Polyethyleneimine-Functionalized Fluorescent Carbon Dots: Water Stability, pH Sensing, and Cellular Imaging

Herein, a facile and one-step hydrothermal route is designed to synthesize fluorescent carbon dots (CDs) with citric acid (CA) as a carbon source and hyperbranched polyethyleneimine (PEI) as a surface passivation agent. The resultant CDs display strong blue fluorescence with a quantum yield up to 24.3 %. The resultant CDs show excellent photoluminescence properties with high photo- and metal-stability, which are attributed to PEI, a surface passivation agent that would form a protecting layer. Due to the water solubility and low toxicity, the as-prepared CDs possess potential applications in biological labelling. Additionally, PEI is a functional polycationic polymer with various amine groups, which makes the CDs exhibit pH-responsive optical properties. The resultant CDs have been used as pH sensors due to the reversible pH-responsive fluorescence from acidic (pH 3) and basic solutions (pH 11). Moreover, the fluorescence intensity increases with increasing concentrations of CDs.