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Full-color emissive carbon dots (CDs) hold a great promise for various applications, especially in light emitting diodes (LEDs). However, the existing synthetic routes for CDs are carried out in solutions, which suffer from low yields, high pressures, various byproducts, large amounts of waste solvents, and complicated photoluminescence (PL) origins. Therefore, it is necessary to explore large scale synthesis of CDs with high quantum yield (QY) across the entire visible range from a single carbon source by a solvent-free method. In this work, a series of CDs with tunable PL emission from 442 to 621 nm, QY of 23%–56%, and production yield within 34%–72%, are obtained by heating o-phenylenediamine with the catalysis of KCl. Detailed characterizations identify that, the differences between these CDs with respect to the graphitization degree, graphitic nitrogen content, and oxygen-containing functional groups, are responsible for their distinct optical properties, which can be modulated by controlling the deamination and dehydrogenation processes during reactions. Blue, green, yellow, red emissive films, and LEDs are prepared by dispersing the corresponding CDs into polyvinyl alcohol (PVA). All types of white LEDs (WLEDs) with high colorrendering- index (CRI), including warm WLEDs, standard WLEDs, and cool WLEDs, are also fabricated by mixing the red, green, and blue emissive CDs into PVA matrix by the appropriate ratios.

As a promising luminescent nanomaterial, carbon dots (CDs) have received tremendous attention for their great potential in biomedical applications, owing to their distinctive merits of ease in preparation, superior optical properties, good biocompatibility, and adjustable modification in structure and functionalities. However, most of the reported CDs exhibit insufficient excitation and emission in red/near-infrared (R/NIR) regions, which significantly limits their practical applications in biomedical assays and therapy. In the latest years, extensive studies have been performed to produce CDs with intensified R/NIR excitation and emission by designed reactions and precise separations. This review article summarizes state-of-the-art progress towards design and manufacture of CDs with long-wavelength or multicolor emissions, involving their synthetic routes, precursors, and luminescence mechanisms. Meanwhile, the applicable availability of CDs in bioimaging, sensing, drug delivery/release, and photothermal/photodynamic therapy, is systematically overlooked. The current challenges concerning feasible controls over optical properties of CDs and their new opportunities in biomedical fields are discussed.

As a class of electrode materials for electric double-layer capacitors (EDLCs), carbon dots (CDs) are able to enlarge specific surface areas, fabricate hierarchical pores, and graft pseudocapacitiv...

Carbon dots (CDs), an emerging class of carbon materials, hold a promising future in a broad variety of engineering fields owing to their high diversity in structure, composition and properties. Recently, their potential applications have spanned from bio-imaging, fluorescent probing and catalysis, to energy storage fields, in particular as materials in the key components of electrochemical energy storage devices. The state-of-the-art research work has revealed that CD-based or modified electrodes exhibit profound improvement in all key functions, such as coulombic efficiency, cycling life, enlarging capacity, etc., in comparison to traditional electrodes. The improvement in all these properties can be realized by introducing a small quantity of CDs to the traditional electrode systems. A comparative optimization in this regard, however, requires incorporation of more carbon nanotubes (CNTs) or graphene or other carbon-based materials, indicating that CD-incorporated electrode materials would maintain their energy density more efficiently. This review will summarize the progress to date in the design and preparation of CD-incorporated energy storage devices, including supercapacitors, Li/Na/K-ion batteries, Li–S batteries, metal–air batteries and flow batteries, and elaborate on the influence of these unique structures and rich properties of CDs on the electrochemical performance of the resulting electrodes and devices. Consequently, the specific functions and the novel working mechanisms of CD-modified electrodes for energy storage units will be discussed, aiming at providing new insights for guidance for design and manufacturing of the next generation of electrode materials for high-performance energy storage.

Non-noble metal-based bifunctional electrocatalysts for both oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) are an essential component of high-performance rechargeable Zn–a...

