Stimulus-responsive polymer materials toward multi-mode and multi-level information anti-counterfeiting: recent advances and future challenges

Cellulose self-erasing material with dual functions in information encryption

Materials capable of multiple encryption, color-changing visualizations, and time-dependent functionalities are ideal for efficient and secure information storage and transmission. In this study, a time-dependent self-erasing material (TDSEM) containing urease developed through the in-situ synthesis of gold nanoclusters on the surface of cellulose nanocrystal (CNC) chemically bonding fluorescein isothiocyanate. CNC not only played an important role as a long-term stabilizer for the material, but also acted as a bridge facilitating the assembly of organic and inorganic interfaces, enabling the integrated system to achieve multi-color regulation and fluorescence resonance energy transfer. The TDSEM demonstrated excellent pH responsiveness, cyclic stability, and durability. The information was encoded by directly introducing HCl/urea into the TDSEM, and automatically erased over time. Additionally, the fluorescence intensity and color (transitioning from red to orange to green) were modulated over a time scale by adjusting the urea concentration, thus precisely controlling the time to retrieve the correct information (22 min). The confidentiality of the encoded information was enhanced by dual "locking" of time and color, ensuring that decryption only occurred "time color key". This time-dependent and color-varying dual functions provide valuable insights for the development of advanced information encryption and anti-counterfeiting technologies.

Information Encryption and Decryption Based on Excitation-Emission Matrix Fluorescence Hyperspectral Imaging and Multiway Chemometrics

As the demand for information integrity and privacy protection grows, interdisciplinary research is becoming increasingly essential for advancing information security technologies. This work proposed an information encryption and decryption strategy based on excitation-emission matrix fluorescence hyperspectral imaging (EEM-HSI) and multiway chemometrics. A novel algorithm, augmented three-directional intersection alternating trilinear decomposition (Augmented TDR-ATLD), was developed to process EEM-HSI data for decrypting information. Initially, the feasibility of this strategy was exemplified using simulated 4D and 5D EEM-HSI data containing encrypted information with 5D data being used for encryption for the first time. Two signal overlap conditions were designed to control the strength of the information encryption. By decrypting mixed signals at the pixel level to extract pure component signals and reconstructing pixels, we successfully decoded the encrypted information. Additionally, the practicality of this strategy was validated through real experimentation. Three rhodamine fluorescent dyes were added to a red watercolor to prepare anticounterfeiting ink, which were used to produce 2D and 3D anticounterfeiting patterns. The excitation-emission matrix fluorescence of each pixel was measured by using the front-face fluorescence instrument to generate EEM-HSI data. Augmented TDR-ATLD was then applied to decrypt mixed signals under scattering and an unknown interference. The results demonstrated that the anticounterfeiting patterns conveyed by different rhodamine fluorescent dyes were accurately decoded. In summary, this strategy, based on EEM-HSI and multiway chemometrics, provides a promising approach for advanced information security technology. It has the potential to be extended to more fields, thereby contributing to enhanced comprehensive information security protection.

Quadra-Mode Luminescent Phosphors for Force/Thermo-Encoded Information Storage and Anticounterfeiting Applications

Multimode luminescent materials possess diverse optical characteristics and play crucial roles in photocommunication and information security. Force-induced mechanoluminescence, as a distinctive excitation mode, exhibits impressive capabilities in the field of anticounterfeiting and information storage. Here, we integrate mechanoluminescence with the conventional luminescence modes (including up-conversion luminescence, down-conversion luminescence, and thermoluminescence (TL)) within LiNbO3 via well-tuned codoping of Er3+ and Pr3+. LiNbO3:Pr3+/Er3+ exhibits excitation-dependent photoluminescence (PL), enabling tunable emission colors ranging from green to red, while it shows green up-converted PL emissions owing to the electronic transitions of Er3+ ions. Both TL and mechanoluminescence emissions are achieved by employing intertrap levels, where mechanical or thermal stimulation releases carriers that transfer energy to Pr3+ ions, resulting in red emissions. Moreover, an underlying competitive interaction between TL and mechanoluminescence is revealed, which can be leveraged for force/thermo-encored information storage. This unique feature positions LiNbO3:Pr3+/Er3+ as novel recordable information storage labels to monitor the freshness of perishable goods such as food and medicine. In addition, quadra-mode luminescence material LiNbO3:Pr3+/Er3+ provides multidimensional codes via emission color and distribution, indicating its significant potential in advanced anticounterfeiting technology.

Multilevel Intelligent Anti-Counterfeiting Label with Spatially Selective Dynamic Aurora Response and 3D Mesoscopic Physical Unclonable Function Fingerprint

With the increasing demand for anti-counterfeiting measures, the efficient integration of multi-level optical anti-counterfeiting information has become a critical challenge. In this study, a novel bottom-up self-assembly technique is introduced for fabricating composite integrated films. This method overcomes the size limitations of phosphors that achieve circularly polarized light (CPL) through co-assembly with cellulose nanocrystals. Specifically, rare earth metal-organic frameworks with a length of 140 µm can generate CPL with an asymmetry factor of 0.65. Moreover, the introduction of random defects in the film imparts unpredictable CPL properties, enabling dynamic auroral anti-counterfeiting within the decryption optical path. Additionally, an innovative two-stage serial decryption process is proposed by leveraging the non-correlation between orthogonal decryption patterns. Notably, the label surface features biomimetic fingerprint textures that exhibit 3D physical unclonable functions (PUFs) at the mesoscopic scale. These textures possess high entropy close to the ideal value of 1, and an encoding capacity in a 175 × 175 µm2 area reaches 262500. In summary, the composite label achieves a high degree of integration by combining three levels optical anti-counterfeiting information: full-chromatographic tunable photoluminescence, spatially selective random dynamic aurora responses, and 3D bionic mesoscopic PUFs fingerprints.

Four-State Photonic Crystal Device with Temperature and Ultraviolet Light Sensitivities

Responsive photonic crystals have garnered significant attention in recent years due to their remarkable capability of exhibiting dynamic color changes in response to external stimuli. Herein, a novel multistage anti-counterfeiting photonic crystal device that integrates chemical (luminescent material) and physical (photonic crystal structure) elements is reported. Based on photochromic materials and thermochromic capsules, a four-state thermochromic/photochromic photonic crystal (TPPC) composite film with dual responsiveness is developed through in situ emulsion polymerization and a straightforward roll shear technology. This innovative approach successfully resolved the issue of short-range order and long-range disorder in conventional photonic crystal films. Through the integration of a multilayer structure and a mask plate process, the dual effects of photochromic and thermochromic are seamlessly combined, enabling the film to exhibit four different optical states under the combined stimuli of temperature and UV light. Unlike tristate systems, the film integrates dual stimuli (UV + heat) for enhanced complexity. Notably, the film demonstrates multilevel responsiveness and dynamic decorative capabilities, allowing flexible switching between four optical states. Furthermore, the TPPC film boasts excellent mechanical properties (with a tensile strength exceeding 2 MPa), emphasizing its strong potential for applications in anti-counterfeiting, information encryption, and dynamic display technologies.

