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

To meet the ever-increasing demand for anticounterfeiting, developing more information encryption strategies is necessary. Herein, we employed carbon dots (C-Dots), fluorescein isothiocyanate (FITC), and rhodamine 6G (R6G), which emit blue, green, and red fluorescence, to construct an anticounterfeiting encryption system. Structural characterizations showed that carbon dots have very small diameters of only a few nanometers and hydrophilic surface functional groups, which makes them easily and stably dispersed in water. Photophysical studies proved the excitation-wavelength-dependent feature of C-Dots and the existence of binary energy transfer between donor/acceptor pairs (C-Dots/FITC, C-Dots/R6G, and FITC/R6G) in solution and on paper, which can be adjusted by the ratio of fluorophores or the excitation wavelength. The aqueous solutions of R6G/FITC/C-Dots as red/green/blue (RGB) inks to form a ternary energy transfer system were used to fill pens or cartridges for hand-drawing or printing out the multicolored patterns as encrypted anticounterfeiting labels. Under natural or different UV lights, both the hand-drawn patterns on paper and the predesigned patterns printed out on paper/banknote displayed significantly different colors and details, which demonstrated the effectiveness of this anticounterfeiting strategy. Moreover, the information carried by the printed quick response (QR) code could be read under certain specific conditions. No significant change was observed in the printed patterns after 20 verifications or 1 h of continuous UV light irradiation. In all, the multidimensional adjustment in the color/detail of the anticounterfeiting patterns has been realized by combining ternary energy transfer among the three fluorophores with the excitation-wavelength-dependent emission characteristic of C-Dots, and this strategy is promising to be used for practical anticounterfeiting applications.