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Graphical abstract Highlights • A strongest multiple fluorescent N-CQDs were synthesized from the P. acidus fruits. • Synthesized N-CQDs showed low-toxic and were employed for in vivo live-cell imaging. • The N-CQDs were uniformly stained within the body of the nematodes (C. elegans). • The live-cell imaging result reveals the usage of N-CQDs in drug delivery. Abstract In this work, nitrogen-doped carbon quantum dots (N-CQDs) have been synthesized successfully by a simple hydrothermal method and demonstrated its application for multicolor imaging in Caenorhabditis elegans (C. elegans) as an in vivo model. The synthesized N-CQDs were characterized by various physicochemical techniques such as XRD, Raman spectroscopy, ATR-FTIR spectroscopy, XPS, HRTEM, UV–vis spectroscopy, and fluorescence spectroscopy. The synthesized N-CQDs exhibited a strong fluorescence due to the uniform size distribution with nitrogen-containing and oxygen-containing functional groups onto the surface of N-CQDs which induce the excellent dispersibility in aqueous media. The N-CQDs has an excitation-dependent fluorescence behavior and the strongest fluorescence appeared at 411 nm (emission peak position) under the excitation of 340 nm. Also, the N-CQDs displayed a high quantum yield (QY) of 12.5. The fluorescence behaviour of the aqueous N-CQDs suspension retains for a long time up to 1 year. The prolonging fluorescent N-CQDs was utilized as a staining agent for bioimaging and toxicity of N-CQDs on C. elegans that was conducted by killing assay. In-vivo studies suggested that the N-CQDs displayed excellent biocompatibility and successfully used for high-contrast imaging of N-CQDs in living and dead C. elegans. Based on the strongest fluorescence along with excellent aqueous dispersibility and biocompatibility, the green synthesized N-CQDs would be an ideal candidate for many biological applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bovine serum albumin and tertbutyl titanate are used to prepare by one-step method. • The photonic conversion function of N-CQDs reduce band gap widths of TiO 2 to 2.84 eV. • N-CQDs/TiO 2 exhibits complete degradation of Congo red and killing of E. coli. • N-CQDs/TiO 2 can degrade of Cr(VI) and the reaction rate reaches 10.7 times of TiO 2. Hexavalent chromium (Cr (VI)) is a typical heavy metal ion caused numerous wastewater, and the photocatalysis is an attractive advanced treatment process for Cr (VI) removal. Biomass material bovine serum albumin and tetrabutyl titanate were used to prepare nitrogen-doped carbon quantum dots and TiO 2 (N-CQDs/TiO 2) nanoparticles by a facile one-step hydrothermal method. The excellent photonic conversion function of N-CQDs narrowed band gap widths of TiO 2 to 2.84 eV due to the N element and the CQDs introduced to the TiO 2 successfully. When the mass ratio of BSA and TiO 2 was 1 and the hydrothermal reaction time was 48 h, the obtained nanoparticle can rapidly and efficiently degrade of Cr (VI) in 40 min of reaction, and the kinetic reaction rate reached 10.7 times that of pure TiO 2 nanoparticles. And the Cr (VI) was reduced in 10 min by 1 mg/L nanoparticle under the pH of 2 or tartaric acid content of 1 g/L due to the electrons were transferred from N-CQDs to the TiO 2 surface to achieve effective electron/hole separation and reacted with the Cr (VI) to reduce it to Cr (III). The N-CQDs/TiO 2 also exhibited degradation of Congo red completely and killing of E. coli under simulated solar light. [ABSTRACT FROM AUTHOR]

