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

Abstract

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; NH 2 -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 Pb 2+ . Notably, the concentration of the Pb 2+ 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 Pb 2+ 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.

Recent advances in process engineering and upcoming applications of metal–organic frameworks

“… MOF compounds Sensing targets LOD Number of recycles Ref a. Metal ion sensors NH 2 -MIL-125(Ti) Pb 2+ 0.70 × 10 −2 nM 5 [405] [Cd(Hcip)(bpea) 0.5 (H 2 O)] n (Cd-CP) Fe 3+ 3.24 µM 5 [406] [Cd(Hcip)(bpea) 0.5 (H 2 O)] n (Cd-CP) Al 3+ 1.31 µM 5 [406] [Cd(Hcip)(bpea) 0.5 (H 2 O)] n (Cd-CP) Cr 3+ 1.84 µM 5 [406] MOF NU-902 Cd 2+ 0.3 ppb – [407] MOF NU-902 PVC membrane Cd 2+ – – [407] CAU-10-V-H Pd 2+ 110 nM 5 [408] MIL-53(Al)–NH2-MPA Fe 3+ 0.36 µM – [416] [[Zn(4,4′-AP)(5-AIA)]. (DMF)0.5] n Hg 2+ 0.01 nM – [409] b. Anion sensors TMU-41 Cr 2 O 7 2− 0.02 µM 4 [410] TMU-41 CrO 4 2− 0.03 µM 4 [410] …”

“…There is therefore an urgent need to develop a suitable sensor to identify high Pb concentrations. Recently, NH 2 -MIL-125(Ti) was employed as a sensing probe for the detection of Pb 2+ ions in real water samples [405] . Among various metal ions, the developed sensor showed high selectivity toward Pb 2+ and detected concentrations as low as 7.7 pM, which is one of the lowest detection limits ever reported.…”

“…46 Encryption using (a) a rubber stamp and (b) an ink pen and subsequent decryption of (i) pure NH 2 -MIL-125(Ti), (ii) with Pb 2+ addition, and (iii) after EDTA addition, as observed in the UV − vis region (260–360 nm). Reprinted with permission from [405] . Copyright 2020 American Chemical Society.…”

Nanoscale Metal-Organic Frameworks as Fluorescence Sensors for Food Safety

“…Detection of Pb 2+ in food and drinking water is of vital importance in our daily life. A Titanium-based NMOF (NH 2 -MIL-125) with 2-aminoterephthalic acid (NH 2 -H 2 BDC) as an organic ligand was recently reported for the ultra-sensitive detection of Pb 2+ [ 113 ]. NH 2 -MIL-125 emits fluorescence derived from NH 2 -H 2 BDC with a high photoluminescence quantum yield through ligand-to-cluster charge transfer (LCCT).…”

“… ( a ) Schematic illustration of the process for encapsulating the lanthanide complex modified Fe 3 O 4 into ZIF-8 and concept for sensing Cu 2+ , reprinted with modifications from Wang et al [ 99 ]; ( b ) schematic illustration of the fabrication of UiO-(OH) 2 @RhB and the sensing process to Al 3+ , reprinted with modifications from Zheng et al [ 110 ]; ( c ) schematic of the fluorescence quenching and recovery mechanisms of NH 2 -MIL-125, reprinted with modifications from Venkateswarlu et al [ 113 ]. …”