Novel PVP/HTA Hybrids for Multifunctional Rewritable Paper

Rapid fabrication of polyoxometalate-enhanced photo responsive films from ethyl cellulose (EC) and polyvinylpyrrolidone (PVP)

Ethyl cellulose (EC), polyvinylpyrrolidone (PVP), and phosphomolybdic acid (PMoA) were the components of a new photochromic hybrid film composed of heteropoly acids (abbreviated EC-PVP/PMoA), created by solvent evaporation. The EC-PVP/PMoA mechanism, visible light photochromic behaviors, and microstructure were closely studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), ultraviolet-visible (UV-visible) spectroscopy, X-ray photo electronics (XPS), and Fourier transmission infrared spectroscopy (FTIR). FTIR spectra show that neither the core structures of EC, PVP, nor the Keggin structure of PMoA in the EC-PVP/PMoA composite hybrid film were damaged during fabrication. In the presence of oxygen, the yellowish transparent hybrid film exhibited reversible photochromism and significant photochromic reactivity, becoming blue in the presence of visible light. Upon heating the film at 90 °C for 1 h its photoresponsive properties were enhanced, making the blue color more vibrant and stable due to polyethylene glycol (PEG). The XPS spectra demonstrated a proton transfer procedure during the photochromic process, and the presence of Mo+5 species indicated a photoreduction interaction between the PMoA particles and the EC-PVP matrix. To the best of our knowledge, this is the first EC-PVP combined hybrid film with POMs with promising prospects for solar UV detection and smart glass windows applications.

Hydrochromic film for dynamic information storage using cellulose nanofibers and silica nanoparticles

A hydrochromic composite film was fabricated by incorporating silica nanoparticles (SiNPs) with cellulose nanofibers (CNFs). The CNF/SiNP composite film underwent a reversible change in transparency in response to external moisture variation. The CNFs improved the dimensional stability of the CNF/SiNP composite film and induced morphological differences in SiNP agglomerates, which control the water vapor condensation in a porous film. The condensed water in the pores reduced the difference in refractive index over the CNF/SiNP film, enhancing its transparency. The selective transparency of the composite film was challenged by printing CNF/SiNP inks at different composition ratios. The differing susceptibility of the printed patterns to moisture provided selective transparency at specific patterns, which can store dynamic information such as QR or numerical codes by simple water vapor adsorption and desorption.

Chelating Coordination Regulated Photochromic Electrospun Nanofibers for Waterproof and Long-Color-Retention Rewritable Wearables

Photochromic materials with rapid color-switching, long color retention times, and rewritability are crucial for meeting the requirements of future rewritable ink-free media. However, these requirements are challenging to satisfy simultaneously due to the inherent constraints among these features. Herein, a novel photochromic nanofiber nonwoven fabric was designed and constructed based on a conjugated organic-inorganic hybrid structure through electrospinning and hot-pressing techniques. The as-prepared fabric can change color in merely 5 s under UV irradiation and can reach saturation within 2 min. In addition, upon the introduction of a potent metal chelator, its color retention time exceeds 14 days under ambient conditions, significantly longer than that of most rewritable materials recently reported (several hours to 5 days). Moreover, the fabric exhibits high writing resolution and can be photoprinted and heat-erased for over 100 cycles while still retaining 96% of its initial reflectivity. Hydrophobic thermoplastic polyurethane provides the fabric with excellent waterproof and antifouling properties, thus preventing the composite from swelling or collecting graffiti due to moisture or dust. This work exploits a competitive approach for designing flexible, rewritable, and superior functional wearables with practical applications.

