A simple and label-free electrochemical DNA hybridization sensor was developed based on a poly(thionine)/gold nanoparticle composite-modified indium tin oxide (PTH/GNPs/ITO) electrode. The PTH/GNPs/ITO electrode was prepared by the electrode position of GNPs on ITO followed by the electropolymerization of thionine (TH). Probe DNA (pDNA) was immobilized on this substrate via covalent linkage between the -NH2 group of PTH and the PO43- group at the 5' end of pDNA. The immobilized single-strand pDNA increased the stiffness of the PTH film and decreased the PTH oxidation current, whereas the current was reduced to a greater extent after the formation of double-strand DNA by the hybridization of target complementary DNA (cDNA). The pDNA/PTH/GNPs/ITO sensor exhibited excellent stability, and selectivity to discriminate DNA base pair mismatches. The semi-log plot of the [cDNA] vs. oxidation current of PTH was linear in the concentration range of 1 mu M - 1 fM with the detection limit (S/N = 3) and sensitivity of ca. 0.3 fM and ca. 2.0 mu A . cm(-2) . fM(-1), respectively. (C) 2016 The Electrochemical Society. All rights reserved.