Exploration of molecular machines in supramolecular soft robotic systems

Peripheral Designed Indigo Bola-Amphiphiles for Supramolecular Assembled Nanoarchitectonics in Aqueous Media

Indigo, an ancient natural dye, featured excellent biodegradability to advance in sustainable polymer. Indigo amphiphiles can pave a way for sustainable supramolecular polymers in aqueous media for potential biomedical functional materials. However, contemporary indigo amphiphiles supramolecular commonly assemble into low aspect ratio nanostructures, which hampers the macroscopic soft scaffolds fabrications and smart functional material applications. In this study, we report a novel peripheral designed indigo bola-amphiphiles (IBAs), which assemble into high aspect ratio supramolecular nanofibers in aqueous media. By employing a shear-flow assembly technique with bio-abundant calcium ions, IBAs assemble across multiple length scales into supramolecular macroscopic scaffolds. The structural characterizations of IBAs macroscopic soft scaffolds show different supramolecular structural packing information by scanning electron microscope and X-ray scattering/diffraction techniques. Our supramolecular nanoarchitectonic approach indicates the feasibility of using IBAs molecular design to construct supramolecular macroscopic materials with higher structural order for the future smart biofunctional materials and sustainable supramolecular polymer under more environmentally friendly conditions.

General Synthetic Methodologies for Building Blocks to Construct Molecular Motors

Here we present a collection of reliable, high yielding, and efficient synthetic methods for the preparation of commonly used building blocks to construct molecular rotary motors based on overcrowded alkenes. The easy access and robust synthesis procedures facilitate new endeavors to employ molecular motors in recently emerging fields toward dynamic chemical systems and responsive materials.

Combined Photopolymerization and Localized Photochromism by Aza-Diarylethene and Hemiindigo Synergy

Molecular photoswitches produce light-controlled changes at the nanometer scale and can therefore be used to alter the states and behavior of materials in a truly bottom-up fashion. Here an escalating photonic complexity of material property control with light is shown using a recently developed aza-diarylethene in combination with hemiindigo (HI) photoswitches. First, aza-diarylethene can be used as a photoswitch in polystyrene (PS) to reversibly inscribe relief-type 3D structures into PS. Second, aza-diarylethene can further be used as a photoinitiator for light-induced polymerization of methyl acrylate (MA), demonstrating for the first time light-controlled chemical reactivity control with its zwitterionic switching state. Third, aza-diarylethene and HIs are implemented into aza-diarylethene polymerized MA, generating photochromic polymers. At the fourth level, a binary mixture allows to synergize aza-diarylethene-induced photopolymerization with localized photochromism changes of the simultaneously entrapped functional HI. With such multilevel light response, the utility of this particular photoswitch combination for applications in advanced photonic materials is demonstrated.

Blue Light Controlled Supramolecular Soft Robotics of Phenylazothiazole Amphiphiles for Rapid Macroscopic Actuations

Nature preprograms sophisticated processes in operating molecular machines at the nanoscale, amplifying the molecular motion across multiple length-scales, and controlling movements in living organisms. Supramolecular soft robotics serve as a new alternative to hard robotics, are able to transform and amplify collective motions of the supramolecularly assembled molecular machines in attaining macroscopic motions, upon photoirradiation. By taking advantage of oriented supramolecular macroscopic soft scaffold, here the first rapid macroscopic movements of supramolecular robotic materials driven by visible light are presented. Head-tail amphiphilic structure is designed with the phenylazothiazole motif as the photoswitching core. Unidirectionally aligned nanostructures of the amphiphilic phenylazothiazoles are controlled by non-invasive blue light irradiation and bends toward the light source, demonstrating a fast macroscopic actuation of supramolecular robotic systems (up to 17° s-1) in aqueous media. Through meticulous X-ray diffraction and electron microscopy analyzes, macroscopic actuation mechanism is illustrated in a tight relation to molecular geometric transformations upon photoisomerization. By elucidating the key macroscopic actuation parameters, this paves the way for the next generation design of supramolecular soft robotic systems with enhanced biomimetic actuating functions.

