A photoelectrochemical DNA sensor for the detection of Hg2+ based on Hg2+-mediated oligonucleotide switching

Abstract

A sensitive photoelectrochemical (PEC) method for Hg²⁺ detection is reported, based on the Hg²⁺-mediated structural switch of an oligonucleotide strand and biocatalytic precipitation (BCP) on a CdS quantum dot electrode. The principle is that, in the absence of Hg²⁺, the oligonucleotide strand forms a stem-loop, thus a G-rich sequence in the strand is partially caged in the stem-loop structure and cannot fold into a G-quadruplex. While in the presence of Hg²⁺, the T–Hg²⁺–T coordination leads to the formation of another stem-loop structure and allows the G-rich sequence to fold properly and bind hemin to form a stable G-quadruplex/hemin complex, which could accelerate oxidation of 4-chloro-1-naphthol (4-CN) by H₂O₂ to yield the insoluble (BCP reaction) and insulating product benzo-4-chlorohexadienone on the photoelectrode, thereby efficiently causing the change in the photocurrent signal. This simple PEC sensor could detect aqueous Hg²⁺ at concentrations as low as 4 nM with high selectivity.