Helpful elements: A facile bottom-up method using citric acid and L-cysteine as a precursor has been developed to prepare graphene quantum dots (GQDs) co-doped with nitrogen and sulfur. A new type and high density of surface state of GQDs arises, leading to high yields (more than 70 %) and excitation-independent emission. FLQY = fluorescence quantum yield.

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A novel sensing system has been designed for Cu(2+) ion detection based on the quenched fluorescence (FL) signal of branched poly(ethylenimine) (BPEI)-functionalized carbon quantum dots (CQDs). Cu(2+) ions can be captured by the amino groups of the BPEI-CQDs to form an absorbent complex at the surface of CQDs, resulting in a strong quenching of the CQDs' FL via an inner filter effect. Herein, we have demonstrated that this facile methodology can offer a rapid, reliable, and selective detection of Cu(2+) with a detection limit as low as 6 nM and a dynamic range from 10 to 1100 nM. Furthermore, the detection results for Cu(2+) ions in a river water sample obtained by this sensing system agreed well with that by inductively couple plasma mass spectrometry, suggesting the potential application of this sensing system.

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Nonemissive tetraphenylethene ͑TPE͒ 1 and its diphenylated derivative 2 were induced to emit intensely by aggregate formation. Crystalline aggregates of the dyes emitted bluer lights than their amorphous counterparts. The emissions of the TPE dyes could be switched off and on continuously and reversibly by wetting and dewetting with solvent vapors, respectively, manifesting their ability to optically sense volatile organic compounds. The light-emitting diodes fabricated from 1 and 2 were turned on at ϳ2.9 and ϳ5 V and emitted blue lights with maximum luminance of ϳ1800 and ϳ11 000 cd/ m 2 , respectively.

Aggregation in poor solvents and complexation with calf thymus DNA and bovine serum albumin turn "on" the fluorescence of tetraphenylethylene derivatives, due to the restriction of intra-molecular rotations of the dyes in the aggregates and complexes.

A facile method has been developed to extract oxidized carbon quantum dots (QDs) directly from activated carbon (AC) by chemical oxidation. The method has several advantages including low cost, high yield of QDs (>10%), and large-scale production. The as-prepared oxidized carbon QDs are mainly graphitic structure nanocrystals of 3-4 nm in diameter, have abundant carboxyl groups at their surfaces, and exhibit strong electrochemiluminescent (ECL) activity, suggesting promising applications in ECL biosensing and imaging. The ECL properities, including ECL activities in the absence and presence of coreactants, effects of the size and surface passivation on the oxidized carbon QDs ECL were investigated and discussed in detail.