Covalent organic frameworks constructed step by step using a [(C 3 + C 2 ) + C 2 ] strategy toward fluorescence detection of Fe 3 .

A series of new covalent organic frameworks (COFs) has been constructed step-by-step using a [(C 3 + C 2 ) + C 2 ] strategy for the first time, all of which show selective fluorescence detection properties toward Fe 3+ ions with the lowest limit of detection (LOD) down to 9.6 × 10 -8 M. The fluorescence quenching is mainly attributed to the absorption competition quenching (ACQ) mechanism.