A Fe3+-triggered multifunctional nitrogen-doped carbon quantum dots on–off-on fluorescent sensing platform for the ultrasensitive detection of biothiols was successfully established. [Display omitted] • A multifunctional fluorescent sensing platform which can be applied to identify and detect Fe3+ and three biothiols was designed. • N -CQDs-Fe3+ fluorescent probe exhibits lower detection limits for GSH (20.39 nM), Hcy (9.31 nM) and Cys (1.93 nM). • The constructed N -CQDs-Fe3+ fluorescent screening platform has strong anti-interference ability, good reproducibility and repeatability. Abnormal levels of biothiols in the human body are closely related to a variety of diseases, thus it is crucial for the selective detection of biothiols in the biological system. In this work, a Fe3+-triggered multifunctional nitrogen-doped carbon quantum dots (N -CQDs) on–off-on fluorescent sensing platform for the ultrasensitive detection of biothiols (GSH, Cys and Hcy) was successfully established. In detail, Fe3+ could adsorb on the surface of N -CQDs through coordination with the phenolic hydroxyl group of N -CQDs, resulting in the quenching of the fluorescence of N -CQDsdue to the photoinduced electron transfer (PET) effect from N -CQDs to Fe3+. After the addition of biothiols, there was stronger coordination between Fe3+ and biothiols than N -CQDs-Fe3+, resulting in fluorescence recovery of N -CQDs. On basis of the on–off-on principle, the N -CQDs-Fe3+ fluorescent probe was applied to determine biothiols. Under optimal parameters, the N -CQDs-Fe3+ fluorescent probe exhibited a relatively lower limit of detection (LOD) of 20.39 nM (GSH), 9.31 nM (Hcy) and 1.93 nM (Cys), respectively, which was better or comparable compared with previous reports. Further, the constructed N -CQDs-Fe3+ fluorescent sensing platform was validated with strong anti-interference ability, good reproducibility and repeatability, and also performed well in serum samples, demonstrating a promising potential in clinical diagnosis for biothiols analysis. [ABSTRACT FROM AUTHOR]

• A simple and novel fluorescence sensing platform was developed. • N-CQDs/DA/Tyr/ALA system was exploited as a sensitive and selective fluorescence "turn off-on" sensor for DA and ALA. • Based on the constructed system, ALA was first determined by spectrometry. • The proposed fluorescence sensor can be used to monitor DA and ALA in human serum and urine with satisfactory results. A simple and reliable fluorescence sensing strategy, which depends on the strong fluorescence emission of nitrogen-doped carbon quantum dots (N-CQDs) and efficient catalytic oxidation of tyrosinase (Tyr), was demonstrated for the sensitive detection of dopamine (DA) and alpha lipoic acid (ALA). Under optimized conditions, the fluorescence intensity of N-CQDs was easily quenched by dopamine oxidation product dopaquinone with the addition of Tyr. When ALA was introduced into the sensing system, ALA could inhibit and reduce the oxidation process of DA, resulting in the fluorescence recovery of N-CQDs. This sensing platform showed a sensitive relationship to the DA and ALA concentrations within certain ranges of 0.05–15 and 0.5−55 μM, and low detection limits of 0.035 and 0.39 μM, respectively. Given the above mechanisms, the proposed biosensor was utilized for the detection of DA and ALA in real samples with satisfactory results. Moreover, we successfully measured ALA through spectrometry, which is expected to provide valuable information in medical and disease diagnosis. [ABSTRACT FROM AUTHOR]

• A novel N-CQDs were prepared by facile solid-heat method using glycine and urea. • The N-CQDs possessed a relatively high fluorescence QY (21.04 %). • The N-CQDs exhibited very high sensitivity and selectivity to dopamine. • This sensor successfully used for dopamine detection in human serum and rat serum. Herein, a novel nitrogen-doped carbon quantum dots (N-CQDs) with bright blue fluorescence were prepared using glycine and urea by a facile solid-heat method. The prepared N-CQDs had a relatively high quantum yield (QY = 21.04 %), excellent photobleaching resistance, acid and alkali resistance. Moreover, it was used as fluorescent probe for the detection of dopamine (DA) with high selectivity and a low limit of detection (LOD = 0.0972 μM). Additionally, this fluorescent probe was successfully used to the determination of DA in human serum and rat serum with satisfactory recoveries (98.24 %–107.86 %), indicating this probe is was a promising biomolecular sensor for quantitation of DA in serum. [ABSTRACT FROM AUTHOR]