发光碳量子点的合成、性质和应用

基于碳量子点具有良好的水溶性、化学惰性、低毒性、易于功能化和抗光漂白性等优异性能,碳量子点和其它的碳纳米材料（如富勒烯、碳纳米管和石墨烯等）同样引起了研究者广泛的关注。 碳量子点可以通过很多较为廉价的一步法进行大规模的制备,包括化学氧化法、超声法、微波法和激光烧蚀法等。 本文主要介绍了不同碳量子点的合成方法,以及依赖于碳量子点尺寸和波长等性质的发光性能,并且讨论了碳量子点在生物成像、光催化、能量转换/储存、光电子、光限幅和传感器等方面的应用。     

Facile one-pot synthesis of PEG-derived carbon dots with enhanced luminescence via L-cysteine-assisted S,N-doping

Carbon nanodots (C-dots) are promising photoluminescent nanomaterials for biomedical applications. Among them, PEG-derived C-dots demonstrate exceptional photoluminescence and passivation properties, making them particularly attractive for use in the biomedical field. In this article, we present the synthesis of photoluminescent S,N-doped PEG-derived carbon dots that are stable at ambient temperature and can be produced using an easy hydrothermal technique. To synthesize the carbon dots, the non-hazardous polymer polyethylene glycol (PEG) was used as the sole precursor rather than any other potentially hazardous compounds. The absence of L-cysteine in the reaction mixture resulted in carbon dots with no significant absorbance in the visible region but exhibited photoluminescence properties with a maximum excitation and emission at 343 and 452 nm, respectively. However, the addition of L-cysteine resulted in a visible absorbance and a red shift in both the maximum excitation and emission, at around 435 and 503 nm, respectively. The Fourier transform infrared spectroscopy (FTIR) analysis provided evidence for the presence of -SH, -SO₂, -NH₂, and CON-H bond stretching after the addition of L-cysteine, suggesting possible S,N-doping of the carbon dots, which likely caused the observed changes in photoluminescence properties. These findings contribute to the understanding of S,N-doping in carbon dots and highlight their potential applications in optoelectronics, sensing, and biomedical imaging.     

Spectroscopic and Isothermal Titration Calorimetry Studies of Binding Interactions Between Carbon Nanodots and Serum Albumins

Carbon nanodots (C-dots) have attracted great attention as a new class of luminescent nanomaterials. In order to better understand the basic behavior of C-dots in biological systems, the binding characteristics of C-dots with bovine serum albumin (BSA) and human serum albumin (HSA) were investigated using spectroscopic approaches and isothermal titration calorimetry at pH 7.4. We found that the intrinsic fluorescence of BSA and HSA was quenched by the C-dots with a dynamic quenching mode. It was proved that the C-dots had little influence on the conformation of BSA and HSA by their UV–vis and circular dichroism spectra. Some important thermodynamic parameters were calculated, and the positive values of ΔH° and ΔS° indicate that the binding process was endothermic, and that the interaction was driven by favorable entropy and unfavorable enthalpy. It also showed that the hydrophobic force played a major role in the binding process.     

Synthesis of fluorescent carbon dots via microwave carbonization of citric acid in presence of tetraoctylammonium ion,and their application to cellular bioimaging

A jelly-like form of carbon dots (C-dots) was prepared by microwave-assisted synthesis from citric acid in the presence of tetraoctylammonium bromide. The effect of the concentration of tetraoctylammonium bromide was examined. The synthesized carbon dots were characterized by UV–vis, XRD, FTIR, fluorescence and HR-TEM. Fluorescence extends from 350 to 600 nm, and the corresponding excitation wavelengths range from 300 to 460 nm. Quantum yields are at around 0.11. A cytotoxicity study showed carbon dots to be cell permeable and biocompatible which renders them appropriate for imaging applications. The dots were used to image HeLa cell lines via the blue fluorescence of the dots.

C-dots were synthesized from citric acid by microwave heating in presence of varying concentrations of tetraoctylammonium bromide (TOAB) as a micellar template. The excellent optical properties of the nanoparticles make them well suitable for bio-imaging of HeLa cells.

