In this work, water soluble carbon quantum dots (wsCQDs) were synthesized from lemon peel waste using a facile and cost effective hydrothermal process. As synthesized wsCQDs were 1–3 nm in size with spherical morphology and oxygen rich surface functionalities. These wsCQDs manifest excellent photoluminescence (PL) properties and exhibited quantum yield (QY) ∼14% with high aqueous stability. wsCQDs were further used to design an economic, green and highly sensitive fluorescent probe for the detection of Cr6+ ions with a detection limit of ∼73 nM. This wsCQDs based fluorescent probe could provide a simple, rapid, convenient technique for the sensitive and selective detection of Cr6+ in water purification processes. Further, wsCQDs were immobilized over electrospun TiO2 nanofibers and the photocatalytic activity for such a TiO2–wsCQDs composite was demonstrated using methylene blue (MB) dye as a model pollutant. Photocatalytic activity for the TiO2–wsCQDs composite was found to be ∼2.5 times more than that of TiO2 nanofibers. The synthesis method for wsCQDs could be easily scaled up for gram scale synthesis of carbon quantum dots.

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Green synthesis of carbon quantum dots from lemon peel waste: applications in sensing and photocatalysis

A fast and facile microwave approach for the synthesis of fluorescent nitrogen-doped carbon dots (N-CDs) is reported. The N-CDs were hydrothermally synthesized using l -ascorbic acid (AA) and β-alanine (BA) as the carbon precursor and the nitrogen dopant, respectively. The morphology of synthesized N-CDs was characterized by high resolution transmission electron microscopy (HR-TEM) and the elemental composition was analyzed using elemental mapping method. The crystallinity and graphitation of N-CDs were examined by X-ray diffraction (XRD) and Raman spectroscopy. The doping of nitrogen over the carbon dots (CDs) was revealed by attenuated total reflection conjunction with Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photo electron spectroscopy (XPS). The optical properties of synthesized N-CDs were examined by UV–Visible (UV–Vis) and fluorescence spectroscopy. The synthesized N-CDs emit strong blue fluorescence at 401 nm under excitation of 325 nm. The excitation dependent emission property of synthesized N-CDs was exposed from fluorescence results. The quantum yield of synthesized N-CDs is about 14% against the reference quinine sulfate. The cytotoxicity of synthesized N-CDs on Madin-Darby Canine Kidney (MDCK) and HeLa cells were evaluated through Cell Counting Kit-8 (CCK-8) cytotoxicity assay. The results implied that the fluorescent N-CDs showed less cytotoxicity, further which was successfully applied as a staining probe for the confocal imaging of MDCK and HeLa cells.

Microwave assisted green synthesis of fluorescent N-doped carbon dots: Cytotoxicity and bio-imaging applications

Facile synthesis of fluorescence carbon dots from sweet potato for Fe 3+ sensing and cell imaging.

Heteroatom doped carbon dots (CDs) have received increasing attention due to their unique properties and related applications. However, previously reported CDs generally show strong emission only in the blue-light region, thus restricting their further applications. And the fundamental investigation on the preparation process is always neglected. Herein, we have developed a simple and solvent-free synthetic strategy to fabricate nitrogen-doped CDs (N-CDs) from citric acid and dicyandiamide. The as-prepared N-CDs exhibited a uniform size distribution, strong yellowish-green fluorescence emission and a high quantum yield of 73.2%. The products obtained at different formation stages were detailedly characterized by transmission electron microscopy, X-ray diffraction spectrometer, X-ray photoelectron spectroscopy and UV absorbance spectroscopy. A possible formation mechanism has thus been proposed including dehydration, polymerization and carbonization. Furthermore, the N-CDs could serve as a facile and label-free probe for the detection of iron and fluorine ions with detection limits of 50 nmol L(-1) and 75 nmol L(-1), respectively.

Synthesis and formation mechanistic investigation of nitrogen-doped carbon dots with high quantum yields and yellowish-green fluorescence.

Sodium-ion batteries are attracting much interest due to their potential as viable future alternatives for lithium-ion batteries, in view of the much higher earth abundance of sodium over that of lithium. Although both battery systems have basically similar chemistries, the key celebrated negative electrode in lithium battery, namely, graphite, is unavailable for the sodium-ion battery due to the larger size of the sodium ion. This need is satisfied by "hard carbon", which can internalize the larger sodium ion and has desirable electrochemical properties. Unlike graphite, with its specific layered structure, however, hard carbon occurs in diverse microstructural states. Herein, the relationships between precursor choices, synthetic protocols, microstructural states, and performance features of hard carbon forms in the context of sodium-ion battery applications are elucidated. Derived from the pertinent literature employing classical and modern structural characterization techniques, various issues related to microstructure, morphology, defects, and heteroatom doping are discussed. Finally, an outlook is presented to suggest emerging research directions.

Hard Carbons for Sodium-Ion Battery Anodes: Synthetic Strategies, Material Properties, and Storage Mechanisms

Photoluminescent carbon dots of 4–5 nm size were prepared from starch (Tapioca Sago) through a solution method under mild conditions. The as-prepared carbon dots were used as photoluminescence probes for highly anion selective fluoride ion detection in aqueous solutions. A ready-to-use device is also demonstrated.

Starch (Tapioca) to carbon dots: an efficient green approach to an on–off–on photoluminescence probe for fluoride ion sensing

We have developed a facile approach for the synthesis of Ag NPs decorated reduced graphene oxide (Ag NPs/rGO) composite via a novel chemical route. This synthetic protocol involves protection, reduction and functionalization of graphene oxide in one step. The as prepared Ag NPs/rGO composite has been characterized by TEM, XRD, SEM, XPS, EDX, UV-Vis, CV, Raman and FT-IR spectra. The results showed that plenty of Ag NPs (∼28 nm) are homogeneously distributed onto the surface of rGO. Further, catalytic application of this fascinating nanomaterial has been utilized for the chemoselective synthesis of pyrrolo[2,3,4-kl]acridin-1-ones via ring-opening or amidic C–N bond cleavage of isatin. The Ag NPs/rGO composite possesses assets of both Ag NPs and reduced graphene oxide, thus, providing enhanced surface acidity with good dispersion ability that is usually missing in Ag NPs alone. After completion of the catalytic reaction, the Ag NPs/rGO composite could be easily recovered by simple filtration and recycled for 7 times without significant loss in its catalytic activity. The recycling and hot-filtration experiments proved the high stability and absence of leaching of the synthesized nanomaterial during the catalytic process. Furthermore, the proposed protocol showed excellent results in terms of green parameters confirming its effectiveness and profound appeal.

Amidic C–N bond cleavage of isatin: chemoselective synthesis of pyrrolo[2,3,4-kl]acridin-1-ones using Ag NPs decorated rGO composite as an efficient and recoverable catalyst under microwave irradiation

Begraj Kumawat

Papers

Starch (Tapioca) to carbon dots: an efficient green approach to an on–off–on photoluminescence probe for fluoride ion sensing

Amidic C–N bond cleavage of isatin: chemoselective synthesis of pyrrolo[2,3,4-kl]acridin-1-ones using Ag NPs decorated rGO composite as an efficient and recoverable catalyst under microwave irradiation

Modern Synthesis of Bioactive Heterocycles via IMCR Modification