Water-triggered, thermally reversible sulfur quantum dots with red-blue luminescence switching for dynamic anti-counterfeiting

For anti-counterfeiting and encryption, there is a scarcity of photoluminescent (PL) stimuli-responsive materials that combine weather resistance, reversibility, color tunability, and dehydration-stress resistance. Here, we report water-triggered, thermally reversible sulfur quantum dots (SQDs) for dynamic anti-counterfeiting. Red phosphorescent SQDs (R-SQDs) rapidly switch to blue fluorescent SQDs (B-SQDs) upon water contact and revert to R-SQDs upon heating (400–700 °C). This reversible luminescence switching originates from the transformation of the SQDs’ environment: they exhibit triplet-state phosphorescence when embedded in a rigid sulfate matrix but show singlet-state fluorescence when dispersed in water. Both forms demonstrate excellent weather resistance and can be stored for one year under ambient conditions. Moreover, the transformation is highly specific to water, as other solvents do not induce this change. This work demonstrates an SQD-based material capable of encryption-decryption-re-encryption cycles through reversible PL color/lifetime switching, offering a robust platform for advanced dynamic anti-counterfeiting.