     

Carbon Dots from Citric Acid and its Intermediates Formed by Thermal Decomposition

Thermal decomposition of citric acid is one of the most common synthesis methods for fluorescent carbon dots; the reaction pathway is, however, quite complex and the details are still far from being understood. For instance, several intermediates form during the process and they also give rise to fluorescent species. In the present work, the formation of fluorescent C-dots from citric acid has been studied as a function of reaction time by coupling infrared analysis, X-ray photoelectron spectroscopy, liquid chromatography/mass spectroscopy (LC/MS) with the change of the optical properties, absorption and emission. The reaction intermediates, which have been identified at different stages, produce two main emissive species, in the green and blue, as also indicated by the decay time analysis. C-dots formed from the intermediates have also been synthesised by thermal decomposition, which gave an emission maximum around 450 nm. The citric acid C-dots in water show short temporal stability, but their functionalisation with 3-aminopropyltriethoxysilane reduces the quenching. The understanding of the citric acid thermal decomposition reaction is expected to improve the control and reproducibility of C-dots synthesis.     

Silicon-hybrid carbon dots strongly enhance the chemiluminescence of luminol

We report on a 4-min microwave pyrolytic method for the preparation of fluorescent and water-soluble silicon-hybrid carbon dots (C-dots) with high fluorescent quantum yield. The material is prepared by preheating aminopropyltriethoxysilane and ethylene diamine tetraacetic acid for 1 min, then adding a mixture of poly(ethylene glycol) and glycerin to the solution and heating for another 3 min. It is found that the hybrid carbon dots strongly enhance the chemiluminescence (CL) of the luminol/N-bromosuccinimide system. A study on the enhancement mechanism via CL, fluorescence and electron paramagnetic resonance spectroscopy showed that the effect most probably is due to electrostatic interaction between the C-dots and the luminol anion which facilitates electron transfer from luminol anion to the N-bromosuccinimide oxidant. CL intensity is linearly related to the concentration of the C-dots in the range between 1.25 and 20 μg mL^?1. The detection limit is 0.6 μg mL^?1 (at an S/N of 3).

Fluorimetric detection of pathogenic bacteria using magnetic carbon dots

A novel and facile approach of pathogenic bacteria detection, which utilizes fluorescent sensing and bacteria capture with Magnetic carbon dots (Mag-CDs), was proposed in this work. Magnetic nanoparticles were synthesized and then decorated with C-dots, and further functionalized with amine groups (chitosan). In this way, bacteria were strongly anchored on the hybrid material Mag-CDs for highly sensitive fluorescent detection. The Mag-CDs were characterized by UV–vis, FT-IR spectra, TEM images, XRD, and EDX. The characterizations validate the fabrication of amine-Mag-CDs and the promising applications of this material. Fluorescence spectroscope and MALDI-MS were used for the detection and identification of bacterial strains, respectively. The limit of detection for Staphylococcus aureus and Escherichia coli was found to be 3 × 10² and 3.5 × 10² cfu mL⁻¹, respectively. With these encouraging results, it is expected that it would open revenues for promising applications of Mag-CDs nanomaterial.     

Usage of coconut coir for sustainable production of high-valued carbon dots with discriminatory sensing aptitude toward metal ions

A novel, remunerative, green synthetic methodology without adding any kind of extra chemical reagent has been proposed for synthesizing highly fluorescent carbon dots (C-dots) by using waste coconut coir as the C source. The as-prepared C-dots possessed a quantum yield of 48% and displayed a high solubility rate in aqueous media. The fluorescence emission intensity of C-dots is found to be excitation dependent with high stability toward variation in the reaction medium. The bio-congenial performance of C-dots has been checked by employing quantitative multi-assay including fungus, plant, aquatic animals, and at the chromosomal level. It is notable that the antibacterial studies of as-prepared C-dots have presented appealing outcomes which advocated that C-dots prepared using coconut coir are biocompatible toward both Gram-positive and Gram-negative bacteria with no zone of inhibition in much higher concentration ranges. The as-prepared C-dots provided a simple sensing podium with high sensitivity and selectivity with a “turn-on and turn-off” fluorescence response for the detection of dual metal ions i.e. cadmium and copper ions in aqueous media with a detection limit of 0.18 nM and 0.28 nM, respectively. The practical utilities of the biocompatible C-dots have been checked over different water resources. The current work provided a simple, easy, bio-matched, harmless, rapid, and cost-effective sensory probe for wastewater remediation.     

