Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

Abstract

Creating a security label that carries entirely distinct information in reflective and fluorescent states would enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but has proven extremely challenging. Efforts to tune the reflection color of luminescent materials by modifying inherent chemical structures remain outweighed by substantial trade-offs in fluorescence properties, and vice versa, which destroys the information integrity of labels in either reflection or fluorescent color. Here, a strategy is reported to design geminate labels by programming fluorescent cholesteric liquid crystal microdroplets (two-tone inks), where the luminescent material is 'coated' with the structural color from helical superstructures. These structurally defined microdroplets fabricated by a capillary microfluidic technique contribute to different but intact messages of both reflective and fluorescent patterns in the geminate labels. Such two-tone inks have enormous potential to provide a platform for encryption and protection of valuable authentic information in anti-counterfeiting technology.

Fig. 1. Design of geminate labels programmed by two-tone microdroplets.

a Schematic illustration to show the design of a geminate label created by using two-tone inks and photographs of the resulting label (47 mm × 17 mm) in reflective and fluorescent states. The label carrying two distinct kinds of information demonstrates a reflective pattern of RGB “FDU” under white light and a fluorescent pattern of cyan “I love U” upon UV irradiation. The blue, green, red, and pale-yellow colors of the blocks represent the reflection colors of the microdroplets and letter “F” represents the microdroplets with fluorescence. b Schematic illustration to show fluorescent cholesteric liquid crystal (FCLC) microdroplets (two-tone inks) combining structural color under white light and fluorescent color upon UV irradiation. c Chemical structures of the materials used to prepare the FCLC mixtures, including fluorescent molecule DC5, (S)-binaphthyl derivative chiral dopant and LC host E7.

Fig. 2. Characterization of the FCLCs.

a Photoluminescence (PL) spectra of DC5 in THF and THF/water mixtures (c = 10⁻⁴ M). The excitation wavelength is 388 nm. b A Plot of the emission intensity at 480 nm versus water fractions (f_w) for DC5 in THF and THF/water mixtures. The inset photograph shows DC5 in THF and THF/water mixtures (f_w increases from 0 to 90 vol%) upon UV irradiation. c PL spectra of FCLC mixtures containing 6, 5, 4, 3 wt% chiral dopants in THF (c = 0.5 M) and chiral dopant in THF (c = 10⁻⁴ M). d CIE chromaticity coordinates of different FCLC mixtures containing 6, 5, 4, 3 wt% chiral dopants in THF (c = 0.5 M), which are (0.159, 0.251), (0.159, 0.252), (0.159, 0.254), and (0.159, 0.254), respectively. e Reflectance spectra of the FCLC mixtures containing 6, 5, 4, 3 wt% chiral dopants in 5 μm thick antiparallel aligned cells. f Photographs of the FCLC mixtures containing 6, 5, 4, 3 wt% chiral dopants in 5 μm thick antiparallel aligned cells to show blue, green, red, transparent (infrared) reflection colors and cyan fluorescent color. R, reflection color under white light; F, fluorescent color upon UV irradiation.

Fig. 3. FCLC microdroplets fabricated by capillary microfluidic technique.

a Schematic illustration to show the capillary microfluidic device used to fabricate the FCLC microdroplets. The hydrophobic and hydrophilic surfaces of the capillaries are denoted in red and blue. The 5 wt% PVA aqueous solution is used as continuous phase to facilitate planar alignment of the LC molecules and radial alignment of the helical axes. b Normalized reflectance spectra of the FCLC microdroplets containing 6, 5, 4, 3 wt% chiral dopants, respectively. c–f Photographs of blue, green, red, and infrared FCLC microdroplets to show striking reflection colors under white light and fluorescent color upon UV irradiation. g–j POM images of blue, green, red, and infrared monodisperse FCLC microdroplets in hexagonally close-packed arrays to show photonic cross-communication and fluorescent patterns. Additional lines caused by double reflection are denoted by white arrows. The scale bar is 100 μm.

Fig. 4. Geminate labels carrying two distinct kinds of information.

a A geminate label with two different messages of a green “2019” reflective pattern and a cyan “2020” fluorescent pattern. In program, the color of the rectangles represents the reflection colors of the microdroplets and the slashes in the rectangles represent the microdroplets with fluorescence. The scale bar is 1 cm. b–g Encryption and decryption of an enhanced-security-level geminate label created by eight kinds of microdroplets in a “pixelated” array (21 × 21). The fluorescent QR code of “LC” is concealed behind a colorful reflective “Christmas tree”, which can be decrypted by smartphones upon UV irradiation. Each pixel is 2.0 mm × 2.0 mm × 400 μm, and the distance between two pixels is 0.5 mm.