The complex [Co(phen) 2 IP]·2ClO 4 ·3H 2 O, where phen=1,10-phenanthroline and IP=imidazo[f] [J. Wang, Anal. Chem. 71 (1999) 328; I.V. Yang, P.A. Ropp, H.H. Thorp, Anal. Chem. 74 (2002) 347] phenanthroline, was synthesized and characterized by infrared spectrometry (IR). Cyclic voltammetry (CV) was used to investigate the interaction between [Co(phen) 2 IP] 2+ and salmon sperm DNA. [Co(phen) 2 IP] 2+ had excellent electrochemical activity on the glassy carbon electrode (GCE) with a couple quasi-reversible redox peaks. In 0.2M pH 4.0 Britton–Robinson (B–R) buffer solution, the binding ratio between [Co(phen) 2 IP] 2+ and salmon sperm DNA was calculated to be 1:1 and the binding constant was 3.74×10 5 Lmol −1 . A human immunodeficiency virus (HIV) DNA biosensor was developed by immobilizing covalently single-stranded HIV DNA fragments to a modified glassy carbon electrode (GCE). The surface hybridization of the immobilized single-stranded HIV DNA fragment with its complementary DNA fragment was evidenced by electrochemical methods using [Co(phen) 2 IP] 2+ as a novel electrochemical indicator, with a detection limit of 27 pmol and a linear range from 1.6×10 −10 to 6.2×10 −9 mol. Selective determination of complementary ssDNA was achieved using differential pulse voltammetry (DPV).