Luminescence of lemon‐derived carbon quantum dot and its potential application in luminescent probe for detection of Mo6+ ions

This article reports on the first attempt of a systematic study on the synthesis of carbon dots (C‐dots) for the potential applications in labeling and detection of molybdenum ion (Mo⁶⁺). Carbon dots (C‐dots) were synthesized directly via a simple hydrothermal method using lemon juices as carbon precursor with different temperatures to control the luminescence of C‐dots. The obtained C‐dots had strong green light emission and the ability to use its luminescence properties as probes for Mo⁶⁺ detection application, which is based on Mo⁶⁺ induced luminescence quenching of C‐dots. This analysis system exhibits strong sensitivity and good selectivity for Mo⁶⁺ ion, and a detection limit as low as 20 ppm is achieved. These results suggest that the present C‐dots have potential application in optoelectronic, labeling and luminescent probing of Mo⁶⁺ ions.     

Carbon dots as a luminescence sensor for ultrasensitive detection of phosphate and their bioimaging properties

Highly blue fluorescence carbon dots were synthesized by one‐step hydrothermal treatment of potatoes. The as‐obtained C‐dots have been applied to bioimaging of HeLa cells, which shows their excellent biocompatibility and low cytotoxicity. The results reveal that C‐dots are promising for real cell imaging applications. In addition, the carbon dots can be utilized as a probe for sensing phosphate. Copyright © 2014 John Wiley & Sons, Ltd.     

Fabrication of nitrogen‐ and phosphorous‐doped carbon dots by the pyrolysis method for iodide and iron(III) sensing

A facile and novel strategy to synthesize nitrogen‐ and phosphorous‐doped carbon dots (NPCDs) by single step pyrolysis method is described here. Citric acid is used as carbon source and di‐ammonium hydrogen phosphate is used as both nitrogen and phosphorous sources, respectively. Through the extensive study on optical properties, morphology and chemical structures of the synthesized NPCDs, it is found that as‐synthesized NPCDs exhibited good excitation‐dependent luminescence property, spherical morphology and high stability. The obtained NPCDs are stable in aqueous medium and possess a quantum yield of 10.58%. In this work, a new assay method is developed to detect iodide ions using the synthesized NPCDs. Here, the inner filter effect is applied to detect the iodide ion and exhibited a wide linear response concentration range (10–60 μM) with a limit of detection (LOD) of 0.32 μM. Furthermore, the synthesized NPCDs are used for the selective detection of iron(III) (Fe³⁺) ions and cell imaging. Fe³⁺ ions sensing assay shows a detection range from 0.2 to 30 μM with a LOD of 72 nM. As an efficient photoluminescence sensor, the developed NPCDs have an excellent biocompatibility and low cytotoxicity, allowing Fe³⁺ ion detection in HeLa cells.     

Investigation on the pH‐independent photoluminescence emission from carbon dots impregnated on polymer matrix

Highly luminescent, polymer nanocomposite films based on poly(vinyl alcohol) (PVA), and monodispersed carbon dots (C‐dots) derived from multiwalled carbon nanotubes (MWCNTs), as coatings on substrates as well as free standing ones are obtained via solution‐based techniques. The synthesized films exhibit pH‐independent photoluminescence (PL) emission, which is an advantageous property compared with the pH‐dependent photoluminescence intensity variations, generally observed for the C‐dots dispersed in aqueous solution. The synthesized C‐dots and the nanocomposite films are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), Fourier transform infra‐red spectroscopy ( FTIR), ultraviolet (UV) ? visible spectroscopy and photoluminescence spectroscopy (PL) techniques. The TEM image provides clear evidence for the formation of C‐dots of almost uniform shape and average size of about 8 nm, homogeneously dispersed in aqueous medium. The strong anchoring of C‐dots within the polymer matrix can be confirmed from the XRD results. The FTIR spectral studies conclusively establish the presence of oxygen functional groups on the surfaces of the C‐dots. The photoluminescence (PL) emission spectra of the nanocomposite films are broad, covering most part of the visible region. The PL spectra do not show any luminescence intensity variations, when the pH of the medium is changed from 1 to 11. The pH‐independent luminescence, shown by these films offers ample scope for using them as coatings for designing diagnostic and imaging tools in bio medical applications. The non‐toxic nature of these nanocomposite films has been established on the basis of cytotoxicity studies.     

