Figure 5 (a) Phosphor spectra of UPy-CDs under 534 nm laser irradiation. (b) Time-resolved Phosphor spectra of UPy-CDs.(c) Schematic diagram of the mechanism of multiple hydrogen bonds induced room temperature phosphorescence of UPy-CDs.
Thanks to the matrix-free afterglow emission of UPy-CDs, their potential application in information security was further explored (Fig. 6). As shown in Fig. 6, the powder of UPy-CDs and EDTA-CDs was made into the shape of ”A H U” (the English abbreviation of the University of AnHui), where the characters “A” and “U” were composed of EDTA-CDs and the character “H” was composed of UPy-CDs. When the powder was exposed under 365 nm UV lamp, bright blue emission was observed for all the characters due to the dominance of fluorescence. However, when the UV excitation resource was removed, the fluorescence of EDTA-CDs vanishes immediately and the characters “A” and “U” cannot be recognized. Concomitantly, the emission color of the characters “H” changed from blue to yellow because of the switch between fluorescence and phosphorescence. This color change could also be used for information security of patterns and numbers. As shown in Fig. 6, because of the interference emission from EDTA-CDs, wrong information (with a blue pattern of “stem and flower” and “888”) was observed under a 365 nm UV lamp. The correct information (“flower” and “123”), however, can only be detected under the phosphorescent mode.