Multicolor Carbon Research Articles

Distinctive optical transitions of tunable multicolor carbon dots.

Three types of carbon dots (CDs) are synthesized from isomers of phenylenediamine to develop multicolor nanomaterials with low toxicity, high stability, and high quantum yield. The distinctive electronic structures of CDs lead to the characteristic optical transitions, such as three colors of blue, green, and red, which are primarily attributed to the difference in configurations, despite the similar basic structures of conjugated systems. The excitation-independent emission and the single exponential decay of CDs indicate the single chromophore-like nature in each type of CD. In addition, the two-photon luminescence of CDs exhibits a comparable shape and time profile to the typical photoluminescence with high photostability. Although the surface-related defect states are observed by intragap excitation, the contribution of defect states is barely observed in the emission profile upon band gap excitation. Consequently, the controllability of optical transitions in CDs enhances the potential of tunable multicolor nanomaterials for various applications as alternatives to quantum dots containing toxic elements.

Rational Design of Surface-State Controlled Multicolor Cross-Linked Carbon Dots with Distinct Photoluminescence and Cellular Uptake Properties.

We disclose for the first time a facile synthetic methodology for the preparation of multicolor carbon dots (CDs) from a single source barring any chromatographic separations. This was achieved via sequential intraparticle cross-linking of surface abundant carboxylic acid groups on the CDs synthesized from a precursor to control their photoluminescence (PL) spectra as well as affect their degree of cellular internalization in cancer cells. The change in PL spectra with sequential cross-linking was projected by theoretical density functional theory (DFT) studies and validated by multiple characterization tools such as X-ray photoelectron spectroscopy (XPS), PL spectroscopy, ninhydrin assay, etc. The variation in cellular internalization of these cross-linked CDs was demonstrated using inhibitor assays, confocal microscopy, and flow cytometry. We supplemented our findings with high-resolution dark-field imaging to visualize and confirm the colocalization of these CDs into distinct intracellular compartments. Finally, to prove the surface-state controlled PL mechanisms of these cross-linked CDs, we fabricated a triple-channel sensor array for the identification of different analytes including metal ions and biologically relevant proteins.

One-step synthesis of nitrogen-doped multi-emission carbon dots and their fluorescent sensing in HClO and cellular imaging.

Tunable multicolor carbon dots (CDs) with a quantum yield reach up to 35% were generated directly from rhodamine and urea via one-step hydrothermal approach and purified through silica gel column chromatography. Transmission electron microscopy images reveal that the as-prepared CDs possess a small size distribution below 10nm with bright blue, green, and yellow color emission, designated as b-CDs, g-CDs, and y-CDs, respectively. The in-depth investigations reveal that the multicolor emission CDs with different fraction displays fluorescence emission wavelength ranges from 398nm (b-CDs), 525nm (g-CDs), to 553nm (y-CDs) which could be well modulated by controlling the amount of heteroatom nitrogen especially amino nitrogen onto their surface structures. Further experiments verify the important role of nitrogen content by using rhodamine solely or substituting urea with sulfur containing compounds as precursors to produce corresponding CDs since the performance is lower than that of urea incorporation. Theoretical calculation results also reveal that the increasing amount of amino nitrogen into their surface structures of b-CDs, g-CDs to y-CDs is responsible for reduced band gaps energy, which result in the redshifted wavelength. Benefiting from the excellent photoluminescence properties, wide pH variation range, high photo stability, and low toxicity, these CDs were employed for HClO sensing at 553nm within the range 5 to 140μM with a limit of detection (LOD) of 0.27 ± 0.025μM (n = 3) and multicolor cellular imaging in HeLa cells. Tunable multicolor carbon dots (CDs) were generated directly from rhodamine and urea via one-step hydrothermal approach and purified through silica gel column chromatography. The as-prepared CDs exhibit bright blue, green, and yellow color emission which could be well modulated by controlling the increasing incorporation of heteroatom nitrogen especially amino nitrogen into their surface structures. These CDs were employed for HClO sensing and demonstrated to multicolor cellular imaging in HeLa cells.

