Chemically Responsive Gels Prepared from Microspheres Dispersed in Liquid Crystals

Liquid‐crystalline materials are a promising class of stimuli‐responsive materials that have been demonstrated to undergo surface‐induced orientational ordering transitions that can be highly sensitive and specific to chemical species. However, past studies demonstrating surface‐induced transitions in liquid crystals (LCs) have employed thin films of low‐molecular‐weight LCs that are difficult to stabilize (due to dewetting of the LC on a surface). Here, it is reported that it is possible to prepare liquid‐crystalline gels using a mixture of polystyrene microspheres and nematic LCs that undergo changes in orientational order, and thus optical appearance, in response to exposure to specific chemical compounds. These colloid‐in‐liquid‐crystal (CLC) gels are mechanically stable and can be molded on chemically functionalized surfaces into thin films containing micrometer‐sized LC‐rich domains that span the two interfaces of the gels. In contrast to other reports of LC gels, where the presence of a polymeric or self‐assembled small‐molecule gelator network within a nematic LC frustrates ordering transitions from propagating through the gels over distances, it is demonstrated that thin films of CLC gels, when supported on chemically functionalized surfaces, do undergo easily visualized ordering transitions upon exposure to organophosphonate compounds. Because these optically responsive CLC gels are mechanically robust and can be molded, this class of composite LC material may be broadly useful for the design of chemically responsive LC devices.     

Dual Light and Temperature Responsive Micrometer‐Sized Structural Color Actuators

Externally induced color‐ and shape‐changes in micrometer‐sized objects are of great interest in novel application fields such as optofluidics and microrobotics. In this work, light and temperature responsive micrometer‐sized structural color actuators based on cholesteric liquid‐crystalline (CLC) polymer particles are presented. The particles are synthesized by suspension polymerization using a reactive CLC monomer mixture having a light responsive azobenzene dye. The particles exhibit anisotropic spot‐like and arc‐like reflective colored domains ranging from red to blue. Electron microscopy reveals a multidirectional asymmetric arrangement of the cholesteric layers in the particles and numerical simulations elucidate the anisotropic optical properties. Upon light exposure, the particles show reversible asymmetric shape deformations combined with structural color changes. When the temperature is increased above the liquid crystal‐isotropic phase transition temperature of the particles, the deformation is followed by a reduction or disappearance of the reflection. Such dual light and temperature responsive structural color actuators are interesting for a variety of micrometer‐sized devices.     

Cholesteric Liquid Crystal Shells as Enabling Material for Information‐Rich Design and Architecture

The responsive and dynamic character of liquid crystals (LCs), arising from their ability to self‐organize into long‐range ordered structures while maintaining fluidity, has given them a role as key enabling materials in the information technology that surrounds us today. Ongoing research hints at future LC‐based technologies of entirely different types, for instance by taking advantage of the peculiar behavior of cholesteric liquid crystals (CLCs) subject to curvature. Spherical shells of CLC reflect light omnidirectionally with specific polarization and wavelength, tunable from the UV to the infrared (IR) range, with complex patterns arising when many of them are brought together. Here, these properties are analyzed and explained, and future application opportunities from an interdisciplinary standpoint are discussed. By incorporating arrangements of CLC shells in smart facades or vehicle coatings, or in objects of high value subject to counterfeiting, game‐changing future uses might arise in fields spanning information security, design, and architecture. The focus here is on the challenges of a digitized and information‐rich future society where humans increasingly rely on technology and share their space with autonomous vehicles, drones, and robots.     

Responsive Liquid‐Crystal‐Clad Fibers for Advanced Textiles and Wearable Sensors

A simple process to clad conventional monofilament fibers with low‐molecular‐weight liquid crystals (LCs) stabilized by an outer polymer sheath is demonstrated. The fibers retain the responsive properties of the LCs but in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC‐core fibers produced by electrospinning or airbrushing. The microscopic structure and the optical properties of round and flattened fibers are reported. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the LCs. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemical concentrations, and pressure. The fabrics can be fashioned into garments that can sense and report the state of health of the wearer or the status of their environment.     

