Reversible Fluorescence Switching of Metal–Organic Framework Nanoparticles for Use as Security Ink and Detection of Pb²⁺ Ions in Aqueous Media

To date, numerous materials, including various quantum dots and dyes, have been widely used for the ultrasensitive detection of toxic metal ions and as security inks to hide information. Nevertheless, because of the poor dispersibility of solid-state materials, security inks based on such materials have been scarcely reported. Herein, a highly dispersible and water-stable metal–organic framework (MOF; NH2-MIL-125(Ti)) is used as an invisible security ink for data coding, encryption, and decryption via its “turn-on/off” switching by treatment with ethylenediaminetetraacetic acid and Pb2+. Notably, the concentration of the Pb2+ solution used to turn off the fluorescence of the MOF was lower than the limit established by several regulatory agencies for drinking water. The MOF was also used as a sensitive probe for the rapid and ultrasensitive detection of Pb2+ ions at a concentration of 7.7 pM which is one of the lowest detection limits reported for such a system. The MOF also shows high selectivity for various transition metal ions that can competitively bound on the ligand. Analyses using Fourier transform infrared spectroscopy,X-ray photoelectron, and UV photoemission spectroscopy clearly revealed the roles of the surface functional groups and the mechanism of the “on/off” switching behavior of the MOF.