Multicolor Carbon Dots Prepared by Single‐Factor Control of Graphitization and Surface Oxidation for High‐Quality White Light‐Emitting Diodes

AbstractIn a reaction system of asparagine and p‐phenylenediamine, blue, green, orange, and red carbon dots (CDs) are successfully prepared by adjusting the ratio of the two reactants. The obtained multicolor CDs have excellent stable properties and high fluorescence quantum yields (up to 74%). Clarifying the luminescence mechanism of multicolor CDs is based on the investigation of the composition, structure, and fluorescence properties of the CDs combined with quantum chemistry calculations. A high‐quality white light‐emitting diode (LED) with CIE color coordinates (0.33, 0.33) is successfully fabricated using a multicolor CD‐epoxy resin composite, and its color temperature and color rendering index are 5612 K and 94, respectively. It is of great significance to prepare multicolor CDs with high fluorescence quantum yields by single‐factor control. Using multicolor CDs to fabricate standard white LEDs close to sunlight will significantly increase the practical application value of CDs.

Multi-color carbon dots from cis-butenedioic acid and urea and highly luminescent carbon dots@Ca(OH)2 hybrid phosphors with excellent thermal stability for white light-emitting diodes

Multi-color carbon dots (CDs) were obtained from cis-butenedioic acid (C-BA) and urea by a simple one-step solvothermal route. This is the first report about using C-BA as the raw material to synthesize CDs. The fluorescent properties of as-prepared CDs vary with the ratio of C-BA to urea. And the sample with 1:1 of C-BA and urea exhibits the optimal yellow emission and highest photoluminescence quantum yield (PLQY) of 16.17%. Further investigations indicate that the variation of PL properties comes from the different content of surface functional groups such as CC, CO, –COOH, and graphitic N. Both of CO and graphitic N cause the PL redshift, whereas the CC and –COOH groups lead to the blueshift. Based on the above, a solid-state luminescent CDs-based phosphor was obtained by adding alkaline Ca(OH)2 powder to as-prepared yellow CDs solution. In this case, PL intensity is increased to 1.78 times, the corresponding emission redshifts from 553 to 568 nm, and PLQY is improved from 16.17% to 35.12%. Simultaneously, CDs@Ca(OH)2 hybrid phosphor exhibits good thermostability. Finally, a white light-emitting diode (LED) was successfully fabricated by combining the yellow-emission CDs-based phosphor with a blue LED chip. It exhibits a high CRI of 86.9, the CCT of 5388 K, and a luminous efficacy of 15.12 lm/W. The CIE coordinate locates at (0.3341, 0.3075) which is very close to the pure white light of (0.33, 0.33). Thus, as-prepared CDs-based solid-state phosphor is a potential candidate for practical lighting devices.

Multicolor carbon dots: Induced by sp2-sp3 hybridized domains and their application in ion detection and WLED

The heteroatom doping to achieve carbon dots has played a crucial part in exploring the fluorescence mechanism of multicolor emissions, which is closely related with the application development of such materials. Herein, a facile method was developed to synthesize three-color CDs via the co-doping of S and N elements with the assistance of ethanol solvent. The achieved CDs mainly emit blue, green and red lights, yet the photoluminescence quantum yields (FLQYs) generally decreases from 54.0% for B-CDs, 11.5% for G-CDs to 8.5% for R-CDs, respectively. Optical and structural characterizations of these CDs show that the variation of sp 2 - sp 3 hybridized domains resulted from the elements doping (S, N) fundamentally determines the color of CDs. The highest FLQYs of B-CDs in all CDs gives it an advantage to detect metal ions, which finds strong quenching only toward Ag + because of the interaction between Ag + and the cores. Likewise, these multicolor CDs attempt to fabricate CDs@PVP composite film emitted white-light as well as a clad material of WLED devices. The results indicate that the correlated color temperatures (CCT) in the ranges of cold and warm white lights can be obtained with different color rendering indexes (CRI) and luminous efficacies. Moreover, CCT and CRI are nearly up to the extremum at the same current value with increasing currents, implying that such CDs are potential as a kind of fluorescent nanomaterial to applicate to WLED device. • A facile strategy to prepare multicolor CDs by means of S and N co-doping. • As-prepared CDs emitted blue, green and red fluorescence respectively. • The B-CDs with high sensitivity for ion detections of Ag +. • Multicolor CDs fabricate the white-light CDs/PVP composite film as well as a clad material of WLED devices.

