Dynamic Coordination of Eu-Iminodiacetate to Control Fluorochromic Response of Polymer Hydrogels to Multistimuli

Engineering Supramolecular Hydrogels via Reversible Photoswitching of Cucurbit[8]uril-Spiropyran Complexation Stoichiometry

The integration of photoswitchable supramolecular units into hydrogels allows for spatiotemporal control over their nanoscale topological network and macroscale properties using light. Nevertheless, the current availability of photoswitchable supramolecular interactions for the development of such materials remains limited. Here, the molecular design of a novel photoswitchable cucurbit[8]uril-spiropyran host-guest complex exhibiting fast and reversible switching of binding ratios between 1:2 and 1:1 is reported. Photoswitchable complexation stoichiometries are rationally exploited as (de)crosslinking units in multiple polymers for the design of supramolecular hydrogels displaying highly dynamic and switchable features that are spatiotemporally controlled by light. The hydrogels exhibit rapid reversible mechanical softening-hardening upon alternating irradiation with blue and UV light, which is used to significantly accelerate and improve the efficiency of self-healing and shape-remolding of hydrogels. Furthermore, spiropyran endows such materials with unique reversible photochromic properties for reproducible patterning/erasing and information storage. Using a dual-light-assisted extrusion process, meter-scale hydrogel fibers with enhanced structural integrity and photoswitchable ionic conductivity are constructed and woven into various slidable knots and fluorescent shapes. This work represents an innovative molecular design strategy for advancing the development of spatiotemporally engineered supramolecular hydrogels using light and opens avenues for their prospective applications in dynamic materials and adaptive systems.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Fabrication of Luminescent Triple-Cross-Linked Gelatin/Alginate Hydrogels through Freezing-Drying-Swelling and Freezing-Thawing Processes

Lanthanide-containing luminescent hydrogels have shown potential for sensing and imaging applications. Nonetheless, integrating lanthanide ions or complexes into the polymer matrix often results in the poor stability and mechanical strength of the hydrogels. This work presents an innovative approach to fabricating luminescent hydrogels with three dynamic cross-links: imine bond, boronate ester bond, and metal-ligand coordination. Europium(III) (Eu3+) ions are incorporated into a dual-cross-linked matrix composed of phenylboronic acid-polyethylenimine-modified gelatin (PPG) and alginate dialdehyde (ADA) through a combined treatment involving freeze-drying-swelling (FDS) and freeze-thawing (FT) processes. The FDS process facilitates the formation of additional europium-carboxylate cross-links within the polymeric network to enhance its luminescence and stability, while the FT process strengthens the network physically. The impact of the FDS-FT cycle number on the microstructures and properties of PPG/ADA-Eu3+ hydrogels is thoroughly investigated, and their potential for monitoring bacterial growth and detecting copper(II) ions is also demonstrated.

Random Copolymers Based on 2-Ethylhexyl Acrylate Exhibit Unusual Glass Transition Breadth and Facile Autonomous Self-Healing over a Broad Composition Range

Statistical copolymers are commercially important because their properties can be tuned by comonomer selection and composition. Rubbery-state styrene (S)/n-butyl acrylate (nBA) copolymers have previously been reported to exhibit facile, autonomous self-healing over a narrow composition band (47/53 to 53/47 mol%). The need for a narrow composition band is explained by alternating comonomer sequences that accommodate interchain secondary bonding. It is hypothesized that copolymers that achieve interchain secondary bonding without alternating sequences can exhibit facile self-healing over a broad composition range. 2-ethylhexyl acrylate (EHA) is identified as yielding sequence-independent secondary bonding interactions. For these interactions it is tested experimentally by glass transition breadth in rubbery-state S/EHA copolymers, with S/n-hexyl acrylate (nHA) and S/nBA copolymers as controls. The n-alkyl acrylate random copolymers exhibit enhanced glass transition breadths over narrow composition bands that correspond to autonomous self-healing. In contrast, S/EHA copolymers exhibit much greater glass transition breadths than S/nHA and S/nBA copolymers at all compositions tested as well as self-healing of damage over a broad composition range with full tensile-property recovery, often in 3-10 h. Characterization of glass transition breadth may serve as a simple screening tool for identifying copolymers that exhibit broad-composition-range, facile, autonomous self-healing and contribute to polymer resilience and sustainability.

Cation-π Interaction-Enhanced Self-Healing Injectable Hydrogels for Gastric Perforation Repair

Surgical operations are the preferred treatment for gastric perforation (GP) but incur postoperative complications such as gastrointestinal adhesions and bacterial infections, leading to inefficient wound healing and serious complications that may even threaten the life of the patient. Developing hydrogel dressings capable of adapting to the gastric environment (acid) and decreasing visceral adhesions and bacterial infections after GP treatment is crucial. In this article, we developed an injectable, self-healing hydrogel using cation-π interactions between protonated amines and aromatic rings under acidic conditions and explored it for GP repair. The hydrogels demonstrate exceptional self-healing capabilities under acidic conditions and can be effectively tailored for the gastric environment. In addition, the hydrogel demonstrated significant efficacy in preventing gastrointestinal adhesion, reducing inflammation, promoting angiogenesis, and effectively facilitating wound healing in a rat GP model. This novel hydrogel demonstrates adaptability to the gastric environment, rendering it highly promising for potential applications in gastric trauma healing.

Full-Color "Off-On" Thermochromic Fluorescent Fibers for Customizable Smart Wearable Displays in Personal Health Monitoring

Responsive thermochromic fiber materials capable of miniaturization and integrating comfortably and compliantly onto the soft and dynamically deforming human body are promising materials for visualized personal health monitoring. However, their development is hindered by monotonous colors, low-contrast color changes, and poor reversibility. Herein, full-color "off-on" thermochromic fluorescent fibers are prepared based on self-crystallinity phase change and Förster resonance energy transfer for long-term and passive body-temperature monitoring, especially for various personalized customization purposes. The off-on switching luminescence characteristic is derived from the reversible conversion of the dispersion state and fluorescent emission by fluorophores and quencher molecules, which are embedded in the matrix of a phase-change material, during the crystallizing/melting processes. The achievement of full-color fluorescence is attributed to the large modulation range of fluorescence colors according to primary color additive theory. These thermochromic fluorescent fibers exhibit good mechanical properties, fluorescent emission contrast, and reversibility, showing their great potential in flexible smart display devices. Moreover, the response temperature of the thermochromic fibers is controllable by adjusting the phase-change material, enabling body-temperature-triggered luminescence; this property highlights their potential for human body-temperature monitoring and personalized customization. This work presents a new strategy for designing and exploring flexible sensors with higher comprehensive performances.

