Deep Eutectic Solvent-Assisted Preparation of Nitrogen/Chloride-Doped Carbon Dots for Intracellular Biological Sensing and Live Cell Imaging

Abstract

A novel approach for the preparation of dual-functional carbon dots, i.e., nitrogen- and chloride-doped carbon dots, abbreviated as N/Cl-CDs, is developed with the assistance of a choline chloride-glycerine deep eutectic solvent (DES). The carbon source is provided by urea and the DES serves as a solvent for controlling the preparation of CDs in the absence of water. The dual-element doped carbon dots are oxygen-rich with hydroxyl and amine groups. They exhibit an average particle size of ca. 3.88 nm and give rise to strong and pH-sensitive fluorescent emission at λ_ex/λ_em = 340/430 nm with a quantum yield of 16.15 ± 1.36%. It is particularly interesting to see that the fluorescence of N/Cl-CDs remains stable in a high-salinity matrix, providing vast potentials for treating real biological sample matrixes with high salinity. The N/Cl-CDs provide an optical probe for intracellular pH sensing and multicolor imaging in HeLa cells. In addition, the N/Cl-CDs show obvious fluorescence response to cytochrome c (cyt- c) with a detection limit of 3.6 mg L^-1 (ca. 0.29 μmol L^-1) within in a range of 10-500 mg L^-1, providing potentials for fluorescence detection of cyt- c as well as facilitating intracellular cyt- c imaging.