Highly fluorescent near-infrared emitting carbon dots derived from lemon juice and its bioimaging application

Fluorescent carbon dots (CDs) have gained intensive interests in the recent years due to their unique optical properties and widespread applications. However, the facile synthesis of highly efficient near-infrared emitting CDs (NIR-CDs) from renewable and low-cost natural resources is still a challenge to be addressed for their further applications, particularly in the bioimaging fields. Herein, NIR-CDs with a high quantum yield of 31% in water were synthesized by heating a formamide solution of pulp-free lemon juice. The as-obtained NIR-CDs possess an average size of 5.7 nm and a nitrogen content of approximately 20%, and exhibit an excitation-independent photoluminescence (PL) emission at 704 nm. Further studies suggest that the intense near-infrared emission of NIR-CDs is originated from their surface states, and that the energy gaps of these surface states are modulated by both the particle size and the nitrogen-doping amount. In addition, characterized with good photostability and low cytotoxicity, these NIR-CDs can be used as an excellent fluorescent probe for bioimaging both in vitro and in vivo.