Development of a split G-quadruplex and DAPI-based fluorescent probe for Hg() and Pb() ions detection

A novel thymine- and guanine-rich oligonucleotide (ODN-7) was engineered explicitly for the detection of Hg( ii ) and Pb( ii ) by a single intercalated dye 4′,6-diamidinyl-2-phenylindole (DAPI). Upon the introduction of Hg( ii ), a rapid formation of T-Hg( ii )-T base pairs takes place, triggering the assembly of a split G-quadruplex structure, resulting in a strong fluorescence signal due to DAPI intercalating into the T-Hg( ii )-T mismatch. The introduction of Pb( ii ) initiates an interaction with the split G-quadruplex, causing a significant conformational change in its structure. Consequently, the altered split G-quadruplex structure fails to facilitate the insertion of DAPI into the T-Hg( ii )-T complexes, leading to fluorescence quenching. This strategy offers a straightforward means of detecting Hg( ii ) and Pb( ii ). Leveraging the split G-quadruplex, the ODN-7 sensor enables the detection limits (3 σ ) for Hg( ii ) and Pb( ii ) to reach an impressive low of 0.39 nM and 4.98 nM, respectively. It exhibited a favorable linear range of 0.39-900 nM for Hg( ii ) detection ( R 2 = 0.9993) and 4.98 nM-5 μM for Pb( ii ) determination ( R 2 = 0.9953), respectively. Furthermore, the proposed sensor had excellent selectivity for detecting Hg( ii ) and Pb( ii ). It was used in milk samples containing mixed Hg( ii ) and Pb( ii ) solutions, yielding recovery rates of 99.3-103.8% for Hg( ii ) detection and 100.1-104.1% for Pb( ii ) detection. Schematic illustration of the designed nucleic acid fluorescent probes (ODN-7) for the detection of Hg( ii ) and Pb( ii ) using T-Hg( ii )-T, G-quadruplex, and DAPI.