We show that nanoporous alumina modified with covalently linked DNA can be used to detect target DNA by monitoring the increase in impedance at the electrode upon DNA hybridization, which resulted from blocking the pores to ionic flow. Using cyclic voltammetry, direct current conductance, and impedance spectroscopy we confirm the importance of pore size: the effect is observed with 20-nm-diameter pores and is absent for 200-nm pores.