An efficient and economic approach for sustainable production of hierarchically interconnected porous carbons are designed and fabricated through the pyrolysis of Rhus typhina fruits and followed by KOH activation to create micropores or mesopores on the nano-sheet wall of macropores. The related N-doped carbon consists of a plenty of micropores and owns high specific surface area (up to 2675 m2 g−1), resulting in high performances for supercapacitors, such as ultrahigh specific capacitance (568 F g−1 at 1 A g−1), remarkable rate capability (310 F g−1 at 20 A g−1, 282 F g−1 at even 30 A g−1 current density) and good long-term stability (capacitance retention of 99% after 10000 cycles at 30 A g−1) in 1 mol L−1 H2SO4. Moreover, the carbon derived from Rhus typhina fruits demonstrates 474 F g−1 at 1 A g−1, 281 F g−1 at 30 A g−1, and capacitance retention of 92% after 10000 cycles at 30 A g−1 in 6 mol L−1 KOH electrolyte. This novel and sustainable biomass-derived carbon material holds a bright future for fabricating high energy supercapacitors.

The development of high-performance nickel-zinc (Ni-Zn) alkaline batteries is mainly plagued by short life span and poor rate performance of ZnO anode materials. To improve the cycling stability and rate capability of Ni-Zn batteries, carbon dots (CDs) are employed to construct clustered ZnO-CDs nanocomposites, coating ZnO with protective shells of carbon layers and providing electron paths to enhance conductivity of the nanocomposites. Univalent zinc species are found at the interfaces between CDs derivatives and ZnO, which are embedded in the nanoclusters and protected well by carbon coating. Theoretical calculations show univalent zinc species change the electronic structures of ZnO surface, so as to accelerate the charging process of ZnO anode materials. Such ZnO-CDs derived nanocomposites exhibit excellent rate capability (95.3%, 84.7% and 75.0% of capacity retention rate at 2, 5 and 10 A g−1, respectively) and outstanding cycling stability with 92.0% of capacity retention rate after 5000 cycles, which is far better than ZnO based anodes without the protection of CDs (39.1% retention rate from 1 to 10 A g−1 and 71.6% of capacity retention rate after 500 cycles).

A series of sponge-like hierarchical porous carbons are prepared using semen cassiae as the precursor and potassium oxalate monohydrate as the activation agent. These sponge-like carbon materials have high surface areas, wide distributions of pore size, and the appropriate forms of nitrogen/oxygen derivatives. All these advantages in structures and surface states contribute to the outstanding performances of such materials as electrodes for high performance electric double-layer capacitors, which include high specific capacitance, high cycling stability, and high energy density. For example, the optimal sample exhibits a high specific capacitance of 401 F g−1, excellent cycling stability (94.5% retention after 10 000 cycles), high energy density (12.01 W h kg−1) and a high power density (10 200 W kg−1) in 1 M H2SO4 solution.

A series of polymer dots/graphene sheets composites with high densities are prepared and tested for supercapacitors. Polymer dots (PDs) are synthesized by one-step method at room temperature. They can effectively increase surface areas of the composites (almost 10 times), and the functional groups from PDs produce high pseudocapacitance, so that the samples exhibit high specific capacitances (e. g., 364.2 F cm−3 at 1 A g−1) and high cycling stability (e. g., more than 95% of the initial capacity retention over 10 000 cycles at different current densities). The optimal sample is employed to fabricate a symmetric supercapacitor, which exhibits an energy density up to 8 Wh L−1 and a power density up to 11 800 W L−1, respectively.

ZnO@polymer core-shell nanoparticles are assembled into novel capsule shells with diameters of about 100 nm to load isotretinoin (ISO) with a capacity as high as 34.6 wt %. Although ISO, a widely used drug for acne treatment, by itself is not suitable for treating cancer because of its hydrophobicity, our ZnO-ISO composite showed much stronger anticancer activity. The improved cytotoxicity is ascribed to the synergistic effects of the ZnO@polymer and ISO, where the ZnO@polymer helps in the accumulation of ISO in cancer cells on the one hand, and on the other hand, ISO is released completely through ZnO decomposition under acidic conditions of cancer cells. Such a pH-triggered drug-delivery system exhibits a much improved killing of cancer cells in vitro in comparison with the benchmarks, Nintedanib and Crizotinib, two commercial drugs clinically applied against lung cancer.

