Covalent Organic Framework Nanosheets for Fluorescence Sensing via Metal Coordination

Covalent organic framework nanosheets (COF NSs) provide well-ordered π–π structures that can be used to develop luminescent materials. However, most COF NSs have problems of weak luminescence and low fluorescence quantum yield. In this work, we prepared covalent organic framework nanosheets (Bpy-NSs) with good water dispersibility, nitrogen-rich functional groups, and regular pore structure. We explored the coordination of Bpy-NSs with Al3+ to eliminate the fluorescence quenching process caused by photoinduced electron transfer (PET). Thus, the fluorescence "turn-on" signal linearly increases with Al3+ concentration, achieving a 15.7-fold improved in fluorescence, and the absolute fluorescence quantum yield increased from 0.15 to 1.74%. Furthermore, this is the first COF fluorescence sensor that can be used for high sensitivity and selectivity detection of Al3+ in the aqueous phase. We anticipate that the expansion of metal ions coordination strategy in the aqueous phase will not only significantly enhance the fluorescence of COF NSs but also extend the functional range of COF NSs.