Carbon dots (CDs), as a new type of carbon-based nanomaterial, have attracted broad research interest for years, because of their diverse physicochemical properties and favorable attributes like good biocompatibility, unique optical properties, low cost, ecofriendliness, abundant functional groups (e.g., amino, hydroxyl, carboxyl), high stability, and electron mobility. In this Outlook, we comprehensively summarize the classification of CDs based on the analysis of their formation mechanism, micro-/nanostructure and property features, and describe their synthetic methods and optical properties including strong absorption, photoluminescence, and phosphorescence. Furthermore, the recent significant advances in diverse applications, including optical (sensor, anticounterfeiting), energy (light-emitting diodes, catalysis, photovoltaics, supercapacitors), and promising biomedicine, are systematically highlighted. Finally, we envisage the key issues to be challenged, future research directions, and perspectives to show a full picture of CDs-based materials.

Carbon Dots: A New Type of Carbon-Based Nanomaterial with Wide Applications

Hydrogen peroxide (H2 O2 ) has received increasing attention because it is not only a mild and environmentally friendly oxidant for organic synthesis and environmental remediation but also a promising new liquid fuel. The production of H2 O2 by photocatalysis is a sustainable process, since it uses water and oxygen as the source materials and solar light as the energy. Encouraging processes have been developed in the last decade for the photocatalytic production of H2 O2 . In this Review we summarize research progress in the development of processes for the photocatalytic production of H2 O2 . After a brief introduction emphasizing the superiorities of the photocatalytic generation of H2 O2 , the basic principles of establishing an efficient photocatalytic system for generating H2 O2 are discussed, highlighting the advanced photocatalysts used. This Review is concluded by a brief summary and outlook for future advances in this emerging research field.

Production of Hydrogen Peroxide by Photocatalytic Processes

In the most recent decade, carbon dots have drawn intensive attention and triggered substantial investigation. Carbon dots manifest superior merits, including excellent biocompatibility both in vitro and in vivo, resistance to photobleaching, easy surface functionalization and bio-conjugation, outstanding colloidal stability, eco-friendly synthesis, and low cost. All of these endow them with the great potential to replace conventional unsatisfactory fluorescent heavy metal-containing semiconductor quantum dots or organic dyes. Even though the understanding of their photoluminescence mechanism is still controversial, carbon dots have already exhibited many versatile applications. In this article, we summarize and review the recent progress achieved in the field of carbon dots, and provide a comprehensive summary and discussion on their synthesis methods and emission mechanisms. We also present the applications of carbon dots in bioimaging, drug delivery, microfluidics, light emitting diode (LED), sensing, logic gates, and chiral photonics, etc. Some unaddressed issues, challenges, and future prospects of carbon dots are also discussed. We envision that carbon dots will eventually have great commercial utilization and will become a strong competitor to some currently used fluorescent materials. It is our hope that this review will provide insights into both the fundamental research and practical applications of carbon dots.

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Novel properties and applications of carbon nanodots

Leaf‐vein‐like g‐C3N4 synthesized via a KBH4‐assisted thermal polycondensation strategy exhibits enhanced optical absorption, efficient charge carrier separation, and ample active sites, accordingly enabling excellent photocatalytic H2O2 evolution. The synergistic effect of B doping and defect sites on the improvement of catalyst performance is fully discussed by experiments and density functional theory calculations.

Synthesis of Leaf‐Vein‐Like g‐C 3 N 4 with Tunable Band Structures and Charge Transfer Properties for Selective Photocatalytic H 2 O 2 Evolution

Carbon dots, an emerging class within the carbon allotrope family, have gained significant attention largely due to their versatile and tunable physico-chemical and optical properties. These quasi-spherical carbon nanomaterials, less than 10 nm in size, can be prepared using numerous synthesis strategies resulting in unique properties that can be exploited for a myriad of applications. This review examines the bottom-up synthesis of these dots with a focus on their microwave-assisted synthesis, which can be used to prepare hydrophilic, hydrophobic or even amphiphilic carbon dots. It also investigates their application to multiple fields including sensing, bioimaging, solar cells and catalysis. Finally, a discussion of the challenges and perspectives are provided.

