Light-induced color changing of redox dyes assisted by a photoinitiator for information encryption on hydrogel paper

Abstract

In this work, we reveal an intriguing mechanism in which free radicals yielded from a commercial cleavage photoinitiator are able to induce rapidly color changing in a methylene blue redox dye under UV irradiation. Leveraging this mechanism, a type of photopolymerized hydrogel rewritable paper through a simple and cost-effective “ink-pumping” strategy for information encryption is proposed. The proposed hydrogel paper exhibits a remarkable ability of transient information encryption and self-erasing decryption (“burn after reading”). Such rewritable paper supports both template-assisted printing and freehand UV writing with stable performance over more than 20 rewrite cycles and a high spatial resolution of up to 3 μm. Moreover, the proposed mechanism and method can be extended to other color redox dyes to produce multi-color patterning. We believe that the proposed mechanism and fabrication strategy open a new pathway for image recording, information storage, rewritable photonic paper, and secure information transmission.

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Introduction

Information encryption on paper has attracted significant attention in an age where paper continues to serve as a vital medium for information delivery.^1–3 However, the inadequacy of information encryption strategies in conventional paper-based documentations renders information security susceptible to information exposure. The non-reusability of traditional pulp paper after only one-time reading or writing incurs significant threats to the depletion of resources.^4–6 For the sake of information protection, it is important to develop a type of rewritable paper that enables multiple writing/erasing cycles and transient information encryption.

Nowadays, many studies on rewritable paper with the ability for information encryption have been reported. Wu et al. designed novel rewritable polymeric paper based on a photochromic monomer for self-erasing time-dependent information encryption.⁷ Xu et al. proposed an alternative strategy that utilizes the swelling properties of a hydrogel to allow information decryption in the form of fluorescence.⁸ Wang et al. employed a commercial phosphor mixed acrylate monomer to fill the hollow cholesteric liquid crystal polymer network film template, thereby fabricating a composite film with anti-counterfeiting capabilities.⁹ Meanwhile, previous other studies have also demonstrated the use of redox dyes to create photocatalytic color-switching systems for rewritable paper, achieving high reversibility and stability.^10,11 The method involving redox materials as appealing candidates in rewritable paper offers several advantages including rapid response, high reversibility, environmental friendliness, and low cost.¹²

Redox dyes, such as methylene blue (MB), can reversibly change color in redox reactions.¹³ MB exhibits redox properties, enabling its reduction from the blue form to a colorless form of leuco-MB (LMB) under UV irradiation. LMB can also be oxidized back to MB, resulting in a reversible photochromic process. However, the reduction of MB under UV light is extremely slow, i.e., it is necessary to shorten the photochromic process for practical use. Wang et al. proposed a series of methods including surface binding of sacrificial capping ligands¹⁴ and creating oxygen vacancies, which effectively scavenge photogenerated holes through doping of metal ions^15,16 in order to accelerate the photochromic process. Han et al. reported a wavelength selective photoreversible color switching system (PCSS) by coupling the photoreductive activity of Sn²⁺-doped SnO_2−x nanocrystals and the color switching properties of redox dyes.¹⁷ Ren et al. reported a PCSS integrating a reducing agent, triethanolamine, a catalyst, β-FeOOH nanorods, and MB, which can be applied for data encoding and reading.¹⁸ Several other studies have also demonstrated that catalysts such as sodium para-toluenesulfinate,¹⁹ SnO₂,²⁰ Ag nanoparticles,^21,22 NiO/ZnO²³ and graphene/Fe₃O₄/Ag nanocomposites²⁴ are able to accelerate this photochromic process. Despite the wide use of redox dyes in information encryption and rewritable paper, the synthesis of catalysts to accelerate the photobleaching is quite complex. Moreover, the solubility of nanoparticles in a polymer matrix, especially in a hydrogel, is still another issue limiting the use of redox dyes in information encryption and rewritable paper. Therefore, it is essential to develop a simple and cost-effective strategy to overcome these issues.

In this work, MB based hydrogel rewritable paper is proposed through a simple and cost-effective method for information encryption. A mechanism for the rapid color change of redox dyes, which is assisted by a commercial photoinitiator, is reported. Based on this mechanism, the ability for transient information encryption and self-erasing decryption of the proposed film is demonstrated. This rewritable paper can be reused for more than 20 writing cycles without causing any rupture of the paper. The rewritable paper can be used for printing text or patterns using a mask and non-contact free writing with a UV laser pen. The mechanism also applies to other redox dyes, such as rhodamine B and malachite green, to achieve multi-color patterning.

