Carbon quantum dots also commonly called carbon dots (abbreviated as CQDs, C-dots or CDs) are carbon

nanoparticles A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...

which are less than

10 nm The following are examples of orders of magnitude for different lengths. __TOC__ Overview Detailed list To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10^ ...

in size and have some form of

surface passivation A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is t ...

History

CQDs were first discovered by Xu et al. in 2004 accidentally during the purification of

single-walled carbon nanotubes file:Chiraltube.png, A scanning tunneling microscopy image of a single-walled carbon nanotube file:Kohlenstoffnanoroehre Animation.gif, Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters ...

. This discovery triggered extensive studies to exploit the fluorescence properties of CQDs. As a new class of fluorescent carbon nanomaterials, CQDs possess the attractive properties of high stability, good conductivity, low toxicity, environmental friendliness, simple synthetic routes as well as comparable optical properties to quantum dots. Carbon quantum dots have been extensively investigated especially due to their strong and tunable fluorescence emission properties, which enable their applications in biomedicine, optronics, catalysis, and sensing. Carbon dots prepared by Paliienko K

The fundamental mechanisms responsible of the fluorescence capability of CQDs are very debated. Some authors have provided evidence of size-dependent fluorescence properties, suggesting that the emission arises from electronic transitions with the core of the dots, influenced by quantum confinement effects, whereas other works have rather attributed the fluorescence to recombination of surface-trapped charges, or proposed a form of coupling between core and surface electronic states. The excitation-dependent fluorescence of CQDs, leading to their characteristic emission tunability, has been mostly linked to the inhomogeneous distribution of their emission characteristics, due to polydispersity, although some works have explained it as a violation of Kasha's rule arising from an unusually slow solvent relaxation.

Properties of CQDs

The structures and components of CQDs determine their diverse properties. Many carboxyl moieties on the CQD surface impart excellent solubility in water and biocompatibility. Such surface moieties enable CQDs to serve as proton conducting nanoparticles. CQDs are also suitable for chemical modification and surface passivation with various organic, polymeric, inorganic or biological materials. By surface passivation, the fluorescence properties as well as physical properties of CQDs are enhanced. Recently, it has been discovered that amine and hydroxamic acid functionalized CD can produce tricolor (green, yellow and red) emission when introduced with different pH environment and this tricolor emission can be preserved in ORMOSIL film matrix. A paper published in 2019 showed that CQD can resist temperatures as high as 800 °C, paving way for applications of CQD in high temperature environments. Based on carbon, CQDs possess such properties as good conductivity, benign chemical composition, photochemical and thermal stability.

Synthesis of CQDs

Synthetic methods for CQDs are roughly divided into two categories, "top-down" and "bottom-up" routes. These can be achieved via chemical, electrochemical or physical techniques. The CQDs obtained could be optimized during preparation or post-treatment. Modification of CQDs is also very important to get good surface properties which are essential for solubility and selected applications.

Synthetic methods

"Top-down" synthetic route refers to breaking down larger carbon structures such as

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

carbon nanotubes A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon nan ...

, and

nanodiamond Nanodiamonds, or diamond nanoparticles, are diamonds with a size below 100 nanometers. They can be produced by impact events such as an explosion or meteoritic impacts. Because of their inexpensive, large-scale synthesis, potential for surfa ...

s into CQDs using

laser ablation Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...

arc discharge An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...

, and electrochemical techniques. For example, Zhou et al. first applied electrochemical method into synthesis of CQDs. They grew multi-walled carbon nanotubes on a carbon paper, then they inserted the carbon paper into an electrochemical cell containing supporting electrolyte including degassed acetonitrile and 0.1 M tetrabutyl ammonium perchlorate. Later, they applied this method in cutting CNTs or assembling CNTs into functional patterns which demonstrated the versatile callability of this method in carbon nanostructure manipulations. "Bottom-up" synthetic route involves synthesizing CQDs from small precursors such as

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...

s, citrate, and polymer-silica nanocomposites through hydrothermal/solvothermal treatment, supported synthetic, and microwave synthetic routes. For instance, Zhu et al. described a simple method of preparing CQDs by heating a solution of poly(ethylene glycol) (PEG) and saccharide in 500 W microwave oven for 2 to 10 min. Recently, green synthetic approaches have also been employed for fabrication of CQDs.

Size control

In addition to post-treatment, controlling the size of CQDs during the preparing process is also widely used. For instance, Zhu et al. reported hydrophilic CQDs through impregnation of citric acid precursor. After pyrolyzing CQDs at 300 °C for 2 hours in air, then removing silica, followed by dialysis, they prepared CQDs with a uniform size of 1.5–2.5 nm which showed low toxicity, excellent luminescence, good photostability, and up-conversion properties.

