A Highly Selective MOF-Based Probe for Turn-On Luminescent Detection of Al³⁺, Cr³⁺, and Fe³⁺ in Solution and Test Paper Strips through Absorbance Caused Enhancement Mechanism

Abstract

Trivalent metal ions (Cr 3+ , Al 3+ , and Fe 3+ ) constitute a major section of the environmental pollutants, and their excess accumulation has a detrimental effect on health, so their detection in trace quantity has been a hot topic of research. A highly scalable 3D porous Zn-based luminescent metal− organic framework (MOF) has been synthesized by exploiting the mixed ligand synthesis concept. The strategic selection of an aromatic π-conjugated organic linker and N-rich spacer containing the azine functionality as metal ion binding sites immobilized across the pore spaces, have made this MOF an ideal turn-on sensor for Al 3+ , Cr 3+ , and Fe 3+ ions with very high sensitivity, selectivity, and recyclability. An in-depth study revealed absorbance caused enhancement mechanism (ACE) responsible for such turn-on phenomena. In order to make the detection process straightforward, convenient, portable, and economically viable, we have fabricated MOF test paper strips (the MOF could be simply immobilized onto the paper strips) for naked eye visual detection under UV light, which, thus, manifests its potential as a real-time smart sensor for these trivalent ions.

Eu(III)-Functionalized MOF-Based Dual-Emission Ratiometric Sensor Integrated with Logic Gate Operation for Efficient Detection of Hippuric Acid in Urine and Serum

“…In addition, the effective encapsulation of Eu 3+ ions in Eu 3+ @NiMOF was also verified by XPS (Figure a), in which the characteristic peaks at 1135.36 and 1164.88 eV can be assigned to the 3d 5/2 and 3d 3/2 states of Eu 3+ ions (Figure b), respectively. , When coordination interaction occurs, the electrons of O atoms will shift to Eu 3+ ions, and charge dispersion will inevitably lead to an enhancement of the O atom binding energy. Thus, the high-resolution O 1s binding energy signals shifted from 531.48 eV (C–O) and 533.13 eV (CO) for NiMOF (Figure c) and to 531.66 eV (C–O) and 533.37 eV (CO) for Eu 3+ @NiMOF (Figure d), and the C–O/CO ratio changes confirmed that the introduced Eu 3+ ions were effectively encapsulated through the coordination interactions with the uncoordinated carboxyl groups, , which was also proved by the desorption experiments and IR spectra. Eu 3+ @NiMOF showed little decrease in fluorescence intensity after stirring in aqueous solution overnight, indicating that the introduction of Eu 3+ ions occurred chemically rather than by simple physical adsorption (Figure S7).…”

Metal–organic framework (MOF)-based fluorescence “turn-on” sensors

“…Although the Fe 3+ ion is a very important element in living organisms, 205,206 its excess leads to harmful health effects such cardiac arrest and Alzheimer's disease. 207,208 Hence, the detection and on-site monitoring of Fe 3+ ions are critical.…”

A water stable and highly fluorescent Zn(ii) based metal–organic framework for fast detection of Hg2+, CrVI, and antibiotics

“…Moreover, some LMOFs with highly π-electron rich ligands prepared by the mixed ligand assembly strategy usually show exclusive luminescence properties. [60][61][62][63] Fluorene derivatives have been widely applied as luminescent materials with high fluorescence quantum yield. 64,65 In addition, as a chromogenic group, the -NH 2 group can act as a Lewis base with the ability to stabilize metal ions because of the strong interaction between the lone pair electrons of -NH 2 and the d-orbitals of metal ions.…”