We have investigated the electrochemical genosensing properties of gold nanoparticle-carbon nanotube hybrid. Thiolated oligonucleotide probes and mercaptohexanol were self-assembled onto the Au-CNT hybrid. The hybridization events of target oligonucleotides are monitored using electrochemical impedance spectroscopy, cyclic voltammetry and a.c. voltammetry techniques. A redox-active mediator Ru(bpy)(3)(2+) is used to detect the oxidation of guanine residues. The as-fabricated genosensor is able to differentiate between complementary and mismatched hybridizations, relying on the oxidation current of the guanine residues mediated via Ru(bpy)(3)(2+). (C) 2004 Elsevier B.V. All rights reserved.