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Interest is growing in the development of artificial enzymes to overcome the drawbacks of natural enzymes. Herein, we have synthesized nitrogen–sulphur dual-doped carbon quantum dots (NS-CQDs) via a one-step hydrothermal method; the NS-CQDs possess excellent optical properties and a high fluorescent quantum yield (46%). Significantly, the NS-CQDs exhibited peroxidase mimetic enzyme activity without support from metals or polymeric materials and efficiently catalyzed the oxidation of peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue solution with an absorption maximum at 652 nm. Mechanistic studies suggest that the small size and high electron density of NS-CQDs play vital roles and accelerate the reduction of H2O2 to generate ṖOH radical, which facilitates the oxidation of TMB. The catalytic activity is based on Michaelis–Menten kinetic behavior, and steady state kinetic analysis suggests that the NS-CQDs exhibit a higher affinity for H2O2 than TMB, similar to the natural enzyme horseradish peroxidase (HRP). Moreover, the catalytic pathway follows a ping-pong mechanism. Therefore, these findings offer a worthy platform for colorimetric detection of H2O2 in a linear range of 0.02 mM to 0.1 mM with a limit of detection of 0.004 mM. Interestingly, the blue colour of oxidized TMB showed excellent selectivity over non-thiolate biological molecules, especially amino acids, and glutathione can be detected up to 0.07 μM via colorimetric and fluorimetric assays. Additionally, this system showed excellent recovery (96.0–108.3%) of GSH from human blood serum. Thus, the proposed sensing system is simple, convenient, and straightforward and can be potentially applied for real time monitoring of H2O2 and glutathione in biological samples. [ABSTRACT FROM AUTHOR]

Highly fluorescent nitrogen and phosphorus-doped carbon quantum dots (N,P-CQDs) were synthesized via a one-step hydrothermal method and fully characterized via various techniques such as TEM, DLS, FT-IR, P-XRD and XPS analysis. The as-synthesized N,P-CQDs showed excellent optical properties and exhibited bright blue colour under UV-light with CIE coordinate (0.20, 0.22) along with a high quantum yield of 73%, due to which they could act as on–off fluorescent nanoprobes for the selective and sensitive detection of highly toxic Cr(vi) below its permissible limit via the inner filter effect (IFE) and static quenching mechanism. In addition, Cr(vi) could be reduced to lower valent chromium species. Therefore, the N,P-CQDs + Cr(vi) system further acted as a selective off–on sensor for reductant ascorbic acid (AA) because it reduced Cr(vi) to Cr(iii) species, resulting in the elimination of IFE and recovery of fluorescence of N,P-CQDs. Notably, this system possesses excellent biocompatibility and negligible cytotoxicity; therefore, it can be potentially applied for fluorescence imaging of intracellular Cr(vi) and ascorbic acid (AA) in living cells and complex biological systems. [ABSTRACT FROM AUTHOR]

Herein, we report a green synthetic approach for the synthesis of water-soluble fluorescent CQDs via a simple one-step hydrothermal treatment using Tamarindus indica leaves for the first time. The prepared CQDs show an excitation-dependent behavior in the range from 260 to 400 nm with a high QY of approximately 46.6%. Further, the prepared CQDs serve as a very sensitive nanoprobe for the turn-off sensing of Hg2+ with a minimum LOD as low as 6 nM in the dynamic range from 0 to 0.1 μM. The attractiveness of the present sensing system is that it further acts as a turn-on sensor for GSH detection with good selectivity. The feasibility of the present sensing system is also examined using pond water samples for the detection of Hg2+. Thus, the present sensing system is reliable for sensing and several another analytical applications. [ABSTRACT FROM AUTHOR]

A novel hybrid fluorophore (FHPY) has been synthesized based on a condensation reaction of two standard fluorescent hydrophobic–hydrophilic molecules, viz. pyrene and fluorescein, with an objective to tune the aggregation-induced emission (AIE) along with the morphology. Owing to the distinct photophysical properties of pyrene and fluorescein, the hybrid FHPY dramatically exhibits a fluorescence change from colourless to yellow-green via a blue colour upon varying the volume fraction of water (poor solvent) in methanol (good solvent). FHPY has exhibited not only AIE, but also an outstanding quantum yield (ΦF) of 97% at a 70% water fraction in methanol (70 : 30, v/v). We attribute the reason behind the tuning of the AIE and quantum yield to the opening of the lactam ring of fluorescein as well as to the amassing of hydrophobic pyrene at a certain water fraction. The mechanism involved in the AIE has been well supported by detailed UV-vis, fluorescence, lifetime, SEM, AFM, DFT, PXRD and 1H NMR experiments. In addition, FHPY serves as a good candidate for the live cell imaging of HeLa cells. [ABSTRACT FROM AUTHOR]

Interest is growing in the development of artificial enzymes to overcome the drawbacks of natural enzymes. Herein, we have synthesized nitrogen-sulphur dual-doped carbon quantum dots (NS-CQDs) via a one-step hydrothermal method; the NS-CQDs possess excellent optical properties and a high fluorescent quantum yield (46%). Significantly, the NS-CQDs exhibited peroxidase mimetic enzyme activity without support from metals or polymeric materials and efficiently catalyzed the oxidation of peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H 2 O 2 to produce a blue solution with an absorption maximum at 652 nm. Mechanistic studies suggest that the small size and high electron density of NS-CQDs play vital roles and accelerate the reduction of H 2 O 2 to generate ˙OH radical, which facilitates the oxidation of TMB. The catalytic activity is based on Michaelis-Menten kinetic behavior, and steady state kinetic analysis suggests that the NS-CQDs exhibit a higher affinity for H 2 O 2 than TMB, similar to the natural enzyme horseradish peroxidase (HRP). Moreover, the catalytic pathway follows a ping-pong mechanism. Therefore, these findings offer a worthy platform for colorimetric detection of H 2 O 2 in a linear range of 0.02 mM to 0.1 mM with a limit of detection of 0.004 mM. Interestingly, the blue colour of oxidized TMB showed excellent selectivity over non-thiolate biological molecules, especially amino acids, and glutathione can be detected up to 0.07 μM via colorimetric and fluorimetric assays. Additionally, this system showed excellent recovery (96.0-108.3%) of GSH from human blood serum. Thus, the proposed sensing system is simple, convenient, and straightforward and can be potentially applied for real time monitoring of H 2 O 2 and glutathione in biological samples.