Strategic Design and Functionalization of an Amine-Decorated Luminescent Metal Organic Framework for Selective Gas/Vapor Sorption and Nanomolar Sensing of 2,4,6-Trinitrophenol in Water

On
the basis of the strategic design of a triazine-based dicarboxylate
ligand with two primary amino groups and one secondary amino group,
an amine-functionalized autofluorescent and polar three-dimensional
metal organic framework (MOF) {[Cd­(ATAIA)]·4H2O}n (1), where H2ATAIA
= 5-((4,6-diamino-1,3,5-triazin-2-yl)­amino)­isophthalic acid, has been
synthesized under two different solvothermal conditions and structurally
characterized. Single-crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic polar space group Fdd2, where each ATAIA ligand acts as a linear linker to connect four
Cd­(II) centers, resulting in the formation of a three-dimensional
framework with a repeat of a double helical metal chain. It has been
further characterized by elemental analysis, UV–vis and Fourier
transform infrared spectroscopy, and thermogravimetric analysis. Its
bulk phase purity and stability in aqueous acid and base solutions
are confirmed by powder X-ray diffraction. Both field emission scanning
electron microscopy and high resolution transmission electron microscopy
images of 1 reflect the formation of microflowers by
self-assembly of nanopetals. With the dehydrated framework of 1, sorption studies of different gases (N2, H2, and CO2) as well as polar and nonpolar solvents,
such as water, benzene (Bz), and cyclohexane (Cy), have been performed.
The CO2 sorption isotherm depicts type I isotherm at 298
and 273 K and type IV isotherm at 195 K. Furthermore, with an uptake
of 129.2 cm3 g–1 (25.62 wt %) at 195
K, sorption of CO2 is selective over N2 (77
K) and H2 (77 K) because of the strong adsorbate–adsorbent
interaction as clearly evident from an isosteric heat of adsorption
(Qst) at zero coverage of 37.5 kJ mol–1, which is exceptionally higher than that of other
functionalized MOFs. Using the ideal adsorption solution theory calculation
for a CO2/N2 (15:85) mixture, selectivity values
are found to be 54.08 (298 K) and 46.96 (273 K) at 100 kPa. For a
major application, activated 1 has been utilized for
selective and ultrafast detection of 2,4,6-trinitrophenol (TNP) in
water with a limit of 0.94 nM (0.2 ppb), which supersedes any previous
reported value. Excellent recyclability and stability of 1 for sensing experiments have been established. Time-resolved fluorescence
studies and density functional theory calculations have been used
to establish its mechanism of action. Furthermore, a prototype experiment
for the real-time sensing of TNP in the vapor phase by fluorescence
microscopy provides an easy colorimetric monitoring.