Rapid sequential detection of Hg²⁺ and biothiols by a probe DNA-MOF hybrid sensory system

A one-dimensional (1D) metal-organic framework (MOF) of [Cu(Cdcbp)(H₂O)₂·2H₂O]_n (1, H₃CdcbpBr = 3-carboxyl-(3,5-dicarboxybenzyl)-pyridinium bromide) has been synthesized and characterized. MOF 1 features a cationic Cu²⁺ center, conjugated tricarboxylate ligand bearing positively charged pyridinium and uncoordinated carboxylate groups within its skeleton. These features enable MOF 1 to tightly adsorb thymine rich (T-rich) single-stranded DNA (ss-DNA) probe labeled with carboxyfluorescein (FAM) (denote as P-DNA) through π-stacking, electrostatic interactions and/or hydrogen bonding to give a hybrid complex (denote as P-DNA@1), and quenches its fluorescence via a photo-induced electron transfer (PET) process. The formed P-DNA@1 hybrid can thus function as a sensing platform for the detection of Hg²⁺, driven by the formation of hairpin-like double-stranded DNA (ds-DNA@Hg²⁺) with a T-Hg-T coordination motif, and subsequently dissociated into the solution due to its more rigid nature than ss-DNA, leading to the recovery of FAM fluorescence. In the presence of biothiols, including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), the strong coordination interaction between Hg²⁺ and the mercapto function serves to sequestrate the Hg²⁺ from the ds-DNA@Hg²⁺ duplex. The released ss-DNA, in turn, are re-adsorbed by MOF 1, leading to the formation of the initial P-DNA@1 state with fluorescence quenching. As such, P-DNA@1 detects Hg²⁺ and biothiols Cys/Hcy/GSH in sequence with detection limits of (2.3 ± 0.8) nM and (29.6 ± 0.1) nM/(19.8 ± 0.5) nM/(10.2 ± 0.1) nM. The sensing process is efficient and selective with instantaneous response time. The detection mechanism was further validated by circular dichroism (CD), and simulation studies using Molecular Operating Environment (MOE) package.