Although heteroatom doping is widely used to promote the optical properties of carbon dots for biological applications, the synthesis process still has problems such as multi-step process, complicating the setting of instrument along with uncontrolled products. In the present study, some elements such as boron, nitrogen, sulfur, and phosphor were intentionally doped into citric acid-based carbon dots by furnace- and microwave-assisted direct and simple carbonization processes. The process produced nanoparticles with an average diameter of 5-9 nm with heteroatoms (B, N, S, and P) placed on the core and surface of carbon dots. Among the doped carbon dots prepared, boron-doped carbon dots obtained by the microwave-assisted (B-CDs2) process showed the highest photoluminescence intensity with a quantum yield (QY) of about 32.96%. All obtained carbon dots exhibit good stability (at pH 6-12 and high ionic strength concentrations up to 0.5 M), whereas cytotoxicity analysis showed that all doped carbon dots are low-toxic with an average cell viability percentage above 80% up to 500 μg mL-1. It can be observed from the CLSM image of all doped carbon dots that the doping process not only increases the QY percentage, but also might accelerate the HeLa uptake on it and produce strong carbon dot emission at the cytoplasm of the cell. Thus, the proposed synthesis process is promising for high-potency bioimaging of HeLa cancer cells.
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