5.1. Anti-counterfeiting applications
Security inks and labels containing fluorophores are increasingly being used to identify counterfeit products. CDs with solid-state PL show great potential for anti-counterfeiting. The surface of CDs is rich in hydrophilic functional groups, enabling CDs-based inks to be inkjet printed on paper with good adhesion. The different PL wavelength and long-life afterglow of solid-state PL, room temperature phosphorescence and delayed fluorescence materials based on CDs offers double or even triple anti-counterfeiting protection. Jiang et al.[111] printed a portrait of Chairman Mao onto a banknote utilizing CDs (Figure 7A). The CDs were colorless under ambient conditions, but showed a blue emission and under 365 nm ultraviolet light. When the excitation light was off, the printed portrait showed green phosphorescence. Tao et al. [106] developed a dual-mode anti-counterfeiting material. A colorful butterfly was drawn using commercial fluorescent materials and long-life afterglow CDs. Under UV light, the butterfly pattern possessed multicolor fluorescence. However, on turning off the UV lamp, a green phosphorescence appeared in the encrypted part of the butterfly. In another multi-mode example[112], adding CDs to a printer’s color ink can caused afterglow emission to appear in the given area of the printed pattern, thus achieving multi-mode anti-counterfeiting. Furthermore, CDs can also be utilized in fingerprint analysis. When the finger touches something, oily sebum and sweat will leave a latent fingerprint, which is not visible to the naked eye and needs to be enhanced. Chen et al.[113] prepared red emission CDs for this purpose, with the red emission negating potential background fluorescence interference from fluorophores in the fingerprints. A coffee ring effect and electrostatic interaction overcame the aggregation-induced fluorescence quenching of CDs. Dong et al. [114]combined CDs and starch to prepare phosphors on a large scale (Figure 7B). A digital processing program was applied to objectively identify the latent fingerprint, with a high degree of matching.