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], showing an UiO-68-type crystal structure, was prepared through direct synthesis under solvothermal conditions. Zr_BTDZ is fully microporous, with pores ranging in the 18 ≤ w ≤ 22 Å size and a BET SSA of 3770 m2/g. The benzothiadiazole ring imparts luminescence to the material, showing a very intense ligand-centered emission band at λmax = 516 nm (upon UV excitation at λ = 328 nm) falling in the light green visible region. Zr_BTDZ was exploited as luminescent sensor and adsorbent of the pharmaceuticals Diclofenac Sodium (DCF) and Ibuprofen Sodium (IBR) in aqueous solutions. No λmax 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 × 10-6 and 1.6 × 10-6 M for DCF and IBR, respectively, while the maximum adsorption capacity at ambient temperature (Xm) is 100 and 161 mg/g 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 μ-OH groups of the [Zr6] inorganic building unit and the DCF/IBR carboxylate groups are present, together with π-π T-shaped / π-cation interactions between the aromatic rings / Na+ ion of IBR/DCF and the MOF organic linkers. The calculated adduct formation energies (ΔEDCF = −192.5 kJ/mol; ΔEIBR = −228.2 kJ/mol) are in line with the experimentally derived binding constants [Kb(DCF) = 12.6 × 104 M-1; Kb(IBR) = 14.9 × 104 M-1], showing a stronger interaction with IBR.