Eu3+-Directed Supramolecular Metallogels with Reversible Quadruple-Stimuli Response Behaviors

Smart luminescent materials that have the ability to reversibly adapt to external environmental stimuli and possess a wide range of responses are continually emerging, which place higher demands on the means of regulation and response sites. Here, europium ions (Eu3+)-directed supramolecular metallogels are constructed by orthogonal self-assembly of Eu3+ based coordination interactions and hydrogen bonding. A new organic ligand (L) is synthesized, consisting of crown ethers and two flexible amide bonds-linked 1,10-phenanthroline moieties to coordinate with Eu3+. Synergistic intermolecular hydrogen bonding in L and Eu3+-L coordination bonding enable Eu3+ and L to self-assemble into shape-persistent 3D coordination metallogels in MeOH solution. The key to success is the utilization of crown ethers, playing dual roles of acting both as building blocks to build L with C2-symmetrical structure, and as the ideal monomer for increasing the energy transfer from L to Eu3+'s excited state, thus maintaining the excellent luminescence of metallogels. Interestingly, such assemblies show K+, pH, F-, and mechano-induced reversible gel-sol transitions and tunable luminescence properties. Above findings are useful in the studies of molecular switches, dynamic assemblies, and smart luminescent materials.

Injectable hydrogel harnessing foreskin mesenchymal stem cell-derived extracellular vesicles for treatment of chronic diabetic skin wounds

Chronic skin wounds are a serious complication of diabetes with a high incidence rate, which can lead to disability or even death. Previous studies have shown that mesenchymal stem cells derived extracellular vesicles (EVs) have beneficial effects on wound healing. However, the human foreskin mesenchymal stem cell (FSMSCs)-derived extracellular vesicle (FM-EV) has not yet been isolated and characterized. Furthermore, the limited supply and short lifespan of EVs also hinder their practical use. In this study, we developed an injectable dual-physical cross-linking hydrogel (PSiW) with self-healing, adhesive, and antibacterial properties, using polyvinylpyrrolidone and silicotungstic acid to load FM-EV. The EVs were evenly distributed in the hydrogel and continuously released. In vivo and vitro tests demonstrated that the synergistic effect of EVs and hydrogel could significantly promote the repair of diabetic wounds by regulating macrophage polarization, promoting angiogenesis, and improving the microenvironment. Overall, the obtained EVs-loaded hydrogels developed in this work exhibited promising applicability for the repair of chronic skin wounds in diabetes patients.

A hyaluronic acid hydrogel as a mild photothermal antibacterial, antioxidant, and nitric oxide release platform for diabetic wound healing

Hyaluronic acid (HA)-based biopolymer hydrogels are promising therapeutic dressings for various wounds but still underperform in treating diabetic wounds. These wounds are extremely difficult to heal and undergo a prolonged and severe inflammatory process due to bacterial infection, overexpression of reactive oxygen species (ROS), and insufficient synthesis of NO. In this study, a dynamic crosslinked hyaluronic acid (HA) hydrogel dressing (Gel-HAB) loaded with allomelanin (AMNP)-N, N'-dis-sec-butyl-N, N'-dinitroso-1, 4-phenylenediamine (BNN6) nanoparticles (AMNP-BNN6) was developed for healing diabetic wounds. The dynamic acylhydrazone bond formed between hydrazide-modified HA (HA-ADH) and oxidized HA (OHA) makes the hydrogel injectable, self-healing, and biocompatible. The hydrogel, loaded with AMNP-BNN6 nanoparticles, exhibits promising ROS scavenging ability and on-demand release of nitric oxide (NO) under near-infrared (NIR) laser irradiation to achieve mild photothermal antibacterial therapy (PTAT) (∼ 48 °C). Notably, the Gel-HAB hydrogel effectively reduced the oxidative stress level, controlled infections, accelerated vascular regeneration, and promoted angiogenesis, thereby achieving rapid healing of diabetic wounds. The injectable self-healing nanocomposite hydrogel could serve as a mild photothermal-enhanced antibacterial, antioxidant, and nitric oxide release platform for the treatment of diabetic wounds.

Reconfiguring hydrogel assemblies using a photocontrolled metallopolymer adhesive for multiple customized functions

Stimuli-responsive hydrogels with programmable shape changes are promising materials for soft robots, four-dimensional printing, biomedical devices and artificial intelligence systems. However, these applications require the fabrication of hydrogels with complex, heterogeneous and reconfigurable structures and customizable functions. Here we report the fabrication of hydrogel assemblies with these features by reversibly gluing hydrogel units using a photocontrolled metallopolymer adhesive. The metallopolymer adhesive firmly attached individual hydrogel units via metal-ligand coordination and polymer chain entanglement. Hydrogel assemblies containing temperature- and pH-responsive hydrogel units showed controllable shape changes and motions in response to these external stimuli. To reconfigure their structures, the hydrogel assemblies were disassembled by irradiating the metallopolymer adhesive with light; the disassembled hydrogel units were then reassembled using the metallopolymer adhesive with heating. The shape change and structure reconfiguration abilities allow us to reprogramme the functions of hydrogel assemblies. The development of reconfigurable hydrogel assemblies using reversible adhesives provides a strategy for designing intelligent materials and soft robots with user-defined functions.

Ion-Cross-Linked Hybrid Photochromic Hydrogels with Enhanced Mechanical Properties and Shape Memory Behaviour

Shape-shifting polymers usually require not only reversible stimuli-responsive ability, but also strong mechanical properties. A novel shape-shifting photochromic hydrogel system was designed and fabricated by embedding hydrophobic spiropyran (SP) into double polymeric network (DN) through micellar copolymerisation. Here, sodium alginate (Alg) and poly acrylate-co-methyl acrylate-co-spiropyran (P(SA-co-MA-co-SPMA)) were employed as the first network and the second network, respectively, to realise high mechanical strength. After being soaked in the CaCl2 solution, the carboxyl groups in the system underwent metal complexation with Ca2+ to enhance the hydrogel. Moreover, after the hydrogel was exposed to UV-light, the closed isomer of spiropyran in the hydrogel network could be converted into an open zwitterionic isomer merocyanine (MC), which was considered to interact with Ca2+ ions. Interestingly, Ca2+ and UV-light responsive programmable shape of the copolymer hydrogel could recover to its original form via immersion in pure water. Given its excellent metal ion and UV light stimuli-responsive and mechanical properties, the hydrogel has potential applications in the field of soft actuators.

