MOF-on-MOF Membrane with Cascading Functionality for Capturing Dichromate Ions and p-Arsanilic Acid Turn-On Sensing

It is very significant that functional porous metal-organic frameworks are used to manufacture hierarchical components to achieve cascading functions that cannot be achieved by a single-layer metal-organic framework (MOF). Here, we report two cases of novel MOFs constructed by the same ligand, Cu(I)-tpt and Cu(II)-tpt (Htpt = 5-[4(1H-1,2,4-triazol-1-yl)]phenyl-2H-tetrazole), and prepared a Cu(II)-tpt-on-Cu(I)-tpt membrane by a layer-by-layer approach ignoring the lattice mismatch problem. The first Cu(I)-tpt layer is grown on an oriented Cu₂O nanostructured array by a "one-pot" approach. The aligned second Cu(II)-tpt layer can be deposited using liquid-phase epitaxy. Notably, the prepared Cu(II)-tpt-on-Cu(I)-tpt membrane combines adsorption and fluorescence sensing, which exhibited significant adsorption for Cr₂O₇^2- (203.25 mg g^-1) as typical highly poisonous ions with a fluorescence quenching response. Hence, based on the oxidation-reduction between Cr₂O₇^2- and p-arsanilic acid (p-ASA), the Cu(II)-tpt-on-Cu(I)-tpt membrane's ability to adsorb Cr₂O₇^2- could be used to design "on-off-on" mode fluorescence probes to detect p-ASA with high sensitivity (limit of detection (LOD) = 0.0556 μg L^-1). p-ASA can be degraded into highly toxic inorganic arsenic compounds in the natural environment and has received widespread attention. Therefore, the integration of adsorption and fluorescence properties makes the Cu(II)-tpt-on-Cu(I)-tpt membrane a feasible multifunctional material for pollution control and detection.