Stimuli‐Responsive Polymeric Systems for Biomedical Applications

Smart polymeric‐based devices and surfaces that reversibly alter their physico‐chemical characteristics in response to their environment are the center of many studies related to the development of materials and concepts in a broad‐range of biomedical fields. Although the initial interests were more focused in systems for the delivery of therapeutic molecules, other applications have been raised in topics ranging from actuators to biomaterials for tissue engineering and regenerative medicine. The general aspects of the different types of stimuli that can be used to modulate the response are reviewed mainly for the case of hydrogels and surfaces, based on natural‐origin or biodegradable macromolecules. Thermosensitive or light responsive surfaces that can modulate cell adhesion or protein adsorption are addressed as well as less conventional smart surfaces, such as substrates onto which biomineralization may be triggered. Injectable liquids that turn to gels by the action of heating (sol‐gel thermo‐reversible hydrogels) or by changing pH or the ionic milieu (bioinspired self‐assembling systems) may find great applicability as temporary scaffolds in non invasive procedures to deliver drugs or cells to particular places in the body. Examples of systems that recognize independently or simultaneously more than one stimulus will also be presented. Besides the typical response to temperature and pH, recent developments on materials that react to biochemical stimuli, including specific enzymes, antibodies or cells, are also highlighted.     

Humidity‐Responsive Single‐Nanoparticle‐Layer Plasmonic Films

2D materials possess many interesting properties, and have shown great application potentials. In this work, the development of humidity‐responsive, 2D plasmonic nanostructures with switchable chromogenic properties upon wetting–dewetting transitions is reported. By exploiting DNA hybridization‐directed anchoring of gold nanoparticles (AuNPs) on substrates, a series of single‐nanoparticle‐layer (SNL) plasmonic films is fabricated. Due to the collective plasmonic responses in SNL, these ultrathin 2D films display rapid and reversible red‐blue color change upon the wetting–dewetting transition, suggesting that hydration‐induced microscopic plasmonic coupling between AuNPs is replicated in the macroscopic, centimeter‐scale films. It is also found that hydration finely tunes the electric field distribution between AuNPs in the SNL film, based on which responsive surface‐enhanced Raman scattering substrates with spatially homogeneous hot spots are developed. Thus it is expected that DNA‐mediated 2D SNL structures open new avenues for designing miniaturized plasmonic nanodevices with various applications.     

Stimuli‐Responsive Self‐Assembled DNA Nanomaterials for Biomedical Applications

Stimuli‐responsive DNA‐based materials represent a major class of remarkable functional nanomaterials for nano‐biotechnological applications. In this review, recent progress in the development of stimuli‐responsive systems based on self‐assembled DNA nanostructures is introduced and classified. Representative examples are presented in terms of their design, working principles and mechanisms to trigger the response of the stimuli‐responsive DNA system upon expose to a large variety of stimuli including pH, metal ions, oligonucleotides, small molecules, enzymes, heat, and light. Substantial in vitro studies have clearly revealed the advantages of the use of stimuli‐responsive DNA nanomaterials in different biomedical applications, particularly for biosensing, drug delivery, therapy and diagnostic purposes in addition to bio‐computing. Some of the challenges faced and suggestions for further development are also highlighted.     

Dynamic Orthogonal Switching of a Thermoresponsive Self‐Organized Helical Superstructure

Controllable manipulation of self‐organized dynamic superstructures of functional molecular materials by external stimuli is an enabling enterprise. Herein, we have developed a thermally driven, self‐organized helical superstructure, i.e., thermoresponsive cholesteric liquid crystal (CLC), by integrating a judiciously chosen thermoresponsive chiral molecular switch into an achiral liquid crystalline medium. The CLC in lying state, in both planar and twisted nematic cells, exhibits reversible in‐plane orthogonal switching of its helical axis in response to the combined effect of temperature and electric field. Consequently, the direction of the cholesteric grating has been observed to undergo 90° switching in a single cell, enabling non‐mechanical beam steering along two orthogonal directions. The ability to reversibly switch the cholesteric gartings along perpendicular directions by appropriately adjusting temperature and electric field strength could facilitate their applications in 2D beam steering, spectrum scanning, optoelectronics and beyond.     