Preparation of photochromic glass nanofiber-reinforced tricarboxylic cellulose hydrogel ink immobilized with phosphor nanoparticles for detection of fingerprints and data encryption

Photochromic inks have been a significant certification approach to improve document anticounterfeiting efficiency. However, the weak photostability and poor durability are two of their major shortcomings. Herein, this article details the development of a photochromic and self-healable hydrogel for advanced anticounterfeiting uses. When immobilized in tricarboxylic cellulose (TCC), electrospun glass nanofibers (90-170 nm) and lanthanide-activated strontium aluminate nanoparticles (LSAN; SrAl2O4:Eu2+, Dy3+; 644 nm) served as reinforcing and photochromic agents, respectively. The tricarboxylic cellulose bearing three carboxylic substituents on the anhydroglucose moiety was synthesized. The nanocomposite hydrogels were developed by the freezing/thawing approach. When illuminated with ultraviolet radiation, the LSAN@TCC hydrogel exhibited remarkable photostability and reversibility. A diverse range of tricarboxylic cellulose hydrogels with variable emission features was generated by varying the quantity of LSA. The LSAN@TCC nanocomposite was transparent in daytime light; however, it has shown a greenish emission under ultraviolet light. The structural and morphological properties of the glass nanofiber-reinforced tricarboxylic cellulose dried films were determined by a wide range of spectroscopic and microscopic methods. The mechanical properties of the LSAN@TCC hydrogel-stamped sheets were examined. The transparency was verified by the excitation peak at 365 nm, whereas the green emission was recorded at 519 nm.

Intramolecular Charge Transfer-Regulated Isomeric Covalent Organic Frameworks for Multiple Solvent-Response

Stimulus-responsive covalent organic frameworks (COFs) own color-switching characteristics when exposed to external stimuli. However, the investigations on the multiple solvent-responsive COFs remain a challenge due to the synthetic difficulties and uncontrollable charge transfer process toward various solvents. In this contribution, two novel isomeric COFs with a regulated intramolecular charge transfer (ICT) process by modulating the distance between the donor/acceptor and the linkage are synthesized. The as-prepared two isomeric COFs exhibited significantly distinct solvatochromic behaviors in water, acid, and halogenated solvents, respectively. These multiple solvent-responsive functions are attributed to the various enhancement degrees of the ICT process by the hydrogen bond interactions, protonation interactions, and halogen/π interactions, respectively. In addition, the two isomeric COFs are employed as stimulation-responsive powder or ink, displaying excellent image and data encryption performances. The work can not only offer a novel viewpoint for the creation of multiple solvent-responsive COFs but also expand the COFs' potential applications in information encryption and anti-counterfeiting.

Spatial & Temporal Dual-Resolved Anti-Counterfeiting Applications in a Long Afterglow System Based on Bismuth Halides

While anticounterfeiting systems based on long persistent luminescence (LPL) materials demonstrate a mature trend, the integration of tunable luminescent lifetimes and emission colors in LPL-based anticounterfeiting systems remains a challenge. Herein, we propose a temporal and spatial anticounterfeiting strategy utilizing novel zero dimensional (0D) metal halides, specifically (PBA)3BiCl6:xSb3+ (PBA = 4-Phenylbenzylamine, x = 0, 0.05, 0.1, 0.15), which exhibit long persistent luminescence characteristics. A controllable emission color transition can be observed from green to white to orange by varying the excitation wavelength, accompanied by different afterglow durations of the LPL emission. Additionally, various emission colors are observed at different delay times. Furthermore, the controlled LPL duration is achieved through Sb3+ doping in (PBA)3BiCl6, attributed to the triplet energy transfer (TET) process. A noticeable afterglow luminescence is observed when using a mobile phone's flashlight as the excitation source, enhancing the portability of the light source. The multifaceted tuning of emission colors and the modulation of the duration of tunable LPL establish a foundation for multicolor and time-resolved anticounterfeiting, significantly elevating the security level of advanced portable anticounterfeiting systems.

Algorithm in chemistry: molecular logic gate-based data protection

Data security is crucial for safeguarding the integrity, authenticity, and confidentiality of documents, currency, merchant labels, and other paper-based assets, which sequentially has a profound impact on personal privacy and even national security. High-security-level logic data protection paradigms are typically limited to software (digital circuits) and rarely applied to physical devices using stimuli-responsive materials (SRMs). The main reason is that most SRMs lack programmable and controllable switching behaviors. Traditional SRMs usually produce static, singular, and highly predictable signals in response to stimuli, restricting them to simple "BUFFER" or "INVERT" logic operations with a low security level. However, recent advancements in SRMs have collectively enabled dynamic, multidimensional, and less predictable output signals under external stimuli. This breakthrough paves the way for sophisticated encryption and anti-counterfeiting hardware based on SRMs with complicated logic operations and algorithms. This review focuses on SRM-based data protection, emphasizing the integration of intricate logic and algorithms in SRM-constructed hardware, rather than chemical or material structural evolutions. It also discusses current challenges and explores the future directions of the field-such as combining SRMs with artificial intelligence (AI). This review fills a gap in the existing literature and represents a pioneering step into the uncharted territory of SRM-based encryption and anti-counterfeiting technologies.

"All-in-One Functionalization and Synergic Ordering" Strategy Enables Multimode Anti-Counterfeiting Patterns

Conventional multimode anticounterfeiting systems usually suffer high cost, complicated structures, and different optical channel interferences. In this study, we demonstrate a simple strategy named "all-in-one functionalization and synergic ordering" to prepare quadruple mode patterns on a photoresponsive alternating copolymer P(DPA-alt-BP) film. Herein, a 9,10-diphenylanthracene (DPA) nonanoate unit serves as the multipurpose moiety with photoresponse and fluorescence, performing photodimerization upon 365 nm UV irradiation. The liquid crystal mesogen biphenyl (BP) caproate units can orderly align along strain for polarized sight and improve the microphase separation. Upon UV irradiation through a photomask followed by solvent annealing, the P(DPA-alt-BP) film is first gradient-cross-linked and then undergoes stress relief and microphase separation, giving rise to a synergic ordering effect that enables fabrication of an anticounterfeiting pattern with quadruple modes, i.e., wrinkled pattern, substructure on a wrinkled surface, distinguishable fluorescent emission, and polarized sight on the film. This strategy is promising for high-level anticounterfeiting applications and provides helpful inspiration to the development of innovative photoresponsive materials.

Continuous porous aromatic framework membranes with acid-/base-induced reversible isomerization for switchable ion conductivity

Stimuli-responsive ion conductor materials are highly sought after in the fields of biological systems, clean energy, and smart devices. However, it remains a huge challenge to achieve acid/base switchable ion conductors owing to their stringent requirements of structural responsive behaviors, high stability and porosity. In this study, porous aromatic frameworks (PAFs) are utilized as a favorable platform to successfully design and prepare ion conductive powders and its continuous membranes based on a commercially available pH indicator. Interestingly, these PAFs possessed structural reversibility in response to acidic and alkaline environments, followed by an apparent ion-conducting switch of about 4 orders of magnitude (from 3.36 × 10-7 S cm-1 to 4.59 × 10-3 S cm-1) under the conditions of 25 °C and 98% RH. Moreover, the continuous PAF membrane exhibited an ultrahigh ion conductivity of 7.29 × 10-1 S cm-1 after 1 mol per L NaOH treatment and good acid/base switchable cycle stability. To our knowledge, this is the first report on exploring ion-conductive porous frameworks and continuous membranes that dynamically respond to acid/base chemical stimuli. This work provides a new research strategy for the application of ion conductors as so-called "smart materials" even in extremely harsh chemical environments.

Rational Construction of a Pyrimido[2,1-b] Benzothiazole-Based Photoswitchable Smart AIE Material: A Theoretical Insight into Fluorescence Switching Mechanism of the Chimeric Dyad

Currently, AIEgen-photochromics conjugates and intrinsic photochromic AIEgens are the two major molecular design strategies for photoresponsive AIE materials. However, these two strategies still have their own limitations. In our previous research, we discovered pyrimido[2,1-b][1,3]benzothiazole (PBT) as a novel core structure of AIEgens. We herein explored a chimeric strategy to rationally integrate photochromic bisthienylethene (BTE) as the orthogonal head group into a new PBT molecule (PBTE). Compared to the conventional design strategies, the compact chimeric design of PBTE not only well maintains the AIE and photochromic properties of the parent fragments but also leads to excellent AIE-photoswitching capability in both films and single crystals. Theoretical calculations revealed that MO energy level arrangement of PBTE reorganized upon photoisomerization and the fast vibrational relaxation (VR) along with internal conversion (IC) from S5 to S1 state may serve as a competitive channel for the fluorescence quenching of PBTE-c. Comparison of the kinetics of the nonradiative decay with those of the excited-state energy transfer (EET) processes clearly showed that the ultrafast intramolecular Förster resonance energy transfer (FRET) is the dominant cause of fluorescence-off state. The applications of PBTE in erasable optical memory material and multi-dimensional anti-counterfeiting have also been demonstrated.