Mechanofluorochromic behaviors and data security protection properties of salicylaldimine-based difluoroboron complexes with different aryl substituents

To further explore the relationship between aryl substituents and mechanofluorochromic (MFC) behaviors, four salicylaldimine-based difluoroboron complexes (ts-Ph BF2, ts-Ph-NA BF2, ts-2NA BF2, and ts-triphenylamine [TPA] BF2), including aromatic substituents with different steric hindrance effects, were designed and successfully synthesized. Four complexes with twisted molecular conformation displayed intramolecular charge transfer and aggregation-induced emission properties. Under external mechanical stimuli, the as-synthesized powders of ts-Ph BF2, ts-Ph-NA BF2, and ts-TPA BF2 exhibited redshift fluorescence emission behaviors, and ts-Ph BF2 and ts-TPA BF2 could be recovered to original shifts by fuming, but ts-Ph-NA BF2 displayed irreversible switching. ts-2NA BF2 had no change during the grinding and fuming processes. The results indicated that the MFC behaviors could be attributed to the phase transformation between the well-defined crystalline and disordered amorphous states by X-ray diffraction measurement. Further research illustrated that ts-TPA BF2 with the most significant MFC phenomenon could be applied in data security protection in ink-free rewritable paper.

A Shape Memory Hydrogel with Excellent Mechanical Properties, Water Retention Capacity, and Tunable Fluorescence for Dual Encryption

Information encryption platforms with reliable encryption performance, excellent mechanical performance, and high water retention capacity are highly desired. In this study, a tough double-network hydrogel is designed using the first network of a polyion complex containing lanthanide complexes via one-pot polymerization and the second network of a poly (N-hydroxyethyl acrylamide) (PHEAA) obtained by deep eutectic solvent (DES)-assisted introduction and subsequent photopolymerization. In this system, the pH-induced shape memory function and pH-/wavelength-dependent fluorescence allow the use of the prepared hydrogel as a dual-encryption platform. Owing to its high response reversibility, the hydrogel-based platform exhibits both a high security level and the advantages of rewritability, reprogrammability, and reusability. Additionally, the excellent mechanical properties and water retention capacity owing to the solvent exchange process involving the low-volatility solvent DES and the resulting introduction of the second network of PHEAA offer high practical application value for the hydrogel-based dual encryption platform, demonstrating its potential for information security protection.

Suppressive Photochromism and Promotive Mechanochromism of Rhodamine Mechanophore by the Strategy of Poly(methyl acrylate)/Polyurethane Interpenetrating Polymer Network

As molecular design and the structure-property relationships of photochemical molecules established in the literature serve as a convenient reference for mechanophore exploration, many typical mechanophores suffer undesired responses to UV light or even sunlight in bulk polymers. We developed a strategy of a poly(methyl acrylate)/polyurethane (PMA/PU) interpenetrating polymer network (IPN) to suppress the photochromic property of the mechanophore and promote its mechanochromic property. A widely used rhodamine mechanophore (Rh-2OH) was first incorporated into polyurethane (P1). Then P1 was swollen in methyl acrylate and photopolymerized to prepare a PMA2.8/PU IPN (P2). Different from photo/force-responsive P1, P2 selectively responded to force because the low free volume in IPN greatly hinders photoisomerization of the rhodamine spirolactam, suggesting that a simple IPN strategy successfully resolves the giant problem of nonselective response to photo/force for photochromic mechanophores. Moreover, PMA/PU IPN enhanced the mechanical property, resulting in a higher mechanochemical activation ratio than PU, and the prestretching effect of PMA/PU IPN promoted the force sensitivity of rhodamine mechanophores significantly. We believe that the strategy can be applied to other mechanophores, promoting their application in more complicated environments.

Bioinspired Multistimuli-Induced Synergistic Changes in Color and Shape of Hydrogel and Actuator Based on Fluorescent Microgels

Fluorescent hydrogels have emerged as one of the most promising candidates for developing biomimetic materials and artificial intelligence owing to their unique fluorescence and responsive properties. However, it is still challenging to fabricate hydrogel that exhibits synergistic changes in fluorescence color and shape in response to multistimulus via a simple method. Herein, blue- and orange-emitting fluorescent microgels (MGs) both are designed and synthesized with pH-, thermal-, and cationic-sensitivity via one-step polymerization, respectively. The two fluorescent MGs are incorporated into transparent doubly crosslinked microgel (DX MG) hydrogels with a preset ratio. The DX MG hydrogels can tune the fluorescent color accompanied by size variation via subjecting to external multistimulus. Thus, DX MG hydrogels can be exploited for multiresponsive fluorescent bilayer actuators. The actuators can undergo complex shape deformation and color changes. Inspired by natural organisms, an artificial morning glory with color and size changes are showcased in response to buffer solutions of different pH values. Besides, an intelligent skin hydrogel, imitating natural calotes versicolor, by assembling four layers of DX MG with different ratios of MGs, is tailored. This work serves as an inspiration for the design and fabrication of novel biomimetic smart materials with synergistic functions.

Polyelectrolyte chain conformation matters in macroscopic supramolecular self-assembly

We demonstrate molecular-conformation-dependent macroscopic supramolecular self-assembly (MSA) driven by electrostatic interactions. Evidence from single molecular force spectroscopy reveals that polyelectrolytes modified on MSA component surfaces make MSA possible with a loop conformation, while those with a flat conformation lead to no assembly, which is attributed to distinct molecular mobility. We believe that this finding is also applicable in fundamental phenomena such as surface adsorption and adhesion regarding polymers.

Tunable Multicolor Lanthanide Supramolecular Assemblies with White Light Emission Confined by Cucurbituril[7]

Macrocyclic confinement-induced supramolecular luminescence materials have important application value in the fields of bio-sensing, cell imaging, and information anti-counterfeiting. Herein, a tunable multicolor lanthanide supramolecular assembly with white light emission is reported, which is constructed by co-assembly of cucurbit[7]uril (CB[7]) encapsulating naphthylimidazolium dicarboxylic acid (G1 )/Ln (Eu3+ /Tb3+ ) complex and carbon quantum dots (CD). Benefiting from the macrocyclic confinement effect of CB[7], the supramolecular assembly not only extends the fluorescence intensity of the lanthanide complex G1 /Tb3+ by 36 times, but also increases the quantum yield by 28 times and the fluorescence lifetime by 12 times. Furthermore, the CB[7]/G1 /Ln assembly can further co-assemble with CD and diarylethene derivatives (DAE) to realize the intelligently-regulated full-color spectrum including white light, which results from the competitive encapsulation of adamantylamine and CB[7], the change of pH, and photochromic DAE. The multi-level logic gate based on lanthanide supramolecular assembly is successfully applied in anti-counterfeiting system and information storage, providing an effective method for the research of new luminescent intelligent materials.

