In this work, ultrabright fluorescent carbon dots (U-CDs) were synthesized by using a strict twice iterative optimization strategy. Their relative photoluminescence (PL) quantum yield is close to 100%, exceeding most of the reported fluorescent CDs and greatly boosting the practical applications of fluorescent CDs in many fields. Then serving as fluorescent anti-counterfeiting ink was taken as an example to briefly introduce the application of the U-CDs. The PL emission of the U-CDs is quenched at the range of pH < 4 or pH > 11 and restored at the range of pH = 5–10. This pH-sensitive PL feature allows the U-CDs to be used as fluorescent ink for pH and UV dual information encryption. The written or printed information is invisible under daylight but visible under UV light. After acid treatment or alkali treatment, the information is invisible even under a UV lamp but visible after neutralization treatment. This work provides a standardized scheme for optimizing the synthesis conditions of fluorescent CDs and paves the way for large-scale production of high-performance fluorescent CDs.

An interesting phenomenon of ultrabright fluorescent carbon dots as a metal ion sensor for Pb2+ ion detection.