Acid–Base Responsive Carbon Nanodots Enabling Reversible Write-Erase Fluorescence Switching for Multilevel Anticounterfeiting

Stimuli-responsive luminescent materials with reversible signal switching are vital for advanced anticounterfeiting and secure optical encryption. However, developing luminescent systems that combine high stability with rapid, controllable responsiveness across diverse environmental conditions remains challenging. Herein, green-emitting carbon nanodots (G-CDs) are synthesized via a solvothermal method. The obtained G-CDs exhibit bright green emission centered at 508 nm with a photoluminescence quantum yield (PLQY) of 27.10%. Notably, the G-CDs display three fully reversible optical states; a nonemissive OFF state under acidic conditions, strong green emission at neutral pH, and a yellow-shifted emission in alkaline environments. These transitions occur rapidly in both liquid and vapor environments, enabling noncontact optical modulation. Benefiting from this property, the G-CDs were incorporated into polymer-based fluorescent inks to construct dynamic anticounterfeiting systems. The resulting patterns exhibit reversible fluorescence switching and reliable optical information encryption with excellent chemical, mechanical, thermal, and photostability during repeated acid-base cycles. These findings establish G-CDs as a robust platform for reversible write-erase fluorescence switching and multilevel optical anticounterfeiting, while providing further insights into surface-state engineering in acid-base switchable carbon nanodots.