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Here, a fast and eco-friendly one-pot hydrothermal technique is utilized for the synthesis of nitrogen/sulfur-co-doped fluorescent carbon quantum dots (NS-CQDs) from a simple precursor of citric acid (CA) and thiosemicarbazide (TSC). The obtained NS-CQDs exhibited strong blue emission under UV light, with fluorescence quantum yield (QY) of ~37.8%. The Commission internationale de l'eclairage (CIE) coordinates originated at (0.15, 0.07), which confirmed the blue fluorescence of the synthesized NS-CQDs. Interestingly, the prepared NS-CQDs were successfully used as a selective nanoprobe for the monitoring of environmentally hazardous explosive picric acid (PA) in different nitro- and non-nitro-aromatic derivatives of PA. The mechanism of the NS-CQDs was also explored, and was posited to occur via the fluorescence resonance electron transfer (FRET) process and non-fluorescent complex formation. Importantly, this system possesses excellent biocompatibility and low cytotoxicity in HeLa cervical cancer cells; hence, it can potentially be used for PA detection in analytical, environmental, and pathological applications. Furthermore, the practical applicability of the proposed sensing system to pond water demonstrated the feasibility of our system along with good recovery. Graphical abstract.

Herein, we were synthesized fluorescent carbon quantum dots via facile one-step hydrothermal treatment of mustard seeds (M-CQDs). It showed excellent optical property with fluorescent quantum yield 4.6%. The as-prepared M-CQDs exhibited peroxidase-like mimetic activity and catalyzed the oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H 2 O 2 to produce a blue color reaction mixture with the prominent peak at 652 nm. Furthermore, the peroxidase-like catalytic performance of M-CQDs followed the steady-state kinetics behavior and exhibited similar catalytic activity as that of natural enzyme Horseradish peroxidase (HRP). In addition to this, the double reciprocal plot showed a parallel line which suggested the occurrence of Ping-Pong type of mechanism. The H 2 O 2 dependent oxidation of TMB was helpful for the colorimetric detection of H 2 O 2 in the linear range of 0.02-0.20 mM with the limit of detection (LOD) of 0.015 mM. Interestingly, the oxidized TMB (ox-TMB) was further reduced to native TMB by the reducing agent ascorbic acid. Hence M-CQDs showed its potential towards the selective and sensitive detection of ascorbic acid in the linear range of 10-70 μM having a correlation coefficient of 0.998 with LOD of 3.26 μM. The practical feasibility of the proposed detection method of AA was also investigated in common fresh fruits., (Copyright © 2018 Elsevier B.V. All rights reserved.)

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]

Nitrogen doped carbon quantum dots (N-CQDs) was efficiently synthesized by one pot hydrothermal method and effectively used for on-off-on sensing of Hg2+ and GSH in pure aqueous medium and live cell imaging application on MCF-7 cancer cell line. • One step hydrothermal synthesis of N-CQDs via simple and cost efficient precursors with fluorescence quantum yield of 41%. • N-CQDs act as fluorescent probe for on-off-on detection of Hg2+ and GSH with good selectivity and sensitivity. • The MTT assay of N-CQDs on MCF-7 cancer cell line showed negligible toxicity and excellent biocompatibility. • The cell imaging result showed intracellular internalization of N-CQDs in MCF-7 cell in dichromic region (blue & green). • The N-CQDs could be utilized for multifunctional applications such as sensing of Hg2+, biothiol and cell imaging agent. The present work describes the synthesis of N-CQDs via one step hydrothermal methods. The synthesized N-CQDs was fully characterized by TEM, P-XRD, FT-IR, XPS and Zeta sizer. It exhibited excitation independent emission behaviour with fluorescent quantum yield of 41% respect to quinine sulphate standard. The excellent optical properties of N-CQDs made it fluorescent probe for turn off detection of toxic heavy metal ion Hg2+ with good selectivity and sensitivity. The limit of detection was found to be 0.08 μM with correlation coefficient (R2 = 0.992). After addition of various reducing agent such as amino acid, EDTA and GSH in the quenched solution of N-CQDs + Hg2+, the fluorescence property was selectively recovered in the presence of GSH and hence the quenched solution further acts as selective turn on sensing of GSH up to lower limit of 2.0 μM with correlation coefficient (R2 = 0.996). The on-off-on sensing behaviour of N-CQDs was utilized for the implication of logic gate in absence and presence of Hg2+ and GSH to construct the molecular switches. The MTT assay of N-CQDs was performed on MCF-7 cancer cell line to check their applicability in biological system and the result showed negligible cytotoxicity and good permeability. The cell imaging on MCF-7 cells visualized in dichromic region (blue and green). Therefore, it could be smart material for sensing of heavy toxic metal as well as cell imaging application. [ABSTRACT FROM AUTHOR]

In this study, we have reported easy and economical synthesis of highly fluorescent nitrogen-rich carbon quantum dots (N-CQDs). The as-prepared N-CQDs displayed strong blue color emission along with CIE co-ordinate index (0.15, 0.14). The N-CQDs possess comparatively high QY of 57% using quinine sulfate as a standard. Furthermore, the as-prepared N-CQDs defend against the high salt strength and longtime photostability over the six months of incubation. Apart from this, the as-prepared N-CQDs responded as a sensor for the 'turn-off' detection of Co2+ along with the detection limit of 0.12 μM over a linear range from 0.5 to 3 μM. The mechanistic study proved the detection of Co2+ was based on the IFE, static quenching, aggregation, and complex formation between the amino group of N-CQDs and Co2+. Moreover, the fluorescence of the quenched N-CQDs can reappear and get 'turn-on' by using GSH, AA, EDTA, and cyst; thus, the prepared N-CQDs could further execute as a probe for the Co2+ detection. Inspired by these outstanding properties, the as-prepared N-CQDs were also successfully employed for the practical application toward monitoring the trace level of Co2+ in a vitamin B-12 sample. • Nitrogen-rich carbon quantum dots (N-CQDs) were synthesized by simple hydrothermal treatment of glycine and PEI. • The synthesized N-CQDs have relatively high QY of 57% using quinine sulfate as a reference. • The N-CQDs were found to be a promising nano-probe to detect Co2+. • The feasibility of the proposed system was checked in a vitamin B-12 sample toward the Co2+ detection. [ABSTRACT FROM AUTHOR]