A conducting polythiophene sensor for direct label-free DNA detection is investigated by electrochemical impedance spectroscopy (EIS) and by quartz crystal microbalance. The end-modified DNA probe is covalently immobilized by using a new anchorage function. The hybridization detection is studied by EIS experiments. An equivalent circuit is suggested and the variations of individual values of parameters deduced from impedance fits are connected to the change of conformation during the hybridization process. (c) 2005 Elsevier B.V. All rights reserved.