Lanthanide-Polyoxometalate-Based Film with Reversible Photochromism and Luminescent Switching Properties for Erasable Inkless Security Printing

Security printing is of the utmost importance in the information era. However, the excessive use of inks and paper still faces many economic and environmental issues. Thus, developing erasable inkless security printing materials is a remarkable strategy to save resources, protect the environment, and improve information security. To this endeavor, a photoresponsive lanthanide-polyoxometalate-doped gelatin film with high transparency was developed through the solution casting method. Attenuated total reflection Fourier-transform infrared spectroscopy confirmed the electrostatic and hydrogen bond interactions between gelatin and lanthanide-polyoxometalate. Absorption spectra, luminescent spectra, and digital images indicated that the film displayed reversible photochromism behavior and was accompanied by luminescent switching property upon exposure to UV irradiation and oxygen (in the dark) alternately, which allowed its potential application as a reprintable medium for inkless security printing. The printed information can be erased upon exposure to oxygen in the dark, and the film can be reused for printing again. The film exhibited excellent erasability, reprintability, renewability, and low toxicity. In addition, multiple encryption strategies were designed to improve information security. This work offers an attractive alternative strategy for constructing a reprintable film for inkless security printing in terms of simplifying the preparation process, saving resources, and protecting the environment.

Reusable Shape‐Memory Photonic Crystal Paper for Pin‐Printing and High‐Resolution Press Printing

Rewritable photonic crystal (PC) paper has the potential to significantly reduce the consumption of forest resources in the printing industry, while also being environmentally friendly and efficient. However, traditional PC papers based on solvent or photothermal responses can lead to diffusion, which can hinder printing accuracy. In this study, a novel rewritable PC paper compatible with pin‐printing is presented based on a pressure‐responsive shape‐memory PC paper. High‐resolution printing can be realized by both computer‐programmed 3D‐printed seals and pin‐printing techniques. The information written on this PC rewritable paper can be erased by water, enabling the paper to be rewritten and reused at least 8 times without any change in performance. Furthermore, the information stored on the PC paper is stable and can be stored in ordinary environments for at least 6 months without fading. The PC paper has the capability of multicolor printing with a precision finer than 100 μm and has potential in office papers, smart price tags, and anti‐counterfeiting labels.

Multiple Light Source-Excited Organic Manganese Halides for Water-Jet Rewritable Luminescent Paper and Anti-Counterfeiting

Rewritable luminescent paper is particularly crucial, considering the ultrahigh paper consumption and confidential information security, but a highly desirable stimuli-responsive smart luminescent material with excellent water solubility has rarely been studied. Herein, a new type of rewritable paper made by highly efficient green light emissive zero-dimensional (0D) organic manganese halides is rationally designed by virtue of the reversible photoluminescence (PL) off-on switching. Specifically, the green emission can be linearly quenched by water vapor in a wide humidity range and again recovered in a dry atmosphere, which make it a smart hydrochromic PL off-on switching and humidity sensor. Benefiting from the reversible luminescence off-on switch and excellent water solubility, rewritable luminescent paper is realized through water-jet security printing technology on 0D halide-coated commercial paper with high resolution. The printed/written information can be easily cleaned by slight heating with outstanding "write-erase-write" cycle capabilities. In addition, multiple light source-induced coincident green light emissions further provide convenience to realize anti-counterfeiting, encryption and decryption of confidential information, and so forth. This work highlights the superiority of dynamic ionic-bonded 0D organic manganese halides as reversible PL switching materials in rewritable luminescent paper, high-security-level information printing, storage and protection technologies, and so forth.

Multi-Component Collaborative Step-by-Step Coloring Strategy to Achieve High-Performance Light-Responsive Color-Switching

Light-responsive color-switching materials (LCMs) are long-lasting hot fields. However, non-ideal comprehensive performance (such as color contrast and retention time cannot be combined, unsatisfactory repeatability, and non-automated coloring mode) significantly hinder their development toward high-end products. Herein, the development of LCMs that exhibit long retention time, good color contrast, repeatability, and the property of automatic coloring is reported. The realization of this goal stems from the adoption of a bio-inspired multi-component collaborative step-by-step coloring strategy. Under this strategy, a conventional one-step photochromic process is divided into a "light+heat" controlled multi-step process for the fabrication of the desired LCMs. The obtained LCMs can effectively resist the long-troubled ambient-light interference and avoid its inherent yellow background, thereby achieving the longest retention time and good repeatability. Multiple colors are generated and ultra-fast imaging compatible with the laser-printing technology is also realized. The application potential of the materials in short-term reusable identity cards, absorptive readers, billboards, and shelf labels is demonstrated. The results reported herein can potentially help in developing and designing various high-performance, switchable materials that can be used for the production of high-end products.