Multiresponsive Self-Healing Lanthanide Fluorescent Hydrogel for Smart Textiles

Lanthanide organometallic complexes exhibit strong luminescence characteristics, owing to their antenna effects. The f-d energy level transition causes this phenomenon, which occurs when ligands and the external electrons of lanthanide metals coordinate. Based on this phenomenon, we used two lanthanide metals, europium (Eu) and terbium (Tb), in the present study as the metal center for iminodiacetic acid ligands. Further, we developed the resulting fluorescent organometallic complex as a smart material. The ligand-metal bond in the material functioned as a metal chelating agent and a cross-linking agent in a dynamically coordinated form, thereby prompting the material to self-heal. Temperature-sensitive poly-N-isopropylacrylamide was incorporated into the material as the polymer backbone. Afterward, we combined it with water-soluble poly(vinyl alcohol) and an additional ligand from poly(acrylic acid) to fabricate a high-performance hydrogel composite material. The shrinkage and expansion of the polymer form a grid between the materials. Because of the different coordination stabilities of Eu3+ and Tb3+, the corresponding material exhibits environmental responses toward excitation wavelength, temperature, and pH, thus generating different colors. When used in fabrics, the cross-linking mechanism of the material effectively looped the material between fabric fibers; furthermore, the temperature sensitivity of the polymer adjusted the size of pores between fabric fibers. At relatively higher temperatures (>32 °C), the polymer structure shrank, fiber pores expanded, and air permeability improved. Thus, this material appears to be promising for use in smart textiles.

Competitive substitution in europium metal-organic gel for signal-on electrochemiluminescence detection of dipicolinic acid

Metal-organic gels (MOGs) emerged as an attractive luminescent soft material for electrochemiluminescence (ECL). In this work, a cathodic ECL-activated europium metal-organic gel (Eu-MOG) has been synthesized by a facile mixing of Eu3+ with 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine (Hcptpy) under mild conditions. The prepared Eu-MOG is highly mesoporous for co-reactant permeation to produce an ultra-stable and high-efficient ECL, based on the antenna effect of Eu3+ coordinating with Hcptpy. Moreover, dipicolinic acid (DPA) can competitively coordinate with Eu3+ instead of water molecules, producing an enhanced ECL signal. Therefore, an ECL enhancement assay was developed for DPA detection. There was a linear relationship between the ECL intensity and the logarithmic concentration of DPA in the 0.01-1 μM range, and the detection limit is 7.35 nM. This work displays the promising application of Eu-MOG in the ECL field, opening a broad inspection for seeking a new generation of ECL luminophores.

Reversible Crosslinking of Commodity Polymers via Photocontrolled Metal-Ligand Coordination for High-Performance and Recyclable Thermoset Plastics

Thermoset plastics, highly desired for their stability, durability, and chemical resistance, are currently consumed in over 60 million tons annually across the globe, but they are difficult to recycle due to their crosslinked structures. The development of recyclable thermoset plastics is an important but challenging task. In this work, recyclable thermoset plastics are prepared by crosslinking a commodity polymer, polyacrylonitrile (PAN), with a small percentage of a Ru complex via nitrile-Ru coordination. PAN is obtained from industry and the Ru complex is synthesized in one step, which enables the production of recyclable thermoset plastics in an efficient way. In addition, the thermoset plastics exhibit impressive mechanical performance, boasting a Young's modulus of 6.3 GPa and a tensile strength of 109.8 MPa. Moreover, they can be de-crosslinked when exposed to both light and a solvent and can then be re-crosslinked upon heating. This reversible crosslinking mechanism enables the recycling of thermosets from a mixture of plastic waste. The preparation of recyclable thermosets from other commodity polymers such as poly(styrene-coacrylonitrile) (SAN) resins and polymer composites through reversible crosslinking is also demonstrated. This study shows that reversible crosslinking via metal-ligand coordination is a new strategy for designing recyclable thermosets using commodity polymers.

Eu3+ Complex-Based Superhydrophobic Fluorescence Sensor for Cr(VI) Detection in Water

Cr(VI) compounds are bioaccumulative and highly toxic pollutants, and there is a need for simple and fast detection methods to monitor their trace levels. In this work, we developed a Eu3+ complex-based fluorescence sensor to easily detect Cr(VI) in water droplets. Our sensor consists of a nanofibrous membrane electrospun with a blend of polyvinylidene fluoride (PVDF), silica particles, and Eu3+ complex. Upon modifying the membrane surface with fluoroalkyl chemistry, the sensor displayed superhydrophobicity. When a water droplet with Cr(VI) was placed on such a superhydrophobic fluorescence sensor, the overlapping absorption of Cr(VI) and Eu3+ complex facilitated the inner filter effect, allowing the selective detection of Cr(VI) down to 0.44 µM (i.e., 45.76 µg L-1). We proposed and designed of new inexpensive and fast sensor for the detection of Cr(VI).

Stabilizing Liquids Using Interfacial Supramolecular Assemblies

Stabilizing liquids based on supramolecular assembly (non-covalent intermolecular interactions) has attracted significant interest, due to the increasing demand for soft, liquid-based devices where the shape of the liquid is far from the equilibrium spherical shape. The components comprising these interfacial assemblies must have sufficient binding energies to the interface to prevent their ejection from the interface when the assemblies are compressed. Here, we highlight recent advances in structuring liquids based on non-covalent intermolecular interactions. We describe some of the progress made that reveals structure-property relationships. In addition to treating advances, we discuss some of the limitations and provide a perspective on future directions to inspire further studies on structured liquids based on supramolecular assembly.

A Photopatternable Conjugated Polymer with Thermal-Annealing-Promoted Interchain Stacking for Highly Stable Anti-Counterfeiting Materials

Photoresponsive polymers can be conveniently used to fabricate anti-counterfeiting materials through photopatterning. However, an unsolved problem is that ambient light and heat can damage anti-counterfeiting patterns on photoresponsive polymers. Herein, photo- and thermostable anti-counterfeiting materials are developed by photopatterning and thermal annealing of a photoresponsive conjugated polymer (MC-Azo). MC-Azo contains alternating azobenzene and fluorene units in the polymer backbone. To prepare an anti-counterfeiting material, an MC-Azo film is irradiated with polarized blue light through a photomask, and then thermally annealed under the pressure of a photonic stamp. This strategy generates a highly secure anti-counterfeiting material with dual patterns, which is stable to sunlight and heat over 200 °C. A key for the stability is that thermal annealing promotes interchain stacking, which converts photoresponsive MC-Azo to a photostable material. Another key for the stability is that the conjugated structure endows MC-Azo with desirable thermal properties. This study shows that the design of photopatternable conjugated polymers with thermal-annealing-promoted interchain stacking provides a new strategy for the development of highly stable and secure anti-counterfeiting materials.