Optical Properties of Self‐Assembled Cellulose Nanocrystals Films Suspended at Planar–Symmetrical Interfaces

Hierarchically structured materials comprising rod‐like, chiral, nanoparticles are commonly encountered in nature as they can form assemblies with exceptional optical and mechanical characteristics. These include cellulose nanocrystals (CNCs), which have a large potential for the fabrication of bioinspired materials mimicking those advanced properties. Fine‐tuning the optomechanical properties of assemblies obtained from CNCs hinges on the transformations from suspensions of liquid crystals to long‐range order in the dry state. So far, associated transitions have been studied using trivial interfaces such as planar substrates. Such transitions are explored as they evolve onto meshed supports. The meshed substrate offers a complex topology, as is encountered in nature, for the formation of CNCs films. The CNCs self‐assembly occurs under confinement and support of the framework bounding the mesh openings. This leads to coexisting suspended and supported nanoparticle layers exhibiting nematic and/or chiral nematic order. Optical microscopy combined with crossed polarizers indicate that the formation of the suspended films occurs via intermediate gelation or kinetic arrest of CNCs across the mesh's open areas. The formation of self‐standing, ultrathin films of CNCs with tunable optical properties, such as selective reflections in the visible range (structural color), is demonstrated by using the presented simple and scalable approach.     

Layer‐by‐Layer Hydrogen‐Bonded Polymer Films: From Fundamentals to Applications

Recent years have seen increasing interest in the construction of nanoscopically layered materials involving aqueous‐based sequential assembly of polymers on solid substrates. In the booming research area of layer‐by‐layer (LbL) assembly of oppositely charged polymers, self‐assembly driven by hydrogen bond formation emerges as a powerful technique. Hydrogen‐bonded (HB) LbL materials open new opportunities for LbL films, which are more difficult to produce than their electrostatically assembled counterparts. Specifically, the new properties associated with HB assembly include: 1) the ease of producing films responsive to environmental pH at mild pH values, 2) numerous possibilities for converting HB films into single‐ or two‐component ultrathin hydrogel materials, and 3) the inclusion of polymers with low glass transition temperatures (e.g., poly(ethylene oxide)) within ultrathin films. These properties can lead to new applications for HB LbL films, such as pH‐ and/or temperature‐responsive drug delivery systems, materials with tunable mechanical properties, release films dissolvable under physiological conditions, and proton‐exchange membranes for fuel cells. In this report, we discuss the recent developments in the synthesis of LbL materials based on HB assembly, the study of their structure–property relationships, and the prospective applications of HB LbL constructs in biotechnology and biomedicine.     

Light‐Driven Reversible Transformation between Self‐Organized Simple Cubic Lattice and Helical Superstructure Enabled by a Molecular Switch Functionalized Nanocage

Self‐organized stimuli‐responsive smart materials with adjustable attributes are highly desirable for a plethora of device applications. Simple cubic lattice is quite uncommon in soft condensed matter due to its lower packing factor. Achieving a stable simple cubic soft lattice and endowing such a lattice with dynamic reconstruction capability solely by a facile light irradiation are of paramount significance for both fundamental studies and engineering explorations. Herein, an elegant stable self‐organized simple cubic soft lattice, i.e., blue phase II, in a chiral liquid crystal (LC) system is disclosed, which is stable down to room temperature and exhibits both reversible lattice deformation and transformation to a helical superstructure, i.e., cholesteric LC, by light stimulation. Such an amazing trait is attained by doping a judiciously designed achiral photoresponsive molecular switch functionalized polyhedral oligomeric silsesquioxane nanocage into a chiral LC host. An unprecedented reversible collapse and reconstruction of such a high symmetric simple cubic blue phase II driven by light has been achieved. Furthermore, a well‐defined conglomerate micropattern composed of simple cubic soft lattice and helical superstructure, which is challenging to fabricate in organic and inorganic crystalline materials, is produced using photomasking technology. Moreover, the promising photonic application based on such a micropattern is demonstrated.     

Thermo‐Switchable Materials Prepared Using the OEGMA‐Platform

We describe here the advantages of oligo(ethylene glycol)‐based (co)polymers for preparing thermoresponsive materials as diverse as polymer‐enzyme bio‐hybrids, injectable hydrogels, capsules for drug‐release, modified magnetic particles for in vivo utilization, cell‐culture substrates, antibacterial surfaces, or stationary phases for bioseparation. Oligo(ethylene glycol) methacrylates (OEGMAs) can be (co)polymerized using versatile and widely‐applicable methods of polymerization such as atom transfer radical polymerization (ATRP) of reversible addition‐fragmentation chain‐transfer (RAFT) polymerization. Thus, the molecular structure and therefore the stimuli‐responsive properties of these polymers can be precisely controlled. Moreover, these stimuli‐responsive macromolecules can be easily attached to–or directly grown from–organic, inorganic or biological materials. As a consequence, the OEGMA synthetic platform is today a popular option for materials design. The present research news summaries the progress of the last two years.     

