Shining a light on cells: amine-passivated fluorescent carbon dots as bioimaging nanoprobes

Abstract

Carbon dots (CDs) have garnered significant interest due to their optical properties and potential applications in cell and tissue imaging. While the physicochemical properties of CDs are predicted to impact their uptake and localization in cells, few studies have directly compared CDs derived from related chemical species, yet such studies are key to revealing structure–function relationships. Herein, CDs synthesized from citric acid and five different amine passivating agents were characterized to determine how an increase in amine composition affects their physicochemical properties, uptake, and localization in HeLa (human cervical carcinoma) and HFF-1 (human foreskin fibroblast) cells. We found that all CDs have similar functional groups and fluorescence properties with monodisperse sizes ranging from 1.3 to 2.7 nm. Their surface charge increases with amine passivation, which correlated with an increase in CD uptake in both cell lines. All CDs exhibited low cytotoxicity and localized to the lysosomes in both HeLa and HFF-1 cells. However, CDs prepared using pentaethylenehexamine (PH6-CDs) were more enriched in the lysosomes compared to the other CDs, while CDs prepared using diethyltriamine (DT3-CDs) were more cytosolic. Indeed, DT3-CDs have amphiphilic properties which may permit passive entry, while hydrophilic PH6-CDs likely rely more heavily on endocytic uptake. Our findings reveal how surface chemistry directs subcellular localization, which can be used to design imaging probes that target specific cellular compartments.