Low-background, highly sensitive DNA biosensor by using an electrically neutral cobalt(II) complex as the redox hybridization indicator.

An electrically neutral cobalt complex, [Co(GA)2(phen)] (GA = glycollic acid, phen = 1,10-phenathroline), was synthesized and its interactions with double-stranded DNA (dsDNA) were studied by using electrochemical methods on a glassy carbon electrode (GCE). We found that [Co(GA)2(phen)] could intercalate into the DNA duplex through the planar phen ligand with a high binding constant of 6.2(±0.2)×10(5)  M(-1). Surface studies showed that the cobalt complex could electrochemically accumulate within the modified dsDNA layer, rather than within the single-stranded DNA (ssDNA) layer. Based on this feature, the complex was applied as a redox-active hybridization indicator to detect 18-base oligonucleotides from the CaMV35S promoter gene. This biosensor presented a very low background signal during hybridization detection and could realize the detection over a wide kinetic range from 1.0×10(-14)  M to 1.0×10(-8)  M, with a low detection limit of 2.0 fM towards the target sequences. The hybridization selectivity experiments further revealed that the complementary sequence, the one-base-mismatched sequence, and the non-complementary sequence could be well-distinguished by the cobalt-complex-based biosensor.