Self-Exfoliating Double-Emission N-Doped Carbon Dots in Covalent Organic Frameworks for Ratiometric Fluorescence “Off–On” Cu<sup>2+</sup> Detection
Shiqi Wang, Lulu Guo, Lili Chen, Li Wang, Yonghai Song
Abstract
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.