In the present work, we report the synthesis of colloidal US nanoparticles with different sizes and the band gaps of these samples exhibit a systematic dependence on the size of particles. We also study the photophysical properties of US nanoparticles and the energy transfer from US nanoparticles to Rhodamme 6G dyes by steady state and time resolved photoluminescence spectroscopy. The experimental results show that the relaxation of the excited state of US nanoparticles is composed of three different components. The calculated energy transfer efficiency from US nanoparticles to dye is 43% for 3.0 nm US nanoparticles. (c) 2005 Elsevier B.V. All rights reserved.