Layer‐by‐Layer Assembly of Light‐Responsive Polymeric Multilayer Systems

Layer‐by‐Layer (LbL) assembly is a simple and highly versatile method to modify surfaces and fabricate robust and highly‐ordered nanostructured coatings over almost any type of substrate. Such versatility enables the incorporation of a plethora of building blocks, including materials exhibiting switchable properties, in a single device through a multitude of complementary intermolecular interactions. Switchable materials may undergo reversible physicochemical changes in response to a variety of external triggers. Although most of the works in the literature have been focusing on stimuli‐responsive materials that are sensitive to common triggers such as pH, ionic strength, or temperature, much less has been discussed on LbL systems which are sensitive to non‐invasive and easily controlled light stimulus, despite its unique potential. This review provides a deep overview of the recent progresses achieved in the design and fabrication of light‐responsive LbL polymeric multilayer systems, their potential future challenges and opportunities, and possible applications. Many examples are given on light‐responsive polymeric multilayer assemblies built from metal nanoparticles, functional dyes, and metal oxides. Such stimuli‐responsive functional materials, and combinations among them, may lead to novel and highly promising nanostructured smart functional systems well‐suited for a wide range of research fields, including biomedicine and biotechnology.     

All‐Optical Control of Shape

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large‐scale, macroscopic deformations when exposed to light. Here, surface‐aligned, azobenzene‐functionalized LCEs are prepared via a radical‐mediated, thiol‐acrylate chain transfer reaction. A long‐lived, macroscopic shape deformation is realized in an LCE composed with an o‐fluorinated azobenzene (oF‐azo) monomer. Under UV irradiation, the oF‐azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF‐azo LCE to analogs prepared from classical and m‐fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light‐induced deformations, oF‐azo LCEs are demonstrated to undergo all‐optical control of shape deformation and shape restoration.     

A Cut‐and‐Weld Process to 3D Architectures from Multiresponsive Crosslinked Liquid Crystalline Polymers

Crosslinked liquid crystalline polymers (CLCPs) have garnered extensive attention in recent years for their significant values in the design of light‐driven soft actuators. However, poor processabilities due to the insoluble and infusible crosslinked networks prevent their practical applications severely. In this study, a weldable azobenzene‐containing CLCP is designed with photo‐ and humidity‐responsive actuations, which enables a cut‐and‐weld process to 3D CLCP architectures. The tensile properties and stability are almost unchanged after welding, much better than those of the films pasted by common adhesive tapes. Meanwhile, the mechanisms of the welding process are clarified on the base of surface hydrogen bonding and further crosslinking. By taking advantage of the cut‐and‐weld process, a 3D “claw” integrated into a robotic arm is realized for grabbing millimeter‐scale objects by remote control. This work enhances significantly not only the processability of CLCP films but also the utilization of leftover pieces, which provides an efficient approach to create functional 3D structures from film precursors for the potential application in the 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.     

The Role of Morphology in Optically Switchable Transistors Based on a Photochromic Molecule/p‐Type Polymer Semiconductor Blend

The correlation between morphology and optoelectronic performance in organic thin‐film transistors based on blends of photochromic diarylethenes (DAE) and poly(3‐hexylthiophene) (P3HT) is investigated by varying molecular weight (M_w = 20–100 kDa) and regioregularity of the conjugated polymer as well as the temperature of thermal annealing (rt‐160 °C) in thin films. Semicrystalline architectures of P3HT/DAE blends comprise crystalline domains, ensuring efficient charge transport, and less aggregated regions, where DAEs are located as a result of their spontaneous expulsion from the crystalline domains during the self‐assembly. The best compromise between field‐effect mobility (μ) and switching capabilities is observed in blends containing P3HT with M_w = 50 kDa, exhibiting μ as high as 1 × 10^?3 cm² V^?1 s^?1 combined with a >50% photoswitching ratio. Higher or lower M_w than 50 kDa are found to be detrimental for field‐effect mobility and to lead to reduced device current switchability. The microstructure of the regioregular P3HT blend is found to be sensitive to the thermal annealing temperature, with an increase in μ and a decrease in current modulation being observed as a response to the light‐stimulus likely due to an increased P3HT‐DAE segregation, partially hindering DAE photoisomerization. The findings demonstrate the paramount importance of fine tuning the structure and morphology of bicomponent films for leveraging the multifunctional nature of optoelectronic devices.     

Liquid Bandage Harvests Robust Adhesive,Hemostatic, and Antibacterial Performances as a First‐Aid Tissue Adhesive

Massive bleeding and wound infection after tissue trauma are the major dangerous factors of casualties in disasters; hence, first‐aid supplies that can greatly achieve wound closure and effectively control the hemorrhage and infection are urgently needed. Although existing tissue adhesives can adhere to the tissue surfaces and achieve rapid wound closure, most of them have limited hemostatic and antibacterial capacities, making them unsuitable as the first‐aid tissue adhesives. In this study, inspired by the inherent hemostatic and antibacterial capacities of chitosan and the excellent tissue integration capacity originating from a Schiff base reaction, liquid bandage (LBA), an in situ imine crosslinking‐based photoresponsive chitosan hydrogel (NB‐CMC/CMC hydrogel), is developed for emergency wound management. Upon UV irradiation, o‐nitrobenzene in modified carboxymethyl chitosan (CMC) converts to o‐nitrosobenzaldehyde that subsequently crosslinks with amino groups on tissue surface, which endows the LBA with superior tissue adhesive performance. LBA's hemostatic and antibacterial properties can be tuned by the mass ratio of NB‐CMC/CMC. Moreover, it exhibits satisfactory biocompatibility, biodegradability, and the capability to enhance wound healing process. This study sheds new light on the development of a multifunctional hydrogel‐based first‐aid tissue adhesive that can achieve robust tissue adhesion, effectively control bleeding, prevent bacterial infection, and promote wound healing.     

Fast and Ample Light Controlled Actuation of Monodisperse All-DNA Microgels

The assembly of adaptive hierarchical soft materials that resemble living tissues requires responsive building blocks of controlled dimensions. While DNA self-assembly provides an exceptional tool for nanoscale architectural control, responsive DNA microstructures remain scarce. Here, two challenges controlling the size of DNA microstructures and embedding them with fast and ample structural response are addressed. For size-control, arrested phase separation and microfluidic confinement are combined to produce monodisperse all-DNA particles. For responsiveness, a light controlled coil-globule transition of the microgel DNA network powered by an azobenzene cationic surfactant is implemented. The photoinduced trans-cis isomerization of the azobenzene moiety reduces its affinity for DNA which results in fast, large amplitudes microgel swelling. Finally, the assembly of light responsive microgel superstructures is demonstrated as proof-of-concept hierarchical all-DNA materials.