Eur. Phys. J. Special Topics 153, 361–364 (2008) c  EDP Sciences, Springer-Verlag 2008 DOI: 10.1140/epjst/e2008-00462-0 T HE EUROPEAN P HYSICAL JOURNAL SPECIAL TOPICS Shape and size dependence of the surface plasmon resonance of gold nanoparticles studied by Photoacoustic technique T.A. El-Brolossy 1 , T. Abdallah 1 , M.B. Mohamed 3 , S. Abdallah 2 , K. Easawi 2 , S. Negm 2 , and H. Talaat 1 1 Physics Department, Faculty of Science, Ain Shams University, Abbsssia Cairo, Egypt 2 Department of Physics and Mathematics, Faculty of Engineering, Zagazig University (Shoubra), Cairo, Egypt 3 NILES, Cairo University, Giza, Egypt Abstract. We report on the optical absorption properties of as prepared gold naoparticles of different shapes and sizes measured by photoacoustic (PA) method. The gold nanoparticles of two different shapes (dots, rods) have been prepared using the seed mediated growth method. The shape and the size of these differ- ent nanoparticles were determined by STM measurements. PA spectra show the splitting of the surface plasmon resonance (SPR) into two modes (transverse and longitudinal) in case of gold nanorods. The increase in the aspect ratio of the nanorods leads to clear redshifts of the longitudinal SPR. These shifts were used to determine the dielectric constant of the surrounding medium and its variation with the aspect ratios. 1 Introduction The reduction of the size and dimensionality of metals results in drastic change in the electronic properties as the spatial length scale of the electronic motion is reduced with decreasing size. A transition from the bulk band structure to individual localized energy levels also occurs in nanometer size, and the quantum size effects with exciting new properties have led the search in metal nanoparticles [1, 2]. Many new areas in the fields of optoelectronics, sensors, nanocatalysis, and nanomedicine make use of the exciting properties of gold nanoparticles. They absorb and scatter light orders of magnitudes stronger than other materials [3]. The enhanced scattering property has been used in imaging and allows for detecting single cancer cell once nanoparticles are conjugated to cancer cell antibodies [4]. The physical origin of the strong light absorption by noble metal nanoparticles is the coherent oscillation of the conduction band electrons (surface plasmon oscillation) induced by interaction with an electromagnetic field. Mie [5] first described theoretically the strong absorption band in the visible region by spherical gold nanoparticles by solving Maxwell’s equations for a radiation field interacting with a spherical metal particle under the appropriate boundary conditions. The resonance frequencies as well as the width of the plasmon absorption band depend on the nanoparticles size and many theories have been developed to explain the observed experimental behavior [6]. Gans [7] extended Mie’s theory to prolate and oblate spheroidal particles averaged over all orientations. In the case of gold nanorod the plasmon resonance splits into two modes: one longitudinal mode parallel to the long axis of the rod and a transverse mode perpendicular to the first. In this work we use photoacoustic (PA) technique to study the shape dependence of surface plasmon resonance of gold nanoparticles (dots and rods) in addition to their dependence on the aspect ratio in the