Nitrogen‐doped carbon dots as a fluorescent probe for the highly sensitive detection of Ag+ and cell imaging

An easy hydrothermal synthesis strategy was applied to synthesize green‐yellow emitting nitrogen‐doped carbon dots (N‐CDs) using 1,2‐diaminobenzene as the carbon source, and dicyandiamide as the dopant. The nitrogen‐doped CDs resulted in improvement in the electronic characteristics and surface chemical activities. N‐CDs exhibited bright fluorescence emission and could response to Ag⁺ selectively and sensitively. Other ions produced nearly no interference. A N‐CDs based fluorescent probe was then applied to sensitively determine Ag⁺ with a detection limit of 5 × 10^?8 mol/L. The method was applied to the determination of Ag⁺ dissolved in water. Finally, negligibly cytotoxic, excellently biocompatibile, and highly fluorescent carbon dots were applied for HepG2 cell imaging and the quenched fluorescence by adding Ag⁺, which indicated its potential applications.     

Highly luminescent nitrogen‐doped carbon dots for simultaneous determination of chlortetracycline and sulfasalazine

Here, we have presented a green and facile strategy to fabricate nitrogen‐doped carbon dots (N‐CDs) and their applications for determination of chlortetracycline (CTC) and sulfasalazine (SSZ). The fluorescent N‐CDs, prepared by one‐step hydrothermal reaction of citric acid and l ‐arginine, manifested numerous excellent features containing strong blue fluorescence, good water‐solubility, narrow size distribution, and a high fluorescence quantum yield (QY) of 38.8%. Based on the fluorescence quenching effects, the as‐synthesized N‐CDs as a fluorescent nanosensor exhibited superior analytical performances for quantifying CTC and SSZ. The linear range for CTC was calculated to be from 0.85 to 20.38 μg ml^?1 with a low detection limit of 0.078 μg ml^?1. Meanwhile, the linear range for SSZ was estimated to be from 0.34 to 6.76 μg ml^?1 with a low detection limit of 0.032 μg ml^?1. Therefore, the N‐CDs hold admirable application potential for constructing a fluorescent sensor for pharmaceutical analysis.     

Metalorganic Quantum Dots and Their Graphene‐Like Derivative Porous Graphitic Carbon for Advanced Lithium‐Ion Hybrid Supercapacitor

Lithium‐ion hybrid supercapacitors (LICs) are considered as a promising candidate in energy storage systems. Taking the factor of sluggish kinetics behavior, battery‐type anode plays a significant role in improving the performance of LICs. Here, onion‐shaped graphene‐like derivatives are synthesized via carbonization of metalorganic quantum dots (MQDs) accompanied with in situ catalytic graphitization by reduced metal. Notably MQDs, exhibiting water‐soluble character and ultrafine particles (2.5–5.5 nm) morphology, are prepared by the amidation reaction. The carbonized sample exhibits highly graphitic tendency with graphitization degree up to 95.6%, and shows graphene‐like porous structure, appropriate amorphous carbon decoration characteristic, as well as N‐doping and defective nature. When employed as anode material in LICs, it shows high energy density of 83.7 Wh kg^–1 and high power density of 6527 W kg^–1 when the mass ratio of cathode to anode is 1:1 and the operating voltage ranges from 2.0 to 4.0 V. It also possesses the long cyclic stability with the energy density retention maintains at 97.3% after 10 000 cycles at 5.0 A g^–1. In addition, the energy density is further increased by altering cathode/anode mass ratio and extending working voltage. This work provides a novel strategy to develop unique carbon materials for energy storage.     

Green synthesis of fluorescent carbon dots using chloroplast dispersions as precursors and application for Fe3+ ion sensing

Water‐soluble carbon dots (CDs) were synthesized using a one‐step hydrothermal treatment of chloroplast dispersions extracted from fresh leaves as a green carbon source. The CD solution showed an emission peak centred at 445 nm when excited at 300 nm. The synthesized CDs were uniform and monodispersed with an average size of 5.6 nm. When adding ferric(III) ions (Fe³⁺) to the solution of the original CDs, the fluorescence intensity decreased significantly. Based on the linear relationship between fluorescence intensity and concentration of Fe³⁺ ions, an effective method for rapid, sensitive and selective Fe³⁺ sensing in aqueous solution could be established. Under optimum conditions, the extent of the fluorescence quenching of prepared CDs strongly depended on the Fe³⁺ ions over a wide concentration range 1.0–100.0 μM with a detection limit (3σ/k) of 0.3 μM. Furthermore, the quantitative determination of Fe³⁺ ions in environmental water samples was realized.     

