Cation exchange in a fluorescent zinc-based metal–organic framework for cadmium ion detection

Abstract

The anion zinc-based metal–organic framework [HDMA]_2n[Zn_2n(DMA)_2n(1,4-BDC)_3n]·2nDMF·6nH₂O (1, DMF = N,N-dimethylformamide, 1,4-H₂BDC = 1,4-benzenedicarboxylic acid, DMA = dimethylamine) with open channels has been used as a luminescent sensor for Cd²⁺ detection, and was obtained by solvothermal synthesis and fully characterized. It shows excellent fluorescence stability in aqueous solution, which was further enhanced with a remarkable fluorescence shift upon introduction of Cd²⁺, while negligible effects occurred for Fe²⁺, Zn²⁺, Fe³⁺, Cu²⁺, Co²⁺, Ni²⁺, Mn²⁺ and Pb²⁺, respectively. The detection limit is 9.9 μM with a linear range between 50 and 600 μM for Cd²⁺. Furthermore, it can be used to detect Cd²⁺ in tap water and rice samples directly. Solid state ¹³C NMR spectra give direct evidence for the ion exchange between protonated dimethylamine and Cd²⁺ hydrates, which results in the increase of fluorescence and blue shift of the wavelength. The coordination of Cd²⁺ with the carboxyl groups of 1 in the interface of nano-channels can be also observed using IR spectra, where a structural model has been suggested in the confined channels. This may provide a new construction method for anion fluorescent MOF materials, where a confined sample has obvious influence on the detection of Cd²⁺. The maximum adsorption capacities are 10.36, 54.53, 103.06 and 8.78 mg g⁻¹ for CH₄, CO₂, O₂ and N₂ at 30 bar respectively.