Spiropyran Photoisomerization Dynamics in Multiresponsive Hydrogels

Photochromic Spiropyran-Based Dual-Emitting Luminescent Hybrid Films for Dynamic Information Anticounterfeiting

Photoluminescent materials are widely used for information storage and anticounterfeiting, while most of them have the disadvantages of static information performance and weak processability, which is still a challenging task in developing dynamic anticounterfeiting materials with high security levels. Herein, we fabricated a novel photostimuli-responsive dual-emitting luminescent material UPTES-SPn-Tb-hfa, which was obtained by introducing the photochromic molecule spiropyran (SP) and lanthanide complex (Tb-hfa) into a siloxane-polyether matrix using the sol-gel process. Due to the conformation-dependent photochromic fluorescence resonance energy transfer between the Tb-hfa donor and SP acceptor, the ring-closing (SP)/ring-opening (MC) isomerization of the SP unit leads to a reversible luminescence switching in UPTES-SPn-Tb-hfa. This composite material has great potential for advanced anticounterfeiting because of the advantage of rapidly repeatable encryption/decryption for at least 8 times and dynamic luminescent colors within 15 s. In addition, due to its two luminescent centers (Tb3+ and MC), the luminescent color of this material can be regulated by 254 and 365 nm UV-light irradiation, which facilitates the design of multicolored anticounterfeiting labels. Our work presents a novel design methodology to fabricate dynamic anticounterfeiting materials, significantly enhancing the security of anticounterfeiting applications.

Light- and Solvent-Responsive Bilayer Hydrogel Actuators with Reversible Bending Behaviors

Light-responsive hydrogel systems have gained significant attention due to their unique ability to undergo controlled and reversible swelling behavior in response to light stimuli. Combining light-responsive hydrogels with nonresponsive polymers offers a unique self-folding feature that can be used in soft robotic actuator designs. However, simple formulation of such systems with rapid response time is still a challenging task. Herein, we demonstrate a simple but versatile bilayer polymeric design combining light-responsive spiropyran-polyacrylamide (SP-PAAm) with polyacrylamide (PAAm) hydrogels. The photochromic spiropyran in our polymer design is a closed-ring, hydrophobic compound and turns into an open-ring, hydrophilic merocyanine isomer under light irradiation. The swelling degree of SP-PAAm and PAAm hydrogels was evaluated using LED lights with different wavelengths and solvent media (e.g., water, ethanol, DMF, and DMSO). We observed that SP-PAAm hydrogels reached a swelling ratio of ∼370% with the illumination of the blue LED in the DMF medium. By combining light-responsive SP-PAAm hydrogels with nonresponsive PAAm, a proof-of-concept demonstration was performed to demonstrate the applicability of our fabricated platforms. Although fabricated one-armed bilayer hydrogels possessed self-folding ability with a folding angle of ∼40° in 30 min, the four-armed bilayer platforms demonstrated more efficient and rapid folding behavior and reached a folding angle of ∼75° in ∼15 min. Given their simplicity and efficiency, we believe that such polymeric designs may offer new avenues for the fields of polymeric actuators and soft robotic systems.

Recent Development of Photochromic Polymer Systems: Mechanism, Materials, and Applications

Photochromic polymer is defined as a series of materials based on photochromic units in polymer chains, which produces reversible color changes under irradiation with a particular wavelength. Currently, as the research progresses, it shows increasing potential applications in various fields, such as anti-counterfeiting, information storage, super-resolution imaging, and logic gates. However, there is a paucity of published reviews on the topic of photochromic polymers. Herein, this review discusses and summarizes the research progress and prospects of such materials, mainly summarizing the basic mechanisms, classification, and applications of azobenzene, spiropyran, and diarylethene photochromic polymers. Moreover, 3-dimensional (3D) printable photochromic polymers are worthy to be summarized specifically because of its innovative approach for practical application; meanwhile, the developing 3D printing technology has shown increasing potential opportunities for better applications. Finally, the current challenges and future directions of photochromic polymer materials are summarized.

