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 Cr2O72– (203.25 mg g–1) as typical highly poisonous ions with a fluorescence quenching response. Hence, based on the oxidation–reduction between Cr2O72– and p-arsanilic acid (p-ASA), the Cu­(II)-tpt-on-Cu­(I)-tpt membrane’s ability to adsorb Cr2O72– 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.