Construction of delayed fluorescence / phosphorescence dual-mode carbon dots and its application in paper anti-counterfeiting systems

Printing encrypted messages and security labels using stimuli-responsive luminescent materials can no longer meet the requirements of protecting high-level confidential information. In this paper, carbon dots (CDs) with near-infrared emission potential at low temperatures were prepared by thermal polycondensation using citric acid as a carbon source and urea as a nitrogen source in a polysuccinimide (PSI) matrix. The prepared CDs@PSI achieved visible (yellow-green) and near-infrared (NIR) dual-mode afterglow emission at room temperature. The C=N covalent bond and hydrogen bond formed during the thermal polycondensation of CDs and PSI, as well as the crystal structure, multiple fixations, and rigidity effect forming during the slow cooling process of PSI are crucial to the activation of bimodal afterglow. By comparing with the samples prepared by the room temperature co-crystallization method, it is found that the effective regulation of the energy gap value can promote the simultaneous generation of intersystem interference and anti-intersystem interference, which is beneficial to RTP and TADF emission behavior. Under direct visual conditions, NIR-RTP is completely covered by yellow-green TADF and can be captured by a camera loaded with a filter with a cut-off wavelength of 700 nm. The enhancement effect of subfluorescent groups (C=O/C=N) on the surface of CDs with PSI polymer crystal bonds is the main reason for the visible light-activated afterglow. Based on the fact that two different afterglow emissions of a single excitation light source and visible afterglow can hide NIR afterglow coding information, a dual-mode delayed fluorescence/phosphorescence anti-counterfeiting system was constructed, and an anti-counterfeiting strategy for visible afterglow to hide NIR afterglow coding information was developed.