Light-triggered transformation of stilbene supramolecular polymers: thermodynamic versus kinetic control

Light irradiation of stilbene supramolecular polymers produces [2+2] cycloadducts in the kinetically trapped state, which convert to the thermodynamically favorable state upon thermal annealing due to the shift of hydrogen bonds from intra- to inter-complexation modes.

Applications of Nanozymes in Chiral-Molecule Recognition through Electrochemical and Ultraviolet-Visible Analysis

Chiral molecules have similar physicochemical properties, which are different in terms of physiological activities and toxicities, rendering their differentiation and recognition highly significant. Nanozymes, which are nanomaterials with inherent enzyme-like activities, have garnered significant interest owing to their high cost-effectiveness, enhanced stability, and straightforward synthesis. However, constructing nanozymes with high activity and enantioselectivity remains a significant challenge. This review briefly introduces the synthesis methods of chiral nanozymes and systematically summarizes the latest research progress in enantioselective recognition of chiral molecules based on electrochemical methods and ultraviolet-visible absorption spectroscopy. Moreover, the challenges and development trends in developing enantioselective nanozymes are discussed. It is expected that this review will provide new ideas for the design of multifunctional chiral nanozymes and broaden the application field of nanozymes.

Light-regulated morphology control in supramolecular polymers

Stimuli-responsive materials have gained significant recent interest owing to their versatility and wide applications in fields ranging from materials science to biology. In the majority of examples, external stimuli, including light, act as a remote source of energy to depolymerize/deconstruct certain nanostructures or provide energy for exploring their functional features. However, there is little emphasis on the creation and precise control of these materials. Although significant progress has been made in the last few decades in understanding the pros and cons of various directional non-covalent interactions and their specific molecular recognition ability, it is only in the recent past that the focus has shifted toward controlling the dimension, dispersity, and other macroscopic properties of supramolecular assemblies. Control over the morphology of supramolecular polymers is extremely crucial not only for material properties they manifest but also for effective interactions with biological systems for their potential application in the field of biomedicine. This could effectively be achieved using photoirradiation which has been demonstrated by some recent reports. The concept as such offers a broad scope for designing versatile stimuli-responsive supramolecular materials with precise structure-property control. However, there has not yet been a compilation that focuses on the present subject of employing light to impact and regulate the morphology of supramolecular polymers or categorize the functional motif for easy understanding. In this review, we have collated recent examples of how light irradiation can tune the morphology and nanostructures of supramolecular polymers and categorized them based on their chemical transformation such as cis-trans isomerization, cycloaddition, and photo-cleavage. We have also established a direct correlation among the structures of the building blocks, mesoscopic properties and functional behavior of such materials and suggested future directions.

Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.

β-Cyclodextrin/Azobenzene Microspheres Loaded with Paraquat Are Safe and Effective

Although paraquat is a widely used herbicide, it is toxic to humans if ingested or absorbed through an open wound. To improve the safety of paraquat, a new formulation of paraquat based on photoresponsive polymers was exploited. Photoresponsive β-cyclodextrin polymer microspheres (AZO-CD) were synthesized via a host-guest interaction between β-cyclodextrin and azobenzene. AZO-CD were characterized by Fourier transform infrared spectrometry, circular dichroism, ultraviolet (UV) spectrophotometry, and thermogravimetric analysis, and their photoresponsiveness was also evaluated. AZO-CD were used to load paraquat, which yielded photoresponsive paraquat-loaded microspheres. For the paraquat-loaded microspheres, irradiation with UV light or sunlight induced the isomerization of azobenzene into the cis form. Then, the cis-azobenzene was liberated from the cavities of the β-cyclodextrin. The paraquat-loaded microspheres released paraquat continuously over time. Furthermore, under UV light, the herbicidal capacity of the paraquat-loaded microspheres against barnyard grass was comparable to that of free paraquat at the same dose. Our findings show that loading paraquat into AZO-CD provides a safe and environmentally friendly herbicide formulation.

Photoregulated Morphological Transformation of Spiropyran Derivatives Achieving the Tunability of Interfacial Hydrophilicity

Regulation of self-assembly morphology is an effective strategy to obtain advanced functional materials with expected properties. However, achieving remarkable morphological transformation by light irradiation is still a challenge. Herein, three simple spiropyran derivatives (SP1, SP2, and SP3) are constructed, achieving different degrees of morphological transformation from nanospheres to hollow tadpole-like structures (SP3), tubular structures (SP2), and microsheets (SP1) after ultraviolet light irradiation. Interestingly, the hollow tadpole-like structures (SP3) can further extend to Y-shaped or T-shaped tubular morphology. In the process, SP1, SP2, and SP3 can be isomerized from a closed-ring form (hydrophobicity) to an open-ring form (hydrophilicity) in different degrees, interacting differently with methanol solvent molecules. The formation of hollow structures or microsheets along with the isomerization of spiropyran derivatives contributes to the adjustment of the hydrophilicity of the interface. Therefore, SP1, SP2, and SP3 with photoregulated morphological transformation show promising applications in tunable interface materials.