Photoresponsive Hydrogels for Tissue Engineering

Hydrophilic and biocompatible hydrogels are widely applied as ideal scaffolds in tissue engineering. The "smart" gelation material can alter its structural, physiochemical, and functional features in answer to various endo/exogenous stimuli to better biomimic the endogenous extracellular matrix for the engineering of cells and tissues. Light irradiation owns a high spatial-temporal resolution, complete biorthogonal reactivity, and fine-tunability and can thus induce physiochemical reactions within the matrix of photoresponsive hydrogels with good precision, efficiency, and safety. Both gel structure (e.g., geometry, porosity, and dimension) and performance (like conductivity and thermogenic or mechanical properties) can hence be programmed on-demand to yield the biochemical and biophysical signals regulating the morphology, growth, motility, and phenotype of engineered cells and tissues. Here we summarize the strategies and mechanisms for encoding light-reactivity into a hydrogel and demonstrate how fantastically such responsive gels change their structure and properties with light irradiation as desired and thus improve their applications in tissue engineering including cargo delivery, dynamic three-dimensional cell culture, and tissue repair and regeneration, aiming to provide a basis for more and better translation of photoresponsive hydrogels in the clinic.

Metal-organic Frameworks in Semiconductor Devices

Metal-organic frameworks (MOFs) are a specific class of hybrid, crystalline, nano-porous materials made of metal-ion-based 'nodes' and organic linkers. Most of the studies on MOFs largely focused on porosity, chemical and structural diversity, gas sorption, sensing, drug delivery, catalysis, and separation applications. In contrast, much less reports paid attention to understanding and tuning the electrical properties of MOFs. Poor electrical conductivity of MOFs (~10-7-10-10 S cm-1), reported in earlier studies, impeded their applications in electronics, optoelectronics, and renewable energy storage. To overcome this drawback, the MOF community has adopted several intriguing strategies for electronic applications. The present review focuses on creatively designed bulk MOFs and surface-anchored MOFs (SURMOFs) with different metal nodes (from transition metals to lanthanides), ligand functionalities, and doping entities, allowing tuning and enhancement of electrical conductivity. Diverse platforms for MOFs-based electronic device fabrications, conductivity measurements, and underlying charge transport mechanisms are also addressed. Overall, the review highlights the pros and cons of MOFs-based electronics (MOFtronics), followed by an analysis of the future directions of research, including optimization of the MOF compositions, heterostructures, electrical contacts, device stacking, and further relevant options which can be of interest for MOF researchers and result in improved devices performance.

Solvation-Tuned Photoacid as a Stable Light-Driven pH Switch for CO2 Capture and Release

Photoacids are organic molecules that release protons under illumination, providing spatiotemporal control of the pH. Such light-driven pH switches offer the ability to cyclically alter the pH of the medium and are highly attractive for a wide variety of applications, including CO2 capture. Although photoacids such as protonated merocyanine can enable fully reversible pH cycling in water, they have a limited chemical stability against hydrolysis (<24 h). Moreover, these photoacids have low solubility, which limits the pH-switching ability in a buffered solution such as dissolved CO2. In this work, we introduce a simple pathway to dramatically increase stability and solubility of photoacids by tuning their solvation environment in binary solvent mixtures. We show that a preferential solvation of merocyanine by aprotic solvent molecules results in a 60% increase in pH modulation magnitude when compared to the behavior in pure water and can withstand stable cycling for >350 h. Our results suggest that a very high stability of merocyanine photoacids can be achieved in the right solvent mixtures, offering a way to bypass complex structural modifications of photoacid molecules and serving as the key milestone toward their application in a photodriven CO2 capture process.

Sustainable lithium extraction enabled by responsive metal-organic frameworks with ion-sieving adsorption effects

Metal-organic frameworks (MOFs) are superior ion adsorbents for selectively capturing toxic ions from water. Nevertheless, they have rarely been reported to have lithium selectivity over divalent cations due to the well-known flexibility of MOF framework and the similar physiochemical properties of Li+ and Mg2+. Herein, we report an ion-sieving adsorption approach to design sunlight-regenerable lithium adsorbents by subnanoporous MOFs for efficient lithium extraction. By integrating the ion-sieving agent of MOFs with light-responsive adsorption sites of polyspiropyran (PSP), the ion-sieving adsorption behaviors of PSP-MOFs with 6.0, 8.5, and 10.0 Å windows are inversely proportional to their pore size. The synthesized PSP-UiO-66 with a narrowest window size of 6.0 Å shows high LiCl adsorption capacity up to 10.17 mmol g-1 and good Li+/Mg2+ selectivity of 5.8 to 29 in synthetic brines with Mg/Li ratio of 1 to 0.1. It could be quickly regenerated by sunlight irradiation in 6 min with excellent cycling performance of 99% after five cycles. This work sheds light on designing selective adsorbents using responsive subnanoporous materials for environmentally friendly and energy-efficient ion separation and purification.

