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

The anion zinc-based metal-organic framework [HDMA](2n)[Zn-2n(DMA)(2n)(1,4-BDC)(3n)]center dot 2nDMF center dot 6nH(2)O (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 mu M with a linear range between 50 and 600 mu M for Cd2+. Furthermore, it can be used to detect Cd2+ in tap water and rice samples directly. Solid state C-13 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, O-2 and N-2 at 30 bar respectively.