Results and discussion

Photoinitiator assisted color changing of redox dyes

A redox reaction is a chemical reaction that involves electron transfer between two chemicals, in which the oxidation state of a molecule, atom, or ion is changed by gaining or losing an electron.²⁵ The photochromic process and molecular structures of MB and LMB are schematically illustrated in Fig. 1a. As mentioned before, an additional component to the redox dye solution is necessary in order to accelerate its photochromic process. Otherwise, the reduction reaction under UV light is extremely slow. A commercial cleavage-type photoinitiator, 2,2-dimethoxy-2-phenylacetophenone (DMPA, C₁₆H₁₆O₃), is chosen in this work. Upon exposure to UV radiation, the DMPA molecules absorb photon energy and transition from the ground state to an excited state, undergoing the Norrish I reaction. Due to the instability of the molecular structure in the excited state, weak covalent bonds within the molecule break forming two primary reactive free radicals, including a benzaldehyde radical (C₆H₅CO˙) and a dimethoxybenzyl radical (C₈H₉O₂˙), which is the so-called Norrish I reaction,^26,27 as shown in Fig. 1a. Unintentionally, we find an interesting phenomenon that MB is able to rapidly turn colorless in the presence of DMPA, as shown in Fig. S1 and the video in the SI. In order to investigate the mechanism of photoinitiator assisted color changing in DMPA/MB solution, a series of experiments were carried out. In the Raman spectra shown in Fig. 1b, the characteristic peaks at 448 and 502 cm⁻¹ that correspond to the bending vibration of the C–N–C framework decrease with the increasing of UV exposure time. This illustrates the photobleaching of MB, which turns into colorless LMB form, consistent with previous research.^18,28–30 The decrease in the intensities at 775, and 1189 cm⁻¹ that correspond to C–H out-of-plane bending vibrations and C–N stretching vibrations, respectively, further indicates the MB consumption in the presence of DMPA during the UV exposure. Importantly, the 882 cm⁻¹ peak, which corresponds to the C–C–O bonds in-plane bending vibrations, shows an increase during the exposure and stays at a similar value for 7 days after exposure. Since only the benzaldehyde radical forms a C=O bond in the DMPA/MB solution, which is the potential contribution to the increasing of the C–C–O bond, such an increase shown by the 882 cm⁻¹ peak is speculated to be caused by the formation of alcohol-related species resulting from the reaction between the benzoyl radical and MB. We further investigate the Raman spectra of the MB solution and DMPA solution separately under UV exposure, as shown in Fig. S2. The results imply that the color change of MB in the presence of DMPA is caused by neither ethanol nor final products of DMPA upon UV exposure (Fig. S3 and S4). Therefore, the results imply that the benzaldehyde radicals induce the reduction of MB leading to a rapid color change.

Fig. 1 Reversible redox reactions involved in the color switching of MB. Mechanism (a) and Raman spectra (b) of photoinitiator assisted MB color changing in DMPA/MB solution.

In order to deepen the understanding of DMPA/MB solution's color change using characterization studies and further use such a solution as an ink for the hydrogel paper, several experiments were carried out. The absorption spectra of DMPA and MB solution under the UV exposure are shown in Fig. 2a. After 7 days at room temperature under ambient air, a partial color recovery of the solution was observed, as illustrated in Fig. 2b. This color recovery originates from the redox cycle between MB and its LMB form. An increase in the Raman spectral signals of MB after UV exposure is clearly seen. This experimental phenomenon is consistent with previous reports.^13,14,16,18 The discoloration process of DMPA/MB solution can be accelerated by a higher molar ratio of DMPA to MB, as shown in Fig. S5 and Fig. 2c. Moreover, it was also demonstrated that a higher UV exposure intensity results in a faster discoloration rate of the MB, as shown in Fig. S6 and Fig. 2d, and experiments. Ethanol, as a necessary solvent, in varying proportions, influences the absorption spectrum of MB solutions, potentially by inhibiting dimer formation (Fig. S7).

Fig. 2 Reversible color switching of the DMPA/MB solution. Absorbance spectra showing the decolouration (a) and recovery color (b) process under UV irradiation and at room temperature under ambient air. Color changing kinetics of the DMPA/MB solution systems with different molar ratios (c) and UV power (d). C/C₀ is used to describe the color changing kinetics of DMPA/MB solutions. C and C₀ represent the absorption intensity of the DMPA/MB solutions at a certain exposure time and in the initial state at 663 nm.

