Synthesis of Core-shell (Pd-Au) bimetallic nanoparticles in microemulsions Eduardo Larios a,c , Lilián Calderón c , Karen Guerrero c , Emanuel Pinedo c , Amir Maldonado b and Judith Tanori a* a Depto. de Investigación en Polímeros y Materiales, Universidad de Sonora. Apdo. Postal 130, 83 000, Hermosillo, Sonora, México. b Depto. de Fisica, Universidad de Sonora, Apdo. Postal 1626, 83000, Hermosillo, Sonora, México. c Departamento de Ingeniería Química y Metalurgia, Universidad de Sonora. Apdo. Postal 130, 83000, Hermosillo, Sonora, México. * Corresponding author. Tel.: 52 + (662) 259-2161; fax: 52 + 662 259-2216 E-mail address: jtanori@polimeros.uson.mx Abstract Palladium-gold core-shell nanoparticles were synthesized in the aqueous domains of water in oil microemulsions by the sequential reduction of H 2 PdCl 4 and HAuCl 4 . The nanoparticles were characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The UV-Vis spectra confirm the presence of palladium nanoparticles after reducing H 2 PdCl 4 . These particles have been used as seeds for the core-shell particles. UV-Vis spectra show that, after reducing HAuCl 4 , the surface plasmon absorption of the nanoparticles is dominated by gold, revealing the encapsulation of the palladium seeds. These results agree with crystallographic analysis performed with High-Resolution TEM pictures, as well as with Selected Area Electron Diffraction. The TEM pictures show the core-shell nanoparticles with an average diameter of 9.1 nm, as compared with 5 nm for the palladium seeds, in good agreement with the used Pd:Au molar ratio. Keywords: microemulsions, palladium-gold, nanoparticles, core-shell.