Bimetallic Strip-Inspired Dual-Layer Covalent Organic Framework Membrane for Smart Organic Vapor Response

Vapor-driven smart materials show significant advantages in areas such as intelligent control, gas detection, and information transmission. However, their typically singular response mechanisms pose challenges for achieving binary response behaviors within a single system. Drawing inspiration from bimetallic strips, a dual-layer covalent organic framework (DL-COF) membrane is developed with a hierarchical pore structure. This membrane exhibits asymmetric expansion or contraction on either side when exposed to morpholine and 1,4-dioxane vapors, enabling binary response behaviors. The driving forces underlying these binary responses are the shifts in hydrogen bond equilibrium caused by chain-like hydrogen bonding and the swelling effects within the two layers, which have different degrees of crystallinity. The hierarchical pore structure further enhances rapid mass transfer, enabling the DL-COF membrane to achieve an impressive response time of just 0.6 s. By leveraging its distinct responsiveness to different vapors, the DL-COF membrane can be effectively utilized for the visual translation of encrypted information, enabling the reliable decoding of gas-encrypted Morse code from continuous programmatic vapor inputs.

Encryption application of a fast stimulus-responsive hydrogen-bonded organic framework based on FRET

A photochromic ionic hydrogen-bonded organic framework (iHOF-51) was synthesized by reacting organic carboxylic acids with amidinium salts. Fast, reversible, and high-contrast stimulus-responsive behavior via anionic radical naphthalenediimide (NDI) is observed. iHOF-51exhibits time-dependent photochromism in the powder state and excellent photoluminescent properties based on the synergistic interaction of FRET and radical anions in the 1%-iHOF-51-PVA composite membrane.

Disturbance-Triggered Instant Crystallization Activating Bioinspired Emissive Gels

Many marine organisms feature sensitive sensory-perceptual systems to sense the surrounding environment and respond to disturbance with intense bioluminescence. However, it remains a great challenge to develop artificial materials that can sense external disturbance and simultaneously activate intense luminescence, although such materials are attractive for visual sensing and intelligent displays. Herein, we present a new class of bioinspired smart gels constructed by integrating hydrophilic polymeric networks, metastable supersaturated salt and fluorophores containing heterogenic atoms. Upon external disturbance, the composite gels undergo an instant and reversible soft-rigid state transition, simultaneously turning on intense fluorescence and activating ultralong afterglow emission with a maximum lifetime of 877.15 ms. The experimental results and molecular dynamics simulations reveal that the disturbance-induced luminescence mainly results from the geometrical confinement of aggregated fluorophores and enhanced molecular interactions to immensely suppress the non-radiative dissipation. Given their versatile and sensitive disturbance-responsiveness, dynamic interactive painting and 3D smart optical displays are demonstrated. This study paves a new avenue to achieve disturbance-sensing soft materials and promotes the development of smart visual sensors and interactive optical displays.

Long-chain crosslinker-induced patterning on an elastic polymer film for robust and reversible information encryption/decryption

While reversible information encryption and decryption are readily achievable with hydrogels, this process presents a significant challenge when applied to elastic polymer films. This is due to the inherent chemical stability of anhydrous polymer films which significantly increases the difficulty of information writing. In this study, we propose a solvent-free radical polymerization method for chemical patterning on the elastic film of poly(styrene-butadiene-styrene) (SBS). Unlike short chain crosslinkers-induced patterning, which increases the brittleness of the film, the long-chain crosslinkers are chemically bonded with the chains of SBS. This not only enhances the mechanical stability of film, but also improves its softness and robustness (the strength increases 1.8 times and the toughness increases 2.3 times), thereby greatly extending its durability for information encryption and decryption. When patterned with a photomask, the crosslinked regions maintain transparency upon acetone absorption, while the non-crosslinked regions become opaque due to an acetone-induced phase change. Upon removal of acetone, these opaque regions can be restored to transparency. Compared with hydrogels liable to water loss and deformation, the patterned films show greater stability, retaining pattern encryption/decryption functions after 30 days in a natural environment without special storage. The rate of this phase transition is directly related to the degree of crosslinking. Therefore, by adjusting the degree of crosslinking, the patterned films can undergo multistage encryption/decryption in response to acetone, providing a promising method for information security and storage.

Temperature and Solvent Dual Switch Photochromic Chiral Ionic Hydrogen-Bonded Organic Framework for Circularly Polarized Luminescence and Advanced Encryption

Multi-response encryption materials with temperature control and time resolution have attracted widespread attention due to their unique response characteristics and higher application security. The design and development of photochromic crystalline materials with multiple stimulus responses remain challenging. In this study, we report a pair of responsive photochromic chiral ionic hydrogen-bonded organic framework (iHOF) R/S-iHOF-19, controlled by both temperature and solvent through charge-assisted synthesis. The chromophore tetrakis(4-sulfophenyl)ethylene (H4TPE) acts as an electron donor and (1R/S,2R/S)-1,2-diphenylethylenediamine (R/S-DPEN) as an electron acceptor and chiral source. Water and methanol molecules connect the donor and acceptor and interact to build a 3D supramolecular framework. Notably, water and methanol molecules form independent hydrogen-bonding channels within the iHOF structural framework, providing a transfer path for the photoinduced electrons. Surprisingly, the formation of a continuous chiral supramolecular framework by R/S-DPEN while generating photo-induced radicals under ultraviolet (UV) irradiation at -20 °C imparts excellent circularly polarized luminescence (CPL) properties to R/S-iHOF-19. The glum values reach -1.8 × 10-3 and +3.75 × 10-3, respectively, and show an enhancement of the circular polarization of light with decreasing temperature. This CPL with unique low-temperature stimulus-responsive photochromism provides new guidance and perspectives for the development of information security and multiple encryption materials.

Multi-mode geometrically gated encryption with 4D morphing hydrogel

Leveraging the rich stimuli-response of polymers represents a promising direction towards optical communication/encryption. Sign language, which relies on specific geometric change for secured communication, has been widely used for the same purpose since ancient time. We report a strategy that combines both in a validated manner with a hydrogel that not only carries encrypted optical information but also has the hidden behavior to morph geometrically. In particular, the shape morphing behavior is programmable by controlling the oriented state of the polymer chain in the thermo-responsive network. Whether the shape morphing direction is positive (bending) or negative (flattening) cannot be predicted when the polymerization methods are not informed, revealing a hidden manner. Through deciphering the coupling of chain elastic stresses and thermo-induced deswelling stress, the hydrogel can perform designed and diversified 4D morphing which represents evolution of 3D geometries with time as the fourth dimension. Consequently, the corresponding optical information can be gated based on these geometric features, thereby decrypting the correct permutation of information. Our approach that utilizes the geometric 4D morphing for gated verification of optical information offers a strategy for enhancing the security of communication in ways that are quite different from existing strategies.

