H+-triggered solid-state fluorescence of carbon dots for dual anti-counterfeiting

Developing luminescent materials with tunable and reversible fluorescence remains challenging, as it requires stable, responsive, and versatile systems capable of switching emission states under precise stimuli. Herein, we prepared carbon quantum dots (CDs) by using 2-aniline-3-methyl-6-(dibutylamine) fluoroalkane as raw material. The optical characterization showed the parent CDs presented a cyan fluorescence resulted from the surface oxidative functional groups dominating the low-energy transitions. Interestingly, stimulated by various acids, this fluorescence phenomenon can be changed into yellow, no matter in aqueous solution or solid state. It can be explained by the fact that H⁺ reduces oxidized surface groups such as nitro to amino groups, creating new emissive traps. Based on this, a reversible dual-switch mode luminescent ink was demonstrated for advanced anti-counterfeiting and dual encryption.