Pathways toward wearable and high-performance sensors based on hydrogels: toughening networks and conductive networks

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors.

Aqueous coordination polymer complexes: From colloidal assemblies to bulk materials

1-dimensional (1D) coordination polymers refer to the macromolecules that have metal ions incorporated in their pendent groups or main chain through metal-binding ligand groups. They have intrinsic advantages over traditional polymers to regulate the polymer structures and functions owing to the nature of the metal-ligand bond. Consequently, they have great potential for the development of smart and functional structures and materials and therapeutic agents. Water-soluble 1D coordination polymers and assemblies are an important subtype of coordination polymers with distinctive interests for demanding applications in aqueous systems, such as biological and medical applications. This review highlights the recent progress and research achievements in the design and use of water-soluble 1D coordination polymers and assemblies. The overview covers the design and structure control of 1D coordination polymers, their colloidal assemblies, including nanoparticles, nanofibers, micelles and vesicles, and fabricated bulk materials such as membraneless liquid condensates, security ink, hydrogel actuators, and smart fabrics. Finally, we discuss the potential applications of several of these coordination polymeric structures and materials and give an outlook on the field of aqueous coordination polymers.

A pillar[5]arene noncovalent assembly boosts a full-color lanthanide supramolecular light switch

A photo-responsive full-color lanthanide supramolecular switch was constructed from a synthetic 2,6-pyridine dicarboxylic acid (DPA)-modified pillar[5]arene (H) complexing with lanthanide ion (Ln3+ = Tb3+ and Eu3+) and dicationic diarylethene derivative (G1) through a noncovalent supramolecular assembly. Benefiting from the strong complexation between DPA and Ln3+ with a 3 : 1 stoichiometric ratio, the supramolecular complex H/Ln3+ presented an emerging lanthanide emission in the aqueous and organic phase. Subsequently, a network supramolecular polymer was formed by H/Ln3+ further encapsulating dicationic G1via the hydrophobic cavity of pillar[5]arene, which greatly contributed to the increased emission intensity and lifetime, and also resulted in the formation of a lanthanide supramolecular light switch. Moreover, full-color luminescence, especially white light emission, was achieved in aqueous (CIE: 0.31, 0.32) and dichloromethane (CIE: 0.31, 0.33) solutions by the adjustment of different ratios of Tb3+ and Eu3+. Notably, the photo-reversible luminescence properties of the assembly were tuned via alternant UV/vis light irradiation due to the conformation-dependent photochromic energy transfer between the lanthanide and the open/closed-ring of diarylethene. Ultimately, the prepared lanthanide supramolecular switch was successfully applied to anti-counterfeiting through the use of intelligent multicolored writing inks, and presents new opportunities for the design of advanced stimuli-responsive on-demand color tuning with lanthanide luminescent materials.

Room-Temperature Self-Healing Glassy Luminescent Hybrid Film

The self-healing of glassy polymer materials on site has always been a huge challenge due to their frozen polymer network. We herein report self-repairable glassy luminescent film by assembling a lanthanide-containing polymer with randomly hyperbranched polymers possessing multiple hydrogen (H) bonds. Because of multiple H bonds, the hybrid film exhibits enhanced mechanical strength, with high glass transition temperature (T_g) of 40.3 °C and high storage modulus of 3.52 GPa, meanwhile, dynamic exchange of multiple H bonds enables its rapid room-temperature self-healing ability. This research provides new insights in preparing mechanical robust yet repairable polymeric functional materials.

A rapid self-healing glassy polymer/metal-organic-framework hybrid membrane at room temperature

The development of repairable MOF-polymer hybrid materials will greatly extend their service life by repairing fractured parts on the spot; however, it is difficult for robust glassy polymers to self-heal below the glass transition temperature (T_g) as the polymer network is frozen. We herein report glassy polyMOF-RHP hybrid membranes by integrating lanthanide polyMOF (polyLnMOF) with randomly hyperbranched polymers (RHP) bearing a high density of hydrogen bonds. Since crystalline lanthanide MOFs act as multiconnected cross-linking agents and cross-link the interpenetrating polymer network, the obtained polyLnMOF-polymer membrane shows enhanced mechanical strength with a storage modulus of 3.09 GPa and a T_g up to 49 °C. Meanwhile, the high intersegment migration ability of the polyLnMOF-polymer network facilitates the exchange of hydrogen-bonded pairs even in the glassy state, leading to an instantaneous room-temperature self-healing ability. The polyLnMOF-polymer membranes inherit the ratiometric temperature-sensing behavior of pristine lanthanide MOFs, resulting in more processable temperature-sensing membranes. This work provides an appealing strategy for the design of mechanically robust, yet self-healing, MOF-polymer functional materials.

Organic‒inorganic semi-interpenetrating networks with orthogonal light- and magnetic-responsiveness for smart photonic gels

Living matter has the ability to perceive multiple stimuli and respond accordingly. However, the integration of multiple stimuli-responsiveness in artificial materials usually causes mutual interference, which makes artificial materials work improperly. Herein, we design composite gels with organic‒inorganic semi-interpenetrating network structures, which are orthogonally responsive to light and magnetic fields. The composite gels are prepared by the co-assembly of a photoswitchable organogelator (Azo-Ch) and superparamagnetic inorganic nanoparticles (Fe₃O₄@SiO₂). Azo-Ch assembles into an organogel network, which shows photoinduced reversible sol-gel transitions. In gel or sol state, Fe₃O₄@SiO₂ nanoparticles reversibly form photonic nanochains via magnetic control. Light and magnetic fields can orthogonally control the composite gel because Azo-Ch and Fe₃O₄@SiO₂ form a unique semi-interpenetrating network, which allows them to work independently. The orthogonal photo- and magnetic-responsiveness enables the fabrication of smart windows, anti-counterfeiting labels, and reconfigurable materials using the composite gel. Our work presents a method to design orthogonally stimuli-responsive materials.

