Methylene Blue as a G‑Quadruplex Binding Probe for Label-Free Homogeneous Electrochemical Biosensing

Herein, G-quadruplex sequence was found to significantly decrease the diffusion current of methylene blue (MB) in homogeneous solution for the first time. Electrochemical methods combined with circular dichroism spectroscopy and UV–vis spectroscopy were utilized to systematically explore the interaction between MB and an artificial G-quadruplex sequence, EAD2. The interaction of MB and EAD2 (the binding constant, K ≈ 1.3 × 10⁶ M^–1) was stronger than that of MB and double-stranded DNA (dsDNA) (K ≈ 2.2 × 10⁵ M^–1), and the binding stoichiometry (n) of EAD2/MB complex was calculated to be 1.0 according to the electrochemical titration curve combined with Scatchard analysis. MB was proved to stabilize the G-quadruplex structure of EAD2 and showed a competitive binding to G-quadruplex in the presence of hemin. EAD2 might mainly interact with MB, a positive ligand of G-quadruplex, through the end-stacking with π-system of the guanine quartet, which was quite different from the binding mechanism of dsDNA with MB by intercalation. A novel signal read-out mode based on the strong affinity between G-quadruplex and MB coupling with aptamer/G-quadruplex hairpin structure was successfully implemented in cocaine detection with high specificity. G-quadruplex/MB complex will function as a promising electrochemical indicator for constructing homogeneous label-free electrochemical biosensors, especially in the field of simple, rapid, and noninvasive biochemical assays.