Introduction: Carbon Dots
Fluorescent carbon dots and carbon nanoparticles have attracted great attention since they were discovered and used by Sun’s group in 2006(Sun et al., 2006). Most of these carbon dots have been synthesized via two approaches: top-down and bottom-up approaches. In top down methods, the nanoparticles are formed by breaking down larger pieces of bulk carbon materials in to the desired nanostructures. In bottom-up approaches, the carbon dots are obtained from assembling molecular precursors under a range of different reaction conditions(Qu et al, 2012). Various precursors have been used as raw materials to synthesize carbon dots, for example, honey, alfalfa and garlic, sucrose, polyethylene glycol, glucose, citric acid, glycerol, coffee grounds, soy milk and grass. Carbon dots have extensive potential applications in bioimaging, biological sensing, photoelectric devices, theranostic agent and catalysis(Wang et al., 2016).
Methods: Dual Emissive Carbon Dots
Colorimetric and fluorescent dual mode detection methods have gained much attention in recent years, however, it is still desirable to develop new colorimetric and fluorescent dual mode nanosensors with more simple preparation procedure, low cost and excellent biocompatibility(Liu et al., 2017). Carbon dots received attention for the construction of dual mode fluorescent sensor because of the unique optical properties, excellent biocompatibility, good water solubility, and convenient surface modification flexibility(Song et al., 2017). According to most of the reports the carbon dots absorb/emit in the visible region with a few exceptions. The green, yellow and red fluorescence emission of CDs has been considered as a key requirement for the broad application in bioimaging but this tunability was achieved only rarely(Parvin et al., 2917). The emission of carbon dots can also be adjusted by doping with other elements, such as sulphur, nitrogen, phosphorus and so on.
Results: Dual Emissive Carbon Dots for Bioimaging
A functional dual emissive carbon dots (dCDs) was prepared by a one−pot hydrothermal carbonization method. because of the low cytotoxicity, good optical and colloidal stability, and excellent wavelength dependent sensitivity and selectivity toward lysine and pH, this probe was successfully applied to monitor the dynamic variation of lysine and pH in cellular systems, demonstrating the promising applicability for biosensing in the future. Hydrothermal treatment of a mixture of ethylene diamine, phosphoric acid and citric acid under ambient pressure generates fluorescent carbon dots that are co-doped with phosphorus and nitrogen(parvin et al., 2017). Recently, the development of new fluorescent probes for the simultaneous ratiometric differentiation of two different targets from dual channels has received more attention.because this sensing strategy can provide further detection information for biological or biomedical analysis. The synthesized nitrogen and phosphorus co-doped carbon quantum dots with high quantum yielding green (QY-30%) and red (QY-78%) dual emission. A highly reproducible, single-pot, rapid hydrothermal process was used to condense citric acid, phosphoric acid and EDA in aqueous solution. The green and red-emissive PN-CQDs have the following characteristics: (1) broad absorption in the region from 400 to 800 nm, (2) active agents in PA and fluorescence imaging, (3) high pH and ionic tolerance strength and (4) visible light excitation (350– 700 nm) for fluorescence and PA imaging in living mice(parvin et al., 2017). Huang et al. developed a nanosensor for ratiometrical detection of O2 •− and pH by conjugating hydroethidine, fluorescein isothiocyanate, and (4−carboxybutyl) triphenylphosphonium bromide onto the silica coated CdSe/ZnS quantum dots(Huang et al., 2016). Chen et al. have designed and synthesized a dual−detection organic fluorescent probe to detect H2S and H2Sn with distinctive fluorescence signals (Chen et al., 2016). Recently, Zhou and coworkers have prepared a two−photon fluorescence probe for ratiometric visualization of NO/H2S(Zhou et al., 2017). Jiang et al. reported the synthesis of red, green, blue emission CDs using phenylenediamines isomers via a solvothermal method, which can be applied in photoluminescence (PL) bioimaging (Jiang et al., 2015). Ding et al obtained red emission CDs, which needed purification via silica column chromatography, and used it for live mice imaging(Ding et al., 2016). Their results proved red emissive CDs showed good performance and hold great potential in bioimaging.
Conclusion: Conclusion
Carbon dots (CDs) have emerged as a new type of fluorescent material due to their multiple advantages, such as high photoluminescence quantum yield (QY), earth abundant raw materials, great environmental sustainability, low production cost, and excellent biocompatibility. Dual emission carbon dots have a high potential for use as fluorescence‐based sensors with higher selectivity and sensitivity.