Transient information encryption rewritable paper

Hydrogel, a specific polymer material with the ability to absorb liquid due to its three-dimensional porous network structure, is considered an ideal dye adsorbent.^31–33 In addition, it takes a couple of days, which is a quite long process, to reach the color recovery of MB. Therefore, we introduce the DMPA/MB solution as an ink into a photopolymerized hydrogel to rapidly produce rewritable paper for the use of transient information encryption. The hydrogel paper is fabricated through a simple photopolymerization and an “ink-pumping” method as illustrated in Fig. 3a. The paper can absorb various molar ratios of DMPA/MB solutions, which is attributed to the hydrogels’ porous network structure, as shown in Fig. S8, leaving enough space for the liquid diffusion from an ambient medium to the hydrogel. Although the decolorization rate of the inked hydrogel paper is slower than the one in solution, the decolorization process still reaches a time as fast as 8 s in the hydrogel paper (Fig. S9, S10, and Fig. 3b). The different rates of decolorization of the hydrogel paper with various DMPA/MB concentrations show its potential for use in display and patterning (Fig. 3c). The DMPA/MB dispersed hydrogel paper exhibits a considerable photochromic performance and surface flatness, as shown in Fig. S11. It should be noted that the solvents used in this paper are 60% anhydrous ethanol and 40% deionized water (DIW) to achieve an optimum ink-pumping ability (Fig. S12).

Fig. 3 The information encryption hydrogel paper. (a) Schematic diagram of the photopolymerization and “ink-pumping” method. (b) Absorbance spectra showing the decolouration process of the film under UV irradiation (the inset is the digital photographs of the hydrogel film before and after exposure under UV irradiation). (c) The digital photographs of hydrogel films immersed in DMPA/MB solutions with different molar ratios changing with varying UV exposure time. The intensity of UV exposure is 250 mW cm⁻². (d) The experimental results of transient information encryption/decryption and self-erasure. Scale bar is 1 cm.

Based on the mechanism underlying the decolorization rate differences at varying DMPA contents, a strategy for instant information encryption was proposed, as illustrated in Fig. S13. Firstly, the paper is prepared by immersing the hydrogel paper in the DMPA/MB solution. Then, the DMPA solution is utilized as an ink for writing a text message into the paper. A brush is used for writing the encrypted text instead of the use of other hard-tipped pens in order to avoid damaging the flatness or integrity of the hydrogel paper and thus the encryption. When the ink is absorbed into the paper, the text message encryption is complete. UV irradiation is applied to the hydrogel paper for decryption. Since the concentration of DMPA in the encrypted area is much higher than in other regions, the text message is bleached more rapidly than the other area, i.e., the encrypted text is revealed. Continued UV exposure leads to self-erasing in which all MB turns into the colorless LMB form. By heating or allowing the paper to stand in the air, the paper can be restored to its colorful state. Fig. 3d shows the experimental results of transient information encryption by using the proposed hydrogel paper. The number “28” written on the hydrogel paper is temporarily visible at the beginning stage “writing password” due to the reflection of the DMPA ink, this phenomenon can be resolved after the absorption of the ink. After the DMPA solution is absorbed by the hydrogel film, the traces adhering on the surface can be washed using ethanol, which makes the text hard to distinguish in visible light. Under UV irradiation, the number “28” gradually becomes visible, thereby decrypting the encoded information within 6 to 24 s of exposure. Prolonged exposure time up to 40 s effectively erases the encrypted information, i.e., the “burn-after-reading” is achieved. After the self-erasure process, the hydrogel film paper can slowly recover its blue color in air, or rapidly colorize by re-immersing in DMPA/MB solution to reload the MB. The hydrogel film paper can thus be reused for subsequent encryption cycles.