Lanthanide Polyoxometalate Based Water-Jet Film with Reversible Luminescent Switching for Rewritable Security Printing

Luminescent security printing is of particular importance in the information era. However, the use of conventional paper still carries a lot of economic and environmental issues. Therefore, developing new environmentally friendly security printing material with a low cost is imperative. To achieve the aforementioned goals, novel lanthanide polyoxometalate doped gelatin/glycerol films with high transparency, high strength, and good flexibility have been developed via a solution-casting method. The electrostatic interaction between zwitterionic gelatin and polyoxometalate was confirmed by attenuated total reflection Fourier transform infrared spectroscopy. Luminescent spectra and digital images indicated that the films exhibited reversible luminescent switching properties through association and dissociation of hydrogen bonds between glycerol and water molecules, allowing its potential application as water-jet rewritable paper for luminescent security printing. Furthermore, the printed information can be conveniently "erased" by heating, and the film can be reused for printing. The film exhibited excellent ability to be both rewritten and re-erased. A QR code pattern and hybrid printing were employed to improve the security of information. In addition, the rewritable films possessed excellent regeneration ability and low toxicity, as well as good stability against UV irradiation and organic solvents. The water-jet rewritable film based on lanthanide polyoxometalate for luminescent security printing, to the best of our knowledge, has not yet been reported up to date. This work provides an attractive alternative strategy on fabricating rewritable films for luminescent security printing in terms of cutting down the cost, simplifying the preparation process, and protecting the environment.

Hydrochromic Visualization of a Keggin-Type Structure Triggered by Metallic Fluids for Liquid Displays, Reversible Writing, and Acidic Environment Detection

Hydrochromic visualization of a liquid interface shows vital potential applications in liquid displays, reversible writing, and acidic environmental detection, which offers a platform for detection and forewarning due to its intuitive and visual characteristics. Herein, we report a hydrochromic display due to the interfacial effect of liquid metal (LM)-triggered ammonium metatungstate (AMT) with instant dual-mode color switching. The double-electron-transfer reaction of the AMT on the surface of gallium-based LM caused the formation of heteropoly blue in the presence of acidic surroundings, resulting in a reversible color switching from being colorless to blue or blue to colorless. This visual interfacial discoloration phenomenon can be applied to the liquid display on diverse patterns of the LM surface. Furthermore, papers with a functional display were prepared, which can be used for writing up to eight times with dual-mode color switching. In addition, the reactive activity of acid triggering make it a potential candidate for use in visualizing an acidic environment with a detection range of pH = 1 to 0 (0.1-1.5 M). Briefly, this interfacial discoloration phenomenon enriches the interfacial engineering of LM and provides a unique prospective and wide-range platform for the application of LM.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Controlling information duration on rewritable luminescent paper based on hybrid antimony (III) chloride/small-molecule absorbates

Controlling the duration that information lasts on paper so that it disappears as desired is crucial for information security. However, this area is rarely studied. Here, we report [TEMA]₂SbCl₅ (1, TEMA⁺ = methyltriethylammonium), [TEA]₂SbCl₅ (2, TEA⁺ = tetraethylammonium), [TEBA]₂SbCl₅ (3, TEBA⁺ = benzyltriethylammonium), and [Ph₄P]₂SbCl₅ (4, Ph₄P⁺ = tetraphenylphosphonium) with structure-dependent reversible photoluminescent switching induced by the absorption and thermal release of small guest molecules including H₂O, methanol, and ethylene glycol. Comparing the structural disorder levels, bond lengths, and luminescent Stokes shifts of the compounds aided in understanding their selective absorption behavior. Our results indicated that the information duration on the rewritable paper coated with the title compounds is easily tuned by changing the cation of the compounds, the type of guest molecules, and laser heating power. Our study opens previously unidentified avenues for information security and extends the potential applications of rewritable paper.

