Uncovering Aggregation-Induced Emission in Carbon Dots for Color-Changing Hydrogels and Information Encryption

Since the discovery of carbon dots (CDs), the typical precursor combination of citric acid (CA)-urea has been widely used in the scientific community to explore the formation mechanism and luminescence behavior of CDs. However, there have only been a few reports on the synthesis of CDs featuring aggregation-induced emission (AIE) characteristics. In this study, CA and urea were used to synthesize hydrophilic red-emissive carbon dots (R-CDs) that exhibit blue fluorescence in water (dispersed state) and red fluorescence in DMF (aggregated state). The study reveals that the photoluminescence of R-CDs is governed by π-π stacking interactions between solute molecules as well as solvent effects between solute and solvent molecules, leading to solvent-responsive emission behavior. By tuning the solvent polarity, the intermolecular distance between R-CDs can be adjusted, thereby influencing their photoluminescent properties. Taking advantage of the solvent-responsive color change, anticounterfeiting printing and information encryption applications were designed. Moreover, by combining R-CDs with poly(vinyl alcohol) (PVA), hydrogel-based fluorescent information-encoding materials were successfully fabricated.