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.