Aqueous Supramolecular Transformations of Motor Bola-Amphiphiles at Multiple Length-Scale

Molecular motor amphiphiles have already been widely attempted for dynamic nanosystems across multiple length-scale for developments of small functional materials, including controlling macroscopic foam properties, amplifying motion as artificial molecular muscles, and serving as extracellular matrix mimicking cell scaffolds. However, limiting examples of bola-type molecular motor amphiphiles are considered for constructing macroscopic biomaterials. Herein, this work presents the designed two second generation molecular motor amphiphiles, motor bola-amphiphiles (MBAs). Aside from the photoinduced motor rotation of MBAs achieved in both organic and aqueous media, the rate of recovering thermal helix inversion step can be controlled by the rotor part with different steric hindrances. Dynamic assembled structures of MBAs are observed under (cryo)-transmission electron microscopy (TEM). This dynamicity assists MBAs in further assembling as macroscopic soft scaffolds by applying a shear-flow method. Upon photoirradiation, the phototropic bending function of MBA scaffolds is observed, demonstrating the amplification of molecular motion into macroscopic phototropic bending functions at the macroscopic length-scale. Since MBAs are confirmed with low cytotoxicity, human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can grow on the surface of MBA scaffolds. These results clearly demonstrate the concept of designing MBAs for developing photoresponsive dynamic functional materials to create new-generation soft robotic systems and cell-material interfaces.

Supramolecular assemblies of amphiphilic donor-acceptor Stenhouse adducts as macroscopic soft scaffolds

In the design of photoharvesting and photoresponsive supramolecular systems in aqueous medium, the fabrication of amphiphilic photoswitches enables a noninvasive functional response through photoirradiation. Although most aqueous supramolecular assemblies are driven by high-energy and biodamaging UV light, we have previously reported a design of amphiphilic donor-acceptor Stenhouse adducts (DASAs) controlled by white light. Herein, we present a series of DASA amphiphiles (DAs) with minor structural modifications on the alkyl linker chain length connecting the DASA motif with the hydrophilic moiety. The excellent photoswitchability in organic medium and the photoresponsiveness in aqueous medium, driven by visible light, were investigated by UV-vis absorption spectroscopy. The assembled supramolecular nanostructures were confirmed by electron microscopy, while the supramolecular packing was revealed by X-ray diffraction analysis. Upon visible-light irradiation, significant transformations of the DA geometry enabled transformations of the supramolecular assemblies on a microscopic scale, subsequently disassembling macroscopic soft scaffolds of DAs. The current work shows promising use for the fabrication of visible-light-controlled macroscopic scaffolds, offering the next generation of biomedical materials with visible-light-controlled microenvironments and future soft-robotic systems.

Localized self-assembly of macroscopically structured supramolecular hydrogels through reaction-diffusion

Localized molecular self-assembly has been developed as an effective approach for the fabrication of spatially resolved supramolecular hydrogels, showing great potential for many high-tech applications. However, the fabrication of macroscopically structured supramolecular hydrogels through molecular self-assembly remains a challenge. Herein, we report on localized self-assembly of low molecular weight hydrogelators through a simple reaction-diffusion approach, giving rise to various macroscopically patterned supramolecular hydrogels. This is achieved on the basis of an acid-catalyzed hydrazone supramolecular hydrogelator system. The acid was pre-loaded in a polydimethylsiloxane (PDMS) substrate, generating a proton gradient in the vicinity of the PDMS surface after immersing the PDMS in the aqueous solution of the hydrogelator precursors. The acid dramatically accelerates the in situ formation and self-assembly of the hydrazone hydrogelators, leading to localized formation of supramolecular hydrogels. The growth rate of the supramolecular hydrogels can be easily tuned through controlling the concentrations of the hydrogelator precursors and HCl. Importantly, differently shaped supramolecular hydrogel objects can be obtained by simply changing the shapes of PDMS. This work suggests that reaction-diffusion-mediated localized hydrogelation can serve as an approach towards macroscopically structuralized supramolecular hydrogels, which may find potential applications ranging from tissue engineering to biosensors.

Photothermal Conversion of the Oleophilic PVDF/Ti3C2Tx Porous Foam Enables Non-Aqueous Liquid System Applicable Actuator

Various physical and chemical reaction processes occur in non-aqueous liquid systems, particularly in oil phase systems. Therefore, achieving efficient, accurate, controllable, and cost-effective movement and transfer of substances in the oil phase is crucial. Liquid-phase photothermal actuators (LPAs) are commonly used for material transport in liquid-phase systems due to their remote operability and precise control. However, existing LPAs typically rely on materials like hydrogels and flexible polymers, commonly unsuitable for non-aqueous liquids. Herein, a 3D porous poly(vinylidene fluoride) (PVDF)/Ti3C2Tx actuator is developed using a solvent displacement method. It demonstrates directional movement and controlled material transport in non-aqueous liquid systems. When subject to infrared light irradiation (2.0 W cm-2), the actuator achieves motion velocities of 7.3 and 6 mm s-1 vertically and horizontally, respectively. The actuator's controllable motion capability is primarily attributed to the foam's oil-wettable properties, 3D porous oil transport network, and the excellent photothermal conversion performance of Ti3C2Tx, facilitating thermal diffusion and the Marangoni effect. Apart from multidimensional directions, the actuator enables material delivery and obstacle avoidance by transporting and releasing target objects to a predetermined position. Hence, the developed controllable actuator offers a viable solution for effective motion control and material handling in non-aqueous liquid environments.

