Fluorescent Carbon Dots with Enhanced Thermal Stability for Multicolor Bioimaging in Hot Conditions

Carbon dots (CDs) have recently garnered considerable attention as remarkable fluorescent nanomaterials. However, the production of thermally stable CDs still remains a significant challenge as it requires a stable structure resistant to thermal oxidation and effective control of aggregation sites. Herein, we have developed a strategy for producing fluorescent nano-CDs with a stable structure and high thermal resistance properties using citrazinic acid and urea via solid-state carbonization. These nano-CDs exhibit excellent performance at high temperatures ranging from 10 to 90 °C. The core structure of nano-CDs is functionalized with various C═O/C═N nitrogen and oxygen groups, which enhance the structural integrity of the fluorescent nano-CDs and protect them against thermal oxidation and the aggregation effect. The thermal stability of the nano-CDs was evaluated using state-of-the-art techniques such as photoluminescent spectroscopy, UV–vis absorption spectroscopy, and decay lifetime measurements. Besides thermal resistance, nano-CDs exhibit green light emission with a maximum excitation of 400 nm and a peak quantum yield of 7.8%. These low-toxicity nano-CDs were successfully incubated with T-ca cancer cells, enabling stable multicolor imaging even at elevated temperatures (25–45 °C).