Electrochemically active DNA ligands for gene detection: present and future

Electrochemical gene sensing methods are gaining attention as diagnostic chips. Here, we review the electrochemically active DNA ligand-based sensing methods. Various DNA ligands have been reported in these studies, among which metal complexes, methylene blue, and ferrocenyl naphthalene diimide (FND) have been studied in detail. DNA probe immobilized electrodes have been created, hybridization reactions on the electrodes with target DNA fragments have been performed, and electrochemical gene detection has been possible using these DNA ligands. An example of the realization of this system is the successful and accurate cancer diagnosis using FND to examine abnormal methylation of the hTERT gene, providing reassurance about the system's reliability. In addition, electrochemical detection of PCR products has been realized using the current decrease due to the double-stranded DNA binding of methylene blue although it is a signal-off system. A naphthalene diimide derivative with ferrocene and β-CD, FNC, increased the current upon double-stranded DNA binding. Using these FNCs, the detection of PCR products in a homogeneous system was realized. Electrochemical qPCR was realized with these ligands. Since FNDs also bind strongly to tetraplex or G-quadruplex (G4) DNA, we succeeded in electrochemically detecting telomerase activity, which is known as a cancer marker, using FNDs to detect the amount of telomeric DNA elongation, which is its substrate, as the amount of G4 DNA. This technique has realized compassionate cancer diagnosis from oral swab fluid. It is known that G4 is also present in viral genome RNA, and a viral testing method using G4 is expected to be a potential alternative to PCR. The first example was the electrochemical detection of novel coronaviruses using incFND as an RNA G4 ligand.