Rewritable Triple-Mode Light-Emitting Display

Despite great progress in developing mode-selective light emission technologies based on self-emitting materials, few rewritable displays with mode-selective multiple light emissions have been demonstrated. Herein, we present a rewritable triple-mode light-emitting display enabled by stimuli-interactive fluorescence (FL), room-temperature phosphorescence (RTP), and electroluminescence (EL). The display comprises coplanar electrodes separated by a gap, a polymer composite with FL inorganic phosphors (EL/FL layer), and a polymer composite with solvent-responsive RTP additives (RTP layer). Upon 254 nm UV exposure, a dual-mode emission of RTP and FL occurs from the RTP and EL/FL layers, respectively. When a polar liquid, besides water, is applied on the display and an AC field is applied between the coplanar electrodes, EL from the EL/FL layer is triggered, and the display operates in a triple mode. Interestingly, when water is applied to the display, the RTP mode is deactivated, rendering the display to operate in a dual mode of FL and EL. By manipulating the evaporation of the applied polar liquids and water, the mode-selective light emission of FL, RTP, and EL is rewritable in the triple-mode display. Additionally, a high-security full-color information encryption display is demonstrated, wherein the information of digital numbers, letters, and Morse code encoded in one optical mode is only deciphered when properly matched with that encoded in the other two modes. Thus, this article outlines a strategy to fulfill the substantial demand for high-security personalized information based on room-temperature multi-light-emitting displays.

Photochromic Metal-Organic Frameworks Based on Host-Guest Strategy and Different Viologen Derivatives for Organic Amines Sensing and Information Anticounterfeiting

The encapsulation of viologen derivatives in metal-organic frameworks (MOFs) to construct host-guest materials has been widely discussed owing to their distinctive spatial arrangement and physical/chemical properties. Herein, three new photochromic MOFs (NWM-1-3) have been successfully synthesized by 1,1,2,2-Tetra(4-carboxylphenyl)ethylene (H4TCPE) ligand as well as three different viologen derivatives based on host-guest strategy. Remarkably, NWM-1-3 exhibit a notable reversible photochromism change from yellow to green under 365 nm UV irradiation. The distance between the electron-deficient N atom in the viologens and the electron-rich carboxylate oxygens satisfies the electron transfer (ET) pathway, and thus ET occurs upon irradiation, producing intermolecular viologen radicals. NWM-1 is able to produce colored responses to different volatile amines by ET and can be recognizable to the naked eye. Differential pulse voltammetry (DPV) analysis and comparative experiments have demonstrated that the host-guest strategy significantly enhances the electron-accepting ability of viologens, thereby achieving superior amine sensing performance. NWM-2 and 3 have been realized in various applications, such as security code, fingerprint, and QR codes for anticounterfeiting. This work provides new host-guest strategy for designing highly sensitive photochromic materials and color-tunable luminescent materials, advancing the development of assembled photochromic materials closer to commercialization.

Dynamic multimodal information encryption combining programmable structural coloration and switchable circularly polarized luminescence

Multimodal optical-materials are highly desirable due to their advantages in enhancing information security, though independent modulation is challenging, especially accurately controlling the orthogonal relationship between the structural coloration (SC) and fluorescence (FL) pattern. Herein, we report a strategy which combines programmable structural coloration and switchable circularly polarized luminescence (CPL) for multimodal information encryption. Using photomask with aligned grating, programmable periodic patterns are fabricated on a polydiacetylene (PDA) gel film, which produces image in tunable structural colors. Introducing a chiral fluorescence layer containing perovskite nanocrystals and twisted-stacking Ag nanowires (NWs) bilayers, which provides stimuli-responsive FL and CPL with high dissymmetry factor (glum, up to 1.3). Importantly, the structural coloration information and FL patterns (including CPL pattern) can be independently modulated without mutual interference, even selectively concealed or exposed by varying microstructure design of the cross-linked PDA gel or by acetonitrile treatment.

High-Security Plastic with Integrated Holographic and Phosphorescent Images

Organic room temperature phosphorescence (ORTP) polymer materials have sparked considerable research interests in recent years, but their optical function is still limited for multi-mode optical imaging. Herein, a feasible and universal approach is proposed to endow ORTP polymer materials with periodic refractive index modulation functions by holographic patterning. The key to this approach is to design a two-stage stepwise crosslinking. Stage-1, with low crosslinking density (≤0.75 mol L-1), is phosphorescence-silent but can provide greater free volume for monomer diffusion and thus facilitate the patterning of refractive index modulated holograms via photopolymerization-induced phase separation. The dense crosslinking at stage-2 can turn on phosphorescence with the intensity rising by 144% when the crosslinking density increases from 3.77 to 4.12 mol L-1. The enhanced phosphorescence is primarily ascribed to the increase of conformational distortion and spin-orbit coupling of organic phosphors based on theoretical calculations. Eventually, the first example is demonstrated of holographic plastic with the unique capability of independently displaying holographic andphosphorescent images. This work not only provides a novel paradigm to impart added optical functions to ORTP polymer materials but also paves the way for the development of high-security optical materials to combat counterfeiting.

Superior Multimodal Luminescence in a Stable Single-Host Nanomaterial with Large-Scale Synthesis for High-Level Anti-Counterfeiting and Encryption

Multimode luminescent materials exhibit tunable photon emissions under different excitation or stimuli channels, endowing them high encoding capacity and confidentiality for anti-counterfeiting and encryption. Achieving multimode luminescence into a stable single material presents a promising but remains a challenge. Here, the downshifting/upconversion emissions, color-tuning persistent luminescence (PersL), temperature-dependent multi-color emissions, and hydrochromism are integrated into Er3+ ions doped Cs2NaYbCl6 nanocrystals (NCs) by leveraging shallow defect levels and directed energy migration. The resulting NCs display strong static and dynamic colorful luminescence in response to ultraviolet, 980-nm laser, and X-ray. Additionally, the NCs exhibit distinct luminescent colors as the temperature increases from 330 to 430 K. Surprisingly, it also demonstrates the ability of the reversible emission modal and color in response to water. Theoretical calculations and experimental characterizations reveal that self-trapped exciton state (STEs), chlorine vacancy defects, and ladderlike 4f energy levels of Er3+ ions contribute to multimodal luminescence. More importantly, it has extremely remarkable environmental stability, which can be stored in the air for more than 18 months, showing promising commercial prospects. This work not only gives new insights into lanthanide-based metal halide NCs but also provides a new route for developing multimodal luminescent nanomaterials for anti-counterfeiting and encryption.

Bioinspired Hierarchical Photonic Structures with Controllable Polarization and Color for Optical-Multiplexed Information Encryption

Optical multiplexing technologies, which integrate multiple optical channels based on photonic structures, offer a significant solution for high-capacity information storage and advanced encryption. However, these photonic materials are limited by their inherent and unswitchable chiral structures, which result in a restricted control over the spatial distribution of light. Here, we propose to construct an integrating optical-multiplexed structure using tunable 1D photonic crystals and orientation texture via a combined self-assembly and shear-aligning approach. In this photonic system, the created diverse orientation structure of ethyl cellulose (EC) offers a wide range of light modulations through phase retardation. When combined with the chromatic layer formed by the self-assembly of EC, tunable wavelength and polarization are achieved. Notably, due to the identical components of the light modulation and chiral photonic crystal layers, the traceless interface between them ensures both high confidentiality and durability. By leveraging these hierarchical structures, photonic slices with well-defined polarization states and structural colors are created, enabling the construction of an advanced photonic platform for multiplexed information storage and multichannel 3D and 4D encryption. This study presents a promising strategy to develop traceless, highly confidential photonic units with controllable polarization and color for advanced encryption technologies.

