Temperature-Dependent Preparation of Hydrogen-Bond Organic Frameworks: Ultrathin and Stable Nanosheets for Fluorescent Sensing toward Uranyl

A series of hydrogen-bond organic frameworks (HOFs) (HOF-TCPB-x, x = 298, 333, 343, 353, and 373) with different crystalline structures were prepared by a temperature-dependent method. Among them, HOF-TCPB-373 shows stable duplex H-bonds and a 2-fold interpenetrating layered structure. HOF-TCPB-373 exhibits high thermal, hydrolytic, acid, and base stabilities. Its thermal decomposition temperature in the air is 450 °C, and it can maintain the original crystal structure after being immersed in 6 M H2SO4, 6 M HCl, 6 M HNO3, 0.01 M NaOH, and boiling water. It is noteworthy that the inborn structural features of HOF-TCPB-373, including the robust and dense layers and the weak interactions between layers, make it easy to be stripped into 2D nanomaterials. Successfully, the bulk HOF-TCPB-373 was exfoliated into uniform ultrathin nanosheets with a thickness of 1.5 nm, which can be used as a sensitive UO22+ fluorescence sensor. The quantum yield of Nano-HOF-TCPB-373 is 58.40%, and the fluorescence lifetime is 1.8 ns. It can exhibit almost identical fluorescence-emission intensities under broad pH 2–9 conditions. The detection limit of Nano-HOF-TCPB-373 aqueous suspension toward uranium is 0.13 μg·L–1. Also, Nano-HOF-TCPB-373 shows apparent selectivity toward UO22+ over the common metal ions in water, such as Na+, K+, Ca2+, Sr2+, Zn2+, Ba2+, and Al3+.