Facile synthesis of fluorescent carbon dots using watermelon peel as a carbon source

The synthesis of water-soluble fluorescent carbon dots (C-dots) has received much attention recently. Here, high quality fluorescent C-dots have been synthesized through low-temperature carbonization and simple filtration using watermelon peel, a waste and reproducible raw resource, as a novel carbon resource. This facile approach allows large-scale production of aqueous C-dots dispersions without any post-treatment process. The as-prepared C-dots possess small particle sizes (~ 2.0 nm), strong blue luminescence, acceptable fluorescence lifetime and good stability in a wide range of pH values (pH 2.0-11.0) and at a high salt concentration. Besides, the obtained C-dots have been successfully applied in live cell imaging, indicating these carbon nanoparticles can serve as high-performance optical imaging probes.     

Versatile immunosensor using CdTe quantum dots as electrochemical and fluorescent labels

A versatile immunosensor using CdTe quantum dots as electrochemical and fluorescent labels has been developed for sensitive protein detection. This sandwich-type sensor is fabricated on an indium tin oxide chip covered with a well-ordered gold nanoparticle monolayer. Gel imaging systems were successfully introduced to develop a novel high-efficient optical immunoassay, which could perform simultaneous detection for the samples with a series of different concentrations of a target analyte. The linear range of this assay was between 0.1 and 500 ng/mL, and the assay sensitivity could be further increased to 0.005 ng/mL with the linear range from 0.005 to 100 ng/mL by stripping voltammetric analysis. The immunosensor showed good precision, high sensitivity, acceptable stability, and reproducibility and could be used for the detection of real sample with consistent results in comparison with those obtained by the ELISA method.     

Silica fluorescent nanospheres containing a large amount of quantum dots prepared using liposomes as templates

Quantum dot (QD) fluorescent spheres have captivated many scientists because of their many potential applications in biomedical research. In this work, QD nanospheres were prepared using a novel method: incorporating QDs into nano-liposomes and then synthesising a silica shell using a lipid membrane as the template. The results showed that the nanocomposites obtained were spherical in shape, and each nanosphere contained a silica shell and the cores consisted of a large amount of QDs. Ultrathin sections of the spheres showed that the thickness of the silica shell was about 50–60?nm. Because the QD cores were coated with liposome and thick silica shell, the bright field of the silica sphere suspension was close to milk white in colour, which was different from that of the red-coloured QD solution. Although the quantum yield of the silica spheres (2.27%) was lower than that of the QDs (23.52%), these nanospheres still emitted a bright fluorescence, and there was no obvious difference between the fluorescent colour of the nanosphere suspension and the QD solution.     

Polyamine-functionalized carbon quantum dots as fluorescent probes for selective and sensitive detection of copper ions

A novel sensing system has been designed for Cu(2+) ion detection based on the quenched fluorescence (FL) signal of branched poly(ethylenimine) (BPEI)-functionalized carbon quantum dots (CQDs). Cu(2+) ions can be captured by the amino groups of the BPEI-CQDs to form an absorbent complex at the surface of CQDs, resulting in a strong quenching of the CQDs' FL via an inner filter effect. Herein, we have demonstrated that this facile methodology can offer a rapid, reliable, and selective detection of Cu(2+) with a detection limit as low as 6 nM and a dynamic range from 10 to 1100 nM. Furthermore, the detection results for Cu(2+) ions in a river water sample obtained by this sensing system agreed well with that by inductively couple plasma mass spectrometry, suggesting the potential application of this sensing system.     