Mass production of highly fluorescent full color carbon dots from the petroleum coke

Heavy oil is treated as an undesirable raw material in traditional refining markets because of its low yield. However, its rich natural aromatic structure and heteroatomic compounds make it possible to be a precursor to large-scale production of carbon materials. Using heavy oil and three SDA products as precursors, we synthesized highly fluorescent multi-color carbon dots (CDs) by hydrothermal method, which can precisely control the photoluminescence wavelength in the range of 350−650nm. The synthesized carbon dots have the advantages of good long-term stability and stability under extreme pH conditions and low price. Importantly, the carbon dots synthesized with asphalt as the precursor have the highest fluorescence quantum yield. X-ray photoelectron spectroscopy (XPS) is used to elucidate the effects of different precursor on emission color change and photoluminescence quantum yield (PLQY), thus providing a controlled tuning of the system for the functionalization of CDs. And we further used the CDs in macrophage labeling. This pathway gives a reliable and repeatable industry possibility and may boost the applications of CDs into reality.

Effect of Solvent-Derived Highly Luminescent Multicolor Carbon Dots for White-Light-Emitting Diodes and Water Detection.

Recently, the multicolor fluorescent carbon dots (CDs) have drawn much attention due to their various applications. Herein, we report multicolor emissive CDs by solvent-controlled and solvent-responded approaches. The blue to red color emissive CDs are obtained by the solvothermal method by varying the solvent during the reaction. The red color emissive CDs (R-CDs) with good quantum yield is obtained in a water medium. The detailed characterization revealed that the solvent controls the particle size, band gap, and nitrogen doping concentration. Specifically, in the protic solvent, the high N content and presence of imine nitrogen are the reason for red emission. However, in an aprotic solvent, the least N doping and a lack of C-O groups are responsible for a blueshift. Interestingly, it was observed that the R-CDs provide a full range of visible color by dispersing in different immiscible solvents. The fluorescence emission in immiscible solvents is redshifted by enhancing the polarity. Moreover, the developed CDs detected the low water concentrations (≤0.2%, v/v) visually and fluorometrically in various organic solvents. Simultaneously, we have employed synthesized CDs in white-light-emitting diodes and fluorescent ink.

Complementary Oligonucleotide Conjugated Multicolor Carbon Dots for Intracellular Recognition of Biological Events.

By using complementary DNA sequences as surface ligands, we selectively allow two individual diffusing "dual-color" carbon dots to interact in situ and in vitro. Spontaneous nanoscale oxidation of surface-abundant nitroso-/nitro-functionalities leads to two distinctly colored carbon dots (CD) which are isolated by polarity driven chromatographic separation. Green- and red-emitting carbon dots (gCD and rCD) were decorated by complementary single-stranded DNAs which produce a marked increase in the fluorescence emission of the respective carbon dots. Mutual colloidal interactions are achieved through hybridization of complementary DNA base pairs attached to the respective particles, resulting in quenching of their photoluminescence. The observed post-hybridization quenching is presumably due to a combined effect from an aggregation of CDs post duplex DNA formation and close proximity of multicolored CDs, having overlapped spectral regions leading to a nonradiative energy transfer process possibly released as heat. This strategy may contribute to the rational design of mutually interacting carbon dots for a better control over the resulting assembly structure for studying different biological phenomenon including molecular cytogenetics. One of the newly synthesized CDs was successfully used to image intracellular location of GAPDH mRNA using an event of change in fluorescence intensity (FI) of CDs. This selectivity was introduced by conjugating an oligonucleotide harboring complementary sequence to GAPDH mRNA. FI of this conjugated carbon dot, rCD-GAPDH, was also found to decrease in the presence of Ca2+, varied in relation to H+ concentrations, and could serve as a tool to quantify the intracellular concentrations of Ca2+ and pH value (H+) which can give important information about cell survival. Therefore, CD-oligonucleotide conjugates could serve as efficient probes for cellular events and interventions.

