Introduction: Carbon Dots
Carbon dots are usually regarded as a group of ultrasmall and photoluminescent carbon nanostructures with a diameter of less than 10 nm, including graphene quantum dots(GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs) and polymer dots. CDs have attracted increasing interest owing to their unique properties, such as small size, excellent biocompatibility and excellent chemical inertness. Recently, massive efforts have been invested in the development of high efficient CDs and these reported methods mainly consist of two categories: topdown treatment and bottomup treatment. However, most of the present CDs emit in the visible region from blue to orange range. This property severely limits their application in the biomedical field due to the tissue absorption, high autofluorescence of biomolecules, and potential photodamage.
Methods: Red-Emitting Carbon Dots
The CDs, which have intense emission in the farred and nearinfrared (far red and NIR) region (>600 nm), are attracting increasing attention owing to their superior properties, such as minimum tissue absorption, deep tissue penetration, low interference from autofluorescence and minimal photodamage of the far red and NIR light to biological sample. And many efforts have been paid on the preparation of farred to NIR CDs and their applications in bioanalysis and biomedicine. The type of reaction solvent, carbon source, particle size, surface state and heteroatom doping are responsible for the optical property regulation toward red CDs. In addition to the above mentioned effects of tuning the optical properties of CDs, the surface modification, separation method and heating manner show important effects on the optical behavior of the CDs
Results: Red-Emitting Carbon Dots Preparation
The type of reaction solvent and carbon source has been regarded as a key factor in the synthesis of red CDs. Many Solvent nature plays significant roles in the degree of dehydration and carbonization of precursors during the fabrication process of CDs. For example, the size of sp2-conjugated extensions in CDs can be tuned by varying solvents during the solvothermal synthesis. In the early stage of CDs preparation, water was used as a prevailing medium because of its safety, convenience, and solubility for small molecules. However, most CDs prepared in aqueous solution show blue emission because of the high polarity of water and thus induced poor dispersibility of conjugated structures, which is unbeneficial to the formation of the conjugated PL center. Thus, the solvents with low polarity are more desirable because CDs with a large conjugated domain size and narrow energy bandgaps would be generated in organic solvents. Suitable carbon sources are crucial to the optical properties of CDs. For red CDs, the precursors always contain (1) a certain number of heteroatoms, which would introduce new energy bandgaps within CDs (discussed in Section ‘Heteroatom doping’); and (2) a suitable aromatic structure, which would inherently lower the energygap by generating large sp2 domains. Both are beneficial to tuning the emission and excitation toward long wavelength.
Conclusion: Red-Emitting Carbon Dots Bio-Applications
As versatile fluorescent nanomaterials, farred to NIR CDs were employed as various sensors in vitro or in vivo because of their obvious merits, like small size, flexible surface modification, excellent chemical stability, etc. Recently, this diverse farred to NIR CDsbased system has been used for the measuring of distinctive metal ions, small biomolecules, gaseous molecules and even fingerprints. In recent times, there has increasing interest in the development of excellent fluorescent CDs as contrast agents for imaging in vivo. Ideally, fluorescent probes for in vivo imaging should possess high QY, NIR excitation and NIR emission, minimum toxicity and photodamage to biological samples. The rationale for the use of inherent farred to NIR emitting CDs in visualizing biological systems both in vitro and in vivo is now a challenging topic. Effective early diagnosis and therapy methods are becoming more and more desirable in the prevention and treatment of cancer, especially photoactivated theragnostic. NIR lighttriggered therapeutic methods, such as photothermal therapy, photodynamic therapy, and chemotherapy, have been actively used owing to relatively low autofluorescence of tissues and mediums in the transparency window (650–950 nm). Farred to NIR emission CDs are very appealing as neonatal theragnostic agents.