Remote On-Paper Electrochemiluminescence-Based High-Safety and Multilevel Information Encryption

The escalating needs in information protection underscore the urgency of developing advanced encryption strategies. Herein we report a novel chemical approach that enables information encryption by on-paper electrochemiluminescence (ECL). Dendritic porous silica nanospheres modified with polyetherimide and bovine serum albumin were prepared as the chemical ink to write the secret message on a paper. Attaching the paper to an electrode, immersing it in a solution containing tris(2,2'-bipyridyl)ruthenium (Ru(bpy)3 2+) and then applying a suitable voltage, a remote "catalytic route" electrochemical reaction produces ECL that functions as the key to decrypt and visualize the message by imaging. In addition, proteins can be also used as the biological ink to write the secret message, which is then decrypted by a combined use of immunochemistry and ECL imaging as two keys. We believe the ECL-based strategy holds great promise in high-safety and multilevel information encryption, as it is protected not only by encoding, like conventional invisible inks, but also by the unique ECL decoding approach.

A rewritable and shape memory hydrogel doped with fluorescein-functionalized ZIF-8 for information storage and fluorescent anti-counterfeiting

The emergence of stimuli-responsive fluorescence anti-counterfeiting technology has garnered increasing attention in the era of intelligent internet. Smart fluorescent hydrogels combine the characteristics of luminous materials with the unique structure of hydrogels, offering the potential for dynamic reversible erasing and multi-tiered data encryption. In this work, a fluorescent hydrogel was constructed by zeolitic imidazolate framework-8 loaded with fluorescein and then mixed with polyvinyl alcohol hydrogel, sodium carboxymethyl cellulose and borax, which could be used for image hiding in visible light. The reversible bonds cross-linked fluorescent hydrogel was stretchable and self-healing with a three-dimensional network structure. The hydrogel presented bright green fluorescence under 365 nm UV light, which was quenched by adding copper ions. Meanwhile, the imprint of the hydrogel could be cleared by L-Cysteine and repeatedly recorded information many times. The alkali-induced shape memory capability was further utilized to achieve multi-tiered data encryption by deforming it to a 3D-specific shape through folding. The rewritable and multi-dimensional encrypted hydrogel is expected to improve data security and reduce resource consumption.

Photo-Controllable Förster Resonance Energy Transfer Based on Dynamic Chiral Self-Assembly of Sequence-Defined Amphiphilic Alternating Azopeptoids

Endowing biomimetic sequence-controlled polymers with chiral functionality to construct stimuli-responsive chiral materials offers a promising approach for innovative chiroptical switch, but it remains challenging. Herein, it is reported that the self-assembly of sequence-defined chiral amphiphilic alternating azopeptoids to generate photo-responsive and ultrathin bilayer peptoidosomes with a vesicular thickness of ≈1.50 nm and a diameter of around ≈290 nm. The photoisomerization of azobenzene moiety facilitates a reversible structural transformation from isotropic peptoidosomes to anisotropic 1D helical nanoribbons (≈80 nm width) under the alternating irradiation with UV and visible lights, consequently leading to the chirality expression and transfer from chiral asymmetric center to achiral azobenzene units. As a biomimetic model with deformation-induced energy transfer, a non-invasive azobenzene-based Förster resonance energy transfer system is unprecedentedly constructed via the introduction of a fluorescent donor of pyrene derivatives and sequentially photo-regulated the donor/acceptor ratio, displaying a reversible gradient fluorescent color variation from blue to yellow (a broad Stokes shift of ≈200 nm) and a high-efficient energy transfer efficiency of 97.2%. The photo-controllable photoluminescence phenomenon endows these chiral aggregates with a proof-of-concept application on multi-colored information encryption. This work provides a prospective strategy to fabricate stimuli-responsive chiral biomimetic materials with a potential on the light-controllable switches.

A Streamlined Approach to Anticounterfeiting Technologies: Patterned AAO Membranes Based on Photonic Crystal Effects with Tunable Color Shifts and pH Responsiveness

Anticounterfeiting technologies have become increasingly crucial due to the growing issue of counterfeit goods, particularly in high-value industries. Traditional methods such as barcodes and holograms are prone to replication, prompting the need for advanced, cost-effective, and efficient solutions. In this work, a practical application of anodic aluminum oxide (AAO) membranes are presented for anticounterfeiting, which addresses the challenges of high production costs and complex fabrication processes. Unlike previous approaches requiring metal coatings for color generation, this method uses commercial aluminum foils to produce colorful AAO membranes without metal layers. Elemental mapping suggests that impurities on the aluminum surface contribute to enhanced reflectivity, aiding photonic crystal formation. A two-step anodization process that creates patterned AAO membranes is further introduced, with the pattern clarity controlled by anodization time. Additionally, a pH-responsive film composed of 2-anilino-6-dibutylaminofluoran (ODB-2) and thermoplastic polyurethane (TPU) is integrated, enabling dynamic color changes under varying pH conditions, further enhancing the anticounterfeiting functionality. This streamlined approach provides a scalable and cost-effective solution for developing versatile AAO membranes for industrial anticounterfeiting applications.

High Defect Tolerance Breaking the Design Limitation of Full-Spectrum Multimodal Luminescence Materials

With the development of optical anti-counterfeiting and the increasing demand for high-level information encryption, multimodal luminescence (MML) materials attract much attention. However, the discovery of these multifunctional materials is very accidental, and the versatile host suitable for developing such materials remains unclear. Here, a grossite-type fast ionic conductor CaGa4O7, characterized by layered and tunnel structure with excellent defect tolerance, is found to meet the needs of various luminescent processes. Almost all luminescent modes, including down/up-conversion luminescence (DCL/UCL), long persistent luminescence (LPL), mechanoluminescence (ML), and X-ray excited optical luminescence (XEOL), are realized in this single host. Full-spectrum (from violet to near-infrared) photoluminescence and ML as well as multicolor XEOL are achieved by simply changing the doped luminescent center. A series of anti-counterfeiting devices, including the quasi-dynamic display of famous paintings, digital information encryption, and multi-color handwritten signatures, are designed to show the encryption of information in temporal and spatial dimensions. This study clarifies the importance of defect tolerance of the host for the development of MML materials, and provides a unique insight into the cross-field applications of special functional materials, which is a new strategy to accelerate the development of novel MML materials.

3D printing of thermochromic multilayer flexible film for multilevel information encryption

Flexible thermal-responsive encryption devices are widely employed in information encryption and anti-counterfeiting due to their cost-effectiveness and dynamic data encryption and decryption capabilities. However, most current devices are limited to a single layer of encryption, resulting in restricted decryption methods and storage capacity, as well as reliance on external heating. In this study, we integrate multiple layers of encryption within a single device and introduce self-heating thermochromic technology along with infrared thermal imaging encryption to establish a novel concept of a multilayer flexible encryption system. By combining infrared encryption and thermochromic encryption in three-dimensional space enhances the difficulty level for decryption while achieving high storage capacity for information. The internally integrated conductive heating layer within the multilayer structure facilitates rapid and adjustable heating for thermochromic patterns, eliminating the need for external heat sources. Furthermore, we employ a low-cost customizable multi-material integrated 3D printing process for manufacturing multilayer flexible encryption devices. This research presents an innovative solution for designing and fabricating high-density multilevel flexible encryption devices.

