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. 

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

Empirical parameters of solvents describing their hydrogen-bond (HB) acidity (e.g., the Kamlet–Taft α parameter) are often difficult to determine for new solvents because they are not directly related to a single definition process. Here, we propose a simple method based on one probe, the betaine dye 30, and one reference process, the solvatochromism of this dye, measured by its first electronic transition energy, ET(30). These ET(30) values are calculated within the time-dependent density functional theory framework, using a polarizable continuum solvent model (PCM). The part of ET(30) values that is not included in the PCM calculation is taken as the HB component of the measured ET(30) values, allowing us to deduce a solvent HB acidity parameter α₁. The validity of this simple model is assessed by good linear correlations between α₁ and a variety of solute properties mainly depending on the solvent’s HB acidity. The quality of fit observed with α₁ is at least comparable with that obtained by previous solvent HB acidity scales. The simplicity of our method is illustrated by the determination of α₁ and of its companion, the electrostatic solvent parameter ES, for some new green solvents derived from glycerol.

Determination of a Solvent Hydrogen-Bond Acidity Scale by Means of the Solvatochromism of Pyridinium-N-phenolate Betaine Dye 30 and PCM-TD-DFT Calculations B

Lubrication behavior of fluorescent graphene quantum dots hybrid polyethylene glycol lubricant

Fluorescence Quenching and Highly Selective Adsorption of Ag+ Using N-Doped Graphene Quantum Dots/Poly(vinyl alcohol) Composite Membrane

Graphene quantum dots via ion modification for improving photoluminescence stability in aqueous solution with heavy metal ions

"On-off-on" fluorescence switch of graphene quantum dots: A cationic control strategy

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"On-off-on" fluorescence switch of graphene quantum dots: A cationic control strategy