Fluorescent Ternary Energy Transfer Systems Using\nCarbon Dots, Fluorescein Isothiocyanate, and Rhodamine 6G for Hand-Drawing/Inkjet-Printing\nAnticounterfeiting Labels

To meet the ever-increasing demand\nfor anticounterfeiting, developing\nmore information encryption strategies is necessary. Herein, we employed\ncarbon dots (C-Dots), fluorescein isothiocyanate (FITC), and rhodamine\n6G (R6G), which emit blue, green, and red fluorescence, to construct\nan anticounterfeiting encryption system. Structural characterizations\nshowed that carbon dots have very small diameters of only a few nanometers\nand hydrophilic surface functional groups, which makes them easily\nand stably dispersed in water. Photophysical studies proved the excitation-wavelength-dependent\nfeature of C-Dots and the existence of binary energy transfer between\ndonor/acceptor pairs (C-Dots/FITC, C-Dots/R6G, and FITC/R6G) in solution\nand on paper, which can be adjusted by the ratio of fluorophores or\nthe excitation wavelength. The aqueous solutions of R6G/FITC/C-Dots\nas red/green/blue (RGB) inks to form a ternary energy transfer system\nwere used to fill pens or cartridges for hand-drawing or printing\nout the multicolored patterns as encrypted anticounterfeiting labels.\nUnder natural or different UV lights, both the hand-drawn patterns\non paper and the predesigned patterns printed out on paper/banknote\ndisplayed significantly different colors and details, which demonstrated\nthe effectiveness of this anticounterfeiting strategy. Moreover, the\ninformation carried by the printed quick response (QR) code could\nbe read under certain specific conditions. No significant change was\nobserved in the printed patterns after 20 verifications or 1 h of\ncontinuous UV light irradiation. In all, the multidimensional adjustment\nin the color/detail of the anticounterfeiting patterns has been realized\nby combining ternary energy transfer among the three fluorophores\nwith the excitation-wavelength-dependent emission characteristic of\nC-Dots, and this strategy is promising to be used for practical anticounterfeiting\napplications.