Strategic engineering of cationic systems for spatial & temporal anti-counterfeiting applications in zero-dimensional Mn(II) halides

While spatial and time-resolved anti-counterfeiting technologies have gained increasing attention owing to their excellent tunable photoluminescence, achieving high-security-level anti-counterfeiting remains a challenge. Herein, we developed a spatial-time-dual-resolved anti-counterfeiting system using zero-dimensional (0D) organic-inorganic Mn(II) metal halides: (EMMZ)2MnBr4 (named M-1, EMMZ=1-Ethyl-3-Methylimidazolium Bromide) and (EDMMZ)2MnBr4 (named M-2, EDMMZ=1-Ethyl-2,3-Dimethylimidazolium Bromide). M-1 shows a bright green emission with a quantum yield of 78 %. It undergoes a phase transformation from the crystalline to molten state with phosphorescence quenching at 350 K. Reversible phase and luminescent conversion was observed after cooling down for 15 s. Notably, M-2 exhibits green light emission similar to M-1 but undergoes phase conversion and phosphorescence quenching at 390 K, with reversible conversion observed after cooling down for 5 s. The photoluminescence switching mode of on(green)-off-on(green) can be achieved by temperature control, demonstrating excellent performance with short response times and ultra-high cyclic reversibility. By leveraging the different quenching temperatures and reversible PL conversion times of M-1 and M-2, we propose a spatial-time-dual-resolved photoluminescence (PL) switching system that combines M-1 and M-2. This system enables multi-fold tuning of the PL switch for encryption and decryption through cationic engineering strategies by modulating temperature and cooling time. This work presents a novel and feasible design strategy for advanced-level anti-counterfeiting technology based on a spatial-time-dual-resolved system.

Hydrogen Bond-Associated Photofluorochromism for Time-Resolved Information Encryption and Anti-counterfeiting

Time-resolved photofluorochromism constitutes a powerful approach to enhance information encryption security but remains challenging. Herein, we report a strategy of using hydrogen bonds to regulate the time for initiating photofluorochromism. In our strategy, copolymers containing negative photochromic spiropyran (NSP), naphthalimide, and multiple hydrogen-bonding (UPy) units are designed, which display photo-switchable fluorescence resonance energy transfer (FRET) process from naphthalimide donor to the NSP acceptor. Interestingly, the FRET is locked via the dynamic hydrogen-bonding interaction between ring-opened NSP and UPy moieties, resulting in time-dependent fluorescence. The change in fluorescence can be finely regulated via UPy fraction in the polymers. Besides the novel time-dependent fluorescence, the polymers also take advantage of visible-light triggerable, excellent photostability, photoreversibility, and processability. We demonstrate that these properties enable them many application opportunities such as fluorescent security labels and multilevel information encryption patterns.

Breaking the Spin-Forbidden Restriction to Achieve Long Lifetime Room-Temperature Phosphorescence of Carbon Dots

Room-temperature phosphorescent (RTP) carbon dots (CDs) demonstrate significant potential applications in the field of information anticounterfeiting due to their excellent optical properties. However, RTP emission of CDs remains significantly limited due to the spin-forbidden properties of triplet exciton transitions. In this work, an in situ nitrogen doping strategy was employed to design and construct strong spin-orbit coupling nitrogen-doped CDs with mesoporous silica with alumina (N-CDs@MS@Al2O3) RTP composites. Both experimental results and theoretical calculations confirmed that the formation of 1(n, π*) following the introduction of nitrogen breaks the spin-forbidden restriction from 1(π, π*) to 3(π, π*), thereby enhancing spin-orbit coupling, which further promotes intersystem crossing and leads to the effective population of triplet excitons. The designed N-CDs@MS@Al2O3 benefiting from an impressive long lifetime of 3.18 s demonstrates potential application prospects in the field of multilevel information encryption. This work provides a new concept to boost the RTP lifetime of CDs.

Biodegradable cellulose nanocrystal composites doped with carbon dots for packaging and anticounterfeiting applications

Developing sustainable and multifunctional materials is imperative for advancing anti-counterfeiting measures, sensing technologies, and intelligent packaging solutions. Concurrently, materials based on carbon dots (CDs) and cellulose nanocrystals (CNCs) are becoming established in such applications. Therefore, herein, we present the fabrication and characterization of water-based CDs and CNCs from Vigna mungo (black lentil: BL). The carbon dots (CDBL) were doped with nitrogen (NCDBL) and sulfur (SCDBL). These CDs were then utilized as anti-counterfeit inks and multifunctional sensor films when loaded in a biodegradable CNCBL matrix. These CDBL, SCDBL, and NCDBL exhibited diameters of 3.7, 5.3, and 5.5 nm, respectively, with bandgap values ranging from 3.65 eV to 2.95 eV. For anti-counterfeiting, CDs/CNCBL-based inks were applied to white sheets, rendering them invisible under normal lighting conditions and visible under UV light (365 nm). NCDBL exhibited sensitivity towards pH changes (2-12), demonstrating the sensing potential of NCDBL/CNCBL films for monitoring food freshness. Additionally, NCDBL/CNCBL-based films have exhibited effective control over microbial load due to nitrogen doping. These films biodegrade within 29 days when buried in soil after use. This innovative approach presents multifunctional films that address critical needs in sensing, anti-counterfeiting, and intelligent packaging and opens new avenues for creating eco-friendly, multifunctional materials.

Single-Luminophore Molecular Engineering for Rapidly Phototunable Solid-State Luminescence

Smart materials enabling emission intensity or wavelength tuning by light stimulus have attracted attention in cutting-edge fields. However, due to the general limitation of the dense molecular stacking (in solid states, especially in crystals) on photoresponsivity, constructing rapidly phototunable solid-state luminescent systems remains challenging. Herein, we present a new luminophore that serves as both a photoresponsive and a luminous group with enhanced conformational freedom to attain this goal, namely, relying on photoexcitation-induced molecular conformational change of an ionized persulfurated arene based on weak intermolecular aliphatic C-H⋅⋅⋅π interaction. Together with the phosphorescence characteristic of the molecule itself, rapidly enhanced phosphorescence upon irradiation can be observed in a series of phase states, like solution state, crystal, and amorphous state, especially with a high photoresponsive rate of 0.033 s-1 in crystal state that is superior to the relevant reported cases. Moreover, a rapidly phototunable afterglow effect is achieved by extending this molecule into some polymer-based doping systems, endowing the system with unique dynamic imaging and fast photopatterning capabilities. This single-luminophore molecular engineering and underlying mechanism have implications for building other condensed functional materials, principally for those with stimuli responses in solid states.

Strong Anisotropy and Giant Photothermoelectricity of 1D Alloy Nb3Se12I-Based Photodetector for Ultrabroadband Light-Detection and Encryption Imaging Application

Securing high-capacity and safe communication holds great potential for ushering in the new era of digital society. In this study, a wide-band photodetector based on the quasi-1D niobium selenide compounds (Nb3Se12I), showing up photothermoelectric (PTE) dominated multiple synergistic effects with high responsivity across a wide wavelength range from deep-ultraviolet (254 nm) to terahertz (0.30 THz), providing an efficient strategy for high-capacity optical processing is introduced. Combined with the polarized-sensitive property of the Nb3Se12I photodetector, an encrypted imaging technology using polarization-resolved PTE current is developed. This technology is capable of transforming encrypted messages into polarization states, which can be restored through a specially designed decryption algorithm, greatly enhancing the concealment and security of the information transmission process. Overall, the Nb3Se12I-based detector manifests in terms of high responsivity (283.2 A W-1 to 520 nm, 1632.4 A W-1 to 980 nm, and 1868.1 A W-1 for short-wave infrared light of 2200 nm), suitable response speed of 43 µs for 0.30 THz wave and polarization anisotropy ratio of 1.83. The versatile abilities of photodetector in the realm of ultrabroadband polarized detection and encryption imaging may advance the use of anisotropic thermoelectric materials in high-capacity and secure information communication applications.

Construction of Heterogeneous Aggregation-Induced Emission Microspheres with Enhanced Multi-Mode Information Encryption

Traditional organic light-emitting materials hinder their anti-counterfeiting application in solid state due to their aggregation-caused quenching effect. A facile and straightforward method was reported to introduce AIE molecules into microspheres and manipulate different reaction parameters to prepare AIE microspheres with different morphologies. In this strategy, fluorescent microspheres with spherical, apple-shaped, and hemoglobin-like types were synthesized. Driven by the photocyclization and oxidation of tetraphenylethene, microspheres can be used as an aqueous fluorescence ink with erasable properties. The fluorescent patterns printed by microsphere ink on paper can be irreversibly erased by prolonged exposure to ultraviolet light (365 nm, 60 mw/cm2). Moreover, the multi-morphology microspheres can be further arranged for multiple-information encryption and anti-counterfeiting of barcodes and two-dimensional codes, in which double validation was carried out through fluorescence spectroscopy and laser confocal microscopy. This approach provides a new method for more reliable anti-counterfeiting and information encryption.

