Improving the Cd2+detection capability of a new anionic rare earth metal-organic framework based on a [RE6(μ3-OH)8]10+secondary building unit: an ion-exchange approach towards more efficient sensors

The synthesis and characterization of a new family of rare earth (RE) MOFs based on a hexanuclear (RE3+)(6)secondary building unit (SBU) and the angular dicarboxylic ligand 4,4 '-oxybisbenzoic acid (H(2)OBA) with the formula {((CH3)(2)NH2)(2)[RE6(mu(3)-OH)(8)(OBA)(6)]}(infinity)(((CH3)(2)NH2)(2)(1)(RE); RE: Y, Tb, Dy, Ho) is reported. Gas sorption studies indicated a moderate Langmuir area of 377 m(2)g(-1). Photoluminescence studies in the visible region of the ((CH3)(2)NH2)(2)(1)(Y) doped with 5% Eu(3+)or 5% Tb(3+)revealed the presence of the characteristic emission bands for these RE(3+)ions. Cd(2+)sensing studies of ((CH3)(2)NH2)(2)(1)(Y) doped with Eu(3+)however, indicated slow exchange kinetics prohibiting the use of this material for sensing applications. Successful counter ion exchange of the as synthesized compound with Na(+)and Mg(2+)led to the isolation of Na-2(1)(Y) and Mg(1)(Y). Interestingly, the exchanged analogue Na-2(1)(Y) doped with 5% Eu(3+)exhibited fast Cd(2+)sorption kinetics and was proven to be a greatly improved sensor for this metal ion both in terms of kinetics and sensitivity. Batch Cd(2+)sorption experiments at room temperature of Na-2(1)(Y) revealed a maximum sorption capacity of 63 +/- 2 mg g(-1)corresponding to the insertion of similar to 1.25 equivalents Cd(2+)per (Y3+)(6)SBU. Overall this work demonstrates the post-synthetic ion exchange of the bulky ((CH3)(2)NH2)(+)counter-ions by inorganic cations as an excellent method for the improvement of the sensing capability of RE-based anionic MOFs.