Polymer composite fluorescent hydrogel film based on nitrogen‐doped carbon dots and their application in the detection of Hg2+ ions

A simple microwave‐assisted solvothermal method was used to prepare fluorescent nitrogen‐doped carbon dots (N‐CDs) with high fluorescence quantum yield (79.63%) using citric acid and N‐(2‐hydroxyethyl)ethylenediamine as starting materials. The PVAm‐g‐N‐CDs grafted products were synthesized by amide bond formation between the carboxylic groups of N‐CDs and amine groups of polyvinylamine (PVAm). Fluorescent hydrogel films (PVAm‐g‐N‐CDs/PAM) were synthesized by interpenetration polymer network polymerization of PVAm‐g‐N‐CDs and acrylamide (AM). When used for ion detection, we found that the fluorescence of the hydrogel films was clearly quenched by addition of Hg²⁺. Repeatability tests on using the hydrogel films for Hg²⁺ detection showed that they could be applied at least three times. The PVAm‐g‐N‐CDs/PAM could serve as an effective fluorescent sensing platform for sensitive detection of Hg²⁺ ions with a detection limit of 0.089 μmol/L. This work may offer a new approach for developing recoverable and sensitive N‐CDs‐based sensors for biological and environmental applications.     

Preparation of carbon quantum dots based high photostability luminescent membranes

Urethane acrylate (UA) was used to prepare carbon quantum dots (C‐dots) luminescent membranes and the resultants were examined with FT‐IR, mechanical strength, scanning electron microscope (SEM) and quantum yields (QYs). FT‐IR results showed the polyurethane acrylate (PUA) prepolymer –C = C‐vibration at 1101 cm^?1 disappeared but there was strong vibration at1687cm^?1which was contributed from the–C = O groups in cross‐linking PUA. Mechanical strength results showed that the different quantity of C‐dots loadings and UV‐curing time affect the strength. SEM observations on the cross‐sections of the membranes are uniform and have no structural defects, which prove that the C‐dots are compatible with the water‐soluble PUA resin. The C‐dot loading was increased from 0 to 1 g, the maximum tensile stress was nearly 2.67 MPa, but the tensile strain was decreased from 23.4% to 15.1% and 7.2% respectively. QYs results showed that the C‐dots in the membrane were stable after 120 h continuous irradiation. Therefore, the C‐dots photoluminescent film is the promising material for the flexible devices in the future applications.     

Dual emission carbon dots from carotenoids: Converting a single emission to dual emission

Dual emission carbon dots have a high potential for use as fluorescence‐based sensors with higher selectivity and sensitivity. This study demonstrated the possibility of conversion of a biological molecular system with a single emission peak to a double emission carbon dots system. This report is the first to describe the synthesis of dual emission carbon dots by tuning the electronic environment of a conjugated system. Here we prepared carbon dots from a natural extract, from which carotenoids were used as a new source for carbon dots. Formation of the carbon dots was confirmed by images obtained under a transmission electron microscope as well as from a dynamic light scattering study. The prepared carbon dots system was characterized and its optical property was monitored. The study showed that, after irradiation with microwaves, the fluorescence intensity of the whole system changed, without any change in the original peak position of the carotenoid but with the appearance of an additional peak. A Fourier transform infrared study confirmed breaking of the conjugated system. When using ethylene glycol as a surface passivating agent added to these carotenoid carbon dots, the dual emission spectra became more distinct.     

Fluorescent probe for detection of Cu2+ using core‐shell CdTe/ZnS quantum dots

Core‐shell CdTe/ZnS quantum dots capped with 3‐mercaptopropionic acid (MPA) were successfully synthesized in aqueous medium by hydrothermal synthesis. These quantum dots have advantages compared to traditional quantum dots with limited biological applications, high toxicity and tendency to aggregate. The concentration of Cu²⁺ has a significant impact on the fluorescence intensity of quantum dots (QDs), therefore, a rapid sensitive and selective fluorescence probe has been proposed for the detection of Cu²⁺ in aqueous solution. Under optimal conditions, the fluorescence intensity of CdTe/ZnS QDs was linearly proportional to the concentration of Cu²⁺ in the range from 2.5 × 10^–9 M to 17.5 × 10^–7 M with the limit of 1.5 × 10^–9 M and relative standard deviation of 0.23%. The quenching mechanism is static quenching with recoveries of 97.30–102.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Nitrogen and sulfur co‐doped carbon nanodots toward bovine hemoglobin: A fluorescence quenching mechanism investigation

