´ ıvar, Caracas, Venezuela 3 Facultad de Ciencias, Escuela de Qu´ ımica, Universidad Central de Venezuela, Venezuela E-mail: lorenzoer@usb.ve (L Echevarria) Received 28 February 2008, accepted for publication 21 April 2008 Published 2 September 2008 Online at stacks.iop.org/JOptA/10/104024 Abstract In this work, we report the thermo-optical properties of nanoparticles in colloidal suspensions. Spherical gold nanoparticles obtained by laser ablation in condensed media were characterized using thermal lens spectroscopy pumping at 532 nm with a 10 ns pulse laser-Nd-YAG system. The obtained nanoparticles were stabilized in the time by surfactants (sodium dodecyl sulfate or SDS) in water with different molar concentrations. The morphology and size of the gold nanoparticles were determined by transmission electron microscopy (TEM) and UV–visible techniques. The plasmonic resonance bands in gold nanoparticles are responsible for the light optical absorption, and the positions of the absorption maximum and bandwidth in the UV–visible spectra are given by the morphological characteristics of these systems. The thermo-optical constants such as thermal diffusion, thermal diffusivity, and (dn/dT ) are functions of the nanoparticle sizes and the dielectric function of the media. For these reasons, the thermal lens (TL) signal is also dependent on nanoparticle sizes. An analysis of the TL signal of the nanoparticles reveals the existence of an inverse dependence between the thermo-optical functions and the size. This methodology can be used in order to evaluate these systems and characterize nanoparticles in different media. These results are expected to have an impact in bioimaging, biosensors, and other technological applications such as cooling systems. Keywords: nanoparticles, thermo-optical properties, colloidal systems 1. Introduction The special interest in the thermal properties of colloidal nanoparticles has been driven by their wide range of applications. The colloidal suspensions formed with nanoparticles and organic solvents can be used as antibacterial medical treatments [1, 2], photo-thermal therapy [3], and cooling systems [4] among others. These systems must be characterized in terms of particle morphology, size distribution, and colloidal stability using techniques such as absorption spectroscopy, transmission electronic microscopy (TEM), and atomic force microscopy (AFM). However, in order to determine the thermal properties of the nanofluids, thermo-optical techniques are required. It is not possible, in 4 Author to whom any correspondence should be addressed. general, to carry out a straightforward relationship between thermo-optical function and morphological characteristics of these systems. Thermal lensing is widely used to determine the thermo-optical coefficients of different materials (gas, liquids, or solids). In a typical pump–probe mode-mismatched thermal lens (TL) experiment, the pump beam is used to generate the temperature distribution in the sample, due to the fact that the sample has both linear and nonlinear absorption at the pump beam wavelength; distortions in the probe beam wavefront at the far field are induced. These distortions are used to calculate the TL signal (TLS) as a function of the intensity of the pump beam by using the Fresnel diffraction approximation. For a nonlinear optical material that has linear and nonlinear absorption coefficients at the wavelength of the pump beam, according to the basic TPA process the beam intensity changes 1464-4258/08/104024+04$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1