Preparation and Biomedical Applications of Multicolor Carbon Dots: Recent Advances and Future Challenges

AbstractMulticolor carbon dots (CDs) as an emerging subclass of carbonaceous nanomaterials have inspired intensive attention due to the fascinating fluorogenic properties of quantum dots, exhibiting great potential applications in the biomedical field. In some cases, reported CDs with blue or green fluorescence are not desirable for further biological applications owing to the conflicting background autofluorescence, low penetration, and relatively large damage to biological tissue. However, multicolor CDs that can work in the longer wavelength region are being developed to address this issue by overcoming the autofluorescence from the cellular components (usually in the blue and green range). In this review, the development of multicolor CDs is described comprehensively, including for red‐emissive or NIR‐emissive CDs. Additionally, the preparation methods of multicolor CDs are summarized. Moreover, the factors affecting the luminescence of multicolor CDs are discussed in detail, and the biomedical applications are emphasized from multiple perspectives.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Summary Highly crystalline and emissive carbon dots (CDs) are firstly synthesized via an ionothermal method in organic solvents. In contrast to well-established approaches, ZnCl2 is utilized as the pyrolysis-promoting agent to prepare emissive CDs with tunable wavelengths, directly by carbonization from different O and N precursors under a mild reaction condition at 210°C. More than 50% synthetic yield and 39% photoluminescent quantum yield (blue CD) are obtained. XAFS measurements, in conjunction with TEM, HAADF-STEM, UPS, and ESR analyses, allow us to obtain energy diagram configuration and confirm the luminescent nature of these hard-core structured CDs. Importantly, the resulting CDs exhibit excellent capability to photogenerate superoxide radical anion (O2⋅–), which can be exemplified by both photodynamic therapy and photooxidative cyclization synthesis of tetrahydroquinolines. Representative pharmaceutical mifepristone can be derivatized in 90% yield within 10 min of irradiation of the CDs in an elegant flow reactor.

Identification of eight pathogenic microorganisms by single concentration-dependent multicolor carbon dots.

The identification of pathogenic microorganisms is crucial to human health and industrial development, however, traditional culture-based methods are laborious and time consuming. We developed a novel method for the identification of pathogenic microorganisms based on single concentration-dependent carbon dots (CDs). The CDs were synthesized using a microwave heating approach without any complicated purification processes and the PL emission wavelength of the CDs could be tuned only by changing the concentration of the solution. The concentration-dependent CDs were applied for multicolor bioimaging and identification of eight kinds of microorganisms within minutes using different fluorescence spectra. This work provides a new approach for the detection of multiplex targets in real samples and lays a foundation for food safety monitoring.

Multicolor carbon dots with concentration-tunable fluorescence and solvent-affected aggregation states for white light-emitting diodes

Multicolor emissive carbon dots (M-CDs) have tremendous potential applications in manifold fields of bioimaging, biomedicine and light-emitting devices. Until now, it is still difficult to produce fluorescence tunable CDs with high quantum yield across the entire visible spectra. In this work, a type of M-CDs with concentration-tunable fluorescence and solvent-affected aggregation states was synthesized by solvothermal treatment of citric acid (CA) and 1-(2-pyridylazo)-2-naphthol (PAN) and the formation mechanism was monitored by different reaction time and raw material ratio. The fluorescence spectra of M-CDs in organic solvents can range from 350 to 750 nm by adjusting the concentration. M-CDs possess different aggregation states in water and organic solvents, accompanied by different fluorescence emission, which is attributed to the different surface states of various component CDs in M-CDs. Moreover, the obtained products can be uniformly dispersed into polymethylmethacrylate (PMMA) solutions as well as epoxy resins to fabricate transparent CDs/PMMA films and CDs/epoxy composites, which can effectively prevent the aggregation and produce multicolor and white light-emitting diodes (WLED). In addition, the prepared WLED with Commission Internationale de L’Eclairage (CIE) of (0.29, 0.31) by using M-CDs/epoxy resin as packages, demonstrating the M-CDs exhibit potential applications for light-emitting devices.

