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 pi-pi 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 mu M), the fluorescence of NCDs is quenched but the fluorescence of COFTAPT-TT remains unchanged. The linear range is 51.9 nM to 0.75 mu M, 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 mu M. This work provides a new idea for application of COFs with weak emission to construct off-on fluorescent sensors.

Automated summary

This research successfully prepared nitrogen-doped carbon dots encapsulated COFTAPT-TT through a one-pot method, in which NCDs led spherical stacked COFTAPT-TT to self-exfoliate into NCCOFTAPT-TT nanosheets, greatly improving its dispersion and stability. In the presence of copper ions (0-10.0 μM), the fluorescence of NCDs was quenched while the fluorescence of COFTAPT-TT remained unchanged, realizing a Cu2+ off-on ratiometric fluorescent sensor.