Photo-Controllable Smart Hydrogels for Biomedical Application: A Review

Nowadays, smart hydrogels are being widely studied by researchers because of their advantages such as simple preparation, stable performance, response to external stimuli, and easy control of response behavior. Photo-controllable smart hydrogels (PCHs) are a class of responsive hydrogels whose physical and chemical properties can be changed when stimulated by light at specific wavelengths. Since the light source is safe, clean, simple to operate, and easy to control, PCHs have broad application prospects in the biomedical field. Therefore, this review timely summarizes the latest progress in the PCHs field, with an emphasis on the design principles of typical PCHs and their multiple biomedical applications in tissue regeneration, tumor therapy, antibacterial therapy, diseases diagnosis and monitoring, etc. Meanwhile, the challenges and perspectives of widespread practical implementation of PCHs are presented in biomedical applications. This study hopes that PCHs will flourish in the biomedical field and this review will provide useful information for interested researchers.

Construction of Multi-Stimuli Responsive Highly Porous Switchable Frameworks by In Situ Solid-State Generation of Spiropyran Switches

Stimuli-responsive molecular systems support within permanently porous materials offer the opportunity to host dynamic functions in multifunctional smart materials. However, the construction of highly porous frameworks featuring external-stimuli responsiveness, for example by light excitation, is still in its infancy. Here a general strategy is presented to construct spiropyran-functionalized highly porous switchable aromatic frameworks by modular and high-precision anchoring of molecular hooks and an innovative in situ solid-state grafting approach. Three spiropyran-grafted frameworks bearing distinct functional groups exhibiting various stimuli-responsiveness are generated by two-step post-solid-state synthesis of a parent indole-based material. The quantitative transformation and preservation of high porosity are demonstrated by spectroscopic and gas adsorption techniques. For the first time, a highly efficient strategy is provided to construct multi-stimuli-responsive, yet structurally robust, spiropyran materials with high pore capacity which is proved essential for the reversible and quantitative isomerization in the bulk as demonstrated by solid-state NMR spectroscopy. The overall strategy allows to construct dynamic materials that undergoes reversible transformation of spiropyran to zwitterionic merocyanine, by chemical and physical stimulation, showing potential for pH active control, responsive gas uptake and release, contaminant removal, and water harvesting.

Breaking the photoswitch speed limit

The forthcoming generation of materials, including artificial muscles, recyclable and healable systems, photochromic heterogeneous catalysts, or tailorable supercapacitors, relies on the fundamental concept of rapid switching between two or more discrete forms in the solid state. Herein, we report a breakthrough in the "speed limit" of photochromic molecules on the example of sterically-demanding spiropyran derivatives through their integration within solvent-free confined space, allowing for engineering of the photoresponsive moiety environment and tailoring their photoisomerization rates. The presented conceptual approach realized through construction of the spiropyran environment results in ~1000 times switching enhancement even in the solid state compared to its behavior in solution, setting a record in the field of photochromic compounds. Moreover, integration of two distinct photochromic moieties in the same framework provided access to a dynamic range of rates as well as complementary switching in the material's optical profile, uncovering a previously inaccessible pathway for interstate rapid photoisomerization.

Multi-stimulus Room Temperature Phosphorescent Polymers Sensitive to Light and Acid cyclically with Energy Transfer

The stimulus-responsive room temperature phosphorescent (RTP) materials have endowed wide potential applications. In this work, by introducing naphthalene and spiropyran (SP) into polyacrylamide as the energy donor and acceptor respectively, a new kind of brilliant dynamic color-tunable amorphous copolymers were prepared with good stability and processibility, and afterglow emissions from green to orange in response to the stimulus of photo or acid, thanks to multi-responsibility of SP and the energy transfer between naphthalene and SP. In addition to the deeply exploring of the inherent mechanism, these copolymers have been successfully applied in dynamically controllable applications in information protection and delivery.

