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

Abstract

The Zr^IV 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 Å 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 λ_max = 516 nm (upon UV excitation at λ = 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 λ_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 × 10⁻⁶ and 1.6 × 10⁻⁶ M for DCF and IBR, respectively, while the maximum adsorption capacity at ambient temperature (X_m) 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 μ-OH groups of the [Zr₆] inorganic building unit and the DCF/IBR carboxylate groups occurs, 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 (ΔE_DCF = −192.5 kJ mol⁻¹; ΔE_IBR = −228.2 kJ mol⁻¹) are in line with the experimentally derived binding constants [K_b(DCF) = 12.6 × 10⁴ M⁻¹; K_b(IBR) = 14.9 × 10⁴ M⁻¹], showing a stronger interaction with IBR.