Protonation-Induced Enhanced Optical-Light Photochromic Properties of an Inorganic-Organic Phosphomolybdic Acid/Polyaniline Hybrid Thin Film

A phosphomolybdic acid/polyaniline (PMoA/PANI) optical-light photochromic inorganic/organic hybrid thin film was successfully synthesized by protonation between the the multiprotonic acid phosphomolybdic acid (H₃PO₄·12MoO₃) and the conductive polymer polyaniline. The stable Keggin-type structure of PMoA was maintained throughout the process. Protonation and proton transfer successfully transformed the quinone structure of eigenstate PANI into the benzene structure of single-polarized PANI in the PMoA/PANI hybridized thin film, and proton transfer transformed the benzene structure of single-polarized PANI back to the quinone structure of eigenstate PANI in the PMoA/PANI hybrid thin film, as verified by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The average distribution of PMoA/PANI was observed by atom force microscopy (AFM). Interestingly, protonation of PMoA caused PANI to trigger transformation of the quinone structure into the single-polarized benzene structure, which enhanced the electron delocalization ability and vastly enhanced the maximum light absorption of the PMoA/PANI hybrid thin film as confirmed by density functional theory (DFT), electrochemistry, and ultraviolet-visible spectroscopy (UV-Vis) studies. Under optical-light illumination, the pale-yellow PMoA/PANI hybrid thin film gradually turned deep blue, thus demonstrating a photochromic response, and reversible photochromism was also observed in the presence of hydrogen peroxide (H₂O₂) or oxygen (O₂). After 40 min of optical-light illumination, 36% of the Mo⁵⁺ species in PMoA was photoreduced via a protonation-induced proton transfer mechanism, and this proton transfer resulted in a structural change of PANI, as observed by XPS, generating a dominant structure with high maximum light absorption of 3.46, when compared with the literature reports.

Environmentally stable, photochromic and thermotropic organohydrogels for low cost on-demand optical devices

HYPOTHESIS

The color-changing ability of creatures widely existed in nature has inspired the development of adaptive allochroic materials, which can respond to various external stimuli. Integrating multi-stimuli responsiveness and long-term stability in allochroic system are urgent for practical applications under complex circumstances.

EXPERIMENTS

The photochromic/thermotropic organohydrogels (PTOs) comprised polyacrylamide and cationic cellulose (JR400) were prepared by facile free-radical polymerization and glycerol displacement. The coexisting covalent bonds and noncovalent interactions collaboratively reinforce the networks, endowing the PTOs with boosted stretchability and toughness.

FINDINGS

The photochromic ammonium molybdate (Mo₇) and thermo-sensitive poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EPE) were made into networks. In these cooperative networks, each constituent performed their own function without disruption, including fast photochromism (10 s) and durable thermo-responsiveness. Importantly, the glycerol-water solvent bestowed the distinct anti-freezing (-30 °C) and anti-dehydration performances on the PTOs. Accordingly, the materials could serve as promising rewritable devices for high-resolution and long-term data storage/encryption. Moreover, on-demand PTO windows integrating UV-prevention and solar energy regulation with T_lum (92.96%) and ΔT_sol (46.02%) could create comfortable and healthy environments for occupants. This work offers a new design strategy for low-cost, environmentally stable smart optical devices.

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified.

