Red carbon dots: Optical property regulations and applications

Recent years have witnessed a rapid development of the fluorescent carbon dots (CDs), due to their distinctive advantages of straightforward synthesis, excellent biocompatibility, low cost, and tunable optical properties. However, the widespread applications of CDs in biomedical theranostics, light harvesting, and photocatalysis are limited by the lack of emission and/or excitation in the red/near-infrared (NIR) region. Extensive explorations have been conducted to synthesize CDs with intensive red/NIR emission/excitation (red CDs) by rational design and ingenious synthesis to broaden their applications. This review emphasizes the most recent efforts in the development of CDs with intensive emission at a long wavelength, with a focus on the regulation methods for the optical properties of CDs, including particle size, surface state, and heteroatom doping. Key factors in processing red CDs, such as reaction solvent and precursors, are demonstrated. More importantly, employing polyaromatic and dye molecules as carbon sources is highlighted, which could further expand emission/excitation to the NIR region. The boosting applications of red CDs in white light-emitting diodes (WLEDs), biosensing, bioimaging, theranostics, and photocatalysis are demonstrated. Finally, the challenges and perspectives of red CDs are also discussed.