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Graphical abstract Highlights • This is first report of synthesis of fluorescent carbon dots from Dunaliella salina. • A-NPCDs were used as fluorescent probe for selective detection of Hg (II), Cr (VI). • A-NPCDs offer excellent multi-color cell imaging. • Limit of detection for Cr (VI) and Hg (II) were 0.018 μM. • MTT assay showed that carbon dots were biocompatible and slightly cytotoxic. Abstract The impact of hazardous chemicals and toxic metal ions in the environment played havoc to the ecosystem. In the present work green chemistry approach was applied for one-step hydrothermal synthesis of nitrogen, phosphorus dual doped carbon dots, utilizing green precursor i.e, biomass of halophilic microalgae Dunaliella salina (algal derived nitrogen phosphorous carbon dots i.e A-NPCDs). Synthesized A-NPCDs were characterized through TEM, FT-IR, P-XRD, DLS and XPS. It showed appreciable optical properties with significant fluorescence quantum yield (8%)and exhibited blue color under UV – light (λ ex = 365 nm). A-NPCDs acted as fluorescent turn off sensor for toxic metal ion such as Hg (II) and Cr (VI) with good selectivity and sensitivity. Interestingly, ANPCDs detected Cr (VI) up to 0.018μM which was below the permissible level of Cr (VI) in drinking water. Such sensing resulted because of combination of inner filter effect and dynamic quenching mechanism. Moreover, it also showed good selectivity (0.018μM) for Hg (II) via dynamic quenching mechanism. MTT assay of A-NPCDs on HEK-293 cell line showed biocompatibility with negligible cytotoxicity. Therefore, these were successfully employed for live cell imaging and intracellular detection of Hg (II) and Cr (VI) in HEK-293 cell line. Thus, green synthesized A-NPCDs may be a good alternative for chemically derived CDs in intracellular detection of Hg (II) and Cr (VI) of a complex biological environment. [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]

An illustration of development of N,S-CQD-MnO 2 nanoprobe for the GSH detection • Highly fluorescent N,S-CQDs is synthesized hydrothermally with high QY. • N,S-CQDs-MnO 2 nano-composite based nanoprobe is designed for the rapid detection of GSH. • The proposed sensing system possesses high sensitivity and selectivity with a detection limit 0.012 μM. • To check the reliability, the developed method is fruitfully applied on human A549 cells and blood serum sample. In present study, the N,S-CQD-MnO 2 nano-composite is designed, which can perform as a sensitive nanoprobe for the 'off-on' sensing of GSH. The fluorescent N,S-CQDs were synthesized from the hydrothermal treatment of tartaric acid and taurine precursors. The synthesized N,S-CQDs exhibit strong blue emission with the high QY up to 47.4 %. MnO 2 nanosheet significantly quenches the blue fluorescence emission of N,S-CQDs by the phenomenon of fluorescence resonance energy transfer and strong electrostatic interactions between the N,S-CQDs and MnO 2 nanosheet. This reaction system could also work as off-on nanoprobe for GSH detection; the introduction of GSH into the reaction system reappears the fluorescence emission of N,S-CQDs by eradicating the fluorescence resonance energy transfer. Moreover, it is also found that the as-prepared N,S-CQD-MnO 2 based nanoprobe show very low cytotoxicity and good biocompatibility. The calculated detection limit is found to be 0.012 μM in a GSH concentration having a linear range of 0.1-0.7 μM. Interference study confirmed that the designed sensing system is specific toward the GSH detection compared to different amino acids along with some other biomolecules and metal interfering ingredients. Furthermore, the proposed method is successfully applied for the in vitro detecting and imaging GSH in A549 human lung cancer cells. Apart from this, the practicability of the proposed sensing system is also successfully applied in the quantification of GSH from human blood serum. Thus, our designed nanoprobe could be trustworthy for the several analytical and disease diagnosis. [ABSTRACT FROM AUTHOR]