Composite materials consisting of poly(3,4-ethylenedioxithiophene) including Au nanoparticles encapsulated by bulky anionic species have been electrogenerated and characterised through spectroscopic and microscopic techniques. The properties of electrodes modified by similar materials have been studied with respect to the oxidation of different benchmark electroactive species, such as negatively charged ascorbic and uric acids, and positively charged dopamine. The responses obtained evidence the dependence of sensitivity and selectivity of the electrochemical responses on the presence and density of the nanoparticles and on the nature of the relevant encapsulating agent. In particular, the insertion of negatively charged NPs activates a sort of electrostatic shield toward negatively charged molecules; however, also positively charged species result to be conditioned in accessing the redox active centres, located either on the nanoparticle's surface or on the conducting polymer's molecules. (C) 2008 Elsevier B.V. All rights reserved.