T-T Mismatch-Based Electrochemical Aptasensor for Ultratrace Level Detection of Hg²⁺ Using Electrochemically Reduced...

Ultratrace levels of mercury ions (Hg²⁺) were quantified utilizing differential pulse voltammetry (DPV) in aqueous solution. This method utilized a thymine (T)-rich, methylene blue-tagged DNA (MBD) and an electrochemically-reduced graphene oxide-modified glassy carbon electrode (ERGO-GCE). The ERGO-GCE was prepared by the direct reduction of graphene oxide (GO) solution on a glassy carbon electrode (GCE) using cyclic voltammetry. The MBD, as a sensing motif, was then coated on the surface of the ERGO-GCE through π-π stacking. In the presence of Hg²⁺, mercury-mediated coordination of the T-Hg²⁺-T base pairs caused folding of the MBD, leading to a hairpin structure. This resulted in subsequent unbinding of MBD from the surface of the ERGO-GCE, which produced a change in redox current of the MB tag. DPV of the modified electrode showed that the MB signal decreased linearly with increases in Hg²⁺ concentrations in the range between 1 fM and 100 nM with a detection limit of 0.16 fM. This sensor also exhibited excellent selectivity for Hg²⁺ against interfering metal ions. In addition, the sensor could be regenerated by decoupling the T-Hg²⁺-T in cysteine, which unfolded the MBD.