Mechanical tough and multicolor aggregation-induced emissive polymeric hydrogels for fluorescent patterning

Aggregation-induced emissive fluorogens (AIEgens) are promising building blocks for fluorescent polymeric hydrogels (FPHs) because intense fluorescence intensities are usually guaranteed by spontaneous aggregates of hydrophobic AIEgens in a hydrophilic polymer network. However, most AIE-active FPHs are single-color fluorescent and cannot display tunable emission colors. Additionally, efforts to produce mechanically strong AIE-active hydrogels have been largely ignored, restricting their potential uses. Herein, we present the synthesis of an AIE-active methyl picolinate-substituted 1,8-naphthalimide monomer (MP-NI) for fabricating mechanical tough and multicolor FPHs. Owing to the introduction of bulky and coordinative methyl picolinate group, these specially designed MP-NI molecules were forced to adopt propeller-shaped conformation that renders them with intense aggregation-induced blue emission. Moreover, the MP-NI moieties grafted in a hydrogel matrix can sensitize red and green fluorescence of Eu³⁺and Tb³⁺ via antenna effect. Consequently, multicolor fluorescent hydrogels that sustain a high stress of 1 MPa were obtained by chemically introducing MP-NI moieties into dually cross-linked alginate polymer networks with high-density metal (Ca²⁺/Tb³⁺/Eu³⁺) coordination and hydrogen bonding crosslinks. Their capacity to enable the writing of arbitrary multicolor fluorescent patterns using Eu³⁺/Tb³⁺ as inks were finally demonstrated, suggesting their potential uses for smart display and information encryption.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Design of a large Stokes shift ratiometric fluorescent sensor with hypochlorite detection towards the potential application as invisible security ink

Hypochlorite (ClO^-) as a well-known highly reactive oxygen species (ROS), is widely used as preservative and household disinfectant in daily life. Although many fluorescence imaging sensors for ClO^- have been reported, the development of ClO^- ratio fluorescence sensors with large Stokes shift is still quite limited. This sensor shows obvious benefits including minimizing environmental intervention and improving signal-to-noise ratio. In the present project, we report an innovative conjugated pyrene-based system, 1-B, as a chlorine fluorescence sensor. The detector exhibits ratio detection performance, large Stokes and emission shifts. Furthermore, the system has desired sensitivity as well as selectivity for ClO^-. Based on these excellent properties, the sensor 1-B was successfully used as ink to encrypt patterns and anti-counterfeiting information through inkjet printing technology. Compared with the existing probes, the probe shows some superior characteristics, which provides a promising tool for exploring the role of ClO^- response sensor in the field of anti-counterfeiting.

Macroscopic Supramolecular Assembly of Rigid Building Blocks Facilitated by Layer-By-Layer Assembled Microgel Film

Macroscopic supramolecular assembly (MSA) of building blocks larger than 1 μm provides new methodology for fabrication of functional supramolecular materials and a platform for mechanism investigation of interfacial phenomena. Most reports on MSA are restricted to soft hydrogels, and supramolecular groups can be directly integrated into a hydrogel matrix to generate sufficient attraction for maintaining macroscopic assemblies. For non-hydrogel stiff building blocks, two layer-by-layer modification processes consisting of flexible spacing coating and additional interacting groups are necessary to enable MSA, which is laborious and time-consuming. Approaches for highly efficient MSA based on flexible spacing coating are desired. In this work, MSA of polydimethylsiloxane (PDMS) building blocks is demonstrated by inducing microgel films that serve as both flexible spacing coating and surface functional groups, thus avoiding a two-step LbL modification process. By the varying bilayer number of microgel films, the MSA probability of modified PDMS increases from 54% at 3 bilayers to 100% at 6 bilayers. Control experiments and in situ force measurement strongly support the obtained MSA results and verify the dominant role of the microgel film as a flexible spacing coating and a supramolecularly interactive layer in achieving MSA. Moreover, the underlying mechanism is interpreted as low Young's modulus microgel films rendering surface groups highly mobile to enhance the multivalent interfacial binding. Taken together, this work has demonstrated the feasibility of MSA of rigid building blocks assisted by microgel films as flexible spacing coating and supramolecularly interactive layer simultaneously, which may extend the application fields of microgel materials to interfacial adhesion and advanced manufacturing with MSA methodology.

Thermoresponsive Self-Healing Zwitterionic Hydrogel as an In Situ Gelling Wound Dressing for Rapid Wound Healing

It is highly desired yet challenging to fabricate biocompatible injectable self-healing hydrogels with anti-bacterial adhesion properties for complex wounds that can autonomously adapt to different shapes and depths and can promote angiogenesis and dermal collagen synthesis for rapid wound healing. Herein, an injectable zwitterionic hydrogel with excellent self-healing property, good cytocompatibility, and antibacterial adhesion was developed from a thermoresponsive ABA triblock copolymer poly[(N-isopropyl acrylamide)-co-(butyl acrylate)-co-(sulfobetaine methacrylate)]-b-poly(ethylene glycol)-b-poly[(N-isopropyl acrylamide)-co-(butyl acrylate)-co-(sulfobetaine methacrylate)] (PZOPZ). The prepared PZOPZ hydrogel exhibits a distinct thermal-induced sol-gel transition around physiological temperature and could be easily applied in a sol state and in situ gelled to adapt complex wounds of different shapes and depths for complete coverage. Meanwhile, the hydrogel possesses a rapid self-healing ability and can recover autonomously from damage to maintain structural and functional integrity. In addition, the CCK-8 and 2D/3D cell culture experiments revealed that the PZOPZ hydrogel dressing shows low cytotoxicity to L929 cells and can effectively prevent the adhesion of Staphylococcus aureus and Escherichia coli. In vivo investigations verified that the PZOPZ hydrogel could increase angiogenesis and dermal collagen synthesis and shorten the transition from the inflammatory to the proliferative stage, thereby providing more favorable conditions for faster wound healing. Overall, this work provides a promising strategy to develop injectable zwitterionic hydrogel dressings with multiple functions for clinic wound management.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Construction and Ion Transport-Related Applications of the Hydrogel-Based Membrane with 3D Nanochannels

Hydrogel is a type of crosslinked three-dimensional polymer network structure gel. It can swell and hold a large amount of water but does not dissolve. It is an excellent membrane material for ion transportation. As transport channels, the chemical structure of hydrogel can be regulated by molecular design, and its three-dimensional structure can be controlled according to the degree of crosslinking. In this review, our prime focus has been on ion transport-related applications based on hydrogel materials. We have briefly elaborated the origin and source of hydrogel materials and summarized the crosslinking mechanisms involved in matrix network construction and the different spatial network structures. Hydrogel structure and the remarkable performance features such as microporosity, ion carrying capability, water holding capacity, and responsiveness to stimuli such as pH, light, temperature, electricity, and magnetic field are discussed. Moreover, emphasis has been made on the application of hydrogels in water purification, energy storage, sensing, and salinity gradient energy conversion. Finally, the prospects and challenges related to hydrogel fabrication and applications are summarized.

