Colorimetric and Photocurrent-Polarity-Switching Photoelectrochemical Dual-Mode Sensing Platform for Highly Selective Detection of Mercury Ions Based on the Split G-Quadruplex–Hemin Complex

Mercury ion (Hg²⁺) is one of the most harmful heavy metal ions with the greatest impact on public health. Herein, based on the excellent catalytic activity toward 3,3',5,5'-tetramethylbenzidine (TMB) and the strong photocurrent-polarity-switching ability to SnS₂ photoanode of the split G-quadruplex-hemin complex, the magnetic NiCo₂O₄@SiO₂-NH₂ sphere-assisted colorimetric and photoelectrochemical (PEC) dual-mode sensing platform was developed for the Hg²⁺ assay. First, the amino-labelled single-stranded DNA1 (S1) was immobilized on NiCo₂O₄@SiO₂-NH₂ and then partly hybridized with another single-stranded DNA2 (S2). When Hg²⁺ was present, the thymine-Hg²⁺-thymine base pairs between S1 and S2 were formed, causing the formation of the split G-quadruplex in the presence of K⁺. After addition of hemin, the split G-quadruplex-hemin complex was obtained and effectually catalyzed the H₂O₂-mediated oxidation of TMB. Thus, the color and absorbance intensity of the TMB solution were changed, resulting in the visual and colorimetric detection of Hg²⁺. The linear response range is 10 pM to 10 nM, and the detection limit is 3.8 pM. Meanwhile, the above G-quadruplex-hemin complex effectively switched the photocurrent polarity of SnS₂-modified indium tin oxide electrode, leading to the sensitive and selective PEC assay of Hg²⁺ with a linear response range of 5 pM to 500 nM and a detection limit of 2.3 pM. Moreover, the developed dual-mode sensing platform provided mutual authentication of detection results in different modes, effectively improving the assay accuracy and confidence, and may have a good potential application in highly sensitive, selective, and accurate determination of Hg²⁺ in environmental fields.