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Herein, a photo-switchable and thermal-enhanced fluorescent hydrogel has been fabricated from N-isopropylacrylamide (NIPAAm) with a mixture of water-soluble acryloyl-α-cyclodextrin/acryloyl-α-cyclodextrin-spiropyran (acryloyl-α-CD/ acryloyl-α-CD-SP) as cross-linkers. The physical properties, photochromic properties, and fluorescent behavior of the hydrogel were characterized. The fluorescence emission of the hydrogel can be reversibly switched ‘on/off’ by UV/visible light irradiation, and meanwhile the fluorescence intensity can be enhanced by increasing the temperature above the volume phase transition temperature (VPTT) of the hydrogel. The hydrogel also shows spatiotemporal fluorescent behavior, excellent cytocompatibility, and fatigue resistance in photochromic and photo-switchable fluorescent behaviors. [ABSTRACT FROM AUTHOR]

• Preparing a novel dual-crosslinked hydrogel with PEG and α-cyclodextrin. • The as-prepared hydrogel possessed high tensile strength. • The hydrogel exhibited real-time reversible photo-switchable color changing property. • Providing a facile method for incorporating spiropyran into the hydrogel in aqueous. Herein, high strength dual-crosslinked hydrogels with photo-switchable color changing behavior were designed and fabricated. In the mixture solution of α-cyclodextrin (α-CD), α-cyclodextrin-spiropyran (α-CD-SP), and poly(ethylene glycol) diacrylate (PEGDA), the PEGDA chains first threaded into the cavity of α-CD/α-CD-SP and form inclusion complexes to generate the physical gel; then, the as-prepared physical gels were subjected to photopolymerization to form a covalently crosslinked network. With the contribution of both no-covalent and covalent bond, the dual-crosslinked hydrogels possess high tensile strength, which is about fifteen times that of the pure PEG hydrogel. And due to the sacrifice of no-covalent bond, hydrogels showed high energy dissipation ratio, up to 75.3% and high fatigue resistance during loading-unloading tests. While the ingenious introduction of SP moieties, enables the as-prepared gels photo-switchable spatiotemporal color changing behavior reversibly. Erasable patterns on hydrogels could be written using UV light and erased by Vis light for multiple cycles. This flexible hydrogel with good physical performance shows high potential in photochromic flexible wearable devices. [ABSTRACT FROM AUTHOR]

Stimuli responsive fluorescence characteristics are significant in some fields, such as sensors, information encryption, and bionic camouflage. In this work, a robust poly(vinyl alcohol) (PVA) hydrogel with photoswitchable dynamic fluorescence behaviors was constructed by incorporating dual-color fluorescent nanoparticles (DCFNs). The DCFNs were composed of the spiropyran-modified β-cyclodextrin (β-CD-SP) and nitrobenzoxadiazolyl (NBD) derivatives, which could exist in the PVA network through non-covalent interaction. The SP moiety functioned as an acceptor and realized the photoisomerization to further quench or recover the fluorescence of the NBD moiety based on the fluorescence resonance energy transfer (FRET) effect. Therefore, under the irradiation of ultraviolet light (UV) or visible light (Vis), the hydrogel showed a fluorescence emission peak of merocyanine (MC) at 660 nm (red) or NBD at 550 nm (green), respectively. Additionally, the pH responsive characteristic of the MC chromophore also endowed the hydrogel with pH responsive fluorescence behaviors. Utilizing these advantages, specific patterns can be displayed on the hydrogel in dual mode through a hollowed-out photomask. The hydrogel could also be utilized as dual-color "building blocks" to construct various 2D or 3D structures. This composite hydrogel has potential application in various fields, such as sensors, optical switch, information storage, and biomimetic components. [Display omitted] • DCFNs are successfully introduced into the hydrogel via the non-covalent interaction. • The non-covalent bond endowed the hydrogel with desired mechanical properties. • The hydrogel could emit dual-color fluorescence based on FRET effect. [ABSTRACT FROM AUTHOR]

For fast and visual detection of multiple metal ions, herein, a re-usable colorimetric polymeric gel with spiropyran moiety was fabricated. Under ultraviolet light, the spiropyran (SP) moiety could isomerize into the colored merocyanine (MC) isomer and chelate with divalent metal ion (M2+) and form MC-M2+ complexes. Chelated with different metal ions (Ca2+, Co2+, Zn2+, and Cu2+ions), the UV–Vis spectra of the gels could display the specific colorimetric response. When the metal-ions chelated gel was irradiated with Vis light, ions dissociated from MC and released from gel due to the MC moiety isomerized to the SP form. Furthermore, after 7 cycles of alternative UV–Vis irradiations, the gel showed excellent reversible chelation/dissociation of metal ion. The competitive tendency of metal ions to chelate with gel was as following: Cu2+ > Co2+ ≈ Zn2+ > Ca2+, which was in accordance with the density functional theory modeling. Using ionoprinting technology, the gel could achieve repeatability simultaneous detection of multiple metal ions. The light-triggered metal ion chelation/dissociation capability of the gel could be harnessed to construct reusable onsite platform for visual, fast, easy detection of multiple metal ions. Herein, a reversible colorimetric polymeric gel with spiropyran moiety for multiple metal ions (Ca2+, Co2+, Zn2+, and Cu2+) detection was constructed. Combining the ionoprinting technique, the polymeric gel would be used repeatedly in the same area for detecting different metal ions after being recovered under Vis light irradiation (a) and multiple metal ions can be ionoprinted and detected on different zone of the gel film simultaneously (b). Unlabelled Image • A reversible colorimetric polymeric gel with spiropyran for multiple metal ions detection was constructed. • The light-triggered binding and dissociation behavior and the relative affinity of the polymeric gel with different metal ions were examined. • The repeatedly and simultaneously detection of multiple metal ions was realized. [ABSTRACT FROM AUTHOR]

Hydrogel actuators, in addition to their programmable shape deformations triggered by external stimuli, are expected to integrate multi-functionality in response to multiple stimuli from the environment and to mimic the biology systems. In this work, inspired by animals (i.e. cephalopods or chameleon) that can change the color of the skin to mimic the environment, a hydrogel actuator was designed to possess both solvent and thermo-responsive bending properties as well as photo-switchable color changing behaviors. This hydrogel actuator consists of a poly (N-isopropylacrylamide) (PNIPAAm) layer and a PNIPAAm layer with spiropyran moiety (PNIPAAm-SP). Spiropyran, a hydrophobic and photochromic compound, endowed the bilayer gel with solvent and temperature triggered reversible bending behaviors as well as the reversible displaying of different patterns that can be "drawn" or "erased" by light. Moreover, accompanying with its shape deformation, the bilayer gel showed a change in fluorescent behavior, i.e. its fluorescent intensity increased upon switching the solvent from DI water to EtOH or increasing the temperature. The potential applications of this bilayer hydrogel in biomimetic devices, gripper, and information storage were demonstrated. With complicated 3D shape-morphing as well as color changing ability integrated in one bilayer gel system, this bilayer hydrogel may find its broad application including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors. The bilayer hydrogel actuator with solvent and temperature triggered reversible bending behaviors as well as the color changing ability was constructed. Image 1 • A new bilayer hydrogel actuator with both solvent and thermo-responsive bending properties. • Showing a change in fluorescent behavior influenced by solvent or temperature. • Reversibly displaying of different patterns that can be "drawn" or "erased" by light. • Used as gripper, biomimetic devices and information storage. [ABSTRACT FROM AUTHOR]