Synthesis and physicochemical properties of visible-light-excited carbon dots: impact of sugar ring structure and surface chemistry

Carbon dots (CDs) are promising tools for in situ tracking in biological systems. However, the majority of CDs are excited by ultraviolet (UV) light, which is harmful and can potentially cause negative effects on biological organisms. Herein, visible-light-excited CDs were developed in order to reduce the cellular toxicity of the excitation wavelength. Three structures of sugar precursors were used to demonstrate the effect of the sugar structure on the carbonisation of carbon dots in a domestic microwave. The results revealed that a sugar with a 5-membered ring more effectively promoted carbon-core formation than a sugar with a 6-membered ring. However, changing the excitation properties of the carbon dots toward the visible region was dominated by the carbon-dot surface, not the carbon core. The carbon dots obtained from our study showed a high quantum yield%, compared with other sugar-derived carbon dots obtained using a microwave, at 20.28%. In addition, doping with different acids caused a change in the core and surface of the carbon dots. The quantum yield% also increased to a maximum of 37.81% when doping with nitric acid, which proved to be the effect of the core crystallinity increasing. However, doping with phosphoric acid gave the best visible-light-excited carbon dots, since the excitation wavelength significantly shifted from 400 to 460 nm. The emission of the phosphoric-acid-doped carbon dots was at 554 nm. The energy band structures of the visible-light-excited carbon dots were illustrated and discussed according to their core and surface chemistry. The fluorescence under visible light enhanced the biocompatibility for biosensing or bioimaging applications. We aimed to enhance the fluorescence quantum yield of sugar-derived carbon dots along with tuning the excitation wavelength to the visible region via a surface modification method with simple acids.