Na⁺-functionalized carbon dots with aggregation-induced and enhanced cyan emission

A new type of carbon dots that emit blue emission in aqueous state while cyan emission in solid state was synthesized by a simple hydrothermal method. The photoluminescence quantum yield of the carbon dots in aqueous state and solid state is 7.6% and 29.2%, respectively. The enhanced and red-shifted emission observed in solid state carbon dots is ascribed to surface state change caused by aggregation. The occurrence of surface state change in solid state carbon dots has been evidenced by concentration dependent steady-state photoluminecence spectra and time-resolved luminescence decay. Surface functionalization by Na⁺ is beneficial for carbon dots to resist luminescence quenching in solid state. A proof-of-concept study was performed to demonstrate the potential application of the obtained carbon dots as inks for anti-counterfeiting and printing high quality fluorescent images.