Photochromic and Photocatalytic Properties of Ultra-Small PVP-Stabilized WO3 Nanoparticles

Tungsten oxide-based bulk and nanocrystalline materials are widely used as photocatalytic and photo- and electrochromic materials, as well as materials for biomedical applications. In our work, we focused our attention on the effect of sodium cations on the structure and photochromic properties of the WO₃@PVP aqueous sols. To establish the effect, the sols were synthesized by either simple pH adjusting of sodium or ammonium tungstates’ solutions, or using an ion exchange technique to remove the cations from the materials to the greatest possible extent. We showed that the presence of sodium cations in WO₃@PVP favors the formation of reduced tungsten species (W⁺⁵) upon UV irradiation of the materials, strongly affecting their photochromic and photocatalytic properties. The pronounced photoreductive properties of WO₃@PVP sols in photocatalytic reactions were demonstrated. Due to photoreductive properties, photochromic sols of tungsten oxide can act as effective photoprotectors in photooxidation processes. We believe that our work provides a considerable contribution to the elucidation of photochromic and redox phenomena in WO₃-based materials.

The regulatory effects of the number of VP(N-vinylpyrrolidone) function groups on macrostructure and photochromic properties of polyoxometalates/copolymer hybrid films

Nanocomposite films were synthesized by introducing phosphotungstic acid (PWA) into the poly (vinylpyrrolidone and methyl acrylate) (P (VP(N-vinylpyrrolidone)coMA(methyl acrylate))). FT-IR curves illustrated that non-covalent bond interaction was built between PWA particles and organic matrixes. In combination the results of SEM, TG-DTA and DSC, it was conducted that the ratio of functional groups of VP had a critical effect on the surface morphology, thermal stability of the hybrid films. Irradiated with ultraviolet light, the transparent thin hybrid films change from colorless to blue. Stored in air, the hybrid films could recover colorless gradually. The analysis of kinetics told us that higher VP content within the different systems always accompanied more superior photochromic performance, faster coloring and fading speed. Appearance of W5+ in ESR spectra indicated that the photo-reduction process between PWA and copolymer matrix occurred.

Easily Prepared and Reusable Films for Fast-Response Rewritable Light Printing

Paper, for writing and printing, is consumed a lot in modern life. However, the production of conventional paper could cause many problems, such as deforestation and environmental pollution. Therefore, it is necessary and urgent to explore novel strategies to solve these problems. In this work, a polyoxometalate-doped gelatin film with high strength, excellent transparency, and fast photochromic properties is designed and prepared. The film can display different colors by using a variety of reagents, exhibiting its potential application as a paper medium. Its rapid photochromic properties allow complex high-resolution images to be displayed by UV light printing. It is found that the mechanical and photochromic properties could be regulated by the introduction of triethylene glycol, and the fading process could be controlled by changing the temperature and humidity. Moreover, it is rewritable, self-repairable, and recyclable, and can also achieve long-term preservation without fading. It is envisioned that the environmentally friendly films with low cost, ease of preparation, and recycling advantages have the potential to be an alternative to conventional paper for writing and printing.

Continuous and rapid fabrication of photochromic fibers by facilely coating tungsten oxide/polyvinyl alcohol composites

Photochromic fibers have attracted great attention due to their wide use in areas of military camouflage, safety warnings, anti-counterfeiting, entertainment, etc. Compared with photochromic organic materials, inorganic photochromic tungsten trioxide (WO₃) materials have been extensively studied, because of their good stability and cost efficiency. In this work, we report the continuous fabrication of photochromic fibers in a simple and low-cost way by dip-coating WO₃/PVA composites. The prepared photochromic fibers show fast and reversible color switch from light yellow to dark blue upon UV irradiation and infrared heating treatment. The obtained photochromic fibers can be produced on a large scale and be woven into various patterns with good mechanical strength and washability, showing great potential in developing photochromic textiles.

Photochromic fibers have attracted great attention due to their wide use in areas of military camouflage, safety warnings, anti-counterfeiting, entertainment, etc.