A Review of Fluorescent Carbon Dots, Their Synthesis, Physical and Chemical Characteristics, and Applications

Abstract

Carbon dots (CDs) are a particularly useful type of fluorescent nanoparticle that demonstrate biocompatibility, resistance to photobleaching, as well as diversity in composition and characteristics amongst the different types available. There are two main morphologies of CDs: Disk-shaped with 1–3 stacked sheets of aromatic carbon rings and quasi-spherical with a core-shell arrangement having crystalline and amorphous properties. They can be synthesized from various potentially environmentally friendly methods including hydrothermal carbonization, microwaving, pyrolysis or combustion, and are then purified via one or more methods. CDs can have either excitation wavelength-dependent or -independent emission with each having their own benefits in microscopic fluorescent imaging. Some CDs have an affinity for a particular cell type, organelle or chemical. This property allows the CDs to be used as sensors in a biological environment and can even provide quantitative information if the quenching or intensity of their fluorescence is dependent on the concentration of the analyte. In addition to fluorescent imaging, CDs can also be used for other applications including drug delivery, quality control, photodynamic therapy, and photocatalysis.

Carbon Dots: Classification, Properties, Synthesis, Characterization, and Applications in Health Care—An Updated Review (2018–2021)

“…(d) Chemical stability and photobleaching properties: Fluorescence bioimaging or biosensing requires long emission lifetimes and stable fluorescence signal. This can be achieved with the help of CDs, since they have the tendency to produce stable signals when stored in an aqueous environment [82,83]. Furthermore, CDs can emit strong fluorescence for long time (i.e., up to a year).…”

“…Under appropriate conditions in redox reaction, CDs can generate CL in aqueous solvents, where the unstable products are produced from intermediate radicals during CL [53]. CDs can generate CL either due to their excitation after direct oxidation or through the enhancement or inhibition of their luminescence [77,82].…”

“…Photoluminescence and Ultraviolet-Visible Spectroscopy PL and UV-Vis spectroscopies are broadly employed methods for studying the optical characteristics of CDs. Additionally, these two spectroscopies can be particularly employed for calculating the QY of CDs [82,155,167].Usually, all forms of CDs show their activities in the UV-Vis region of the electromagnetic spectrum. Furthermore, fluorescence of CDs reveals λ ex -dependent emission.…”

“…Furthermore, fluorescence of CDs reveals λ ex -dependent emission. PL spectroscopy is most commonly used to determine the photoluminescent lifetime of CDs [82,155,167,168]. Figure 7 shows illustrations of UV-Vis absorption spectra, PL spectra at different λ ex , and time-resolved PL spectra of sulfur-doped carbon dots (S-CDs) [169].…”

pH-Sensitive Silver-Containing Carbon Dots Based on Folic Acid

“…Carbon dots (CDs), a kind of fluorescent carbon-based materials, have attracted great attention recently for a wide variety of chemical and biological applications, such as biosensing, bioimaging, drug delivery, photodynamic therapy, photocatalysis and electrocatalysis, due to their good fluorescence quantum yield, excellent solubility, high biocompatibility, nontoxicity, and good photochemical stability [ 1 , 2 , 3 , 4 ]. Generally, almost all CDs show excitation-dependent fluorescence [ 5 ].…”