Scheme 1.

Schematic illustration of the ligation based electrochemical DNA biosensor for the SNP analysis on an ONS electrode. (1) Target DNA (or PCR amplicons) and ligase; (2) Washing step; (3) Avidin-HRP. Capture probes assembled on the ONS electrode had different end bases at 3′ terminals, only when the target DNA could perfectly hybridize to the capture probes and form a “sandwich” structure with the signal probes, the ligation between two probes could be performed and let the signal probes survive in the following washing step. The avidin-HRP was then combined by the biotin label and catalyzed the redox reaction of the TMB substrate.