Ratiometric fluorescence detection of mercuric ion based on the nanohybrid of fluorescence carbon dots and quantum dots

A novel nanohybrid ratiometric fluorescence probe comprised of carbon dots (C-dots) and hydrophilic CdSe@ZnS quantum dots (QDs) has been developed by simply mixing the blue-emission C-dots with red-emission carboxylmethyldithiocarbamate modified CdSe@ZnS QDs (GDTC-QDs). The nanohybrid ratiometric fluorescence probe exhibits dual emissions at 436 nm and 629 nm under a single excitation wavelength. Due to the strong chelating ability of GDTC on the surface of QDs to mercuric ion (Hg²⁺), the fluorescence of the GDTC-QDs in the nanohybrid system could be selectively quenched in the presence of Hg²⁺ while the fluorescence of the C-dots remained constant, resulting in an obviously distinguishable fluorescence color evolution (from red to blue) of the nanohybrid system. The detection limit of this method was found to be as low as 0.1 μM. Furthermore, the recovery result for Hg²⁺ in real samples including tap water and lake water by this method was satisfying, suggesting its potential application for Hg²⁺ sensing.     

Electrochemical preparation of a novel type of C-dots/ZrO2 nanocomposite onto glassy carbon electrode for detection of organophosphorus pesticide

A selective, sensitive novel electrochemical sensor for detection of methyl parathion on the preparation of a carbon dots (C-dots)/ZrO₂ nanocomposite was developed. The C-dots/ZrO₂ nanocomposite was fabricated using electrochemical deposition onto a glassy carbon electrode and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and cyclic voltammetry. The optimum parameters such as effect of pH, accumulation time, accumulation potential, scan rate, effect of amount of C-dots and effect of amount of ZrO₂ were investigated. The C-dots/ZrO₂ modified glassy carbon electrode allowed rapid, selective determination of methyl parathion in rice samples by adsorptive stripping voltammetry. The stripping response was highly linear over the methyl parathion concentrations ranging from 0.2 ng mL^?1 to 48 ng mL^?1, with a detection limit of 0.056 ng mL^?1. This novel electrochemical nanocomposite-based electrochemical sensor was successfully applied for the detection of methyl parathion in rice samples.     

Carbon dots-based fluorescent probes for sensitive and selective detection of iodide

We report on a simple method for the determination of iodide in aqueous solution by exploiting the fluorescence enhancement that is observed if the complex formed between carbon dots and mercury ion is exposed to iodide. Fluorescent carbon dots (C-dots) were treated with Hg(II) ion which causes quenching of the emission of the C-dots. On addition of iodide, the Hg(II) ions are removed from the complex due to the strong interaction between Hg(II) and iodide. This causes the fluorescence to be restored and enables iodide to be determined in the 0.5 to 20 μM concentration range and with a detection limit of ~430 nM. The test is highly selective for iodide (over common other anions) and was used for the determination of iodide in urine.

Aminophenol-based carbon dots with dual wavelength fluorescence emission for determination of heparin

We describe the preparation of carbon quantum dots (C-dots) by a one-step hydrothermal method starting from o-aminophenol as the precursor. The C-dots exhibit bright both blue fluorescence (with excitation/emission peaks at 300/410 nm and with quantum yield of 0.40) and green fluorescence (420/500 nm; QY 0.28) without any other element doping. The unique emission properties are attributed to a synergistic effect of amino and hydroxy groups on the surface of the C-dots. The C-dots are shown to be viable fluorescent probes for heparin. The positively charged surface amino groups are assumed to interact with sulfate and carboxy groups in heparin via electrostatic interactions and hydrogen bonding. This causes the blue fluorescence of C-dots to be turned off (quenched). Fluorescence is strongest at a pH value of 6. The fluorometric calibration plot is linear in the 10 to 100 nM concentration range, with an 8.2 nM detection limit (at a signal-to-noise ratio of 3).

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Graphical abstract Carbon quantum dots with dual fluorescence emission bands were synthesized and are shown to be a viable fluorescent probe for heparin.

