Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbon nanomaterials that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots. In addition to their comparable optical properties, CQDs have the desired advantages of low toxicity, environmental friendliness low cost and simple synthetic routes. Moreover, surface passivation and functionalization of CQDs allow for the control of their physicochemical properties. Since their discovery, CQDs have found many applications in the fields of chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This article reviews the progress in the research and development of CQDs with an emphasis on their synthesis, functionalization and technical applications along with some discussion on challenges and perspectives in this exciting and promising field.

Carbon quantum dots and their applications

Fluorescent carbon-based materials have drawn increasing attention in recent years owing to exceptional advantages such as high optical absorptivity, chemical stability, biocompatibility, and low toxicity. These materials primarily include carbon dots (CDs), nanodiamonds, carbon nanotubes, fullerene, and fluorescent graphene. The superior properties of fluorescent carbon-based materials distinguish them from traditional fluorescent materials, and make them promising candidates for numerous exciting applications, such as bioimaging, medical diagnosis, catalysis, and photovoltaic devices. Among all of these materials, CDs have drawn the most extensive notice, owing to their early discovery and adjustable parameters. However, many scientific issues with CDs still await further investigation. Currently, a broad series of methods for obtaining CD-based materials have been developed, but efficient one-step strategies for the fabrication of CDs on a large scale are still a challenge in this field. Current synthetic methods are mainly deficient in accurate control of lateral dimensions and the resulting surface chemistry, as well as in obtaining fluorescent materials with high quantum yields (QY). Moreover, it is important to expand these kinds of materials to novel applications. Herein, a facile and highoutput strategy for the fabrication of CDs, which is suitable for industrial-scale production (yield is ca. 58%), is discussed. The QY was as high as ca. 80%, which is the highest value recorded for fluorescent carbon-based materials, and is almost equal to fluorescent dyes. The polymer-like CDs were converted into carbogenic CDs by a change from low to high synthesis temperature. The photoluminescence (PL) mechanism (high QY/PL quenching) was investigated in detail by ultrafast spectroscopy. The CDs were applied as printing ink on the macro/micro scale and nanocomposites were also prepared by polymerizing CDs with certain polymers. Additionally, the CDs could be utilized as a biosensor reagent for the detection of Fe in biosystems. The CDs were prepared by a hydrothermal method, which is described in the Supporting Information (Figure 1a; see also the Supporting Information, Figure S1). The reaction was conducted by first condensing citric acid and ethylenediamine, whereupon they formed polymer-like CDs, which were then carbonized to form the CDs. The morphology and structure of CDs were confirmed by analysis. Figure 1b shows transmission electron microscopy (TEM) images of the CDs, which can be seen to have a uniform dispersion without apparent aggregation and particle diameters of 2–6 nm. The sizes of CDs were also measured by atomic force microscopy (AFM; Figure S2), and the average height was 2.81 nm. From the high-resolution TEM, most particles are observed to be amorphous carbon particles without any lattices; rare particles possess well-resolved lattice fringes. With such a low carbon-lattice-structure content, no obvious D or G bands were detected in the Raman spectra of the CDs (Figure S3). The XRD patterns of the CDs (Figure 1c) also displayed a broad peak centered at 258 (0.34 nm), which is also attributed to highly disordered carbon atoms. Moreover, NMR spectroscopy (H and C) was employed to distinguish sp-hybridized carbon atoms from sp-hybridized carbon atoms (Figure S4). In the H NMR spectrum, sp carbons were detected. In the C NMR spectrum, signals in the range of 30–45 ppm, which correspond to aliphatic (sp) carbon atoms, and signals from 100–185 ppm, which are indicative of sp carbon atoms, were observed. Signals in the range of 170– 185 ppm, which correspond to carboxyl/amide groups, were also present. In the FTIR analysis of CDs, the following were observed: stretching vibrations of C OH at 3430 cm 1 and C H at 2923 cm 1 and 2850 cm , asymmetric stretching vibrations of C-NH-C at 1126 cm , bending vibrations of N H at 1570 cm , and the vibrational absorption band of C=O at 1635 cm 1 (Figure S5). Moreover, the surface groups were also investigated by XPS analysis (Figure 1d). C1s analysis revealed three different types of carbon atoms: graphitic or aliphatic (C=C and C C), oxygenated, and nitrous (Table S1). In the UV/Vis spectra, the peak was focused on 344 nm in an aqueous solution of CDs. In the fluorescence spectra, CDs have optimal excitation and emission wavelengths at 360 nm and 443 nm, and show a blue color under a hand-held UV lamp (Figure 2a). Excitation-dependent PL behavior was [*] S. Zhu, Q. Meng, Prof. J. Zhang, Y. Song, Prof. K. Zhang, Prof. B. Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University Changchun, 130012 (P. R. China) E-mail: byangchem@jlu.edu.cn

Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging

Graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be produced on a large scale at low cost. Thus, it has been regarded as an important component for making various functional composite materials. Especially, graphene-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. This critical review summarizes the recent progress in the design and fabrication of graphene-based semiconductor photocatalysts via various strategies including in situ growth, solution mixing, hydrothermal and/or solvothermal methods. Furthermore, the photocatalytic properties of the resulting graphene-based composite systems are also discussed in relation to the environmental and energy applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation and photocatalytic disinfection. This critical review ends with a summary and some perspectives on the challenges and new directions in this emerging area of research (158 references).

Graphene-based semiconductor photocatalysts

Carbon nanostructures are attracting intense interest because of their many unique and novel properties. The strong and tunable luminescence of carbon materials further enhances their versatile properties; in particular, the quantum effect in carbon is extremely important both fundamentally and technologically. Recently, photoluminescent carbonbased nanoparticles have received much attention. They are usually prepared by laser ablation of graphite, electrochemical oxidation of graphite, electrochemical soaking of carbon nanotubes, thermal oxidation of suitable molecular precursors, vapor deposition of soot, proton-beam irradiation of nanodiamonds, microwave synthesis, and bottom-up methods. Although small (ca. 2 nm) graphite nanoparticles show strong blue photoluminescence (PL), definitive experimental evidence for luminescence of carbon structure arising from quantum-confinement effects and size-dependent optical properties of carbon quantum dots (CQDs) remains scarce. Herein, we report the facile one-step alkali-assisted electrochemical fabrication of CQDs with sizes of 1.2– 3.8 nm which possess size-dependent photoluminescence (PL) and excellent upconversion luminescence properties. Significantly, we demonstrate the design of photocatalysts (TiO2/CQDs and SiO2/CQDs complex system) to harness the use of the full spectrum of sunlight (based on the upconversion luminescence properties of CQDs). It can be imagined that judicious cutting of a graphite honeycomb layer into ultrasmall particles can lead to tiny fragments of graphite, yielding CQDs, which may offer a straightforward and facile strategy to prepare high-quality CQDs. Using graphite rods as both anode and cathode, and NaOH/EtOH as electrolyte, we synthesized CQDs with a current intensity of 10–200 mAcm . As a reference, a series of control experiments using acids (e.g. H2SO4/EtOH) as electrolyte yielded no formation of CQDs. This result indicates that alkaline environment is the key factor, and OH group is essential for the formation of CQDs by the electrochemical oxidation process. Figure 1a shows a trans-

Water‐Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design

Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to provide further insight into their controversial emission origin (particularly the upconverted photoluminescence) and to stimulate further research into their potential applications, especially in photocatalysis, energy conversion, optoelectronics, and sensing.

Carbon nanodots: synthesis, properties and applications

[*] Prof. Z.-H. Kang, Dr. N.-B. Wong, Prof. S.-T. Lee, Dr. Y. Liu C. H. A. Tsang, Dr. D. D. D. Ma, Dr. X. Fan Center of Super Diamond and Advanced Films (COSDAF) Hong Kong SAR (P.R. China) E-mail: zhkang@suda.edu.cn; bhnbwong@cityu.edu.hk apannale@cityu.edu.hk Prof. Z.-H. Kang, Dr. N.-B. Wong, Dr. Y. Liu, C. H. A. Tsang Department of Biology and Chemistry City University of Hong Kong Hong Kong SAR (P.R. China)

Water‐Soluble Silicon Quantum Dots with Wavelength‐Tunable Photoluminescence

The 1−2 nm SiQDs can photocatalyze CO2 reduction and dye (methyl red) degradation, while 3−4 nm SiQDs can photocatalyze selective oxidation of benzene to phenol, owing to tunable band gap and excellent photoconductive properties of SiQDs.

