Constructing a luminescent covalent organic framework (LCOF) via an ultrasound-assisted one-step reaction used for “one to many” recognition of targets is a challenge. In this work, we report a fluorescent sp2c-covalent organic framework (COF) (TzDA–BTAN–COF) containing a regular nanoporous structure, uninterrupted π-electron delocalization, and an electron-deficient unit. We explore the use of the TzDA–BTAN–COF for the visual inspection and fluorescent quantitative analyses of hazardous substances. The TzDA–BTAN–COF, as a Janus sensor, exhibited double nature with different fluorescence properties: “turn-on” and “turn-off.” Fluorescence enhancement was exhibited in the presence of electron-rich ofloxacin (OFLX) and fluorescence quenching in the presence of electron-deficient picric acid (PA) and aristolochic acid (AAS). The fluorescence-on and -off sensing mechanism might be attributed to the photoinduced electron transfer by the electron transfer from the donor to the acceptor based on theoretical calculations. In addition, the TzDA–BTAN–COF has been used for the highly sensitive fluorescence detection of PA, AAS, and OFLX. It exhibited good linearity toward PA, AAS, and OFLX over the range from 0.1 to 80, 0.01 to 100, and 0.01 to 1 μg mL–1 with a limited detection of 32.1, 3.3, and 3.0 ng mL–1, respectively. Meanwhile, the real samples were measured, with satisfactory recoveries (95.0–105.5%) and reliable analytical results. The TzDA–BTAN–COF is the first nanoporous COF Janus fluorescence sensor for the “one to many” recognition and fluorescence detection of different hazardous substances by dual functional fluorescence-on and -off sensing.

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