A spiropyran-decorated nanocoating for dynamically regulating bacteria/cell adhesion and detachment

Microorganism adhesion and the resulting contamination of the biomaterial is one of the major causes of biomedical device failure. Stimuli-responsive materials based on dynamically regulating interactions with reversible characteristics of on-off states have attracted increasing attention. Here, a facile self-assembled biomaterial nanocoating constructed using acidity- and photoregulated spiropyran-modified nanoparticles was developed for reversibly regulating bacteria or mammalian cell adhesion-and-detachment. The coating was formed by coating a solution of spiropyran-conjugated nanoparticles around the surface of a silica gel followed by curing and drying at 60 °C for 30 min. Importantly, efficient adhesion-and-detachment of bacteria or cells could be controlled even after 8 cycles owing to the excellent acidity- and light-switched ability. Collectively, this well-defined self-assembled nanocoating as a dynamical and reversible agent provides promising insight for the development of biomedical devices, especially for biomaterial medical coatings.

Recyclable soft photonic crystal film with overall improved circularly polarized luminescence

Existing circularly polarized luminescence materials can hardly satisfy the requirements of both large luminescence dissymmetry factor and high luminescent quantum yield, which hinders their practical applications. Here, we present a soft photonic crystal film embedded with chiral nanopores that possesses excellent circularly polarized luminescence performance with a high luminescence dissymmetry factor as well as a large luminescent quantum yield when loaded with various luminescent dyes. Benefitting from the retention of chiral nanopores imprinted from a chiral liquid crystal arrangement, the chiral soft photonic crystal film can not only endow dyes with chiral properties, but also effectively avoid severe aggregation of guest dye molecules. More importantly, the soft photonic crystal film can be recycled many times by loading and eluting guest dye molecules while retaining good stability as well as circularly polarized luminescence performance, enabling various applications, including smart windows, multi-color circularly polarized luminescence and anticounterfeiting.

Stimuli-responsive dynamic hydrogels: design, properties and tissue engineering applications

The field of tissue engineering and regenerative medicine has been evolving at a rapid pace with numerous novel and interesting biomaterials being reported. Hydrogels have come a long way in this regard and have been proven to be an excellent choice for tissue regeneration. This could be due to their innate properties such as water retention, and ability to carry and deliver a multitude of therapeutic and regenerative elements to aid in better outcomes. Over the past few decades, hydrogels have been developed into an active and attractive system that can respond to various stimuli, thereby presenting a wider control over the delivery of the therapeutic agents to the intended site in a spatiotemporal manner. Researchers have developed hydrogels that respond dynamically to a multitude of external as well as internal stimuli such as mechanics, thermal energy, light, electric field, ultrasonics, tissue pH, and enzyme levels, to name a few. This review gives a brief overview of the recent developments in such hydrogel systems which respond dynamically to various stimuli, some of the interesting fabrication strategies, and their application in cardiac, bone, and neural tissue engineering.

Fascinating Pathway to Facilitate the Photoisomerization of Spiropyran-Based Nanocomposites

Recently, spiropyran-based composites have gained more attention on account of their stimuli-responsive essence, especially of the fascinating and green photo stimulus. However, the great dipole moment change between the ring-opened merocyanine and ring-closed spiropyran requires a large free volume available for isomerization, which significantly restrains the photoisomerization of spiropyran-based nanocomposites. Herein, a fascinating pathway by regulating the states both of spiropyran and the immobilized nanoparticle supports was put forward to facilitate the photoisomerization. The results demonstrated that the spiropyran grafting percentage of 5.18% and immobilized supports with less aggregation, high specific surface area, large pore size, and noncrystalline structure were suitable to fabricate spiropyran-based nanocomposites, which showed a significant improvement for Pb2+ and Cr3+ removal from aqueous solution on account of free photoisomerization of spiropyran on the support's surface. This work will pave the pathway to extend the exploitation of spiropyran-based nanocomposites in various fields such as biotechnology, physiology, and electronics to photonics and environmental-friendly fields.

Ligand-protected nanocluster-mediated photoswitchable fluorescent nanoprobes towards dual-color cellular imaging

