Strategic Construction of Highly Stable Metal–Organic Frameworks Combining Both Semi-Rigid Tetrapodal and Rigid Ditopic Linkers: Selective and Ultrafast Sensing of 4‑Nitroaniline in Water

In
this work, we have designed two new 3D metal–organic
frameworks (MOFs), {Zn4(TPOM)­(1,4-NDC)4}n (1) and {Ni2(TPOM)­(1,4-NDC)2(H2O)2}n (2), utilizing both semi-rigid tetrapodal neutral linker,
tetrakis­(4-pyridyloxymethylene)­methane (TPOM) and rigid ditopic anionic
linker, 1,4-naphthalene dicarboxylic acid (H2(1,4-NDC)).
On the basis of the single-crystal X-ray diffraction, 1 has a 3D structure with star-shaped pores arising from four-fold
symmetry due to the presence of a paddle-wheel core [Zn2(O2CC12H6)4(C6H4N)2] as a subunit, whereas 2 consists of a zig-zag 3D framework with strong hydrogen bonding
between the coordinated water molecules and coordinated carboxylate
groups. Their thermogravimetric analysis indicates an extraordinary
thermal stability: 1 up to 400 °C and 2 up to 350 °C. In addition to elemental microanalysis and spectroscopic
characterization (UV–vis and infra-red spectroscopy), the bulk
phase purity of 1 and 2 as well as hydrolytic
stability of 1 are established by powder X-ray diffraction.
Exploiting the luminescence nature of 1, both solvent-dependent
fluorescence properties and sensing of various amines in aqueous medium
are demonstrated. It exhibits good sensing ability toward 4-nitroaniline
(4-NA) and 2,6-dichloro-4-nitroaniline (2,6-DCNA; a broad spectrum
pesticide belonging to toxicity class III) with the lowest detection
limit of 88 ppb and 0.28 ppm, respectively. The mechanism of action
has been established through Stern–Volmer plots, time-resolved
fluorescence studies, spectral overlap, and density functional theory
calculations. The recyclability and stability of 1 after
sensing experiments also reveal no change in its crystallinity. Furthermore,
selectivity test and time-dependent detection for 4-NA have been successfully
demonstrated. For practical applications, naked eye detection of 4-NA
using test paper strips is also displayed.