Self-Exfoliating Double-Emission N-Doped Carbon Dots in Covalent Organic Frameworks for Ratiometric Fluorescence “Off–On” Cu²⁺ Detection

Covalent organic frameworks (COFs) usually have weak emission because of π–π stacking between layers, linkage bond rotation, or photoinduced electron transfer (PET) between monomer parts, which gives a chance to prepare “off–on” fluorescent sensors based on these weakly emissive COFs. Here, nitrogen-doped carbon dots (NCDs) encapsulated in COFTAPT-TT (NCCOFTAPT-TT) are successfully prepared by a one-pot method, in which COFTAPT-TT is synthesized by ammonaldehyde condensation between 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and thieno[3,2-b]thiophene-2,5-dicarboxaldehyde (TT) in the presence of NCDs. NCCOFTAPT-TT emits a strong NCD peak at 445 nm and a weak peak of COFTAPT-TT at 337 nm due to PET between electron-deficient TAPT and electron-rich TT. The NCDs lead spherical stacked COFTAPT-TT to self-exfoliate into NCCOFTAPT-TT nanosheets, which greatly improves the dispersion and stability of NCCOFTAPT-TT. In the presence of Cu2+ (0–10.0 μM), the fluorescence of NCDs is quenched but the fluorescence of COFTAPT-TT remains unchanged. The linear range is 51.9 nM to 0.75 μM, and the detection limit is 17.3 nM. Then, Cu2+ inhibits the PET process to result in an enhancement of COFTAPT-TT fluorescence and the fluorescence of NCDs remains unchanged, thus realizing a Cu2+ off–on ratiometric fluorescent sensor with high sensitivity, good selectivity, and good stability. This off–on fluorescent sensor has a linear range of 18–26 μM. This work provides a new idea for application of COFs with weak emission to construct off–on fluorescent sensors.