Microwave-assisted synthesis of carbon dots and their applications

The infection of bacteria and fungus is one of the most challenging global threats to human health. With the recent advancement in nanoscience and nanotechnology, much progress has been achieved in the development of antimicrobial nanomedicine; however, these nanomaterial-based antibacterial agents still suffer from potential biological toxicity, poor degradation, and various secondary pollution. Here, we demonstrate the fabrication of low-toxic and degradable carbon dots (CDs) from vitamin C by one-step electrochemical method. These newly generated CDs display a strong broad-spectrum antibacterial activity and antifungal activity even at low concentrations, as they destroy the bacterial walls during the diffusive entrance, perturb secondary structures of DNA/RNAs of bacteria and fungus, and inhibit important gene expressions to finally kill the bacteria and fungus. We also show that these well-characterized CDs can be completely degraded into CO2, CO and H2O under visible light in air (or at very mild te...

Degradable Carbon Dots with Broad-Spectrum Antibacterial Activity

Carbon dots promote the growth and photosynthesis of mung bean sprouts

A series of ∼5 nm sized carbon dots (CDs) with different oxygen contents were fabricated and employed as a model material with which to explore the impacts of carbon nanoparticles on rice-plant growth. We show that CDs can penetrate into all parts of rice plants, including the cell nuclei. Systematic investigations provide insight into the different processes by which seed germination, root elongation, seedling length, enzyme (RuBisCO) activity, and carbohydrate generation are increased. CDs are capable of entering the cell, reaching the nucleus, loosening the DNA structure, and increasing the thionin (Os06g32600) gene expression, which finally enhanced the rice-plant disease-resistance ability. CDs can be degraded by plant to form plant-hormone analogues and CO2, and then the hormone analogues promote the rice-plant growth, while the CO2 is converted into carbohydrates through the Calvin cycle of photosynthesis. The outcome of these processes is a 14.8% enhancement of the total rice yield and an increase...

Impacts of Carbon Dots on Rice Plants: Boosting the Growth and Improving the Disease Resistance

Chiral compounds/materials have important effects on the growth of plants. Chiral carbon dots (CDs), as an emerging chiral carbon nanomaterial, have great potential in bio-application and bio-nanotechnology. Herein, we report a hydrothermal method to synthesize chiral CDs from cysteine (cys) and citric acid. These chiral CDs were further demonstrated to have systemic effects on the growth of mung bean plants, in which case both L- and D-CDs can promote the growth of the root in mung bean plants, stem length of mung bean sprouts and water absorption of bean seeds. The elongation of mung bean sprouts presented an increasing trend with the treatment of chiral CDs of increasing concentration (below 500 μg mL−1). Furthermore, in the optimal concentration (100 μg mL−1), the L-CDs can improve root vigor and the activity of the Rubisco enzyme of bean sprouts by 8.4% and 20.5%, while the D-CDs increased by 28.9% and 67.5%. Due to more superior properties in improving root vigor and the activity of the Rubisco enzyme of mung bean sprouts, D-CDs are able to enhance photosynthesis better and accumulate more carbohydrate in mung bean plants.

Yuxiang Song

Papers

Degradable Carbon Dots with Broad-Spectrum Antibacterial Activity

Carbon dots promote the growth and photosynthesis of mung bean sprouts

Impacts of Carbon Dots on Rice Plants: Boosting the Growth and Improving the Disease Resistance

One-step hydrothermal synthesis of chiral carbon dots and their effects on mung bean plant growth.

Photocatalytic H2O2 and H2 Generation from Living Chlorella vulgaris and Carbon Micro Particle Comodified g-C3N4