Polymer‐Based Stimuli‐Responsive Recyclable Catalytic Systems for Organic Synthesis

The introduction of stimuli‐responsive polymers into the study of organic catalysis leads to the generation of a new kind of polymer‐based stimuli‐responsive recyclable catalytic system. Owing to their reversible switching properties in response to external stimuli, these systems are capable of improving the mass transports of reactants/products in aqueous solution, modulating the chemical reaction rates, and switching the catalytic process on and off. Furthermore, their stimuli‐responsive properties facilitate the separation and recovery of the active catalysts from the reaction mixtures. As a fascinating approach of the controllable catalysis, these stimuli‐responsive catalytic systems including thermoresponsive, pH‐responsive, chemo‐mechano‐chemical, ionic strength‐responsive, and dual‐responsive, are reviewed in terms of their nanoreactors and mechanisms.     

Ultrafast Laser‐Shock‐Induced Confined Metaphase Transformation for Direct Writing of Black Phosphorus Thin Films

Few‐layer black phosphorus (BP) has emerged as one of the most promising candidates for post‐silicon electronic materials due to its outstanding electrical and optical properties. However, lack of large‐scale BP thin films is still a major roadblock to further applications. The most widely used methods for obtaining BP thin films are mechanical exfoliation and liquid exfoliation. Herein, a method of directly synthesizing continuous BP thin films with the capability of patterning arbitrary shapes by employing ultrafast laser writing with confinement is reported. The physical mechanism of confined laser metaphase transformation is understood by molecular dynamics simulation. Ultrafast laser ablation of BP layer under confinement can induce transient nonequilibrium high‐temperature and high‐pressure conditions for a few picoseconds. Under optimized laser intensity, this process induces a metaphase transformation to form a crystalline BP thin film on the substrate. Raman spectroscopy, atomic force microscopy, and transmission electron microscopy techniques are utilized to characterize the morphology of the resulting BP thin films. Field‐effect transistors are fabricated on the BP films to study their electrical properties. This unique approach offers a general methodology to mass produce large‐scale patterned BP films with a one‐step manufacturing process that has the potential to be applied to other 2D materials.     

Graphene‐Based Smart Platforms for Combined Cancer Therapy

The extensive research of graphene and its derivatives in biomedical applications during the past few years has witnessed its significance in the field of nanomedicine. Starting from simple drug delivery systems, the application of graphene and its derivatives has been extended to a versatile platform of multiple therapeutic modalities, including photothermal therapy, photodynamic therapy, magnetic hyperthermia therapy, and sonodynamic therapy. In addition to monotherapy, graphene‐based materials are widely applied in combined therapies for enhanced anticancer activity and reduced side effects. In particular, graphene‐based materials are often designed and fabricated as “smart” platforms for stimuli‐responsive nanocarriers, whose therapeutic effects can be activated by the tumor microenvironment, such as acidic pH and elevated glutathione (termed as “endogenous stimuli”), or light, magnetic, or ultrasonic stimuli (termed as “exogenous stimuli”). Herein, the recent advances of smart graphene platforms for combined therapy applications are presented, starting with the principle for the design of graphene‐based smart platforms in combined therapy applications. Next, recent advances of combined therapies contributed by graphene‐based materials, including chemotherapy‐based, photothermal‐therapy‐based, and ultrasound‐therapy‐based synergistic therapy, are outlined. In addition, current challenges and future prospects regarding this promising field are discussed.     

Visible‐Light‐Induced Self‐Organized Helical Superstructure in Orientationally Ordered Fluids

Light‐induced phenomena occurring in nature and in synthetic materials are fascinating and have been exploited for technological applications. Here visible‐light‐induced formation of a helical superstructure is reported, i.e., a cholesteric liquid crystal phase, in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible‐light‐driven chiral molecular switch. The cyclic‐azobenzene‐based chiral molecular switch exhibits reversible photoisomerization in response to visible light of different wavelengths due to the band separation of n–π* transitions of its trans‐ and cis‐isomers. Green light (530 nm) drives the trans‐to‐cis photoisomerization whereas the cis‐to‐trans isomerization process of the chiral molecular switch can be caused by blue light (440 nm). It is observed that the helical twisting power of this chiral molecular switch increases upon irradiation with green light, which enables reversible induction of helical superstructure in nematic liquid crystals containing a very small quantity of the molecular switch. The occurrence of the light‐induced helical superstructure enables the formation of diffraction gratings in cholesteric films.     