UV printing on rewritable paper

In this section, the proposed DMPA/MB dispersed hydrogel paper is applied to the UV printing, as schematically illustrated in Fig. 4a and its rewritable ability is also demonstrated in Fig. 4b. A flower-shaped hollow mask is placed on the hydrogel paper, and then the masked area is bleached to reveal the flower pattern by UV exposure. Clear patterns and details can be printed in just 20 s as shown in Fig. 4b. It can be erased either by heating it on a heating plate at 150 °C for 25 min or by exposing the film to air for 6 days, allowing it to return to its colorful state and making it suitable for rewriting. Notably, the sequence of these two erasing methods does not affect the rewriting efficiency (Fig. S14). Then, a new pattern of a butterfly mask can be printed on the hydrogel paper. Additionally, the rewritable paper on patterning shows a resolution of 3 μm according to the 1951 USAF resolution test chart, as shown in Fig. 4c. Moreover, the DMPA/MB dispersed hydrogel paper can be used as a medium for contactless and free text writing by using a homemade UV laser pen, as illustrated in Fig. 4d, e and Fig. S15, where the schematic and experimental results are presented.

Fig. 4 Printing patterns on the rewritable paper. (a) Schematic diagram of UV printing on the rewritable paper. (b) Rapid or slow recoloration of the hydrogel film by heating or by exposing it to ambient air. The purple, red, and gray color blocks indicate UV exposure, heating, and exposure to air, respectively. (c) Digital photograph of the patterns of the 1951 USAF resolution test chart in hydrogel paper. The inset is the optical microscopy image of the 1951 USAF resolution test chart. Scale bars, 5 mm. (d) The schematic diagram and (e) experimental results of non-contact freely writing using a homemade UV laser pen. Scale bar is 1 cm.

The quality of the repetitive writing with a single immersion into DMPA/MB solution may be compromised due to the consumption of DMPA and MB, as shown in Fig. S16. This phenomenon is consistent with that observed in the DMPA/MB solution tested in Fig. 2b. However, through the proposed “ink-pumping” strategy, the proposed rewritable paper exhibits excellent reusability and maintains an excellent performance after 20 cycles, as illustrated in Fig. 5a. It should be noted here that the rewritable paper will appear slightly yellow because of the accumulation of chromophore structures produced by the cleavage of DMPA with the increasing of cycles as shown in Fig. S17.³⁴ Nevertheless, it has barely affected the rewritable paper's ability to achieve precise patterning, as illustrated in Fig. 5b. The obtained results show that the proposed “ink-pumping” strategy is more practical and cost-effective for use in information encryption and UV printing.

Fig. 5 Repeatability and RGB color light printing. (a) The absorbance of the rewritable paper at 663 nm recorded continuously for 20 cycles between MB and LMB states. (b) The prints were produced after >20 consecutive writing–erasing cycles. (c)–(f) Patterns photoprinted with RGB colors. The hydrogel films are immersed in solutions of (c) rhodamine B (RB), (d) malachite green, (e) MB and (f) a mixture of RB and MB. Photoprinted patterns with various colors were achieved by DMPA under UV exposure combined with a mask. Scale bars, 5 mm.

UV-printing with multiple colored dyes

We conduct an experiment using other redox dyes, such as rhodamine B and malachite green, to print the logo of Beijing University of Technology with multiple colors. The patterns are clearly illustrated in Fig. 5c–e. A purple pattern is also produced by mixing MB and RB dyes, as shown in Fig. 5f. This suggests that the proposed mechanism and method hold potential for multi-color printing applications. It not only broadens the color palette but also contributes to the advancement of smart materials with adjustable optical characteristics, suitable for dynamic displays, intelligent windows, and color-shifting devices.

Experimental

Chemicals and materials

Monomer acrylamide (AAM, C₃H₅NO) and monomer 2-hydroxyethyl methacrylate (HEMA, C₆H₁₀O₃) are purchased from Sigma Aldrich Chemicals Pvt. Ltd and Sahn Chemical Technology Co., Ltd, respectively. Photoinitiator 2, 2-dimethoxy-2-phenylacetophenone (DMPA, C₁₆H₁₆O₃), cross-linking agent ethylene glycol dimethacrylate (EGDMA, C₁₀H₁₄O₄), ethanol (EG, 98%), MB, RB and malachite green are all purchased from Shanghai Macklin Biochemical Technology Co., Ltd. The molecular structures of the monomers and crosslinker used for the hydrogel synthesis are shown in Fig. S18. All chemicals and reagents used in this work were of analytical grade and used directly without further purification. The microscope slide and PVC spacer are purchased from a relevant entity store. The photomask used in UV printing is self-prepared.