Silicon Quantum Dots: A General Photocatalyst for Reduction, Decomposition, and Selective Oxidation Reactions

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200890277

Inside Cover: Ultrastable, Highly Fluorescent, and Water‐Dispersed Silicon‐Based Nanospheres as Cellular Probes (Angew. Chem. Int. Ed. 1/2009)

Fluorescent cellular probes are powerful tools for studying cellular morphology, behavior, and physiological functions. For optimum imaging and tracking of biological cells, these probes should be water-dispersible, antibleaching, luminescent, and biocompatible. In the last century, organic dyes and fluorescent proteins were mostly used as fluorescent probes in biological and biomedical research; however, they suffer from severe photobleaching that restricts their applications for long-term in vitro or in vivo cell imaging. This shortcoming has led to an intense interest in colloidal fluorescent semiconductor quantum dots (QDs) since II/VI QDs were shown to be promising for cell imaging in 1998. Compared to fluorescent dyes, QDs possess unique advantages such as size-tunable emission wavelengths, broad photoexcitation, narrow emission spectra, strong fluorescence, and high resistance to photobleaching. Consequently, QDs have been widely used as a new class of fluorescent probes in cellular research, for example, in vitro imaging, cell labeling, and tracking cell migration. However, recent investigations have shown that these QDs are cytotoxic, especially under harsh conditions (e.g., UV irradiation), because heavy metal ions (such as Cd or Pb ions) are often released in oxidative environments, which leads to severe cytotoxicity by conventional mechanisms of heavy metal toxicity. Surface modification of QDs, such as epitaxial growth of the ZnS shell, silica coating, or polymer coating, has been used to alleviate the cytotoxicity problem. While these techniques are viable to a certain extent, they are relatively complicated and require additional processing steps. Notably, the risk of cytotoxic problems still remains since heavy-metal ions contained in the modified QDs may invariably be released in physiological environments. Consequently, the inherent problems, that is, severe photobleaching and cytotoxicity, associated with the traditional dyes and the fluorescent II/VI QDs remain unsolved, and have fueled a continual and urgent search for new cellular probes that are more photostable and biocompatible. Silicon is the leading semiconductor material for technological applications because of its wide-ranging applications by the electronics industry. Silicon-based nanostructures, such as nanoribbons, nanowires, and nanodots, are being intensely investigated. The quantum confinement phenomenon in silicon QDs (SiQDs) is a particular focus of research since it would increase the probability of irradiative recombination by indirect-to-direct band-gap transitions, which lead to enhanced fluorescent intensity and the prospect of longawaited optical applications. The biocompatibility and noncytotoxic properties of SiQDs are much better than those of traditional II/VI QDs, although their photoluminescence (PL) intensity is weaker. As such, silicon-containing materials may be useful for biological applications such as cellular imaging and labeling because of their favorable biocompatibility, if water-dispersed silicon nanomaterials with adequate stability and strong fluorescence are properly developed. Herein we report a new variety of silicon-based nanospheres for use as cellular probes, which possess excellent water-dispersibility, strong photoluminescence, and robust photostability. Our design of these nanospheres was formulated from calculations performed by using the B3LYP/6-31G method, a classic calculation method based on hybrid density functional theory. Figure 1a shows that the calculated free energy of the products PSi O ( 15.2 kcal mol ) is higher than that of PSi C ( 33.1 kcalmol ), whereas the energy barrier of forming the Si O bond (ESi O = 41.5 kcalmol ) is much lower than that of the Si C bond (ESi C = 59.5 kcal mol ). The energy consideration implies that an acrylic acid monomer (AAc) would react with the surface of the SiQDs via the carbonyl or alkene groups by forming Si O or Si C bonds, as previously reported. Furthermore, the calculations yield the energy relation of ESi O (41.5 kcal mol )<Eblue (58.3 kcalmol )< ESi C (59.5 kcal mol )<EUV (79.4 kcalmol ). It shows that the energy value of blue light (Eblue) is larger than the energy barrier of formation ESi O of the Si O bond, but less than the [*] Prof. C.-H. Fan Shanghai Institute of Applied Physics Chinese Academy of Sciences Shanghai 201800 (P.R. China) E-mail: fchh@sinap.ac.cn

Chi Him A. Tsang

Papers

Water‐Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design

Water‐Soluble Silicon Quantum Dots with Wavelength‐Tunable Photoluminescence

Silicon Quantum Dots: A General Photocatalyst for Reduction, Decomposition, and Selective Oxidation Reactions

Inside Cover: Ultrastable, Highly Fluorescent, and Water‐Dispersed Silicon‐Based Nanospheres as Cellular Probes (Angew. Chem. Int. Ed. 1/2009)

Ultrastable, Highly Fluorescent, and Water‐Dispersed Silicon‐Based Nanospheres as Cellular Probes