Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets: Preparation and Application in Highly Sensitive and Selective DNA Detection

The ability to prepare ultrathin
two-dimensional (2D) covalent
organic framework (COF) nanosheets (NSs) in high yield is of great
importance for the further exploration of their unique properties
and potential applications. Herein, by elaborately designing and choosing
two flexible molecules with C3v molecular symmetry as building units, a novel imine-linked
COF, namely, TPA-COF, with a hexagonal layered structure and sheet-like
morphology, is synthesized. Since the flexible building units are
integrated into the COF skeletons, the interlayer stacking becomes
weak, resulting in the easy exfoliation of TPA-COF into ultrathin
2D NSs. Impressively, for the first time, the detailed structural
information, i.e., the pore channels and individual building units
in the NSs, is clearly visualized by using the recently developed
low-dose imaging technique of transmission electron microscopy (TEM).
As a proof-of-concept application, the obtained ultrathin COF NSs
are used as a novel fluorescence sensing platform for the highly sensitive
and selective detection of DNA.