Construction of a pH-sensitive self-assembly in aqueous solutions based on a dansyl-modified β-cyclodextrin

Here we present a pH-responsive self-assembly based on a β-cyclodextrin (β-CD) derivative bearing a dansyl terminus (βCD-C₆-Dns). Vesicular structures were formed over the entire studied pH range (8.5-0.7); however, the molecular configuration and packing within the vesicles were different at different pH values. Intramolecular host-guest complexation occurred mainly between the dansyl group and β-CD at pH values where the dansyl group was not protonated. The alkyl chain also acted as a competitive guest to form host-guest inclusions as confirmed by 2D ¹H NMR measurements. The pH-responsive βCD-C₆-Dns vesicles have potential application prospects in pH-controlled drug release based on the low cytotoxicity of βCD-C₆-Dns.

UV Accelerated Assemblies Constructed Using Calixpyridinium in Aqueous Solution

In this work, an irregular calixpyridinium-suramin sodium supramolecular assembly was constructed by the strong host-guest electrostatic interactions. More interestingly, a novel regular spherical supramolecular assembly was also fabricated by the hydrogen bonding interactions between suramin sodium and the UV accelerated addition product of deprotonated calixpyridinium in water. The same principle was also applied to construct a UV accelerated regular spherical self-assembly by the addition product of deprotonated calixpyridinium in water. Compared with the complicated and irreversible covalent connection of the light-responsive groups to the building block, which is one of the common means of obtaining light-responsive supramolecular systems, this finding not only provides a smart, facile, and universally applicable method to construct deprotonated calixpyridinium-based light-responsive host-guest systems but also provides a new idea for the development of other novel light-responsive building blocks.

Fabrication of a β-cyclodextrin-based self-assembly containing a redox-responsive ferrocene

The current research involves fabrication of a redox-responsive self-assembly system based on a ferrocene (Fc)-containing β-cyclodextrin (β-CD) derivative (βCD-EG-Fc). βCD-EG-Fc was synthesized, and its redox-sensitive self-assembly behavior was investigated using various techniques. On the basis of the intermolecular host-guest recognition between the β-CD group and the Fc moiety, βCD-EG-Fc primarily formed network-like structures and then vesicles following aging for a specified time. The formation of these structures was primarily driven by hydrogen bonding. Conversely, the oxidized molecules (βCD-EG-Fc⁺) self-assembled into cationic vesicles with the absence of host-guest complexation. Upon controlling the oxidation and reduction of Fc/Fc⁺, reversible aggregate transformation was achieved. The current study resulted in a deeper understanding of β-CD/Fc redox-responsive self-assemblies and contributed to the development of a single-component host-guest inclusion model.

Highly efficient photocontrolled targeted delivery of siRNA by a cyclodextrin-based supramolecular nanoassembly

A binary supramolecular nanoassembly that can efficiently load siRNA into A549 cancer cells and inhibited cell growth by photo-irradiation was fabricated using α-CD-modified hyaluronic acid and an azobenzene-modified diphenylalanine derivative.

Cyclodextrin-Based Multistimuli-Responsive Supramolecular Assemblies and Their Biological Functions

Cyclodextrins (CDs), which are a class of cyclic oligosaccharides extracted from the enzymatic degradation of starch, are often utilized in molecular recognition and assembly constructs, primarily via host-guest interactions in water. In this review, recent progress in CD-based supramolecular nanoassemblies that are sensitive to chemical, biological, and physical stimuli is updated and reviewed, and intriguing examples of the biological functions of these nanoassemblies are presented, including pH- and redox-responsive drug and gene delivery, enzyme-activated specific cargo release, photoswitchable morphological interconversion, microtubular aggregation, and cell-cell communication, as well as a geomagnetism-controlled nanosystem for the suppression of tumor invasion and metastasis. Moreover, future perspectives and challenges in the fabrication of intelligent CD-based biofunctional materials are also discussed at the end of this review, which is expected to promote the translational development of these nanomaterials in the biomedical field.

Photolysis of a calixpyridinium-based supramolecular amphiphilic assembly and its selective turn-on fluorescence recognition of lysine in water

A new calixpyridinium-based light-responsive host-guest recognition motif was found in this work. This host-guest recognition motif was further discovered to be applied as a selective turn-on fluorescent sensor for lysine over other natural amino acids.

Construction of Chiral Nanoassemblies Based on Host-Guest Complexes and Their Responsive CD and CPL Properties: Chirality Transfer From 2,6-helic[6]arenes to a Stilbazolium Derivative

A couple of water-soluble chiral 2,6-helic[6]arene derivatives P -H1 and M -H1 were synthesized, and they could form 1:1 stable complexes with 4-[(4'-N, N-diphenylamino)-styryl]-N-methylpyridinium iodide (G) in water. Compared with G, the host-guest complexes exhibited enhanced fluorescence, which might be attributed to the spatial confinement of G and restriction of aggregation-caused quenching (ACQ) effects. Based on the host-guest complexation, the first helic[6]arene-based chiral assemblies were then constructed, and they showed rectangular or hexagonal nanostructures by scanning electron microscopy (SEM) images. Interestingly, the assemblies showed clear mirror-image circular dichroism (CD) and circularly polarized luminescence (CPL) spectra in aqueous solution, revealing a consecutive chirality transfer from the chiral macrocyclic cavities of the hosts to G. Moreover, the supramolecular chirality of the assemblies could also show responsiveness to the pH values and temperatures of the system.

