Lawrence Berkeley National Laboratory

We have designed and synthesized an isoreticular series of luminescent metal-organic frameworks (LMOFs) by incorporating a strongly emissive molecular fluorophore and functionally diverse colinkers into Zn-based structures. The three-dimensional porous networks of LMOF-261, -262, and -263 represent a unique/new type of nets, classified as a 2-nodal, (4,4)-c net (mot-e type) with 4-fold, class IIIa interpenetration. All compounds crystallize in a body-centered tetragonal crystal system (space group I4₁/a). A systematic study has been implemented to analyze their interactions with heavy metals. LMOF-263 exhibits impressive water stability, high porosity, and strong luminescence, making it an excellent candidate as a fluorescent chemical sensor and adsorbent for aqueous contaminants. It is extremely responsive to toxic heavy metals at a parts per billion level (3.3 ppb Hg²⁺, 19.7 ppb Pb²⁺) and demonstrates high selectivity for heavy metals over light metals, with detection ratios of 167.4 and 209.5 for Hg²⁺/Ca²⁺ and Hg²⁺/Mg²⁺, respectively. Mixed-metal adsorption experiments also show that LMOF-263 selectively adsorbs Hg²⁺ over other heavy metal ions in addition to light metals. The Pb²⁺ K_SV value for LMOF-263 (55,017 M^-1) is the highest among LMOFs reported to date, and the Hg²⁺ K_SV value is the second highest (459,446 M^-1). LMOF-263 exhibits a maximum adsorption capacity of 380 mg Hg²⁺/g. The Hg²⁺ adsorption process follows pseudo-second-order kinetics, removing 99.1% of the metal within 30 min. An in situ XPS study provides insight to help understand the interaction mechanism between Hg²⁺ and LMOF-263. No other MOFs have demonstrated such a high performance in both the detection and the capture of Hg²⁺ from aqueous solution.