In Situ Synthesis of Amino Acid Functionalized Carbon Dots with Tunable Properties and Their Biological Applications

Emissive carbon dots (C-Dots) are known for displaying versatile properties, which have been widely utilized in many applications such as bioimaging, light-emitting devices, and photocatalysis. Further functionalization can tune their physical and chemical properties, which play the key role in many biologically and chemically relevant applications. Amino acids provide an attractive means for introducing functionality with a structural diversity. In this work, water-soluble emissive C-Dots were synthesized from pyrolysis of citric acid in the presence of various amino acids under hydrothermal conditions. We established that the diverse functionality-bearing polyamides and the polyester backbone of C-Dots are the origin of optical properties with a high quantum yield. The amino acid based C-Dots were systematically characterized using various analytical methods to confirm the core structure as well as the functionality. The results show that, depending on functionalization, the quantum yield can be varied and enhanced up to 62% and surface charge and hydrophobicity can be tuned. Most importantly, we observed a correlation between the quantum yield and properties of the side chain residue of amino acids such as hydrophobicity index and volume. Furthermore, it was found that the synthesized, diversely functionalized C-Dots exhibit no cellular toxicity and can be used for cell imaging. Due to variable surface functionality, we have also applied these C-Dots for array-based protein sensing. Overall, based on this reported method, we can easily tune the optical as well as surface properties of C-Dots, which will be suitable for many future applications.