Acid-reagent-engineered tunable multicolor carbon dots via solid-phase pyrolysis for advanced information encryption and anti-counterfeiting

Multicolor fluorescent carbon dots (CDs) have attracted considerable attention because of their prospect applications in advanced information encryption and anti-counterfeiting. Herein, we describe a simple solid-phase pyrolysis that uses o-phenylenediamine as the carbon source and exploits the deliberate selection of different solids. The resulting CDs emit bright blue, yellow, or red fluorescence, are accordingly designated as B-CDs, Y-CDs, and R-CDs, all of which exhibit excellent optical properties and outstanding photothermal stability. Detailed characterization indicates that the distinct photoluminescence arises from differences in particle size, surface functional groups, and heteroatom (N, S) doping. Density functional theory (DFT) calculations reveal that the emission redshift originates from bandgap narrowing induced by surface functional groups and expanded π-conjugated domains. Total and projected density of states (TDOS/PDOS) analyses further confirm that heteroatom incorporation generates localized states and surface defects, which collectively reduce the bandgap and induce the observed redshift. Furthermore, these multicolor CDs serve as efficient fluorescent inks, demonstrating superior performance in anti-counterfeiting and information encryptions. This study not only offers a new strategy for synthesizing multicolor-emitting CDs, but also underscores their potential in advanced security technologies.