Red-light-controlled supramolecular assemblies of indigo amphiphiles at multiple length scales

Amphiphilic molecules functionalized with photoresponsive motifs have attractive prospects for applications in smart functional bio-material ranging from cell-material interfaces to drug delivery systems owing to the precisely controllable functionality of self-assembled hierarchical supramolecular structures in aqueous media by a non-invasive light stimulation with high temporal- and spatial-resolution. However, most of reported photoresponsive amphiphiles are triggered by bio-damaging UV-light, which greatly limits the potential in bio-related applications. Herein, we present newly designed red-light controlled N,N'-diaryl-substituted indigo amphiphiles (IA), exhibiting excellent photoswitchablity and photostability with dual red-/green-light in organic media. Meanwhile, aqueous solutions of IA assembled into supramolecular structures in both microscopic and macroscopic length-scale, though the photoresponsiveness of IA is slightly compromised in aqueous media. At macroscopic length-scale, morphological changes of IA macroscopic scaffold prepared by a shear-flow method can be fine adjusted upon red-light irradiation. Moreover, the preferential attachment of live h-MSCs to IA macroscopic scaffold surface also indicates a good biocompatibility of IA macroscopic scaffold. These results provide the potential for developing the next generation of red-light controlled soft functional materials with good biocompatibility.

Controlled Supramolecular Assemblies of Chiral Cyclometalated Gold (III) Amphiphiles in Aqueous Media

Gold (III) cyclometalated based amphiphiles in aqueous media have been revealed with excellent supramolecular transformations to external stimuli to open new pathways for soft functional material fabrications. Herein, we report a new chiral cyclometalated gold (III) amphiphile (GA) assembling into lamellar nanostructures in aqueous media confirmed with transmission electron microscopy (TEM). Counterion exchange with D-, L-, or racemic-camphorsulfonates features the significant supramolecular helicity enhancements, enabling transformations of GA from lamellar structure to vesicles and to nanotubes with multi-equivalents of counterion. The limited cytotoxicity of GA in aqueous media exhibits good biocompatibility.

Tailoring Multicontrolled Supramolecular Assemblies of Stiff-Stilbene Amphiphiles into Macroscopic Soft Scaffolds as Cell-Material Interfaces

Biocompatible synthetic supramolecular systems have shed light on biomedical and tissue-regenerative material applications. The intrinsic functional applicability, tunability, and stimuli-responsiveness of synthetic supramolecular systems allow one to develop various multicontrolled supramolecular assemblies in aqueous media. However, it remains highly challenging to use state-of-the-art supramolecular assemblies of photoresponsive amphiphiles controlled by multiple stimulations in fabricating macroscopic materials. Herein, we demonstrate a stiff-stilbene amphiphile (SA) multicontrolled supramolecular assembling system that comprises two different charged end groups. The excellent photoswitchabilities of SA in both organic and aqueous media are demonstrated. Furthermore, multiple stimuli, i.e., light, pH, and counterions, are applied to control the supramolecular assembling behaviors, which are monitored by circular dichroism spectroscopy and electron microscopies. This multicontrolled supramolecular system can be systematically assembled into macroscopic soft functional scaffolds, whose structural parameters are investigated by electron microscopies and X-ray diffraction techniques, suggesting the large aspect ratio of SA nanostructures assembled into macroscopic soft scaffolds. The fabricated soft functional scaffold is highly biocompatible for photocontrolled biotarget encapsulation/release selectively, as well as a cell-material interface for diverse cells' attachment. This new synthetic multicontrolled soft functional material provides a new strategy toward the development of next-generation controllable and biocompatible soft functional materials.

Highly stretchable and self-healing photoswitchable supramolecular fluorescent polymers for underwater anti-counterfeiting

Thanks to the non-destructiveness and spatial-controllability of light, photoswitchable fluorescent polymers (PFPs) have been successfully applied in advanced anti-counterfeiting and information encryption. However, most of them are not suitable for use in harsh underwater environments, including high salinity seawater. In this study, by integrating photochromic molecules into a hydrophobic polymer matrix with the fluorine elastomer, including dipole-dipole interactions, we describe a class of novel photoswitchable supramolecular fluorescent polymers (PSFPs) that can adaptively change their fluorescence between none, green and red by the irradiation of different light. The PSFPs not only exhibited excellent photoswitchable properties, including fast photo-responsibility, prominent photo-reversibility, and photostability, but also exhibited some desired properties, including exceptional stretchability, hydrophobicity, antifouling, self-healing ability, simple preparation process, and processability. We thus demonstrated their applications in underwater data encryption and anti-counterfeiting labels.