     

Dually emissive P,N-co-doped carbon dots for fluorescent and photoacoustic tissue imaging in living mice

Hydrothermal treatment of a mixture of ethylene diamine, phosphoric acid and citric acid under ambient pressure generates fluorescent carbon dots that are co-doped with phosphorus and nitrogen. These have features such as (a) both green fluorescence (peaking at 430 nm; 30% quantum yield) and red fluorescence (peaking at 500 nm, quantum yield 78%), (b) wavelength-dependent emission peaks, and (c) insensitivity to changes of pH values, dot concentration and ionic strength. The C-dots are useful for both fluorescent (FL) and photoacoustic (PA) imaging of living tissue. PA imaging warrants better spatial resolution and allows deeper tissues to be imaged compared to most optical imaging techniques. It is essential to assign a photoacoustic contrast agent as most of the diseases do not show a natural photoacoustic contrast in their early stage. The dually emitting C-dots are shown to be a useful contrast agent for PA and FL imaging of mice tumors. Intravenous administration of the C-dots resulted in strong signals in both PA and FL imaging.

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Graphical abstract Photographs of the excitation wavelength-dependent fluorescence of P,N-doped C-dots obtained from ethylenediamine, phosphoric acid and citric acid. Intravenous administration of the C-dots resulted in strong signals in both photoacoustic (PA) and fluorescent (FL) imaging.

     

Carbon Dot Nanozymes: How to Be Close to Natural Enzymes

The design, catalytic process, and property study of nanozymes are of importance for both fundamental research and application demand. Here, the peroxidase-mimicking properties of a series of carbon dots (C-dots) was systematically investigated and they were found to be probably closer to their natural counterparts, as compared to the known corresponding nanozymes. Firstly, four kinds of metal-free and surface-modulated C-dots were bottom-up fabricated using glucose, α-cyclodextrin (CD), β-CD, and γ-CD as precursors, respectively, and their formation processes, structures, as well as surface chemistry were investigated. Secondly, in the peroxidase-mimicking catalytic system, no hydroxyl radicals were produced, which indicates a different and special catalytic mode. By employing a joint experimental–theoretical study, a probable catalytic mechanism is proposed. Thirdly, the present C-dots maintained well their catalytic activity even in complicated serum matrices because their catalytic performances are completely irrelevant of any cation-related binding sites. Finally, the catalytic performances of the as-prepared C-dots were modulated by either pre-engineering NP surface structures or subsequently introducing photo-regulated host–guest reactions.     

Immobilised cytochrome c on the carbon dots functionalised MWCNTs and its application to hydrogen peroxide detection

We have prepared the carbon dots (C-dots) by sucrose carbonisation in the presence of sulphuric acid and characterised by transmission electron microscopy, Fourier transform infrared spectra, fluorescence spectroscopy and UV–vis absorption spectra. The resultant C-dots could be used for the activation of CNTs. Cytochrome c immobilised on the functionalised multi-walled carbon nanotube-modified glass carbon electrode was found to promote electron transfer that was investigated by electrochemical methods. The obtained electrode can also be used as a biocatalyser to catalyse the reduction of H₂O₂. Under optimal conditions, the constructed H₂O₂ sensor showed two linear segments and the detection limits were 1.02 and 2.59 µM (S/N = 3). The modified electrode displayed high stability, good reproducibility and excellent selectivity.     

Enhanced visible-light-driven photocatalytic activities of 0D/1D heterojunction carbon quantum dot modified CdS nanowires

Zero-dimensional carbon dots (0D C-dots) and one-dimensional sulfide cadmium nanowires (1D CdS NWs) were prepared by microwave and solvothermal methods, respectively. A series of heterogeneous photocatalysts that consisted of 1D CdS NWs that were modified with 0D C-dots (C-dots/CdS NWs) were synthesized using chemical deposition methods. The mass fraction of C-dots to CdS NWs in these photocatalysts was varied. The photocatalysts were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. Their photocatalytic performance for the spitting of water and the degradation of rhodamine B (RhB) under visible light irradiation were investigated. The photocatalytic performance of the C-dots/CdS NWs was enhanced when compared with that of the pure CdS NWs, with the 0.4% C-dots/CdS NWs exhibiting the highest photocatalytic activity for the splitting of water and the degradation of RhB. The enhanced photocatalytic activity was attributed to a higher carrier density because of the heterojunction between the C-dots and CdS NWs. This heterojunction improved the electronic transmission capacity and promoted efficient separation of photogenerated electrons and holes.     

Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices

Similar to the popular older cousins, luminescent carbon dots (C-dots), graphene quantum dots or graphene quantum discs (GQDs) have generated enormous excitement because of their superiority in chemical inertness, biocompatibility and low toxicity. Besides, GQDs, consisting of a single atomic layer of nano-sized graphite, have the excellent performances of graphene, such as high surface area, large diameter and better surface grafting using π-π conjugation and surface groups. Because of the structure of graphene, GQDs have some other special physical properties. Therefore, studies on GQDs in aspects of chemistry, physical, materials, biology and interdisciplinary science have been in full flow in the past decade. In this Feature Article, recent developments in preparation of GQDs are discussed, focusing on the main two approaches (top-down and bottom-down). Emphasis is given to their future and potential development in bioimaging, electrochemical biosensors and catalysis, and specifically in photovoltaic devices that can solve increasingly serious energy problems.     

Enhanced voltammetric determination of dopamine using a glassy carbon electrode modified with ionic liquid-functionalized graphene and carbon dots

The authors describe a dopamine (DA) sensor based on a glassy carbon electrode modified with a composite film composed of carbon dots (C-dots) and graphene functionalized with an ionic liquid. The C-dots were functionalized with carboxy groups whose negative charge promotes electrostatic attraction to the protonated amino groups in DA. The presence of an imidazole cation in the IL facilitates interaction with the C-dots and DA via electrostatic interactions and π-stacking forces. Under optimal conditions, the modified GCE display improved electrochemical response to DA compared to a bare GCE, or a GCE modified with C-dots or IL-graphene only. The oxidation current, measured best at a potential of 0.22 V (vs. Ag/AgCl) is linearly related to the DA concentration in the 0.1 to 600 μM range, with a 30 nM detection limit at a signal-to-noise ratio of 3. Ascorbic acid does not interfere even in large excess, and the sensor is stable for at least a month. The modified GCE was applied to the determination of DA in spiked fetal bovine serum and gave satisfactory results.     

Strong enhancement of the chemiluminescence of the cerium(IV)-thiosulfate reaction by carbon dots,and its application to the sensitive determination of dopamine

We show that the very weak chemiluminescence (CL) of the Ce(IV)-thiosulfate system is enhanced by a factor of ~150 in the presence of fluorescent carbon dots (C-dots). The C-dots were prepared by a solvothermal method and characterized by fluorescence spectra and transmission electron microscopy. Possible mechanisms that lead to the effect were elucidated by recording fluorescence and CL spectra. It is found that dopamine at even nanomolar levels exerts a diminishing effect on the enhancement of CL. This was exploited to design a method for the determination of dopamine in the concentration range from 2.5 nM to 20 μM, with a limit of detection (at 3 s) of 1.0 nM. Dopamine was determined by this method in spiked human plasma samples with satisfactory results.

Polymerization-Driven Photoluminescence in Alkanolamine-Based C-Dots

Carbonized polymer dots (CPDs), a peculiar type of carbon dots, show extremely high quantum yields, making them very attractive nanostructures for application in optics and biophotonics. The origin of the strong photoluminescence of CPDs resides in a complicated interplay of several radiative mechanisms. To understand the correlation between CPD processing and properties, the early stage formation of carbonized polymer dots has been studied. In the synthesis, citric acid monohydrate and 2-amino-2-(hydroxymethyl)propane-1,3-diol have been thermally degraded at 180 °C. The use of an oil bath instead of a more traditional hydrothermal reactor has allowed the CPD properties to be monitored at different reactions times. Transmission electron microscopy, time-resolved photoluminescence, nuclear magnetic resonance, infrared, and Raman spectroscopy have revealed the formation of polymeric species with amide and ester bonds. Quantum chemistry calculations have been employed to investigate the origin of CPD electronic transitions. At short reaction times, amorphous C-dots with 80 % quantum yield, have been obtained.