Brightly red fluorescent carbon dots are synthesized hydrothermally and dissolved in diluted hydrochloric acid solution. Such carbon dots exhibit excitation-independent emission at about 620 nm with quantum yield over 10%, which is visible in daylight. After the carbon dots solution is sprayed to the fingerprints on various solid substrates and dried in air, clear fingerprints can be seen under an ultraviolet lamp and stay stable for 1 day. Detailed characterizations suggest that during the drying process, the coffee-ring effect and the electrostatic interactions between the carbon dots and the fingerprint residues prevent the typical aggregation-induced fluorescence quenching of carbon dots.

In this work, red-emitting carbon dots (R-CDs) with a high quantum yield (QY) of 28% in water were synthesized for the first time by heating an ethanol solution of pulp-free lemon juice. The obtained R-CDs were mono-dispersed with an average diameter of 4.6 nm, and exhibited excitation-independent emission at 631 nm. Meanwhile, these R-CDs featured low cytotoxicity and good photostability, which allow R-CDs to be employed as luminescent probes for in vitro/in vivo bioimaging. In addition, a detailed study on the physical properties and structural compositions of the sodium borohydride (NaBH4) reduced R-CDs with orange emission suggested that surface states on the R-CD surfaces and nitrogen-derived structures in the R-CD cores synergistically caused their intense red luminescence. The low-cost and eco-friendly synthesis method and favorable optical properties of R-CDs make these carbon dots promising for further applications, such as bioimaging and light-emitting diodes.

A series of carbon dots/NiCo2 O4 composites with various morphologies are prepared and tested for supercapacitors. These samples have good electrical conductivities and efficient ions transport paths, so they exhibit high specific capacitances, superior rate performances, and high cycling stabilities. The optimal composite for hybrid supercapacitor exhibits a high energy density up to 62.0 Wh kg-1 .

Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states.

Carbon materials doped with heteroatoms are a class of cost-effective and stable electrocatalysts for oxygen reduction reactions (ORR), whose activities are mainly based on the heteroatom-related active sites. Besides the widely reported one-dimensional carbon nanotubes and two-dimensional graphene materials, carbon dots (CDs), as a new kind of zero-dimensional carbon materials, exhibit a range of unique structures and promising catalytic activities for ORR. In order to optimize the complex conditions of carbon-based catalysts, composites consisting of doped CDs and reduced graphene oxide (rGO) (designated as CD/rGO) are prepared hydrothermally, in comparison with directly doped rGO. All produced composites outperform their corresponding directly doped rGO counterparts in ORR measurements. It is noted that nitrogen and sulfur co-doped samples perform better than those doped by individual N or S. Mechanistic relationships between the ORR catalytic activities and the catalyst features are proposed, including type, location, bonding, fraction and synergistic effects of dopants, as well as the composition and structure of the carbon substrates. It is apparent that doping heteroatoms and constructing carbon substrates play a synergistic role in yielding high-performance carbon based catalysts.

A series of nitrogen and oxygen enriched porous carbons are prepared from poly-N-phenylethanolamine (PNPEA) and polyaniline (PANI) conducting polymers through pyrolysis, chemical activation, and oxidation processes. Ar or N2-adsorption, Fourier transform infrared, and X-ray photoelectron spectroscopy are used to characterize the surface areas, pore volumes, surface chemical compositions, and oxygen and nitrogen content. Mikhail and Brunauer micropore analytical method (MP method) is successfully used to analyze the micropore size distribution of the samples. The electrochemical behavior of the samples is studied in two- and three-electrode cells. The contribution of pseudocapacitance is confirmed by cyclic voltammetry and galvanostatic tests performed in acidic (H2SO4) and basic (KOH) media. The potential drop and the equivalent series resistance value certify that the samples with wide micropore size distribution possess low interface resistances. A sample with a Brunauer–Emmett–Teller (BET) surface area...

