Effective detection of Ag+, Hg2+ and dye adsorption properties studies of Ln-MOFs based on a benzimidazole carboxylic acid ligand

Abstract

Five new lanthanide metal–organic frameworks (Ln-MOFs), namely {[Ln(L)]·Cl}_n, [Ln = Pr(1), Nd(2), Eu(3), Ho(4), Ce(5)], based on a benzimidazole carboxylic acid ligand [H₂L = 2-(2-carboxyphenyl)-1H-benzo[d]imidazole-6-carboxylic acid] were synthesized by a solvothermal method. Ln-MOFs 1–5 have the same two-dimensional layered structures. Interestingly, 1–5 exhibit excellent adsorption performance to anionic dye Congo red (CR), with adsorption capacities of 2724 mg g⁻¹, 2719 mg g⁻¹, 2718 mg g⁻¹, 327 mg g⁻¹, and 2273 mg g⁻¹, respectively. Adsorption kinetics experiments showed that this kind of adsorption belonged to chemisorption; the hydrogen bonding and π–π interactions between the 1–5 host and CR guest molecules resulted in a high adsorption capacity. Luminescence and sensing experiments showed that 5 can be considered a promising multifunctional fluorescent sensor with good reusability and a high sensitivity toward Ag⁺ and Hg²⁺ ions, with detection limits of 1.7 × 10⁻⁷ and 2.5 × 10⁻⁶ M, respectively.