Reactions of La(NO)·6HO with the polar, tritopic quaternized carboxylate ligands N-carboxymethyl-3,5-dicarboxylpyridinium bromide (HCmdcpBr) and N-(4-carboxybenzyl)-3,5-dicarboxylpyridinium bromide (HCbdcpBr) afford two water-stable metal-organic frameworks (MOFs) of {[La(Cmdcp)(HO)]} (1, 3D) and {[La(Cbdcp)(HO)]} (2, 2D). MOFs 1 and 2 absorb the carboxyfluorescein (FAM)-tagged probe DNA (P-DNA) and quench the fluorescence of FAM via a photoinduced electron transfer (PET) process. The nonemissive P-DNA@MOF hybrids thus formed in turn function as sensing platforms to distinguish conservative linear, single-stranded RNA sequences of Sudan virus with high selectivity and low detection limits of 112 and 67 pM, respectively (at a signal-to-noise ratio of 3). These hybrids also exhibit high specificity and discriminate down to single-base mismatch RNA sequences.

A 2D metal-organic framework (MOF) of {[Cu(Dcbb)(Bpe)]·Cl} (1, HDcbbBr = 1-(3,5-dicarboxybenzyl)-4,4'-bipyridinium bromide, Bpe = trans-1,2-bis(4-pyridyl)ethylene)) has been prepared. MOF 1 associates with the thymine-rich (T-rich), single-stranded probe DNA (ss-DNA, denoted as P-DNA) labeled with fluorophore FAM (FAM = carboxyfluorescein) and quenches the FAM emission to give a nonemissive P-DNA@1 hybrid (off state). The P-DNA in the hybrid subsequently captures the Hg to give a rigid double-stranded DNA featuring T-Hg-T motif (ds-DNA@Hg) and detach from MOF 1, triggering the recovery of the FAM fluorescence (on state). Upon subsequent addition of I, Hg was further sequestrated from the ds-DNA@Hg duplex, driven by the stronger Hg-I coordination. The released P-DNA is resorbed by MOF 1 to regain the initial P-DNA@1 hybrid (off state). The P-DNA@1 sensor thus detects Hg and I sequentially via a fluorescence "off-on-off" mechanism. The sensor is highly selective and sensitive, yielding detection limits of 3.2 and 3.3 nM, respectively. The detection process was conformed by circular dichroism (CD) and the detection mechanism was verified by fluorescence anisotropy, binding constant, and simulation of the binding free energy at each stage.

A 3D Tb(iii)-based metal–organic framework has been synthesized as a luminescent sensor for the selective and reversible detection of Fe3+ and ascorbic acid in water. Such sensing event showed good potential in biologically relevant applications.

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We herein report an efficient Ag+ and S2− dual sensing scenario by a three-dimensional Cu-based metal–organic framework shielded with a 5-carboxytetramethylrhodamine-labeled C-rich single-stranded DNA as a fluorescent probe.