Carbon quantum dot
[6] Carbon quantum dots have been extensively investigated especially due to their strong and tunable fluorescence emission properties,[7] which enable their applications in biomedicine, optronics, catalysis, and sensing.
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,[10][11] whereas other works, including single particle measurements,[12] have rather attributed the fluorescence to recombination of surface-trapped charges,[13][14] or proposed a form of coupling between core and surface electronic states.
[19] CQDs are also suitable for chemical modification and surface passivation with various organic, polymeric, inorganic or biological materials.
[24] 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.
[25] By varying the molar ratio of citric acid and urea (two common precursor molecules) of the mixture that is subjected to pyrolysis, a number of distinct fluorescent materials in both liquid and solid state can be synthesised, predominantly comprising Carbon dots with embedded fluorophores.
[25] 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.
[38] 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.
[43] By injecting solvents containing CQDs into a living body, images in vivo can be obtained for detection or diagnosis purposes.
Moreover, the conjugation process not only accounts for dual-mode bioimaging but also passivates the rhodium nanoparticle surface, resulting in reduced cytotoxicity.
[45] CQDs were also applied in biosensing as biosensor carriers for their flexibility in modification, high solubility in water, nontoxicity, good photostability, and excellent biocompatibility.
[1] The biosensors based on CQD and CQs-based materials could be used for visual monitoring of cellular copper,[46] glucose,[47] pH,[48] trace levels of H2O2[42] and nucleic acid.
The discriminating tags on the amplicons are recognized by their respective antibodies and fluorescence signals provided by the attached CQDs.
[7] More generally, the fluorescence of CQDs efficiently responds to pH,[50] local polarity,[15] and to the presence of metal ions in solution,[51] which further expands their potential for nanosensing applications,[52] for instance in the analysis of pollutants.
[57] 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.