Mimicking Color-Changing Organisms to Enable the Multicolors and Multifunctions of Smart Fluorescent Polymeric Hydrogels

Fluorescent polymer hydrogels (FPHs) are of significant interest for diverse emerging applications such as visualized sensing, smart display, camouflaging skins, soft actuators/robots, because they can synergize the features of classic fluorescent polymers and hydrogels. With great efforts in the past decades, the major challenge in this field has been believed to be not whether a given FPH of interest can be prepared but how to fabricate robust FPHs with multicolor tunability and multifunctional synergy. Such materials will conceptually minimize the contribution of passive materials to the mass and size of the final system, holding great potential to facilitate multiple applications. To this end, one promising way is to learn from the Nature that has superb capability to forge delicate or sometimes beyond-imagination materials. Chameleons and cephalopods serve as typical examples, which are famous for not only diverse skin color adaptability under changing environmental demands, but also synergistic skin color and body gesture changes to communicate, warn, camouflage, etc. Biological studies revealed their structural color-changing capacity derives from different types of skin chromatophores and their rational multilayer arrangement in under-skin tissues. Besides, their superb ability to heterogeneously integrate soft tissues with disparate functions into topology-optimized architectures has led to various multifunctional performances. Such natural strategies, if replicated and implemented in artificial systems, would significantly benefit and advance the development of robust FPHs for various applications.In this Account, we summarizes the key advances of smart FPHs mainly achieved by our groups. We start by introducing the unique hierarchical multilayer structures of skin chromatophores in structural color-changing reptiles, followed by an in-depth discussion on how a rational integration of bioinspiration and man-made design makes it possible to largely expand the fluorescence color-changing range of smart FPHs to almost cover the whole visible spectrum. Then, to closely mimic the multifunctional behaviors of chameleons and cephalopods, we further develop efficient strategies to introduce supramolecular interactions or heterogeneously integrating smart FPHs with other soft materials with disparate functions, producing a number of multifunctional fluorescent polymeric hydrogel systems. These robust FPHs can find many frontier applications, including bioinspired synergistic color/shape switchable hydrogel actuators/robots, smart systems with on-demand fluorescent patterning capacities for displaying or information encryption, as well as robust chemosensors for important food or environmental analytes. We expect that the discussion presented in this Account would promote better understanding of the discoloration systems in nature, and advance the development of bioinspired color-changing materials.

Designing Rewritable Dual-Mode Patterns using a Stretchable Photoresponsive Polymer via Orthogonal Photopatterning

The fabrication of dual-mode patterns in the same region of a material is a promising approach for high-density information storage, new anti-counterfeiting technologies, and highly secure encryption. However, dual-mode patterns are difficult to achieve because the two patterns in one material may interfere with each other during fabrication and usage. The development of noninterfering dual-mode patterns requires new materials and patterning techniques. Herein, a novel orthogonal photopatterning technique is reported for the fabrication of noninterfering dual-mode patterns on an azopolymer P1. P1 is a unique material that exhibits both photoinduced reversible solid-to-liquid transitions and good stretchability. In the first step of orthogonal photopatterning, patterned photonic structures are fabricated on a P1 film via masked nanoimprinting controlled by photoinduced reversible solid-to-liquid transitions. In the second step, the P1 film is stretched and irradiated with polarized light through a photomask, which generates a chromatic polarization pattern. In particular, the photonic structures and chromatic polarization in the dual-mode pattern are noninterfering. Another feature of dual-mode patterns is that they are rewritable via photo-, thermal, or solution reprocessing, which are useful for recycling and reprogramming. This study opens an avenue for the development of novel materials and techniques for photopatterning.

Double hydrophilic copolymers - synthetic approaches, architectural variety, and current application fields

Solubility and functionality of polymeric materials are essential properties determining their role in any application. In that regard, double hydrophilic copolymers (DHC) are typically constructed from two chemically dissimilar but water-soluble building blocks. During the past decades, these materials have been intensely developed and utilised as, e.g., matrices for the design of multifunctional hybrid materials, in drug carriers and gene delivery, as nanoreactors, or as sensors. This is predominantly due to almost unlimited possibilities to precisely tune DHC composition and topology, their solution behavior, e.g., stimuli-response, and potential interactions with small molecules, ions and (nanoparticle) surfaces. In this contribution we want to highlight that this class of polymers has experienced tremendous progress regarding synthesis, architectural variety, and the possibility to combine response to different stimuli within one material. Especially the implementation of DHCs as versatile building blocks in hybrid materials expanded the range of water-based applications during the last two decades, which now includes also photocatalysis, sensing, and 3D inkjet printing of hydrogels, definitely going beyond already well-established utilisation in biomedicine or as templates.

A review of recent advances in metal ion hydrogels: mechanism, properties and their biological applications

The mechanisms, common properties and biological applications of different types of metal ion hydrogels are summarized.

A tunable full-color lanthanide noncovalent polymer based on cucurbituril-mediated supramolecular dimerization

The construction of lanthanide multicolor luminescent materials with tunable photoluminescence properties has been developed as one of the increasingly significant topics and shown inventive applications in miscellaneous fields. However, fabricating such materials based on synergistically assembly-induced emission rather than simple blending of different fluorescent dyes together still remains a challenge. Herein, we report a europium-based noncovalent polymer with tunable full-color emission, which is constructed from the 2,6-pyridinedicarboxylic acid-bearing bromophenylpyridinium salt. This rationally designed bifunctional component can concurrently serve as a guest molecule and a chelating ligand to associate with cucurbit[8]uril and europium ions, thus leading to the formation of a trichromatic (red–green–blue, RGB) photoluminescent polypseudorotaxane-type noncovalent polymer in aqueous solution. Meanwhile, the full-color emission enclosed within the RGB color triangle could be readily produced by simply tuning the molar ratio of cucurbit[8]uril and europium ions. The lanthanide supramolecular polymer featuring tricolor emission, long lifetime, high photoluminescence efficiency and low cytotoxicity could be further applied in multicolor imaging in a cellular environment. These results provide a new and feasible strategy for the construction of full-color single lanthanide self-assembled nanoconstructs.

A lanthanide noncovalent polymer is constructed by integrating host–guest complexation and metal–ligand coordination, and can exhibit tunable trichromatic emission and multiple excited-state lifetimes under single wavelength excitation.

Photoinduced synthesis of fluorescent hydrogels without fluorescent monomers

A fluorescent monomer-free one-step strategy is developed for the synthesis of fluorescent acrylamide gels, using inexpensive and commercially available rhodamine B as the hydrogen donor in type II photoinitiation system....

