One-step Synthesis of Specific pH-responsive Carbon Quantum Dots and Their Luminescence Mechanism

Nitrogen-doped carbon quantum dots (CDs) were synthesized via a facile one-step solvothermal reaction using citric acid, formamide and water. Subsequent characterizations indicate that the nitrogen-doped CDs are featured by good water solubility, bright blue emission and typical excitation-dependent emission. However, unlike from conventional pH response of most CDs, the as-prepared CDs possess unique pH response. Apart from that fluorescence intensity varies with the pH value, the prepared CDs can generate excitation-independent red emission in alkaline conditions. Through scientific research towards fluorescent property of prepared CDs in different alkaline environments as well as solution with different hydroxyl content, combined with chemical composition and surface states based on Raman spectrum, FT-IR and XPS, the fluorescent mechanism of the unique pH response are attributed to a large number of hydroxyl groups in strong alkaline environment bonded to the surface of CDs, thus changing the surface states of CDs, further forming a new stable luminescence center. Finally, cytotoxicity tests and cell imaging experiments showed that the prepared CDs have low cytotoxicity, and can be used as fluorescent probe for cell imaging, indicating their potential application in the field of biological imaging.