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

Abstract

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, <b>Cu(I)</b>-<b>tpt</b> and <b>Cu(II)</b>-<b>tpt</b> (Htpt = 5-[4(1<i>H</i>-1,2,4-triazol-1-yl)]phenyl-2<i>H</i>-tetrazole), and prepared a <b>Cu(II)</b>-<b>tpt</b>-<b>on</b>-<b>Cu(I)-tpt membrane</b> by a layer-by-layer approach ignoring the lattice mismatch problem. The first <b>Cu(I)</b>-<b>tpt</b> layer is grown on an oriented Cu₂O nanostructured array by a "one-pot" approach. The aligned second <b>Cu(II)</b>-<b>tpt</b> layer can be deposited using liquid-phase epitaxy. Notably, the prepared <b>Cu(II)</b>-<b>tpt</b>-<b>on</b>-<b>Cu(I)-tpt membrane</b> 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 <i>p</i>-arsanilic acid (<i>p</i>-ASA), the <b>Cu(II)</b>-<b>tpt</b>-<b>on</b>-<b>Cu(I)-tpt membrane's</b> ability to adsorb Cr₂O₇^2- could be used to design "on-off-on" mode fluorescence probes to detect <i>p</i>-ASA with high sensitivity (limit of detection (LOD) = 0.0556 μg L^-1). <i>p</i>-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 <b>Cu(II)</b>-<b>tpt</b>-<b>on-Cu(I)</b>-<b>tpt membrane</b> a feasible multifunctional material for pollution control and detection.