Green synthesis of nitrogen-doped fluorescent carbon quantum dots for selective detection of iron

Carbon quantum dots (CQDs) were synthesized by a simple and economic hydrothermal method using Lycium barbarum (LB-CQDs) as precursor. Ammonia was used as a dopant to prepare nitrogen modified LB-CQDs (N-LB-CQDs). The characterizations of atomic force microscope, transmission electron microscopy, X-ray photoelectron spectroscopy showed that N-LB-CQDs were spherical in shape with an average diameter of 2～5 nm. Its surface was rich in nitrogen-containing groups. The fluorescence spectrophotometer showed that N-LB-CQDs exhibited double peak emission. The two peak positions in photoluminescence were about 462 nm and 512 nm. The as-prepared N-LB-CQDs are high-fluorescent quantum yield and water soluble. In addition, iron selectively results in a strong fluorescence quenching of N-LB-CQDs. Theoretical research results verified that iron reduced energy gap between HOMO and LUMO of N-LB-CQDs greatly, which leads to its emission wavelength shift out of the visible range. Results from the study may shed light on the production of fluorescent and biocompatible CQDs with simple, economic and environmental benign strategy in which Lycium barbarum was used as a carbon source.     

Cell uptake and intracellular visualization using quantum dots or nuclear localization signal-modified quantum dots with gold nanoparticles as quenchers

Understanding and controlling the interactions between nanoscale objects and living cells is of great importance for diagnostic imaging and therapeutic applications. Quantum dots (QDs) have remarkable optical characteristics, such as uniquely feature bright, photostable, tunable and narrow fluorescence emissions, as well as broad absorption spectra. Here we report a platform of using quantum dots to investigate the cell uptake and the interactions between nanoscale objects and cells. QDs are uptaken by BHK cells easily through endocytosis. We could clearly differentiate the QDs outside the cell or inside the cell by quenching the QDs with similar sized gold nanoparticles and reduce the noise of fluorescent image. Microscopic images show that QDs are homogeneously distributed within the whole cell except the nucleus. However, unmodified QDs could not penetrate the nuclear membrane and move into the nucleus. Coupling QDs with Nuclear Localization Signal (NLS, CGGGPKKKRKVGG) can significantly enhance the translocation amount of QDs into the cell and cell nucleus. This method combined with microscopy imaging system can visualize the particle delivery routes and provide valuable information in the drug/gene delivery and tumor diagnosis.     

One-pot synthesis of hydrophilic and hydrophobic fluorescent carbon dots using deep eutectic solvents as designer reaction media

Carbon dots are often synthesized in the presence of a carbon source and passivating agents in which they are crucial for an enhanced fluorescence. The solvent choice and/or combination to be used in the synthesis of these nanoparticles can influence their surface chemical composition, morphology, and fluorescence properties. In this study, highly fluorescent carbon dots were synthesized using deep eutectic solvents of different compositions as green solvent media and doping agent. Resulting carbon dots were then separated by their hydrophilicity/hydrophobicity using a three-phase solvent system (water/acetone/chloroform) and compared with traditional centrifugation-based separation method. Carbon dots with a size below 20 nm and quantum yield reaching 50% were obtained. Many properties of them including surface functional groups, optical, fluorescence, and electric properties were shown to be determined by the deep eutectic solvent composition.     

Enhanced and tunable fluorescent quantum dots within a single crystal of protein

The design and synthesis of bio-nano hybrid materials can not only provide new materials with novel properties, but also advance our fundamental understanding of interactions between biomolecules and their abiotic counterparts. Here, we report a new approach to achieving such a goal by growing CdS quantum dots （QDs） within single crystals of lysozyme protein. This bio-nano hybrid emitted much stronger red fluorescence than its counterpart without the crystal, and such fluorescence properties could be either enhanced or suppressed by the addition of Ag（I） or Hg（II）, respectively. The three-dimensional incorporation of CdS QDs within the lysozyme crystals was revealed by scanning transmission electron microscopy with electron tomography. More importantl~ since our approach did not disrupt the crystalline nature of the lysozyme crystals, the metal and protein interactions were able to be studied by X-ray crystallography, thus providing insight into the role of Cd（II） in the CdS QDs formation.     