Silylated Carbon Nanofiber/Polydimethylsiloxane Based Printable Electrorheological and Sensor Inks for Flexible Electronics

Electrorheological fluids (ERF) have garnered significant attention for their potential to provide actuation on demand. Similarly, developing stimuli-responsive printable inks for flexible electronics is also gaining antecedence. However, developing a material that demonstrates both functionalities is far and few. Accordingly, a printable ink is made using silylated carbon nanofiber (SiCNF)-polydimethylsiloxane (PDMS). The viscosity of the ink increased by 43%, when subjected to an electric field (E). Robust stability for 20 cycles under E = 300 V mm-1 is noted. The yield stress (τy) value increased by 1600% (E = 600 V mm-1) compared to zero-field yield stress. Applying temperature with E further increased the τy. In the absence of E, applying temperature not only slowed down the relaxation modulus but also counterintuitively augmented the extent of sluggishness with an increase in temperature. A comprehensive study on the waiting time also indicated a structure build-up within the ink composition happening as the waiting time increases. Accordingly, the time-temperature and time-waiting time superposition principle is applied to predict the long-term behavior of the inks. Further, the printability index of the ink check is studied and used for printing designs using direct ink writing. The printed ink demonstrated pressure sensing capability with a sensitivity of 6.3%/kPa and is stable over 60 cycles.

Brush-Like Polymeric Gels Enabled Photonic Crystals toward Ultrasensitive Cosolvent Chromism

Responsive photonic crystals (RPCs) exhibit dynamic chromism upon trigger by various solvents, showing potential applications in qualitative identification and quantitative analysis of multicomponent solvents. However, distinguishing similar solvents, especially traces of cosolvents, remains challenging due to the limited sensitivity of RPCs. To address this, we herein introduce brush-like polymeric gels inside photonic crystals, forming a brush-like polymeric photonic gel (BPPG) that can trace tiny component changes. In this BPPG system, the acrylate backbones and polyethylene glycol (PEG) side-chains stretch incrementally due to the cosolvency of ethanol-water mixtures, resulting in highly sensitive chromatic responses within ethanol-rich concentrations. With water content varying slightly from 0 to 1 vol %, the reflection wavelength of BPPG can sharply redshift over 30 nm, leading to very noticeable changes in structural color. This enhanced sensitivity makes BPPG suitable for real-time, in situ purity monitoring of absolute ethanol during storage, transportation, and other applications.

Water-Vapor-Triggered Dual-Mode Optical Responses in Rare-Earth-Doped Hollow Nanospheres

Multimode responsive optical materials are garnering ever-increasing attention due to their diverse applications. This work showcases a film assembled with rare-earth-doped CaF2 hollow nanospheres that exhibit water-vapor-triggered dual-mode optical responses. Upon exposure to flowing water vapor, the film rapidly (less than 1.5 s for a 7.7 μm thickness) transitions to a transparent state and simultaneously undergoes a sharp decrease in the photoluminescence intensity. Both of these changes fully reverse upon water evaporation, demonstrating an impressive reversibility over at least 200 cycles. The water-vapor-induced dual-mode responses are attributed to the altered incident light propagation path stemming from the similar refractive indices between CaF2 and water, coupled with the water-induced energy loss of the rare-earth ions. The fabrication of encryption patterns displaying separate signals in multiple channels, as well as the demonstration of noncontact sensing for water vapor distribution, underscore the promising application potential of this dual-mode responsive system.

Hydrochromic Effect of Perovskite-Polymer Composites

Hydrochromic materials undergo magical color changes when interacting with water and are receiving widespread attention for their frontier applications such as sensing and information security. The hydrochromic effect is observable in perovskite materials via the mechanism of water-induced fluorescence quenching. However, due to water isolation, achieving a hydrochromic effect in perovskite-polymer composite remains elusive, notwithstanding its importance as a potentially commercial-ready material. Here, we demonstrate a hydrochromic effect of perovskite-polymer-based porous composite via a nonsolvent-induced phase separation method, comprising of FA2PbBr4/poly(vinylidene fluoride) (FA = formamidinium). The naturally formed pores serve as microchannels, facilitating moisture diffusion. The penetrated water induces a phase transition of perovskite material from the nonfluorescent two-dimensional FA2PbBr4 to the fluorescent three-dimensional FAPbBr3. This work has developed the hydrochromic perovskite-polymer composites, enabling various commercial-ready chromatic applications as conceptually demonstrated custom-made fingerprint labels, quick response code anticounterfeiting labels, encrypted document protections, and water-ink inkjet printing.

Novel core-shell materials SiO2@Tb-MOF for the incorporation of spiropyran molecules and its application in dynamic advanced information encryption

Dynamic fluorescent switches with multiple light outputs offer promising opportunities for advanced security encryption. However, the achievement of dynamic emission, particularly when based on the timing of external stimuli, continues to present a significant challenge. Herein, a unique dynamic fluorescent switch was developed by integrating spiropyran molecules (SP) into a core-shell structure (SiO2@Tb-MOF). The core-shell structure, derived from lanthanide complexes and silica microspheres, was synthesized under solvothermal conditions. This structure not only preserves the green fluorescence emission of Tb-MOF, but also results in a substantial specific surface area and mesoporous pore size from SiO2, which is advantageous for incorporating SP molecules to create a dynamic fluorescent switch, SP ⊂ SiO2@Tb-MOF. Upon exposure to ultraviolet light, SP gradually transitions into the merocyanine form (MC), displaying a pronounced absorption band at approximately 550 nm. Concurrently, a fluorescence resonance energy transfer (FRET) process is initiated between Tb3+ and the merocyanine isomers. With prolonged exposure to UV light, the fluorescence color shifts progressively from green to red, facilitated by the ongoing FRET process. Moreover, SP ⊂ SiO2@Tb-MOF is doped with polydimethylsiloxane to fabricate a film. Utilizing time-dependent fluorescence, dynamic encryption patterns and advanced information encryption were investigated. This work provides a design basis for how to better construct core-shell structures and combine them with SP molecules to prepare dynamic fluorescent materials, and paves a way for constructing advanced encryption materials with higher safety requirements.

Polyacrylic Acid-Reinforced gelatin hydrogels with enhanced mechanical properties, temperature-responsiveness and antimicrobial activity for smart encryption and salmon freshness monitoring

Hydrogels hold great potential for use in intelligent packaging, yet they often suffer from limited functionality and inadequate mechanical strength when applied to anticounterfeiting and freshness monitoring. In this study, we present a straightforward method to create a multifunctional hydrogel by in-situ polymerizing acrylic acid (PAA) within a gelatin-Al3+ system. The resulting hydrogels exhibited an elongation at break of over 1200 %, a tensile stress of 1.20 MPa, and impressive toughness reaching 5.15 MJ/m3, significantly outperforming traditional gelatin-based hydrogels that typically achieve less than 800 % strain and below 1 MPa stress. These hydrogels also showed exceptional antifatigue and tear resistance, with a tearing energy of 5200 J/m2, greatly exceeding the 1000 J/m2 standard of typical double network hydrogels, and were capable of supporting weights 1560 times their own mass. The strong hydrogen bonding between the -COOH groups of PAA and the -NH2 groups of gelatins contributed to an upper critical solution temperature above 40°C, with adaptable PAA content allowing for anticounterfeiting applications. The hydrogel could encode information such as self-erasing numbers, QR codes, and ASCII binary codes, changing its encoded data with temperature shifts and erasing at room temperature to enhance data security. Additionally, it exhibited potent antibacterial properties against S. aureus and E. coli, immobilized anthocyanin as an ammonia-responsive indicator, and accurately tracked salmon spoilage by correlating color changes with total volatile basic nitrogen content. These characteristics make the hydrogel highly suitable for smart packaging applications within the food industry.