High-Purity Carbon Dots Prepared by Modulating Morphology of Carbon Nano-Crystals: In Vitro and In Vivo Multi-Color Bioimaging

Due to the unique optical properties and excellent biological applications, carbon-based luminescent materials have become a research hotspot in recent years. In this paper, we report a synthesis method under mild condition, in which Carbon NanoCrystals (CNCs) were first prepared, and then high-purity multi-color carbon dots (HPMCCDs) were synthesized by modulating crystal morphology. The prepared nonfluorescent CNCs are, respectively, dissolved in a strong polar solvent and a weak polar solvent to prepare HPMCCDs. It exhibits the multi-excitation and multi-emission features (from green to yellow). The high-purity carbon dots (HPCDs) prepared by using this method demonstrate the higher quantum yields (QYs) (53.7%) than that of the carbon dots (CD) by traditional hydrothermal methods with dialysis, and make full use of the atomic economic model. Besides, it shows the unique up-conversion luminescence property (UCLP) at lower wavelength excitation and, which can also be placed for a long time at room temperature without being oxidized or deteriorated. Furthermore, we explored the regulation of polar-chromism mechanism of the CNCs. Finally, the HPMCCDs can be used for multi-color bioimaging in vivo and in vitro and as well as many other potential applications.

Synthesis of Multicolor Carbon Dots Based on Solvent Control and Its Application in the Detection of Crystal Violet.

The adjustment of the emitting wavelength of carbon dots (CDs) is usually realized by changing the raw materials, reaction temperature, or time. This paper reported the effective synthesis of multicolor photoluminescent CDs only by changing the solvent in a one-step solvothermal method, with 1,2,4,5-tetraaminobenzene as both the novel carbon source and nitrogen source. The emission wavelengths of the as-prepared CDs ranged from 527 to 605 nm, with quantum yields (QYs) reaching 10.0% to 47.6%, and it was successfully employed as fluorescence ink. The prepared red-emitting CDs (R-CDs, λem = 605 nm) and yellow-emitting CDs (Y-CDs, λem = 543 nm) were compared through multiple characterization methods, and their luminescence mechanism was studied. It was discovered that the large particle size, the existence of graphite Ns, and oxygen-containing functional groups are beneficial to the formation of long wavelength-emitting CDs. Y-CDs responded to crystal violet, and its fluorescence could be quenched. This phenomenon was thus employed to develop a detection method for crystal violet with a linear range from 0.1 to 11 µM and a detection limit of 20 nM.

Betel-derived nitrogen-doped multicolor carbon dots for environmental and biological applications

An innovative, facile, low-cost and one-pot hydrothermal carbonization method was developed for the synthesis of bright fluorescence nitrogen-doped carbon dots (NCDs) using Piper betle (Betel) leaf as a carbon and nitrogen precursor. Various advanced physicochemical characterization techniques have been performed to confirm the structural and optical properties of the resulting Betel-derived NCDs (B-NCDs). The as-synthesized B-NCDs exhibited unique excitation-dependent fluorescence behavior, high fluorescence/photochemical stability, exceptional solubility in hydrophilic solvents like water, and good biocompatibility. On the basis of the astounding optical behavior of B-NCDs, they were utilized as a nanoprobe for determination of Fe3+ ion by the fluorometric method. Notably, this nanoprobe was performed by a linear relationship between the fluorescence intensity and the concentration of Fe3+ ion ranging from 5 to 30 μM with a detection limit as 0.43 μM. Highly selective and sensitive determination of Fe3+ ion was carried out through the fluorescence quenching of B-NCDs due to the resonance energy/electron transfer (RET) mechanism. Besides, the resulting B-NCDs employed as a biocompatible probe in multicolor imaging applications of HCT-116 human colon cancer cells. B-NCDs is not only efficient nanosensor for the determination of Fe3+ ion and biocompatible probe for the multicolor imaging. It also would be a substantial alternative for the traditional fluorescent inks due to its extraordinary fluorescence stability, biocompatibility, pollution-free, and easily washable. In this report for the first time, Betel leaf as carbon and nitrogen source was used to synthesize B-NCDs without adding any chemical reagent. Thus it was applied for diverse analysis field.