Applications of carbon quantum dots in lubricant additives: a review

Advances in lubricants are vital to the pursuit of energy efficiency and sustainable development. It is well known that the essence of lubricating oil is lubricant additives, especially the friction-reducing and anti-wear additives. Carbon quantum dots (CQDs), a novel zero-dimensional carbon-based nanomaterial, have attained growing expectations in material and chemical sciences because of their extraordinary properties such as low toxicity and environmentally friendly, high chemical and thermal stability, and good designability. Since their discovery, CQDs have shown great potential in many applications including sensors, medicine, photovoltaic devices, biology, and tribology. The latest application of CQDs as the high-performance friction-reducing and anti-wear additives has garnered increasing attention. With the in-depth study, CQDs have gradually exhibited their excellent tribological properties, especially acted as additives in lubricating base oils. This paper has reviewed the progress in the research and development of CQDs-based lubricant additives by introducing lots of successful applications of CQDs-based additives in the present work and then highlighted the friction-reducing and anti-wear property, superiority, as well as the lubrication mechanism of CQDs as an additive, along with some discussion on challenges and perspectives in this significant and promising field. Finally, we offered a series of suggestions for developing the next-generation high-performance CQDs-based lubricant additives.

Graphical abstract

This work presented the carbon quantum dots as the lubricant additives of lubricating base oils.

     

Synthesis of polyethylene glycol modified carbon dots as a kind of excellent water-based lubricant additives

The polyethylene glycol (PEG) modified carbon dots (CDs-PEG) were facile synthesized by one-pot pyrolysis of the mixture of gluconic acid and PEG. When the average molecular weight of PEG was 200, 600, 1000 and 4000?g/mol, the CDs-PEG were denoted as CDs-PEG200-4000, respectively. As the water-based lubricant additives, the tribological properties of CDs-PEG200 under four-ball mode and steel/steel contact were far superior to PEG200, CDs derived from gluconic acid only, and CDs-PEG600-4000, reflecting the carbon cores and surface groups of CDs-PEG200 should play a synergetic lubrication effect. Specifically, at load of 40?N, adding 0.20?wt% of CDs-PEG200 into base liquid resulted in the largest friction coefficient and wear volume reductions up to 83.5% and 90.9%. Meanwhile, the load carrying capacity of base liquid increased from 50?N to at least 200?N. Amazingly, the lubrication ability of base liquid containing 0.20?wt% of CDs-PEG200 sometimes was even comparable to some lubricant oils. The results of worn surface analyses revealed that the distinguished friction-reducing and antiwear performance of CDs-PEG200 as additives could be attributed to the formed composite lubrication film composed of CDs-PEG200 and Fe₃O₄ on the rubbing surfaces.     

Preparation and characterisation of fluorescent polystyrene spheres coated with different sized quantum dots

Abstract

Water soluble CdSe quantum dots (QDs) coated polymer sphere with single and multiple fluorescent peaks have been prepared through the layer by layer self-assembly method. Polystyrene (PS) spheres ranging mainly from 330 to 750 nm and from 1·0 to 3·5 μm were used as the templates for deposition of three sized fluorescent QDs. The results of transmission electron microscopy and fluorescent confocal optical slice showed that the PS/CdSe composites were spherical in shape with core–shell structures. Emission colours of the mixture of spheres with single colour signal emitting at different wavelengths stimulated by a single light source could be distinguished clearly. Single spheres with separated, multiple fluorescence peaks could be easily obtained by controlling the ratios of the three sized CdSe QDs.     

Synthesis and modulation of the optical properties of carbon quantum dots using microwave radiation