Flexible self-supporting photonic crystals: Fabrications and responsive structural colors

Photonic crystals (PCs) play an increasingly significant role in anti-counterfeiting, sensors, displays, and other fields due to their tunable structural colors produced by light manipulation of photonic stop bands. Flexible self-supporting photonic crystals (FSPCs) eliminate the requirement for conventional structures to rely on the existence of hard substrates, as well as the problem of poor mechanical qualities caused by the stiffness of the building blocks. Meanwhile, diverse production techniques and materials provide FSPCs with varied stimulus-responsive color-changing capacities, thus they have received an abundance of focus. This review summarizes the preparation strategies and variable structural colors of FSPCs. First, a series of preparation strategies by integrating polymers with PCs are summarized, including assembly of colloidal spheres on flexible substrates, polymer packaging, polymer-based direct assembly, nanoimprinting, and 3D printing. Subsequently, variable structural colors of FSPCs with different stimulations, such as viewing angle, chemical stimulation (solvents, ions, pH, biomolecules, etc.), temperature, mechanical/magnetic stress, and light, are described in detail. Finally, the outlook and challenges regarding FSPCs are presented, and several potential directions for their fabrication and application are discussed. It's believed that FSPCs will be a valuable platform for advancing the practical implementation of optical metamaterials.

Engineered Shape-Morphing Transitions in Hydrogels Through Suspension Bath Printing of Temperature-Responsive Granular Hydrogel Inks

4D printing of hydrogels is an emerging technology used to fabricate shape-morphing soft materials that are responsive to external stimuli for use in soft robotics and biomedical applications. Soft materials are technically challenging to process with current 4D printing methods, which limits the design and actuation potential of printed structures. Here, a simple multi-material 4D printing technique is developed that combines dynamic temperature-responsive granular hydrogel inks based on hyaluronic acid, whose actuation is modulated via poly(N-isopropylacrylamide) crosslinker design, with granular suspension bath printing that provides structural support during and after the printing process. Granular hydrogels are easily extruded upon jamming due to their shear-thinning properties and their porous structure enables rapid actuation kinetics (i.e., seconds). Granular suspension baths support responsive ink deposition into complex patterns due to shear-yielding to fabricate multi-material objects that can be post-crosslinked to obtain anisotropic shape transformations. Dynamic actuation is explored by varying printing patterns and bath shapes, achieving complex shape transformations such as 'S'-shaped and hemisphere structures. Furthermore, stepwise actuation is programmed into multi-material structures by using microgels with varied transition temperatures. Overall, this approach offers a simple method to fabricate programmable soft actuators with rapid kinetics and precise control over shape morphing.

Ionic Hydrogen-Bonded Organic Frameworks with a Two-Photon Synergistic Color Change and Their Information Encryption Applications

Photochromic hydrogen-bonded organic frameworks (HOFs) can introduce different luminescent functional groups to achieve synergistic controlled multiple color change properties, which are in great demand for diverse information encryption applications. We report in this paper switchable photochromic and photoluminescent dual luminescent functional group HOFs constructed with synergistic effects by N,N'-bis(2-phenylalanine)-1,4,5,8-naphthalenediimine (H2PheNDI) and benzenecarboximidamide 4,4'-azobis(hydrochloride) (AZBH). The crystal powder of iHOF-41 is orange-red in color, which can be changed to black under the irradiation of a 365 nm ultraviolet (UV) light source for 15 min. The photoisomerization rate of the crystal solution under continuous UV irradiation for 5 h was close to 99%. The composite membranes can achieve the properties of photochromism and photoluminescence when they are discolored under 365 nm UV irradiation and, at the same time, excite red bright fluorescence. This work achieves the construction of HOFs based on switching biluminescent functional groups and explores the synergistic mechanism of the photoisomerization process and photochromism as well as its practical application in information encryption.

Dynamic metal-ligand coordination enables a hydrogel with rewritable dual-mode pattern display

The realization of dual-mode information display in the same material is of great significance to the expansion of information capacity and the improvement of information security. However, the existing systems lose the ability to re-encode information once they are constructed. Here, dynamic metal-ligand coordination is introduced into a novel hydrogel-based optical platform that allows rewritable dual-mode information display. The hydrogel system consists of a hard lamellar structure of poly(dodecylglyceryl itaconate) (pDGI) and soft double networks of poly(acrylamide)/poly(acrylic acid) (PAAm/PAAc) containing fluorescent carbon dots (CDs). As the carboxylic acid groups can coordinate with metal ions such as Al3+, the layer spacing of the lamellar structure is reduced while CDs aggregate, leading to the blue shift of the structural color and the red shift of the fluorescent color. Additionally, the metal chelating agent, ethylenediaminetetraacetic acid (EDTA), is able to strip away Al3+ ions and restore the two colors, realizing an erasable dual-mode information display. This study opens up a path for the development of new materials and technologies for rewritable dual-mode information protection.

Multi-Stimuli Responsive Viologen-Imprinted Polyvinyl Alcohol and Tricarboxy Cellulose Nanocomposite Hydrogels

Photochromic inks have shown disadvantages, such as poor durability and high cost. Self-healable hydrogels have shown photostability and durability. Herein, a viologen-based covalent polymer was printed onto a paper surface toward the development of a multi-stimuli responsive chromogenic sheet with thermochromic, photochromic, and vapochromic properties. Viologen polymer was created by polymerizing a dialdehyde-based viologen with a hydroxyl-bearing dihydrazide in an acidic aqueous medium. The viologen polymer was well immobilized as a colorimetric agent into a polyvinyl alcohol (PVA)/tricarboxy cellulose (TCC)-based self-healable hydrogel. The viologen/hydrogel nanocomposite films were applied onto a paper surface. The coloration measurements showed that when exposed to ultraviolet light, the orange layer printed on the paper surface switched to green. The photochromic film was used to develop anti-counterfeiting prints using the organic hydrogel composed of a PVA/TCC composite and a viologen polymer. Reversible photochromism with strong photostability was observed when the printed papers were exposed to UV irradiation. A detection limit was monitored in the range of 0.5-300 ppm for NH3(aq). The exposure to heat (70 °C) was found to reversibly initiate a colorimetric change.

Excitation-wavelength-dependent persistent luminescence from single-component nonstoichiometric CaGaxO4:Bi for dynamic anti-counterfeiting

Materials capable of dynamic persistent luminescence (PersL) within the visible spectrum are highly sought after for applications in display, biosensing, and information security. However, PersL materials with eye-detectable and excitation-wavelength-dependent characteristics are rarely achieved. Herein, a nonstoichiometric compound CaGaxO4:Bi (x < 2) is present, which demonstrates ultra-long, color-tunable PersL. The persistent emission wavelength can be tuned by varying the excitation wavelength, enabling dynamic color modulation from the green to the orange region within the visible spectrum. Theoretical calculations, in conjunction with experimental observations, are utilized to elucidate the thermodynamic charge transitions of various defect states, thereby providing insights into the relationship between Bi3+ emitters, traps, and multicolored PersL. Furthermore, the utility of color-tunable PersL materials and flexible devices is showcased for use in visual sensing of invisible ultraviolet light, multicolor display, information encryption, and anti-counterfeiting. These discoveries create new opportunities to develop smart photoelectric materials with dynamically controlled PersL for various applications.