A deep understanding of the molecular interactions of carbon nanodots with biomacromolecules is essential for wider applications of carbon nanodots both in vitro and in vivo. Herein, nitrogen and sulfur co‐doped carbon dots (N,S‐CDs) with a quantum yield of 16% were synthesized by a 1‐step hydrothermal method. The N,S‐CDs exhibited a good dispersion, with a graphite‐like structure, along with the fluorescence lifetime of approximately 7.50 ns. Findings showed that the fluorescence of the N,S‐CDs was effectively quenched by bovine hemoglobin as a result of the static fluorescence quenching. The mentioned quenching mechanism was investigated by the Stern‐Volmer equation, temperature‐dependent quenching, and fluorescence lifetime measurements. The binding constants, number of binding sites, and the binding average distance between the energy donor N,S‐CDs and acceptor bovine hemoglobin were calculated as well. These findings will provide for valuable insights on the future bioapplications of N,S‐CDs.     

Facile preparation of high fluorescent carbon quantum dots from orange waste peels for nonlinear optical applications

A facile and eco‐friendly hydrothermal method was used to prepare carbon quantum dots (CQDs) using orange waste peels. The synthesized CQDs were well dispersed and the average diameter was 2.9 ± 0.5 nm. Functional group identification of the CQDs was confirmed by Fourier transform infrared spectrum analysis. Fluorescence properties of the synthesized CQDs exhibited blue emission. The fluorescence quantum yield of the CQDs was around 11.37% at an excitation wavelength of 330 nm. The higher order nonlinear optical properties were examined using a Z‐scan technique and a continuous wave laser that was operated at a wavelength of 532 nm. Results demonstrated that the synthesis of CQDs can be considered as promising for optical switching devices, bio‐scanning, and bio‐imaging for optoelectronic applications.     

Superior Oxygen Reduction Reaction on Phosphorus‐Doped Carbon Dot/Graphene Aerogel for All‐Solid‐State Flexible Al–Air Batteries

Carbon dots have been recognized as one of the most promising candidates for the oxygen reduction reaction (ORR) in alkaline media. However, the desired ORR performance in metal–air batteries is often limited by the moderate electrocatalytic activity and the lack of a method to realize good dispersion. To address these issues, herein a biomass‐deriving method is reported to achieve the in situ phosphorus doping (P‐doping) of carbon dots and their simultaneous decoration onto graphene matrix. The resultant product, namely P‐doped carbon dot/graphene (P‐CD/G) nanocomposites, can reach an ultrahigh P‐doping level for carbon nanomaterials. The P‐CD/G nanocomposites are found to exhibit excellent ORR activity, which is highly comparable to the commercial Pt/C catalysts. When used as the cathode materials for a primary liquid Al–air battery, the device shows an impressive power density of 157.3 mW cm^?2 (comparing to 151.5 mW cm^?2 of a similar Pt/C battery). Finally, an all‐solid‐state flexible Al–air battery is designed and fabricated based on our new nanocomposites. The device exhibits a stable discharge voltage of ≈1.2 V upon different bending states. This study introduces a unique biomass‐derived material system to replace the noble metal catalysts for future portable and wearable electronic devices.     

Carbon quantum dots fluorescence quenching for potassium optode construction

Photophysical phenomena associated with carbon nanoparticles in combination with biocompatibility and readily functionalizable properties have attracted significant interest for sensing and imaging applications. A potassium ion optode based on the fluorescence quenching of carbon quantum dots (CQDs) was constructed. The CQDs were synthesized using a microwave method, citric acid and 2,2′‐(ethylene‐dioxy)bis(ethylamine). A quantum yield of 7.1% was calculated for the synthesized CQDs. A linear dynamic range of about one‐order of magnitude with a correlation coefficient of 0.99 was obtained. The optode was applied on real samples and a 0.60–1.60% error range was obtained relative to the ion‐selective electrode.     