A type of hierarchical porous carbon material was prepared using a Schiff-base network as the precursor and ZnCl2 as the activation agent, and their electrochemical performances were investigated in acid and alkaline aqueous solutions, respectively. The as-prepared materials have high surface areas, appropriate distributions of hierarchical pore sizes, and various forms of nitrogen/oxygen derivatives. These structural advantages guarantee the outstanding performances of such carbon materials as electrodes for supercapacitors, which include high specific capacitances, fast current responses, and high cycling stabilities.

Carbon dots (CDs) have tremendous potential applications in bioimaging, biomedicine, and optoelectronics. By far, it is still difficult to produce photoluminescence (PL) tunable CDs with high quantum yield (QY) across the entire visible spectrum and narrow the emission peak widths of CDs close to those of typical quantum dots. In this work, a series of CDs with tunable emission from 443 to 745 nm, quantum yield within 13-54%, and narrowed full width at half maximum (FWHM) from 108 to 55 nm, are obtained by only adjusting the reaction solvents in a one-pot solvothermal route. The distinct optical features of these CDs are based on their differences in the particle size, and the content of graphitic nitrogen and oxygen-containing functional groups, which can be modulated by controlling the dehydration and carbonization processes during solvothermal reactions. Blue, green, yellow, red, and even pure white light emitting films (Commission Internationale de L'Eclairage (CIE)= 0.33, 0.33, QY = 39%) are prepared by dispersing one or three kinds of CDs into polyvinyl alcohol with appropriate ratios. The near-infrared emissive CDs are excellent fluorescent probes for both in vitro and in vivo bioimaging because of their high QY in water, long-term stability, and low cytotoxicity.

Carbon dots (CDs) with tunable photoluminescence (PL) and a quantum yield of up to 35% in water were hydrothermally synthesized in one pot and separated via silica column chromatography. These separated CDs emitted bright and stable luminescence in gradient colors from blue to red under a single-wavelength UV light. They exhibited high optical uniformity; that is, every sample showed only one peak in the PL excitation spectrum, only one peak in the excitation-independent PL emission spectrum, and similar monoexponential fluorescence lifetimes. Although these samples had similar distributions of particle size and graphite structure in their carbon cores, the surface state gradually varied among the samples, especially the degree of oxidation. Therefore, the observed red shift in their emission peaks from 440 to 625 nm was ascribed to a gradual reduction in their band gaps with the increasing incorporation of oxygen species into their surface structures. These energy bands were found to depend on the surface groups and structures but not on the particle size, not as in traditional semiconductor quantum dots. In addition, because of their excellent PL properties and low cytotoxicity, these CDs could be used to image cells in different colors under a single-wavelength light source, and the red-emitting CDs could be used to image live mice because of the strong penetration capability of their fluorescence.

Sulfur-doped carbon dots (S-CDs) with a quantum yield (QY) of 5.5% and nitrogen, sulfur co-doped carbon dots (N,S-CDs) with a QY of 54.4% were synthesized, respectively, via the same hydrothermal route using α-lipoic acid as the carbon source. The obtained S-CDs and N,S-CDs had similar sizes but different optical features. The QY of N,S-CDs was gradually enhanced when extending the reaction time to increase the nitrogen content. After careful characterization of these CDs, the doped nitrogen element was believed to be in the form of C=N and C-N bonds which enhanced the fluorescence efficiency significantly. Meanwhile, the co-doped sulfur element was found to be synergistic for nitrogen doping in N,S-CDs. The optimal N,S-CDs were successfully employed as good multicolor cell imaging probes due to their fine dispersion in water, excitation-dependent emission, excellent fluorescence stability and low toxicity. Besides, such N,S-CDs showed a wide detection range and excellent accuracy as fluorescent sensors for Fe(3+) ions.