Sample characterization studies

The absorption spectra of hydrogel paper and MB solution are recorded using a Hitachi U-4100 spectrophotometer and a 8453A spectrophotometer. SEM images are taken on a JEOL2100F electron microscope, in order to study the large macrospore structure of the hydrogel film. The flatness of the hydrogel film is characterized using a NanoMap-Scanning 3D surface profilometer. The model of the UV lamp is NBET-LED4 from Beijing NBeT Technology Co., Ltd (China), and its exposure time and power can be freely adjusted. All Raman spectra are collected using a confocal micro-Raman system (Renishaw) with an excitation laser at 785 nm. The excitation power is 250 mW, and the acquisition time is 4 s. An objective lens (×20) and 1200 gratings are used to test the samples and collect the Raman signals. All displayed Raman spectra are baseline-corrected. All digital photographs are captured using a Redmi K70 Pro smartphone.

Preparation of the liquid hydrogel solution

Equal masses of HEMA and AAM monomers are introduced into a brown reagent bottle to prepare a liquid hydrogel solution. The mixture is stirred for 5 min, followed by the addition of 2 wt% of EGDMA, 1 wt% of DMPA, and 25 wt% of DIW. Then, the mixture is stirred for an additional 30 min until the solution became transparent and homogeneous. An ultrasonic oscillator is used to remove any bubbles generated during the stirring process under dark conditions. A PVC spacer with a specific thickness is placed between two glass slides. The liquid hydrogel solution is injected into the template, and then UV light is applied for 1 min to cure the hydrogel. After curing, the top glass slide is removed, leaving the film on the glass substrate. In order to fabricate the rewritable paper with the photochromic ability, the hydrogel film is immersed in the DMPA/MB solution for 30 min, which is the so-called “ink-pumping”. Finally, the inked hydrogel film is washed with ethanol to remove surface solution and placed at room temperature for 30 min to evaporate the excess ethanol on the surface of the film.

Preparation of the DMPA/MB ink

An appropriate amount of DMPA and MB is dissolved in a 10 mL mixture of DIW and ethanol, and subsequently stirred thoroughly to ensure that DMPA and MB are fully mixed.

Conclusions

We report a mechanism by which free radicals, generated from a commercial cleavage-type photoinitiator, trigger rapid and controllable color changing in redox dyes under UV irradiation. Based on this mechanism, we have developed a simple and straightforward strategy to fabricate rewritable hydrogel paper with excellent performance in transient information encryption and self-erasing decryption. The proposed rewritable paper exhibits a high spatial resolution up to 3 μm and supports both template-assisted printing and freehand UV writing with a stable performance over more than 20 rewrite cycles. Moreover, the underlying mechanism has been successfully extended to other redox dyes such as rhodamine B and malachite green, which enables multi-color patterning. In comparison to previous works, our work offers multifunctional and multi-color abilities with a rapid response and provides a straightforward fabrication strategy (Table 1). These findings not only validate the proposed photoreduction mechanism but also offer a versatile and practical approach for application in rewritable photonic media, secure information storage, and temporary data display.

Table 1 Comparison of this work with previously reported studies

Number	Catalyst	Substrate	Time of decoloration and method	Monocolor or multi-color printing	Transient information encryption	Ref.
1	TEOA + β-FeOOH	Plastic	300 s (Red light)	Monocolor (Blue)	Yes	18
2	TiO2−x	Agarose	6 s (UV light)	Monocolor (Blue)	Yes	14
3	SnO2−x	Plastic	10 s (Blue light)	Monocolor (Blue)	No	17
4	TiO2−x	Plastic	1 min (UV light)	Multi-color (Red, Green, and Blue)	No	16
5	TiO2	Paper	30 s (UV light)	Multi-color (Red, Green, and Blue)	No	10
6	Pd/C3N4	Solution	3 min (H2)	Monocolor (Blue)	No	13
7	Ba-doped TiO2	Glass	10 s (UV light)	Monocolor (Blue)	No	15
8	DMPA	Hydrogel	3 s in solution (UV light)	Multi-color (Red, Green, Blue, and Purple)	Yes	This work
			8 s in substrate (UV light)

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Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. P. L., L. Z., and J. G. conceived and designed the experiments. P. L., J. W., and Z. Q. performed the experiments. Y. Z., Y. F., and X. F. contributed to sample analysis. P. L., J. G., and X. Z. analyzed the results and completed the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc02952j.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant no. 62175005, 61735002, 62275005, and 12074020) and the Beijing Municipal Education Commission (Grant no. KM202410005015).

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