Ultrathin Fluorescent Covalent Organic Framework Nanosheets with Tunable Thickness for Highly Selective and Visual Detection of Cu ²⁺

The synthesis of ultrathin two-dimensional covalent organic framework (COF) nanosheets is of great significance for enhancing their fluorescence properties and expanding their applications in fluorescence sensing. Herein, we report a bottom-up synthesis strategy for the in situ preparation of ultrathin two-dimensional fluorescent COF (TpBpy) nanosheets. By introducing a sterically hindered trimethylaniline modulator during the synthesis, the thickness of the COF nanosheets could be precisely tuned, yielding a series of TpBpy-TEA-x nanosheets (x = 20, 25, 30, 35) with thicknesses ranging from 1.5 to 5.6 nm. All nanosheets exhibit red fluorescence centered at 620 nm, and their fluorescence intensity shows a positive correlation with the nanosheet thickness. Among them, TpBpy-TEA-30 possesses the thinnest structure (∼1.5 nm) and the strongest fluorescence emission. This material demonstrates high selectivity toward Cu2+ ions, with a linear detection range from 0.2 to 10 μmol/L and a limit of detection (LOD) as low as 0.0508 μmol/L. Moreover, TpBpy-TEA-30 maintains excellent fluorescence response performance over three consecutive sensing cycles. To enable visual detection, the COF nanosheets were deposited onto filter paper, forming a fluorescent test strip that retains strong solid-state emission and allows for the visible detection of Cu2+. This work not only provides a viable approach for fabricating highly fluorescent two-dimensional COF nanosheets but also offers insights into the application of fluorescent COFs in heavy metal ion sensing.