A transition metal complex as an electrochemical probe of a DNA sensor must have an applicable redox potential, high binding affinity and chemical stability. Some complexes with the dipyrido[3,2-a:2 ' ,3 ' -c]phenazine (DPPZ) ligand have been reported to have high binding affinity for DNA. However, it was difficult to detect the targeted DNA electrochemically using these complexes because of the relatively high redox potential. In this work, a combination of bipyridine ligands with functional groups (-NH2, -CH3 and -COOH) and the DPPZ ligand were studied. The introduction of electron-donating groups was effective for controlling the redox potential of the DPPZ-type osmium complex. The [Os(DA-bPY)(2)DPPZ](2+) complex (DA-bpy; 4,4 ' -diamino-2,2 ' -bipyridine) had a lower half-wave potential (E-1,E-2) of 147 mV (vs. Ag\AgCl) and higher binding affinity with DNA {binding constant, K = 3.1 x 10(7) M-1 in 10 mmol dm(-3) Tris-HCl buffer with 50 mmol dm(-3) NaCl (pH 7.76)} than those of other complexes. With the single stranded DNA (ssDNA) modified gold electrode, the hybridization signal (DeltaI) of the [Os(DA-bpy)(2)DPPZ](2+) complex was linear in the concentration range of 1.0 pg ml(-1)-0.12 mug ml(-1) for the targeted DNA with a regression coefficient of 0.999. The detection limit was 0.1 pg ml(-1).