A simple method for the preparation of multi-color carbon quantum dots by using reversible regulatory color transformation.

It is known that polychromatic carbon quantum dots (CQDs) can be obtained by doping and surface modification. The layer-wise synthesis of blue and green emitting CQDs (with typical sizes between 3 and 6nm) is described here by adding oxalic acid and by introducing polycarboxy groups. By changing the external environment, the emission of CQDs can be adjusted in the blue-green spectral region (469-527nm) under photoexcitation at 405nm. The findings presented here provide new directions for the reversible regulatory transformation of polychromatic CQDs. The luminescence also is affected by a variety of conditions such as surface defects, degree of polymerization, polarity of external solutions, and hydrogen bonding which is studied in some detail. Graphical abstract Schematic presentation of the blue-green fluorescent transformation of the green carbon quantum dots(G-CQDs) to blue carbon quantum dots(B-CQDs).

Metal-triggered fluorescence enhancement of multicolor carbon dots in sensing and bioimaging

In this paper, two multicolor CDs have been prepared with eco-friendly folic acid and glucose as precursors by means of a facile hydrothermal strategy. The microscopic and spectroscopic results comprehensively confirmed that both of them exhibited well-defined spherical particles, amorphous structures and similar surface chemistry properties, but with different lateral dimensions and element contents. Moreover, CDs1 and CDs2 show the favorable biocompatibility and low cytotoxicity, in vitro and in vivo CLSM images of HeLa cells and zebrafish larvae tagged with CDs1 and CDs2 revealed multicolor emission (blue, green and red) at different excitation wavelengths. More importantly, the fluorescence of CDs1 could be remarkably enhanced with the increase of Mn2+ concentration in aqueous solution through the bonding of Mn2+ with C–OH and N–H groups on the surface of CDs1. A wide linear range was obtained from 0.5 to 90 μM with a detection limit of 0.42 μM.

Polymer-Assisted Self-Assembly of Multicolor Carbon Dots as Solid-State Phosphors for Fabrication of Warm, High-Quality, and Temperature-Responsive White-Light-Emitting Devices.

White-light-emitting devices (WLEDs) are considered to be a promising illumination source; especially, the WLEDs based on carbon dots (CDs) with white fluorescence have attracted extensive research interest. Herein, we report the design and implementation of solid white-light-emitting phosphors (WCDs@PS), which combine blue and orange emissive CDs (BCDs and OCDs) assisted by polystyrene (PS) through a self-assembly technique. Based on these phosphors (OCDs/BCDs = 1.2:1), the obtained WLEDs display a warm white light with International Commission on Illumination (CIE) coordinates of (0.35, 0.36), a high color rendering index of 93.2, a low correlated color temperature of 4075 K, and a luminous efficiency of up to 14.8 lm·W-1. Interestingly, these WLEDs exhibit temperature-dependent emission performance, whose light-emission spectrum can be adjusted in situ from white (λ ∼ 400-730 nm) to blue (λ ∼ 440 nm) in the range of 20-80 °C. A change in CIE coordinates from (0.35, 0.36) to (0.32, 0.23) was also observed. The temperature-driven tunable LEDs as a thermochromism device could broaden the application of CDs-based lighting systems in special displays.

Concentration-tuned multicolor carbon dots: microwave-assisted synthesis, characterization, mechanism and applications

A new and simple way to obtain multicolor-emission carbon dots and an exploration of their mechanism and applications.