Facile Preparation of a Rhodamine B Derivative-Based Fluorescent Probe for Visual Detection of Iron Ions

Iron ions play an important role in our lives. Excessive or lack of iron ion intake leads to many diseases. At the same time, the water environment is easily polluted by these metal ions with the acceleration of industrialization. Therefore, the detection of iron ions in the water environment and the human body is particularly important. In this paper, we prepared a RhB-EDA fluorescent probe by condensing rhodamine B (RhB) with ethylenediamine (EDA) for high recognition of Fe3+. A RhB-EDA molecule itself is colorless and has no fluorescence emission in an alcohol solution. When Fe3+ was added, the lactam ring structure of the fluorescent probe opened, and the UV and fluorescence spectra changed. At the same time, the color of the mixed solution gradually deepened toward pink. Therefore, dual spectral detection and naked-eye observation of Fe3+ were realized. In addition, with the decrease of the pH value and the prolongation of chelating time, the ultraviolet absorbance and fluorescence emission intensity were enhanced and the color of the mixed solution deepened. The RhD-EDA fluorescent probe is simple and accurate and provides good technical support for the detection of Fe3+.

Metal Cation-Responsive and Excitation-Dependent Nontraditional Multicolor Fluorescent Hydrogels for Multidimensional Information Encryption

Fluorescent polymeric hydrogels especially multicolor fluorescent polymeric hydrogels (MFPHs) have important applications in information storage, encryption, and encoding. MFPHs are generally prepared by incorporating multiple traditional fluorescent materials into hydrogels. In recent years, nontraditional luminescent polymers without any traditional π-conjugated chromophores have received increasing attention. Here, we report a novel type of nontraditional MFPHs prepared by in situ polymerization of acrylamide (AAm) in the presence of poly(itaconic acid) (PITAc). The hydrogen-bonded mechanically strong PAAm/PITAc hydrogels show strong intrinsic fluorescence, and the fluorescence emission is excitation-dependent and metal cation-responsive. More impressively, the hydrogels treated with metal cations also possess excitation-dependent fluorescence. We developed a multi-ion inkjet printing (MIIP) technique to print texts or designed patterns onto the hydrogel surface using different metal cation solutions as inks, and then variable texts or patterns appear under the irradiation of UV, violet, and blue lights. Patterns can be further changed by selective printing, erasing, or reprinting on some regions. Therefore, multidimensional information encryption is achieved. This work provides a new strategy for preparing MFPHs for wide applications.

Ru-Se Coordination: A New Dynamic Bond for Visible-Light-Responsive Materials

Photodynamic bonds are stable in the dark and can reversibly dissociate/form under light irradiation. Photodynamic bonds are promising building blocks for responsive or healable materials, photoactivated drugs, nanocarriers, extracellular matrices, etc. However, reactive intermediates from photodynamic bonds usually lead to side reactions, which limit the use of photodynamic bonds. Here, we report that the Ru-Se coordination bond is a new photodynamic bond that reversibly dissociates under mild visible-light-irradiation conditions. We observed that Ru-Se bonds form via the coordination of a selenoether ligand with [Ru(tpy)(biq)(H₂O)]Cl₂ (tpy = 2,2':6',2″-terpyridine, biq = 2,2'-biquinoline) in the dark, while the Ru-Se bond reversibly dissociates under visible-light irradiation. No side reaction is detected in the formation and dissociation of Ru-Se bonds. To demonstrate that the Ru-Se bond is applicable to different operating environments, we prepared photoresponsive amphiphiles, surfaces, and polymer gels using Ru-Se bonds. The amphiphiles with Ru-Se bonds showed reversible morphological transitions between spherical micelles and bowl-shaped assemblies for dark/light irradiation cycles. The surfaces modified with Ru-Se-bond-containing compounds showed photoswitchable wettability. Polymer gels with Ru-Se cross-links underwent photoinduced reversible sol-gel transitions, which can be used for reshaping and healing. Our work demonstrates that the Ru-Se bond is a new type of dynamic bond, which can be used for constructing responsive, reprocessable, switchable, and healable materials that work in a variety of environments.

Smart luminescent hydrogel with superior mechanical performance based on polymer networks embedded with lanthanide containing clay nanocomposites

Due to their inherent biocompatibility and unique optical properties, luminescent hydrogels have recently garnered tremendous interest. However, most of the explored luminescent hydrogels cannot well meet the requirements of both sufficient mechanical properties and smart luminescence. Here we report a smart luminescent hydrogel with superior mechanical performance by in situ copolymerization of the acrylamide monomers and lanthanide loaded clay nanosheets. The Eu-DPA@clay nanosheets emitting center can not only be well dispersed in aqueous solution, but also maintain robust luminescence. Interestingly, the additional interaction between the Eu-DPA@clay nanosheets and the polymer network endows the hydrogel with excellent mechanical properties. Moreover, a luminescence on/off switch is also achieved by alternative acid and base stimuli, which may have potential applications in smart luminescent devices.

Carboxymethyl chitosan‒promoted luminescence of lanthanide metallogel and its application in assay of multiple metal ions

In this work, the luminescence of lanthanide supramolecular metallogel formed by the self-assembly of 5,5',5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate (H₆L) and Tb³⁺ was efficiently promoted by carboxymethyl chitosan (CMCS). The total quantum yield of the resultant metallogel (denoted as H₆L/Tb³⁺/CMCS gel) was 9 times higher than the gel without CMCS. The average lifetime of H₆L/Tb³⁺/CMCS gel increased from 0.51 ms to 1.20 ms. More importantly, the aqueous dispersion of H₆L/Tb³⁺/CMCS xerogels showed a stable and pH-dependent luminescence. Based on the selective affinity of CMCS to different metal ions as well as with the aid of principal component analysis, H₆L/Tb³⁺ /CMCS can be used as a sensor array to distinguish 11 metal ions (P < 0.05). This work provides a new strategy for the design and development of bio-based functional luminescent lanthanide supramolecular metallogels.

Promotion of Color-Changing Luminescent Hydrogels from Thermo to Electrical Responsiveness toward Biomimetic Skin Applications

The active color-changing ability of many living species has inspired scientists to replicate the optical property into soft wet and tissue-like hydrogel materials. However, the color-changing processes of most reported examples are controlled by the traditional stimuli (e.g., pH, temperature, and ions), which may suffer from the residual chemical product accumulation, and have difficulty in achieving local control and integration into the commercial robots, especially when applied as biomimetic skins. Herein, inspired by the nervous (bioelectricity) control of skin color change in cephalopods, we present an electrically powered multicolor fluorescent hydrogel system with asymmetric configuration that couples thermoresponsive fluorescent hydrogel with stacked graphene assembly (SGA)-based conductive paper through luminous paint as the middle layer. Owing to the highly controllable electrical stimulus in terms of amplitude and duration, the Joule heat supplied by SGA film can be regulated locally and in real time, leading to precise and local emission color control at low voltage. It also avoids the addition of any chemicals. Furthermore, the electrically powered color-changing hydrogel system can be conveniently integrated into the commercial robots as biomimetic skins that help them achieve desirable camouflage, display, or alarming functions.