Abstract

Carbon quantum dots have exhibited highly fluorescent characteristics as nanomaterials. Soluble in water and easily synthesized by multiple simple techniques, there are immense fabrication possibilities by permuting their properties via changing precursors, synthesis route, reaction parameters, etc. As economic and environment-friendly seed material, they are being viewed as an alternative to conventional fluorescent materials in myriad of applications including displays, cancer detection, drug delivery carriers in biomedicine, absorbing material in photovoltaics, etc. In this work, the hydrophilic carbon quantum dots were synthesized from the aqueous solution of citric acid and urea through microwave radiation for varying heat durations. The method is facile, faster and friendly to the environment without any need for high temperature and complicated chemical techniques. It was observed that the bandgap of the fabricated carbon quantum dots and its optical properties namely absorbance, photoluminescence enhanced with an increase in exposure of samples to heat up to an optimum limit, owing to the increase in density of states. However, further exposure to heat for longer duration degraded the absorbance and bandgap while photoluminescence gets saturated. Stokes’ shift revealed that all the synthesized carbon quantum dots possess stable emission. This was reconfirmed from consistent emission peak positions under varying excitation in the samples. The absorbance and PL spectrum exhibited by the synthesized dots makes it a suitable material for boosting the performance of organic solar cell.     

Synthesis of fluorescent carbon quantum dots from aqua mesophase pitch and their photocatalytic degradation activity of organic dyes

A synthesis strategy of fluorescent carbon quantum dots (CQDs) with high quantum yield (QY) using aqua mesophase pitch (AMP) as the carbon source has been developed via the hydrothermal method in this study. The hydrothermal temperature and soaking time have important effects on the morphology and QY of CQDs. As-prepared CQDs at 120 °C holding for 24 h (CQDs-120-24) have the uniform size of about 2.8 nm, and the QY can reach 27.6%. The obtained CQDs are successfully modified with ammonia and thionyl chloride, respectively, and they exhibit an excellent photocatalytic performance on degrading rhodamine B (Rh B), methyl blue (MB) and indigo carmine (IC). Importantly, the degradation percentage of N-CQDs on Rh B under natural light for 4 h reaches 97% with the degradation rate constant of 0.02463 min⁻¹ and it can maintain 93% after repetitively used 5 times. The results indicate that these as-prepared CQDs have the potential application in degrading organic dyes.     

Selecting single quantum dots from a bundle of single-wall carbon nanotubes using the large current flow process

We demonstrate selecting a Coulomb peak which originates from a single quantum dot, from a bundle of many single-wall carbon nanotubes. The method uses the previously reported current flowing process, by which the number of nanotubes can be reduced for the transport. By adjusting the gate voltage in an appropriate range, the single peak belonging to the single quantum dot has been selected. The effect of the high frequency application on the peak has been investigated, and it is shown that the basic response can be explained by the adiabatic response of the single dot to the high frequency signal.     

Preparations of bifunctional polymeric beads simultaneously incorporated with fluorescent quantum dots and magnetic nanocrystals

Bifunctional polystyrene beads simultaneously incorporated with fluorescent CdTe quantum dots (Q-dots) and superparamagnetic Fe(3)O(4) nanocrystals were prepared by a modified mini-emulsion polymerization method, in which polymerizable surfactants were used as both phase transfer agent for aqueous colloidal nanoparticles and emulsifier. In addition, silica coating was also introduced to Fe(3)O(4) nanocrystals for regulating the internal structure of the composite beads. Transmission electron microscopy, confocal fluorescence microscopy and conventional spectroscopy were used to characterize the composite beads, as well as the polymerizable surfactant-coated CdTe Q-dots and silica-coated Fe(3)O(4) nanoparticles. Different mixing methods were also attempted in order to vary the size of the resultant bifunctional beads.     

Highly sensitive fluorescent analysis of dynamic glycan expression on living cells using glyconanoparticles and functionalized quantum dots