Carbon‐Sheathed MoS2 Nanothorns Epitaxially Grown on CNTs: Electrochemical Application for Highly Stable and Ultrafast Lithium Storage

Molybdenum disulfide (MoS₂), which possesses a layered structure and exhibits a high theoretical capacity, is currently under intensive research as an anode candidate for next generation of Li‐ion batteries. However, unmodified MoS₂ suffers from a poor cycling stability and an inferior rate capability upon charge/discharge processes. Herein, a unique nanocomposite comprising MoS₂ nanothorns epitaxially grown on the backbone of carbon nanotubes (CNTs) and coated by a layer of amorphous carbon is synthesized via a simple method. The epitaxial growth of MoS₂ on CNTs results in a strong chemical coupling between active nanothorns and carbon substrate via C? S bond, providing a high stability as well as a high‐efficiency electron‐conduction/ion‐transportation system on cycling. The outer carbon layer can well‐accommodate the structural strain in the electrode upon lithium‐ion insertion/extraction. When employed as an anode for lithium storage, the prepared material exhibits remarkable electrochemical properties with a high specific capacity of 982 mA h g^?1 at 0.1 A g^?1, as well as excellent long‐cycling stability (905 mA h g^?1 at 1 A g^?1 after 500 cycles) and superior rate capability, confirming its potential application in high‐performance Li‐ion batteries.     

A dual‐readout nanosensor based on biomass‐based C‐dots and chitosan@AuNPs with hyaluronic acid for determination of hyaluronidase

A dual‐signal strategy is proposed based on fluorescent biomass‐based carbon dots (BC‐dots) and chitosan stabilized AuNPs (CS@AuNPs) to determine hyaluronidase (HAase). BC‐dots can induce aggregation of CS@AuNPs nanoparticles with a colour change from red to blue. Positively charged CS@AuNPs interacted with the negatively charged hyaluronic acid (HA) through electrostatic adsorption, and CS@AuNPs maintained stability due to the semirigid coil conformation of HA. However, in the presence of HAase, due to enzymatic hydrolysis of HA by HAase, the CS@AuNPs agglomerated. Based on the change of fluorescence and colour, quantitative analysis of HAase was achieved. Linear ranges for the fluorometric and colorimetric determinations were 2.0–70 U mL^?1 and 8–60 U mL^?1, respectively, with a detection limit of 0.27 U mL^?1. This dual‐signal sensing system possesses high potential for determination of HAase in biological matrices.     

Dual doped biocompatible multicolor luminescent carbon dots for bio labeling,UV‐active marker and fluorescent polymer composite

We report on metal–non‐metal doped carbon dots with very high photoluminescent properties in solution. Magnesium doping to tamarind extract associated with nitrogen‐doping is for the first time reported here which also produce very high quantum yield. Our aim is to develop such dual doped carbon dots which can also serve living cell imaging with easy permeation towards cells and show non‐cytotoxic attributes. More importantly, the chemical signatures of the carbon dots unveiled in this work can support their easy solubilization into water; even in sub‐ambient temperature. The cytotoxicity assay proves the almost negligible cytotoxic effect against human cell lines. Moreover, the use of carbon dots in UV‐active marker and polymer composites are also performed which gave clear distinguishable features of fluorescent nanoparticles. Hitherto, the carbon dots can be commercially prepared without adopting any rigorous methods and also can be used as non‐photo‐bleachable biomarkers of living cells.     

Fluorescence sensing of nitric oxide in aqueous solution by triethanolamine‐modified CdSe quantum dots

The water‐soluble luminescent CdSe quantum dots were prepared by ligand exchange with triethanolamine (TEA). Oxygen can reversibly enhance the fluorescence of the synthesized quantum dots (TEA‐CdSe‐QDs) in aqueous solution. Nitric oxide radical (NO) can react easily with dissolved oxygen in water and was found to have a significant quenching effect on the fluorescence of the TEA‐CdSe‐QDs. The fluorescence responses were concentration‐dependent and can be well described by the typical Stern–Volmer equation. A good linear relationship (R² = 0.9963) was observed over the range 5.92 × 10^?7 to 1.85 × 10^?5 mol/L nitric oxide. Above this concentration was a second linear region ranging from 2.12 × 10^?5 to 1.12 × 10^?4 mol/L NO with a gentler slope. The detection limit, calculated following the 3σ IUPAC criteria, was 3.02 × 10^?7 mol/L. The interference effect of some common interferents such as nitrite (NO₂^?), nitrate (NO₃^?), glucose and l ‐ascorbic acid on the detection of NO was negligible for the proposed system, demonstrating the potential utility of this probe for the detection of NO in biological systems. The possible mechanism was also discussed. Copyright © 2009 John Wiley & Sons, Ltd.