A Dual‐Stimuli‐Responsive Sodium‐Bromine Battery with Ultrahigh Energy Density

Stimuli‐responsive energy storage devices have emerged for the fast‐growing popularity of intelligent electronics. However, all previously reported stimuli‐responsive energy storage devices have rather low energy densities (<250 Wh kg^–1) and single stimuli‐response, which seriously limit their application scopes in intelligent electronics. Herein, a dual‐stimuli‐responsive sodium‐bromine (Na//Br₂) battery featuring ultrahigh energy density, electrochromic effect, and fast thermal response is demonstrated. Remarkably, the fabricated Na//Br₂ battery exhibits a large operating voltage of 3.3 V and an energy density up to 760 Wh kg^?1, which outperforms those for the state‐of‐the‐art stimuli‐responsive electrochemical energy storage devices. This work offers a promising approach for designing multi‐stimuli‐responsive and high‐energy rechargeable batteries without sacrificing the electrochemical performance.     

Directed Self‐Assembly of Star‐Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism

Exploring the ordering mechanism and dynamics of self‐assembled block copolymer (BCP) thin films under confined conditions are highly essential in the application of BCP lithography. In this study, it is aimed to examine the self‐assembling mechanism and kinetics of silicon‐containing 3‐arm star‐block copolymer composed of polystyrene (PS) and poly(dimethylsiloxane) blocks as nanostructured thin films with perpendicular cylinders and controlled lateral ordering by directed self‐assembly using topographically patterned substrates. The ordering process of the star‐block copolymer within fabricated topographic patterns with PS‐functionalized sidewall can be carried out through the type of secondary (i.e., heterogeneous) nucleation for microphase separation initiated from the edge and/or corner of the topographic patterns, and directed to grow as well‐ordered hexagonally packed perpendicular cylinders. The growth rate for the confined microphase separation is highly dependent upon the dimension and also the geometric texture of the preformed pattern. Fast self‐assembly for ordering of BCP thin film can be achieved by lowering the confinement dimension and also increasing the concern number of the preformed pattern, providing a new strategy for the design of BCP lithography from the integration of top‐down and bottom‐up approaches.     

Self‐Regulating Iris Based on Light‐Actuated Liquid Crystal Elastomer

The iris, found in many animal species, is a biological tissue that can change the aperture (pupil) size to regulate light transmission into the eye in response to varying illumination conditions. The self‐regulation of the eye lies behind its autofocusing ability and large dynamic range, rendering it the ultimate “imaging device” and a continuous source of inspiration in science. In optical imaging devices, adjustable apertures play a vital role as they control the light exposure, the depth of field, and optical aberrations of the systems. Tunable irises demonstrated to date require external control through mechanical actuation, and are not capable of autonomous action in response to changing light intensity without control circuitry. A self‐regulating artificial iris would offer new opportunities for device automation and stabilization. Here, this paper reports the first iris‐like, liquid crystal elastomer device that can perform automatic shape‐adjustment by reacting to the incident light power density. Similar to natural iris, the device closes under increasing light intensity, and upon reaching the minimum pupil size, reduces the light transmission by a factor of seven. The light‐responsive materials design, together with photoalignment‐based control over the molecular orientation, provides a new approach to automatic, self‐regulating optical systems based on soft smart materials.     

Gold Nanoparticles Thin Films with Thermo‐ and Photoresponsive Plasmonic Properties Realized with Liquid‐Crystalline Ligands

Robust synthesis of large‐scale self‐assembled nanostructures with long‐range organization and a prominent response to external stimuli is critical to their application in functional plasmonics. Here, the first example of a material made of liquid crystalline nanoparticles which exhibits UV‐light responsive surface plasmon resonance in a condensed state is presented. To obtain the material, metal cores are grafted with two types of organic ligands. A promesogenic derivative softens the system and induces rich liquid crystal phase polymorphism. Second, an azobenzene derivative endows nanoparticles with photoresponsive properties. It is shown that nanoparticles covered with a mixture of these ligands assemble into long‐range ordered structures which exhibit a novel dual‐responsivity. The structure and plasmonic properties of the assemblies can be controlled by a change in temperature as well as by UV‐light irradiation. These results present an efficient way to obtain bulk quantities of self‐assembled nanostructured materials with stability that is unattainable by alternative methods such as matrix‐assisted or DNA‐mediated organization.