Facile synthesis of efficient red-emissive carbon quantum dots as a multifunctional platform for biosensing and bioimaging

The red-emissive carbon quantum dots (r-CQDs) have gained considerable interest in the field of fluorescence imaging as they can effectively overcome short-wave background interference. However, the development of efficient and simple synthetic pathways for the preparation of r-CQDs with high production yield and fluores-cence quantum yield remains a great challenge. Herein, we prepared the intense red-emissive carbon quantum dots (r-CQDs) through one-pot hydrothermal treatment of methylene violet. The morphology and structure of the as-prepared r-CQDs were characterized through transmission electron microscopy, Raman spectra, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. r-CQDs were well dispersed in water with an average size of 2.39 nm. The surface of r-CQDs was full of oxygen-containing functional groups, which contributed to their high solubility in water. Under ultraviolet light (365 nm) irradiation, both the aqueous solution and powder of r-CQDs emitted intense red luminescence. r-CQDs exhibited the excitation-independent emission property with the maximum fluorescence peak at 596 nm. The fluorescence quantum yield of r-CQDs was calculated to be 26.46% in water using rhodamine B as a reference. Due to the significantly different fluorescence intensity in water and organic solvents, r-CQDs could be applied as the fluorescence sensor to quickly detect the trace amounts of water in various organic solvents. Moreover, the fluorescence of r-CQDs showed selective response to hypochlorite (ClO-) through an on-off sensing mechanism. The good biocom-patibility of r-CQDs was verified by the cell counting kit-8 (CCK-8) method against mouse hepatocellular car-cinoma (Hepa 1-6) cells. Finally, the r-CQDs were successfully used in cellular imaging and they were mainly located in the lysosomes of cancel cells. Thus, r-CQDs could be served as a powerful fluorescence nano-platform for biosensing and bioimaging.

Automated summary

Intense red-emissive carbon quantum dots (r-CQDs) were synthesized through one-pot hydrothermal treatment of methylene violet. The r-CQDs showed high solubility in water and exhibited excitation-independent emission with a maximum fluorescence peak at 596 nm. They could be used as fluorescence sensors for detecting trace amounts of water in organic solvents and showed selective response to hypochlorite. The good biocompatibility of r-CQDs was verified and they were successfully used in cellular imaging, mainly located in lysosomes.