A benzothiadiazole-decorated UiO-68 metal-organic framework for diclofenac and ibuprofen luminescence sensing and adsorption in wastewater

The ZrIV mixed-linker metal-organic framework (MIXMOF) [Zr6O4(OH)7(H2O)3(TPDC)3(BTDZ)1.5] [Zr_BTDZ; H2TPDC = (p-terphenyl)-4,4 ''-dicarboxylic acid, H2BTDZ = 4,4 '-(benzothiadiazole-4,7-diyl)dibenzoic acid], exhibiting an UiO-68-type crystal structure, was prepared through direct synthesis under solvothermal conditions. Zr_BTDZ is fully microporous, with pore sizes in the range of 18 <= w <= 22 & Aring; and a BET SSA of 3770 m2 g-1. The benzothiadiazole ring imparts luminescence to the material, showing a very intense ligand-centered emission band at lambda max = 516 nm (upon UV excitation at lambda = 328 nm) falling in the light green visible region. Zr_BTDZ was exploited as a luminescent sensor and an adsorbent of the pharmaceuticals diclofenac sodium (DCF) and ibuprofen sodium (IBR) in aqueous solutions. No lambda max variation occurs upon interaction with the drugs; while for DCF an emission intensity decrease is observed with increasing pollutant concentration in solution, the opposite holds for IBR. The limit of detection is 4.1 x 10-6 and 1.6 x 10-6 M for DCF and IBR, respectively, while the maximum adsorption capacity at ambient temperature (Xm) is 100 and 161 mg g-1 for DCF and IBR, respectively. The non-covalent host-guest interactions were disclosed through DFT optimizations of the [DCF@Zr_BTDZ] and [IBR@Zr_BTDZ] adducts; hydrogen bonding between the mu-OH groups of the [Zr6] inorganic building unit and the DCF/IBR carboxylate groups occurs, together with pi-pi T-shaped/pi-cation interactions between the aromatic rings/Na+ ion of IBR/DCF and the MOF organic linkers. The calculated adduct formation energies (Delta EDCF = -192.5 kJ mol-1; Delta EIBR = -228.2 kJ mol-1) are in line with the experimentally derived binding constants [Kb(DCF) = 12.6 x 104 M-1; Kb(IBR) = 14.9 x 104 M-1], showing a stronger interaction with IBR.