Carbon and graphene quantum dots are prepared using top-down and bottom-up methods. Sustainable synthesis of quantum dots has several advantages such as the use of low-cost and non-toxic raw materials, simple operations, expeditious reactions, renewable resources and straightforward post-processing steps. These nanomaterials are promising for clinical and biomedical sciences, especially in bioimaging, diagnosis, bioanalytical assays and biosensors. Here we review green methods for the fabrication of quantum dots, and biomedical and biotechnological applications.

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Green synthesis, biomedical and biotechnological applications of carbon and graphene quantum dots. A review

Carbon dots, a class of zero dimensional nanomaterials have recently emerged as excellent candidates for versatile applications such as bioimaging, catalysis, sensing and drug delivery. They possess outstanding optical properties, low toxicity, high biocompatibility, and require simple low-cost synthesis methods. Both top-down and bottom-up approaches have been developed for the synthesis since its discovery. Various techniques such as laser ablation, chemical and electrochemical oxidations, thermolysis and sonolysis of carbon precursors have been reported for the synthesis of these key materials. Recently, the attention has been shifted to the synthesis of carbon dots from eco-friendly sources like biomass especially waste biomass as well as other waste, as these precursors provide a partial solution for the mitigation of the omnipresent problem of waste management. This review presents an overview of the various methods and precursors used in the synthetic strategies of carbon dots, with emphasis on green synthesis.

Recent trends in the use of green sources for carbon dot synthesis–A short review

Quantum dots are semiconductor nanocrystals that inherently fluoresce at specific wavelengths in the visible, enabling a number of potential applications to be realized. However, conventional quantum dots are based on metallic elements, which has raised concerns over toxicity, stability and high cost. As a result, the search for more benign substitutes is a worthwhile yet challenging undertaking. Recently a new type of visible emitters has been reported exclusively based on functionalized carbon nanoparticles. The carbon dots were 5 nm in diameter and were produced via laser ablation of graphite. Surface oxidation with nitric acid and subsequent covalent grafting of organic moieties afforded light-emitting derivatives. Notably, the light emitted by these dots depends on the wavelength of light used for excitation. It was suggested that the tethered modifier stabilizes the surface of the carbon nanoparticles helping to generate energy traps that emit light when stimulated, an effect described as emission from passivated surfaces. Because of its origin the emission is size-dependent, i.e., the smaller the size of the dots the better their photoluminescence efficiency. In another intriguing approach, photoluminescent carbon dots 3 nm in size were directly fabricated by electrochemical shocking of multi-wall carbon nanotubes. The demonstrated photoluminescence adds another dimension to the versatility of carbon-based

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-005201.pdf

Surface Functionalized Carbogenic Quantum Dots

The copper doped fluorescent carbon dots (Cu-CDs) with excellent biocompatibility were prepared from l -tryptophan and copper chloride as precursors through a facilely hydrothermal method. As-prepared Cu-CDs exhibited the bright blue fluorescence and excellent optical stability. In the presence of tetracyclines antibiotics (TCs), the intensely fluorescent quenching of Cu-CDs occurred from internal filtration effect (IFE), providing the foundation of TCs quantification. Meanwhile, the fluorescence intensity of Cu-CDs decreased gradually along with the increasing of pH and a sigmoidal curve was well fitted with the pKa12.49 and pKa2 6.21, respectively. The Cu-CDs as a multi-functional fluorescence sensing platform demonstrated great accuracy and selectivity for tetracyclines (TCs) and pH. On account of practical application, as-synthesized Cu-CDs not only were effectively employed for determination of TCs in antibiotic tablets and milk products, but also could trace nuclear and had been extended to sense and visualize pH fluctuations in cells due to low toxicity and biocompatibility. As a facile and effective multi-functional fluorescent sensor, the proposed Cu-CDs have a significant potential for biosensing and biomedical imaging.

Copper doped carbon dots as the multi-functional fluorescent sensing platform for tetracyclines and pH

The emergence of carbon quantum dots (CQDs) opens up new opportunities in different branches of science and technology primarily because of their conducive biocompatibility, tunable bandgaps, and u...

Syntheses of N-Doped Carbon Quantum Dots (NCQDs) from Bioderived Precursors: A Timely Update

In this study, we report a green and economical hydrothermal synthesis of fluorescent-nitrogen-doped carbon quantum dots (NCQDs) using citrus lemon as a carbon source. The prepared NCQDs possess high water solubility, high ionic stability, resistance to photobleaching, and bright blue color under ultraviolet radiation with a high quantum yield (∼31%). High-resolution transmission electron microscopy (HRTEM) results show that the prepared NCQDs have a narrow size distribution (1–6 nm) with an average particle size of 3 nm. The mercury ion (Hg2+) sensing efficiency of the NCQDs was studied, and the result indicated that the material has high sensitivity, high precision, and good selectivity for Hg2+. The limit of detection (LOD) is 5.3 nM and the limit of quantification (LOQ) is 18.3 nM at a 99% confidence level. The cytotoxicity was evaluated using MCF7 cells, and the cell viabilities were determined to be greater than 88% upon the addition of NCQDs over a wide concentration range from 0 to 2 mg/mL. Based ...

https://pubs.acs.org/doi/pdf/10.1021/acsomega.0c00832

Fluorescent-Nitrogen-Doped Carbon Quantum Dots Derived from Citrus Lemon Juice: Green Synthesis, Mercury(II) Ion Sensing, and Live Cell Imaging.

