Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles were prepared by reverse micelle method, which could be well dispersed in water or alcoholic solvent with ultrasonic treatment. The formation and doped state of PEDOT nanoparticles were confirmed by UV-vis-near IR absorption spectrum, X-ray photoelectron spectroscopy (XPS) and FT-IR spectrum. The nanoparticles with the size of ca. 20-40 run were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Compare to electrical conductivity of conventional PEDOT particles (ca. 0.5 S/cm), the as-prepared palletized nanoparticles have higher conductivity (ca. 10.2 S/cm) and the conductivity varies with the doped and de-doped states of PEDOT. As for sensing property, it has been found that QCM device coated with nanoparticles shows faster response and recovery to 20 ppm ammonia gas (ca. 50 s) than that of conventional PEDOT particles. The PEDOT nanoparticle covered device exhibits almost linear relation to lower NH3 gas concentration and shows a saturate tendency of gas sensitivity with the increase of gas concentration over 500 ppm. The mechanisms of electrical conductivity and gas sensitivity of PEDOT nanoparticles were also discussed. (c) 2007 Elsevier Ltd. All rights reserved.