Abstract

Carbon dots (CDs) have garnered significant attention in the past decade owing to their fascinating luminescent properties and potential use in cell and tissue imaging, as well as for the study of biological processes. These quasi-spherical amorphous carbon-based nanomaterials can be tailored to emit light from the UV-NIR regions of the spectrum while offering low cytotoxicity and good biocompatibility. The physico-optical and -chemical properties of CDs are determined by the precursors, methods of preparation, and reaction conditions. These properties then influence their cellular uptake, cytotoxicity, and sub-cellular localization.
In this work, CDs synthesized from citric acid and five different amine passivating agents were characterized to reveal their properties, and determine how these properties affect their uptake and localization in cultured human cells (HFF-1; human male foreskin fibroblasts, and HeLa; human female cervical carcinoma). All CDs exhibited fluorescence with a corresponding max λ_ex at 350 nm and max λ_em at 450 nm. We found that these spherical amine passivated CDs ranged in size from 1.29 to 2.74 nm, and each had similar functional groups such as carboxyl, amines and amides. There was an increase in surface charge from -21 mv to +8 mv that correlated with the increase in amine passivation. All of the CDs had low cytotoxicity in both HFF-1 and HeLa cells, with an increase in uptake that correlated with an increase in their surface charge. While all of the CDs localized to the lysosomes, suggesting entry and trafficking via the endomembrane system, there was a difference in their enrichment; PH6-CDs (with the most amine-passivation) were the most strongly co-localized with lysosomes in both cell lines. Interestingly, DT3-CDs also localized to the cytosol, which could be explained by their amphiphilic properties causing (some) CDs to enter cells passively. These findings indicate that CDs could be made to have diverse optical properties and subcellular localization, which could be explored for applications as bioimaging nanotools, and/or delivery agents.