Development of robust multi-color photoswitchable fluorescent probes is critical for many optical applications, but it remains a challenge to rationally design these probes. Here, we report a new design of Förster resonance energy transfer-based dual-color photoswitchable fluorescent nanoparticles (DPF NPs) by taking advantage of the distinct properties of ligand-protected gold nanoclusters (AuNCs). Detailed photophysical studies revealed that ultrasmall-sized AuNCs not only act as the FRET donors due to their intrinsic fluorescence properties, but also play a significant role in regulating the photochromic and aggregate properties of spiropyran through ligand-spiropyran interactions. These DPF NPs exhibit a high fluorescence on/off ratio (∼90%) for both green and red fluorescence emission, and good reversibility during cycled photo-stimulation. Cell imaging experiments showed that DPF NPs could specifically accumulate in lipid droplets, and enable photoswitchable dual-color imaging in living cells. Moreover, by labeling mitochondria with a green-emitting marker, we demonstrated that DPF NPs can distinguish different targets based on dynamic and static fluorescence signals at the sub-cellular level in two emission channels reliably. This study provides a new strategy for designing robust photoswitchable fluorescent probes by modulating the properties of photochromic dyes through ligand-protected nanoclusters, which can be generalized for the development of other photoswitch systems towards advanced optical applications.

Light, pH, and Temperature Triple-Responsive Magnetic Composites for Highly Efficient Phosphopeptide Enrichment

Smart materials can dynamically and reversibly change their structures and functions in response to external stimuli. In this study, we designed a smart magnetic composite (MNP-pSPA-b-pNIPAm) with a triple response to ultraviolet (UV) light, pH, and temperature. Relying on the response of spiropyranyl acrylate (SPA) and N-isopropylacrylamide (NIPAm) to external stimuli (light, pH, and temperature), MNP-pSPA-b-pNIPAm was used for the controlled capture and release of phosphopeptides. The established phosphopeptide enrichment platform exhibits high sensitivity (detection limit of 0.04 fmol), high selectivity (BSA/β-casein, 1000:1), and good reusability (6 cycles). In addition, the method was also applied to the enrichment of phosphopeptides in real samples (skim milk, human saliva, and serum), demonstrating the feasibility of this method for phosphoproteomic analysis. After enriching from human nonsmall cell lung cancer cell (A549) lysates with MNP-pSPA-b-pNIPAm, 2595 phosphopeptides corresponding to 2281 phosphoproteins were identified. The novel responsive enrichment probe is highly specific for phosphoproteomic analysis and provides an effective method for studying the significance of protein phosphorylation in complex biological samples.

Controllable Wetting Transitions on Photoswitchable Physical Gels

Softness plays a key role in the deformation of soft elastic substrates at the three-phase contact line, and the acting forces lead to the formation of a wetting ridge due to elastocapillarity. The change in wetting ridge and surface profiles at different softness has a great impact on the droplet behavior in different phenomena. Commonly used materials to study soft wetting are swollen polymeric gels or polymer brushes. These materials offer no possibility to change the softness on demand. Therefore, adjustable surfaces with tunable softness are highly sought-after to achieve on-demand transition between wetting states on soft surfaces. Here, we present a photorheological physical soft gel with adjustable stiffness based on the spiropyran photoswitch that shows the formation of wetting ridges upon droplet deposition. The presented photoswitchable gels allow the creation of reversibly switchable softness patterns with microscale resolution using UV light-switching of the spiropyran molecule. Gels with varying softness are analyzed, showing a decrease in the wetting ridge height at higher gel stiffness. Furthermore, wetting ridges before and after photoswitching are visualized using confocal microscopy, showing the transition in the wetting properties from soft wetting to liquid/liquid wetting.

Tuning the Photochromism of Spiropyran in Functionalized Nanoporous Silica Nanoparticles for Dynamic Anticounterfeiting Applications

Here, we report a novel invisible ink with different decay times based on thin films with different molar ratios of spiropyran (SP)/Si, which allows the encryption of messages over time. Nanoporous silica has been found to be an excellent substrate to improve the solid photochromism of spiropyran, but the hydroxyl groups of silica have a serious effect on fade speeds. The density of silanol groups in silica has an influence on the switching behavior of spiropyran molecules, as they stabilize the amphiphilic merocyanine isomers and thus slow down the fading process from the open to the closed form. Here, we investigate the solid photochromic behavior of spiropyran by sol-gel modification of the silanol groups and explore its potential application in UV printing and dynamic anticounterfeiting. To extend its applications, spiropyran is embedded in organically modified thin films prepared by the sol-gel method. Notably, by using the different decay times of thin films with different SP/Si molar ratios, time-dependent information encryption can be realized. It provides an initial "false" code, which does not display the required information, and only after a given time will the encrypted data appear.

