Photoresponsive Luminescent Polymeric Hydrogels for Reversible Information Encryption and Decryption

Abstract

Conventional luminescent information is usually visible under either ambient or UV light, hampering their potential application in smart confidential information protection. In order to address this challenge, herein, light-triggered luminescence ON-OFF switchable hybrid hydrogels are successfully constructed through in situ copolymerization of acrylamide, lanthanide complex, and diarylethene photochromic unit. The open-close behavior of the diarylethene ring in the polymer could be controlled by UV and visible light irradiation, where the close form of the ring features fluorescence resonance energy transfer with the lanthanide complex. The hydrogel-based blocks with tunable emission colors are then employed to construct 3D information codes, which can be read out under a 254 nm UV lamp. The exposure to 300 nm UV light leads to the luminescence quenching of the hydrogels, thus erasing the encoded information. Under visible light (>450 nm) irradiation, the luminescence is recovered to make the confidential information readable again. Thus, by simply alternating the exposure to UV and visible lights, the luminescence signals could become invisible and visible reversibly, allowing for reversible multiple information encryption and decryption.

Keywords: information encryption and decryption; luminescence; photoresponsive materials; polymeric hydrogels; tunable emission.

Scheme 1

Chemical structure of the polymer consisting of AAm, lanthanide complex, and diarylethene photochromic unit for the formation of hydrogels.

Scheme 2

Illustrated luminescent ON‐OFF photoswitchable behavior of lanthanide‐containing hydrogels under alternating UV and visible light irradiation.

Figure 1

A) Tensile stress–strain curves of the Eu³⁺‐containing hydrogel (a) before and (b) after UV light irradiation, and (c) freshly cut Eu³⁺‐containing hydrogel after self‐healing for 10 h. B) Frequency (ω) sweep tests at ω = 0.01–100 rad s⁻¹ and strain (γ) = 1.0% for Eu³⁺‐containing hydrogel before (black squares) and after (red circles) irradiation with UV at 25 °C. C) Strain sweep tests of Eu³⁺‐containing hydrogel at γ = 0.1–100 000% and ω = 1.0 Hz. D) Continuous step strain tests of Eu³⁺‐containing hydrogel at γ = 0.1 and 50 000% with ω = 1.0 Hz.

Figure 2

A) Luminescence emission spectra of the hydrogels with varied Eu³⁺/Tb³⁺ molar ratios (λ_ex = 280 nm). Tb³⁺/Eu³⁺ = 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, 0:10; B) The corresponding CIE 1931 chromaticity diagram with varied Eu³⁺/Tb³⁺ molar ratios.

Figure 3

UV–vis spectral changes of 1 aqueous solution (2.0 × 10⁻⁵ m) upon irradiation with 300 nm UV light for up to 60 s. Inset: corresponding photographic images of 1 aqueous solution upon alternating UV and visible light irradiation.

Figure 4

A,B) Partial ¹H NMR spectra (DMSO, 400 MHz, 25 °C) of compound 1 before and after irradiated at 300 nm for 10 min.

Figure 5

A) Normalized absorption spectra of (a) OF‐1 and (b) CF‐1, as well as emission spectra of (c) Eu³⁺‐containing and (d) Tb³⁺‐containing hydrogels. B) Luminescence emission spectra (λ_ex = 280 nm) and emission intensity changes at 615 nm (upper inset) for Eu³⁺‐containing hydrogel upon 300 nm UV light irradiation. The lower inset shows the photographs of Eu³⁺‐containing hydrogel under daylight upon alternating irradiation with 300 nm UV light and visible light (λ > 450 nm). C) Luminescence emission spectra and intensity changes at 615 nm (inset) for Eu³⁺‐containing hydrogel upon repeatedly alternating irradiation with UV and visible light. D) Luminescence emission color changes of (a) Eu³⁺‐containing, (b) Tb³⁺‐containing, and (c) Tb³⁺/Eu³⁺ codoped (Tb³⁺/Eu³⁺ = 1:1) hydrogels under 254 nm light upon alternating irradiation with 300 nm UV and visible light (λ > 450 nm). The size of the hydrogel blocks is 2.5 cm × 1 cm × 0.2 cm.

Figure 6

A,C) Photographs of a pattern (Code A) made up from the assembly of red hydrogel, green hydrogel, yellow hydrogel, and nonhydrogel on a black substrate under daylight before and after irradiation with 300 nm UV light. B,D) Under these conditions, the information could be read out or masked. E,F) Photographs showing the transformation of Code A into Code B under daylight and 254 nm UV light by means of a reassembly strategy. Hydrogels were all prepared on the same substrate. Photographs of (A), (C), and (E) were taken under daylight, and photographs of (B), (D), and (F) were taken under 254 nm UV light. The size of each hydrogel block in the code pattern is 0.5 cm × 0.5 cm × 0.2 cm.