Development of a split G-quadruplex and DAPI-based fluorescent probe for Hg(II) and Pb(II) 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 (3σ) 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 (R² = 0.9993) and 4.98 nM–5 μM for Pb(II) determination (R² = 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.