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.

本研究以含两个伯氨基与一个仲氨基的三嗪基二羧酸配体的定向设计为基础，通过两种不同溶剂热条件合成了一例胺功能化、兼具自发荧光与极性特征的三维金属有机框架（metal organic framework, MOF）{[Cd(ATAIA)]·4H2O}n（1），其中H2ATAIA为5-((4,6-二氨基-1,3,5-三嗪-2-基)氨基)间苯二甲酸，并对其完成了结构表征。单晶X射线衍射分析显示，化合物1结晶于正交晶系极性空间群Fdd2，每个ATAIA配体作为线性连接体与四个镉(II)中心相连，构筑出以双螺旋金属链为重复单元的三维框架结构。通过元素分析、紫外-可见光谱、傅里叶变换红外光谱及热重分析对化合物1进行了进一步表征；粉末X射线衍射证实了其体相纯度以及在酸性、碱性水溶液中的稳定性。场发射扫描电子显微镜与高分辨透射电子显微镜图像表明，化合物1由纳米花瓣自组装形成微花结构。采用脱溶剂活化后的框架材料，开展了氮气（N2）、氢气（H2）、二氧化碳（CO2）等不同气体以及水、苯（Bz）、环己烷（Cy）等极性与非极性溶剂的吸附性能研究。CO2吸附等温线在298 K与273 K下呈现I型等温行为，在195 K下则表现为IV型等温行为。在195 K时其CO2吸附量可达129.2 cm³·g⁻¹（25.62 wt%）；由于吸附质-吸附剂间存在强相互作用，零覆盖度下吸附等容热（Qst）为37.5 kJ·mol⁻¹，远高于其他功能化MOF，因此其对CO2的吸附选择性优于氮气（77 K）与氢气（77 K）。通过理想吸附溶液理论对CO2/N2（15:85）混合气体进行计算，在100 kPa下其选择性值分别为54.08（298 K）与46.96（273 K）。作为核心应用场景，活化后的化合物1可用于水中2,4,6-三硝基苯酚（TNP）的选择性超快检测，检出限低至0.94 nM（0.2 ppb），优于此前所有已报道的检测结果。传感实验证实了该材料优异的循环稳定性与可重复性；通过时间分辨荧光研究与密度泛函理论计算阐明了其传感作用机理。此外，利用荧光显微镜开展的气相TNP实时传感原型实验，可实现简便的比色可视化监测。