A double signal amplification strategy was designed for highly sensitive and selective in situ monitoring of carbohydrate on living cells. The double signal amplification included the multiplex sandwich binding of functionalized quantum dots (QDs) to both glycan groups on the cell surface and glyconanoparticles and a cadmium cation sensitized fluorescence emission of Rhod-5N. Using the sialic acid-phenylboronic acid recognition system as a model, the 3-aminophenylboronic acid functionalized QDs (APBA-QDs) were synthesized by covalently binding APBA to mercaptopropionic acid capped CdS QDs, and the glyconanoparticles, polysialic acid stabilized gold nanoparticles (PSA-AuNPs), were prepared by a one-pot procedure. The APBA-QDs first recognized the sialic acid (SA) groups on BGC-823 human gastric carcinoma (BGC) cells and then the PSA on AuNPs, which were further used to bind more APBA-QDs on the cell surface for signal amplification. After the bound QDs were dissolved to release the Cd(2+), a Cd(2+)-sensitized fluorescence method was developed for the detection of BGC cells in a linear range from 5.0 × 10(2) to 1.0 × 10(7) cells mL(-1) with a limit of detection down to 210 cells mL(-1) (8 cells in 40 μL of solution) and the dynamic monitoring of SA expression variation on the cell surface. The monitoring result was identical with that from flow cytometric analysis. This approach showed high specificity and acceptable reproducibility. This strategy provided a promising platform for highly sensitive cytosensing and cytobiologic study.     

Facile consecutive solvothermal growth of highly fluorescent InP/ZnS core/shell quantum dots using a safer phosphorus source

The work presents a facile, stepwise synthetic approach for the production of highly fluorescent InP/ZnS core/shell quantum dots (QDs) by using a safer phosphorus (P) precursor. First, InP quantum dots (QDs) were solvothermally prepared at 180?°C for 24 h by using a P source of P(N(CH(3))(2))(3). The as-grown InP QDs were consecutively placed in another solvothermal condition for ZnS shell overcoating. In contrast to the almost non-fluorescent InP QDs, due to their highly defective surface states, the ZnS-coated InP QDs were highly fluorescent as a result of effective surface passivation. After the shell growth, the resulting InP/ZnS core/shell QDs were subjected to a size-sorting processing, by which red- to green-emitting QDs with quantum yields (QYs) of 24-60% were produced. Solvothermal shell growth parameters such as the reaction time and Zn/In solution concentration ratio were varied and optimized toward the highest QYs of core/shell QDs.     

Growth of InGaN quantum dots with AlGaN barrier layers via modified droplet epitaxy

The growth of c-plane InGaN quantum dots via modified droplet epitaxy with AlGaN barrier layers is reported. The growth of the AlGaN layer underlying the InGaN quantum dot layer was carried out under H₂ at 1050 °C, while the capping AlGaN layer was grown at the same temperature (710 °C) and using the same carrier gas (N₂) as that used to grow the InGaN quantum dot layer to prevent decomposition of the InGaN. Atomic force microscopy of InGaN epilayers grown and annealed on high temperature AlGaN using identical growth conditions used for the quantum dot samples highlighted a narrower distribution of nanostructure heights than that obtained for similar growth on GaN. Scanning transmission electron microscopy (STEM) imaging combined with energy dispersive X-ray (EDX) analysis revealed the presence of a thin high aluminium content layer at the surface of both AlGaN layers, which is believed to be related to loss of Ga during temperature ramping processes. Micro-photoluminescence studies carried out at low temperature revealed near resolution-limited peaks while time-resolved measurements on these peaks demonstrated mono-exponential decay times between 1 and 4 ns, showing that quantum dots had successfully been formed between the AlGaN barriers. Temperature-dependant measurement of the emission lines revealed that quenching of the peak often occurred at ∼60–70 K, with some of the peaks exhibiting significant line broadening whilst others remained narrow.     

荧光碳点的制备和性质及其应用研究进展

荧光碳点是继碳纳米管、纳米金刚石和石墨烯之后,最受关注的碳纳米材料之一。与传统半导体量子点相比,碳点具有优异的荧光性能、小尺寸特性、良好的生物相容性、低毒性以及表面易于化学修饰等特点,在环境检测、生物成像、药物载体、光催化及电催化技术等领域具有很好的潜在应用价值。总结了碳点合成方法、结构与性能及应用面进展,剖析了目前制约碳点应用发展的瓶颈问题,并展望了其未来的研究发展重点方向。