Light and pH dual-responsive spiropyran-based cellulose nanocrystals

Reversibly light and pH dual-responsive spiropyran-based cellulose nanocrystals (SP-CNCs) is synthesized by the attachment of carboxyl-containing spiropyran (SP-COOH) onto cellulose nanocrystals (CNCs). The resulting structure and properties of SP-CNCs are examined by Fourier transform infrared spectroscopy (FT-IR), elemental analysis, transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic laser light scattering (DSL), ζ-potential measurements and ultraviolet-visible (UV-Vis) light absorption spectroscopy. SP-CNCs exhibit excellent photochromic and photoswitching properties. Spiropyran moieties on SP-CNCs can be switched between open-ring merocyanine (MC) and closed ring spiropyran (SP) forms under UV/Vis irradiation, leading to color changes. Moreover, SP-CNCs display improved photoresponsiveness, photoreversibility, fatigue resistance, and stability in DMSO than in H₂O. We further investigate the pH-responsive behavior of SP-CNCs in H₂O. SP-CNCs aqueous solution display different colors at different pH values, which can be directly observed by naked eye, indicating that SP-CNCs can function as a visual pH sensor. These results suggest that light and pH dual-responsive SP-CNCs possess great potential for applications in reversible data storage, sensing, optical switching and light-controlled nanomaterials.

Maskless, Reusable Visible-Light Direct-Write Stamp for Microscale Surface Patterning

We report a straightforward method for creating large-area, microscale resolution patterns of functional amines on self-assembled monolayers by the photoinduced local acidification of a flat elastomeric stamp enriched with photoacid. The limited diffusivity of the photoactivated merocyanine acid in poly(dimethylsiloxane) (PDMS) enabled to confine efficient deprotection of N-tert-butyloxycarbonyl amino group (N-Boc) to line widths below 10 μm. The experimental setup is very simple and is built around the conventional HD-DVD optical pickup. The method allows cost-efficient, maskless, large-area chemical patterning while avoiding potentially cytotoxic photochemical reaction products. The activation of the embedded photoacid occurs within the stamp upon illumination with the laser beam and the process is fully reversible. Preliminary positive results highlight the possibility of repeatable use of the same stamp for the creation of different patterns.

Porous Electrospun Films with Reversible Photoresponsive Microenvironmental Humidity Regulation: A Controllable Hydrogen-Bonding Synergistic Effect Exhibited by Acrylic Acid Segments

Suitable relative humidity is essential for the preservation of cultural relics, food storage, and so on. A special material that can regulate the relative humidity in the microenvironment is particularly important. In this work, several innovative electrospun films with reversible photoresponsive wettability and the ability to regulate microenvironmental relative humidity were prepared. The spiropyran unit of the synthesized copolymer played the most important role in humidity regulation due to its reversible transition between a nonpolar ring-closed state and a polar ring-opened state induced by alternating ultraviolet/visible illumination. More interestingly, the introduction of acrylic acid segments exhibited a controllable hydrogen bond synergistic effect for increasing the range of humidity regulation. The color change and the reversible change ranges of wettability and microenvironmental relative humidity under ultraviolet/visible irradiation are all closely related to the number of acrylic acid segments. Cassie theory, density functional theory (DFT), and interaction region indicator (IRI) analysis were used to characterize this phenomenon. Electrospinning is a promising method to achieve large-scale production that can put such material into practical applications.

Controlling the LCST-Phase Transition in Azobenzene-Functionalized Poly (N-Isopropylacrlyamide) Hydrogels by Light

Poly(N-isopropylacrylamide) PNIPAAm hydrogels were modified with a new azobenzene-containing co-monomer. In this work, light responsiveness as an additional functionality, is conceptualized to induce two phase transitions in the same material, which can be controlled by light. For a hydrogel with merely 2.5 mol% of this co-monomer, the lower critical solution transition temperature (LCST) was lowered by 12 °C (to 20 °C) compared to PNIPAAm (LCST at 32 °C), as analyzed by differential scanning calorimetry (DSC). The untreated unimodal endotherm split into a bimodal peak upon irradiation with UV-light, giving a second onset due to the switched (Z) isomer-rich regions, LCST*_H2.5%-(Z) = 26 °C. On irradiation with 450 nm, leading to the reverse (Z) to (E) isomerization, the endotherm was also reversible. Thus, a photo-switchable hydrogel whose LCST and structure are tunable with the hydrophobicity-hydrophilicity of the (E) and (Z) isomeric state of azobenzene was obtained. The influence of the increase in the mol% of azoacrylate on the LCST was evaluated via DSC, in combination with NMR studies, UV-vis spectroscopy and control experiments with linear polymers. The large light-driven modulation of the LCST adds bistability in thermoresponsive hydrogels, which may open diverse applications in the field of soft robotics actuators.