Facile and green one pot hydrothermal method was used for synthesis of fluorescent carbon quantum dots (CQDs) using citrus lemon juice as precursor. The synthesized CQDs were characterized using UV–Vis spectrophotometer, fluorescence spectrometer, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), field emission scanning electron microscope equiped with energy dispersive X-ray spectroscopy (FESEM-EDS) and fluorescence microscopy. The obtained CQDs have high photoluminescence of 10.20% quantum yield. The photoluminescence intensity of CQDs depends on pH of the solution and maximum intensity obtained at pH of 6. The particle size of the carbon dots were distributed in narrow range of 2–10 nm with an average of 5.8 nm. The highly water soluble CQDs have high cell viability even at high concentration which rich up to 85%. MTT assay was used to investigate the potential application of CQDs and the results indicated that the material can be used as florescent probe in the cell imaging.

http://www.ajnanomat.com/article_57712_c180ebf10d888464520073be893327fb.pdf

Facile green synthesis of fluorescent carbon quantum dots from citrus lemon juice for live cell imaging

Carbon quantum dots (CQD) as the result of their exceptional physical and chemical properties show tremendous potential in various field of applications like cell imaging and doping of CQDs with elements like nitrogen and phosphorous increase its fluorescence property. Herein, we have synthesized fluorescent nitrogen and phosphorous codoped carbon quantum dots (NPCQDs) via a one-pot hydrothermal method. Sesame oil, L-Aspartic acid, and phosphoric acid were used as carbon, nitrogen, and phosphorous sources, respectively. UV-Vis spectrophotometer, fluorescence spectrometer, Fourier transform infrared spectrometer (FTIR), X-ray diffraction spectrometer (XRD), field emission scanning microscopy (FESEM), and transmission electron microscopy (TEM) were employed to characterize the synthesized fluorescent NPCQDs. The as-synthesized NPCQDs with a particle size of 4.7 nm possess excellent water solubility, high fluorescence with high quantum yield (46%), high ionic stability, and resistance to photobleaching. MTT assay indicated the biocompatibility of NPCQDs and it was used for multicolor live-cell imaging. Besides, the NPCQDs show an effective probe of iron ions (Fe3+) in an aqueous solution with a high degree of sensitivity and selectivity. The DPPH assay showed its good antioxidant activity.

Synthesis of Fluorescent Nitrogen and Phosphorous Co-doped Carbon Quantum Dots for Sensing of Iron, Cell Imaging and Antioxidant Activities.

In order to explore an alternative photocatalyst for environmental remediation, we report a two-step process for the synthesis of zinc oxide/nitrogen doped carbon quantum dot nanocomposites (ZnO@NCQD NCs). In the first step, graphitic amorphous nitrogen doped carbon quantum dots (NCQDs) were synthesised using sesame oil and urea as precursors via a hydrothermal method. A facile one pot coprecipitation method was used to synthesise white-brown ZnO@NCQD NCs by mixing zinc acetate and NCQD solution. The ZnO@NCQD NCs were characterized by advanced techniques to confirm their morphological, structural and optical features. TEM (transmission electron microscopy) analysis corroborated the formation of well-defined uniformly distributed quasi-spherical NCQDs with an average particle size of about 5.5 nm. Spherical shaped nanostructures of ZnO@NCQD NCs were observed from the SEM (scanning electron microscopy) micrographs. The XRD (X-ray diffraction) result revealed the hexagonal wurtzite polycrystalline structure of ZnO@NCQD NCs. A decrease in the band gap energy of ZnO@NCQD NCs (3.01 eV) was observed due to the insertion of NCQDs into the structure of zinc oxide nanoparticles (ZnO NPs). The FTIR spectra confirmed the presence of hydroxyl and amine functional groups on the surface of NCQDs which are expected to have played a significant role in the process of functionalization and synthesis of ZnO@NCQD NCs. A photocatalytic degradation efficiency of 92% was recorded for Rhodamine B (Rh-B) dye by ZnO@NCQD NCs as a consequence of the reduced band gap. In addition, ZnO@NCQD NCs exhibited highly impressive antibacterial and antioxidant properties. Thus ZnO@NCQD NCs can serve as better materials for environmental as well as biomedical applications.

Enhanced photocatalytic degradation of Rhodamine B, antibacterial and antioxidant activities of green synthesised ZnO/N doped carbon quantum dot nanocomposites

Mebrahtu Hagos

Papers

Fluorescent-Nitrogen-Doped Carbon Quantum Dots Derived from Citrus Lemon Juice: Green Synthesis, Mercury(II) Ion Sensing, and Live Cell Imaging.

Facile green synthesis of fluorescent carbon quantum dots from citrus lemon juice for live cell imaging

Synthesis of Fluorescent Nitrogen and Phosphorous Co-doped Carbon Quantum Dots for Sensing of Iron, Cell Imaging and Antioxidant Activities.

Enhanced photocatalytic degradation of Rhodamine B, antibacterial and antioxidant activities of green synthesised ZnO/N doped carbon quantum dot nanocomposites

Facile disinfection of river and pond water using natural solar radiation at shoa robit and neighboring rural kebeles.