Photocontrolled morphological conversion and chiral transfer of a snowflake-like supramolecular assembly based on azobenzene-bridged bis(dibenzo-24-crown-8) and a cholesterol derivative

A snowflake-like supramolecular clockwise-helical assembly was fabricated via the host–guest interaction, while a snowflake-like supramolecular non-helical assembly can be obtained upon UV-irradiation.

Light-induced morphological transition between unconjugated bilirubin photoisomers

Morphology switching by an external stimulus creates the possibility to detect and control the activity and functionality of bio-molecules. Unconjugated bilirubin (UCB), a waste product resulting from heme catabolism, is highly sensitive towards blue light induced configurational conversion from (4Z,15Z) to (4Z,15E)-bilirubin. UCB has a distinct elongated nanostructure which is readily photoswitchable to spherical by external blue light (470 nm) irradiation. Herein, the morphology alteration by blue light was nicely correlated with the photoisomerisation of UCB, using different microscopic and spectroscopic techniques. To restrict the other photo-incidents during phototreatment on UCB, a suitable time frame was calibrated by monitoring the absorption, HPLC, lifetime distribution and ¹H NMR studies. Furthermore, by the help of this morphological transition as a marker, UCB early stage photoisomerisation has also been triggered by two-photon irradiation (940 nm).

A Natural Glycyrrhizic Acid-Tailored Light-Responsive Gelator

The construction of stimuli-responsive materials by using naturally occurring molecules as building blocks has received increasing attention owing to their bioavailability, biocompatibility, and biodegradability. Herein, a symmetrical azobenzene-functionalized natural glycyrrhizic acid (trans-GAG) was synthesized and could form stable supramolecular gels in DMSO/H₂ O and MeOH/H₂ O. Owing to trans-cis isomerization, this gel exhibited typical light-responsive behavior that led to a reversible gel-sol transition accompanied by a variation in morphology and rheology. Additionally, this trans-GAG gel displayed a distinct injectable self-healing property and outstanding biocompatibility. This work provides a simple yet rational strategy to fabricate stimuli-responsive materials from naturally occurring, eco-friendly molecules.

A hyaluronidase/temperature dual-responsive supramolecular assembly based on the anionic recognition of calixpyridinium

We have successfully constructed a supramolecular assembly based on the anionic recognition of calixpyridinium for the first time employing native biocompatible polysaccharide hyaluronan as the guest, which showed hyaluronidase-responsive disassembly and temperature-responsive morphological conversion from a nanosphere to a nanosquare upon increasing the temperature.

Photocontrolled Coumarin-diphenylalanine/Cyclodextrin Cross-Linking of 1D Nanofibers to 2D Thin Films

Using coumarin-modified diphenylalanine (CO-FF) and γ-cyclodextrin (γ-CD) as building blocks, we successfully constructed one-dimensional nanofibers with several nanometers in width and tens of micrometers in length. Through the photodimerization of coumarin units in CO-FF⊂γ-CD complexes, the resultant nanofibers could be cross-linked to organic 2D thin films with a lateral dimension of tens of micrometers and a thickness of nanometers. The resultant thin films exhibited a significant fluorescence enhancement for twisted intramolecular charge-transfer (TICT) molecules and effective removal of pollutant from water through filtration. This 1D → 2D morphological conversion controlled by light may provide a novel strategy to construct the highly ordered nanostructures that can be used as templates for making nanoscaled materials with defined sizes and shapes.

Photocontrolled reversible conversion of a lamellar supramolecular assembly based on cucurbiturils and a naphthalenediimide derivative

Lamellar and helical supramolecular assemblies were constructed using cucurbiturils and a naphthalenediimide derivative. The formation of the lamellar assembly could be reversibly photocontrolled.

Modulating the Phe–Phe dipeptide aggregation landscape via covalent attachment of an azobenzene photoswitch

A covalently modified trans-H-(l)AzoPhe–Phe–OH dipeptide shows reversible photo-switching between its native fibril and vesicle aggregate morphology.

Hierarchical supramolecular hydrogels: self-assembly by peptides and photo-controlled release via host–guest interaction

Using photoresponsive host–guest chemistry, three different payloads can be photo-released successively from the same peptide hydrogel.

Effects of side chains of oxatub[4]arene on its conformational interconversion, molecular recognition and macroscopic self-assembly

The side-chain length of oxatub[4]arenes was shown to affect its conformational interconversion, molecular recognition and macroscopic self-assembly behavior.