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

Abstract

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 (H₂OBA) with the formula {((CH₃)₂NH₂)₂[RE₆(μ₃-OH)₈(OBA)₆]}_∞ (((CH₃)₂NH₂)₂(1)(RE); RE: Y, Tb, Dy, Ho) is reported. Gas sorption studies indicated a moderate Langmuir area of 377 m² g⁻¹. Photoluminescence studies in the visible region of the ((CH₃)₂NH₂)₂(1)(Y) doped with 5% Eu³⁺ or 5% Tb³⁺ revealed the presence of the characteristic emission bands for these RE³⁺ ions. Cd²⁺ sensing studies of ((CH₃)₂NH₂)₂(1)(Y) doped with Eu³⁺ 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 Mg²⁺ led to the isolation of Na₂(1)(Y) and Mg(1)(Y). Interestingly, the exchanged analogue Na₂(1)(Y) doped with 5% Eu³⁺ exhibited fast Cd²⁺ sorption kinetics and was proven to be a greatly improved sensor for this metal ion both in terms of kinetics and sensitivity. Batch Cd²⁺ sorption experiments at room temperature of Na₂(1)(Y) revealed a maximum sorption capacity of 63 ± 2 mg g⁻¹ corresponding to the insertion of ∼1.25 equivalents Cd²⁺ per (Y³⁺)₆ SBU. Overall this work demonstrates the post-synthetic ion exchange of the bulky ((CH₃)₂NH₂)⁺ counter-ions by inorganic cations as an excellent method for the improvement of the sensing capability of RE-based anionic MOFs.