Monochromatic Blue and Switchable Blue-Green Carbon Quantum Dots by Room-Temperature Air Plasma Processing

Carbon quantum dots (CQDs, C-dots or CDs) are an emerging type of nanomaterial which has received immense attention due to their numerous applications. However, most of the reported CQDs in literature typically emit single emission peak under an excitation. Multipeak emissions without any complicated techniques will be ideal for various applications in the fields of ratiometric sensing, optoelectronics, and multifunctional bio-imaging systems. Here, a fast, effective, and single-step method is developed for the bulk synthesis of CQDs using atmospheric pressure air plasmas. Structural, morphological, and chemical properties are characterized by advanced analytical techniques. The CQDs have an average diameter of about 3 nm with a narrow size distribution. Emission wavelengths of 470 nm for blue emissive CQDs and 515 nm for green emissive CQDs are observed. Concentration dependency of the CQDs suggests that the switchable mechanism is due to the formation of PTSA excimers. Dual-emissive CQDs have the potential to be used in bi-channel ratiometric determination for metal ions, pH sensing, tumor diagnosis and detection, and solid-state lighting materials. The proof-of-principle demonstration of the use of dual-emissive CQDs (DCQDs) as a fluorescent sensor of Cu²⁺ ions is also presented to highlight the possible applications.