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Abstract 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.
Wang et al. (Mon,) studied this question.