Easy-to-perform organic-solvent-free synthesis of carbon dots with strong green photoluminescence

Carbon dots (CDs) have been extensively studied owing to their fascinating optical properties and wide potential applications. Here, we report an easy-to-perform and organic-solvent-free synthesis strategy for green-emissive CDs (G-CDs) possessing high photoluminescence (PL) quantum yield (QY). The G-CDs are synthesized by heating the homogeneous precursors of citric acid and cyanamide in an open vessel, circumventing the use of organic solvents, complex operations, high-pressure reactors, and expensive instruments in the synthesis process. The effect of various reaction variables on the formation and the optical properties of G-CDs are systematically investigated. The resultant G-CDs show bright PL emission at 521 nm with PL QY up to 73%. Then a white light-emitting diode (LED) with Commission Internationable de L'Eclairage (CIE) coordinates of (0.33, 0.34) and color rendering index (CRI) of 92 is constructed based on G-CDs/thermoplastic polyurethane (TPU) composite. Moreover, a visual microfluidic detection platform is designed by using G-CDs as fluorescent probes for rapid quantitative detection of Fe³⁺, Cu²⁺, and Mn²⁺ metal ions, which can realize synchronized testing of multiple samples. This study might promote the development and preparation methods of high-performance CDs with various optical applications.