A 3D smart wood membrane with high flux and efficiency for separation of stabilized oil/water emulsions

Oily sewage discharged from indiscriminate industrial and frequent oil spills have become a serious global problem. There is an urgent need to separate stable oil/water emulsions by efficient and environmentally friendly methods. Membrane separation technology has the advantages of low energy consumption and low cost, thus is an effective solution to the problems of oily wastewater. However, the manufacture of multifunctional membranes with high efficiency, high flux and self-cleaning using renewable materials remains a challenge. Herein, three-dimensional (3D) smart membranes with switchable superhydrophobic-hydrophilic surfaces were prepared by grafting photo-responsive poly-spiropyran (PSP) on wood-based substrates via surface atom transfer radical polymerization. This novel membrane can efficiently separate stabilized water-in-oil and oil-in-water emulsions due to reversible hydrophilic-hydrophobic transition by switching UV and visible light irradiation. Remarkably, after immobilization, the PSP grafted on the wood substrate exhibited a faster photo response effect than the free spiropyran (SP). More importantly, the prepared 3D smart membranes showed exceptional high flux (4392 L•m^-2•h^-1) and efficiency (above 99.99 %), good cycle stability (99.99 % after 12 times) and durability (available for at least 60 days) for the separation of surfactant-stabilized water-in-oil emulsions. This work opens a new avenue for the design of functional biomass-derived membranes for efficient and sustainable oily wastewater treatment with high flux, easy scale-up, and green regeneration.

Regulation of Bacterial Behavior by Light and Magnetism Mediated by Mesoporous Silica-Coated MnFe2O4@CoFe2O4 Nanoparticles

Unicellular bacterial cells exhibit diverse population behaviors (i.e., aggregation, dispersion, directed assembly, biofilm formation, etc.) to facilitate communication and cooperation. Suitable bacterial behaviors are required for efficient nutrient uptake, cell recycling, and stress response for environmental and industrial application of bacterial populations. However, it remains a great challenge to artificially control bacterial behaviors because of complicated genetic and biochemical mechanisms. In this study, we designed facile mesoporous silica nanoparticle (MSN)-based assemblies to intelligently regulate bacterial behaviors with the help of light and magnetic field. This system was composed of magnetic MSNs, i.e., MnFe₂O₄@CoFe₂O₄@MSN modified by photoactive spiropyran (SP), and the chitosan-based polymers ChiPSP, i.e., chitosan grafted by triphenylphosphine and SP. The assembly strongly bound bacterial cells, inducing reversible bacterial aggregation by visible-light irradiation and dark. Moreover, the formed bacterial aggregates could be further governed by a directed magnetic field (DMF) to form microfibers and by an alternating magnetic field (AMF) to form biofilms. This study realized stimulus-triggered regulation of bacterial behaviors by MSNs and implied the great power of chemical strategies in intelligent control of diverse biological processes for environmental and industrial applications.

Light-responsive self-assembled microstructures of branched polyethyleneimine at low pH

Soft materials comprising polyethyleneimine (PEI), which integrate low pH-stimulated higher-order assemblies (fibres and sheets) with light responsiveness, have been shown. Excitation wavelength light-driven interactions enable the formation of bead-necklace-type structures in fibres or volume collapse of sheets. This work can have significant implications for transfection or encapsulation through PEI-based complexes.

Touch-Responsive Hydrogel for Biomimetic Flytrap-Like Soft Actuator

Stimuli-responsive hydrogel is regarded as one of the most promising smart soft materials for the next-generation advanced technologies and intelligence robots, but the limited variety of stimulus has become a non-negligible issue restricting its further development. Herein, we develop a new stimulus of "touch" (i.e., spatial contact with foreign object) for smart materials and propose a flytrap-inspired touch-responsive polymeric hydrogel based on supersaturated salt solution, exhibiting multiple responsive behaviors in crystallization, heat releasing, and electric signal under touch stimulation. Furthermore, utilizing flytrap-like cascade response strategy, a soft actuator with touch-responsive actuation is fabricated by employing the touch-responsive hydrogel and the thermo-responsive hydrogel. This investigation provides a facile and versatile strategy to design touch-responsive smart materials, enabling a profound potential application in intelligence areas.