Modification

Being a new type of fluorescent nanoparticles, applications of CQD lie in the field of bioimaging and biosensing due to their biological and environmental friendly composition and excellent biocompatibility. In order to survive the competition with conventional semiconductor quantum dots, a high quantum yield should be achieved. Although a good example of CQDs with ~80% quantum yield was synthesized, most of the quantum dots synthesized have a quantum yield below 10% so far. Surface-passivation and doping methods for modifications are usually applied for improving quantum yield. To prevent surfaces of CQDs from being polluted by their environment, surface passivation is performed to alleviate the detrimental influence of surface contamination on their optical properties. A thin insulating layer is formed to achieve surface passivation via the attachment of polymeric materials on CQDs surface treated by acid. In addition to surface passivation, doping is also a common method used to tune the properties of CQDs. Various doping methods with elements such as N, S, P have been demonstrated for tuning the properties of CQDs, among which N doping is the most common way due to its great ability in improving the photo luminescence emissions. The mechanisms by which Nitrogen doping enhances the fluorescence quantum yield of CQDs, as well as the structure of heavily N-doped CDs, are very debated issues in the literature. Zhou et al. applied XANES and XEOL in investigating the electronic structure and luminescence mechanism in their electrochemically produced carbon QDS and found that N doping is almost certainly responsible for the blue luminescence. Synthesis of new nanocomposites based on CDs have been reported with unusual properties. For example, a new nanocomposite has been designed by using of CDs and magnetic Fe₃O₄ nanoparticles as precursors with nanozymetic activity.

Applications

Bioimaging

CQDs can be used for bioimaging due to their fluorescence emissions and biocompatibility. By injecting solvents containing CQDs into a living body, images in vivo can be obtained for detection or diagnosis purposes. One example is that organic dye-conjugated CQDs could be used as an effective fluorescent probes for H₂S. The presence of H₂S could tune the blue emission of the organic dye-conjugated CQDs to green. So by using a fluorescence microscope, the organic dye-conjugated CQDs were able to visualize changes in physiologically relevant levels of H₂S. Another example can be dual-mode bioimaging using their highly accessible surface functional groups to conjugate them ''via'' EDC-NHS chemistry. Saladino ''et al''. demonstrated the concept using MW-assisted synthesized nitrogen-doped excitation-independent CQDs. These were conjugated with rhodium nanoparticles – X-ray fluorescence contrast agents – leading to dual-mode nanohybrids with both optical and X-ray fluorescent properties. Moreover, the conjugation process not only accounts for dual-mode bioimaging but also passivates the rhodium nanoparticle surface, resulting in reduced cytotoxicity.

Sensing

CQDs were also applied in biosensing as biosensor carriers for their flexibility in modification, high solubility in water, nontoxicity, good photostability, and excellent biocompatibility. The biosensors based on CQD and CQs-based materials could be used for visual monitoring of cellular copper, glucose, pH, trace levels of H₂O₂ and nucleic acid. A general example is about nucleic acid lateral flow assays. The discriminating tags on the amplicons are recognized by their respective antibodies and fluorescence signals provided by the attached CQDs. More generally, the fluorescence of CQDs efficiently responds to pH, local polarity, and to the presence of metal ions in solution, which further expands their potential for nanosensing applications, for instance in the analysis of pollutants.

Drug delivery

The nontoxicity and biocompatibility of CQDs enable them with broad applications in biomedicine as drug carriers, fluorescent tracers as well as controlling drug release. This is exemplified by the use of CQDs as photosensitizers in photodynamic therapy to destroy cancer cells.

Catalysis

The flexibility of functionalization with various groups CQDs makes them possible to absorb lights of different wavelengths, which offers good opportunities for applications in photocatalysis. CQDs-modified P25 TiO₂ composites exhibited improved photocatalytic H2 evolution under irradiation with UV-Vis. The CQDs serve as a reservoir for electrons to improve the efficiency of separating of the electron-hole pairs of P25. In the recent times, metal-free CQDs have been found to improve the kinetics of hydrogen evolution reaction (HER), making CQDs a sustainable choice for catalysis.

Optronics

CQDs possess the potential in serving as materials for

dye-sensitized solar cell A dye-sensitized solar cell (DSSC, DSC, DYSC or Grätzel cell) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a '' photoelectr ...

s, organic solar cells,

supercapacitor A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable ba ...

, and light emitting devices. CQDs can be used as photosensitizer in dye-sensitized solar cells and the photoelectric conversion efficiency is significantly enhanced. CQD incorporated hybrid silica based sol can be used as transparent

Fluorescent paint Luminous paint or luminescent paint is paint that exhibits luminescence. In other words, it gives off visible light through fluorescence, phosphorescence, or radioluminescence. There are three types of luminous paints: fluorescent paint, phospho ...