Directed Nanoscale Self-Assembly of Low Molecular Weight Hydrogelators Using Catalytic Nanoparticles

Microfluidic-Assisted Self-Assembly of Molecular Hydrogelator at Water-Water Interfaces for Continuous Fabrication of Supramolecular Microcapsules

Control over the self-assembly of small molecules at specific areas is of great interest for many high-tech applications, yet remains a formidable challenge. Here, how the self-assembly of hydrazone-based molecular hydrogelators can be specifically triggered at water-water interfaces for the continuous fabrication of supramolecular microcapsules by virtue of the microfluidic technique is demonstrated. The non-assembling hydrazide- and aldehyde-based hydrogelator precursors are distributed in two immiscible aqueous polymer solutions, respectively, through spontaneous phase separation. In the presence of catalysts, hydrazone-based hydrogelators rapidly form and self-assemble into hydrogel networks at the generated water-water interfaces. Relying on the microfluidic technique, microcapsules bearing a shell of supramolecular hydrogel are continuously produced. The obtained microcapsules can effectively load enzymes, enabling localized enzymatic growth of supramolecular fibrous supramolecular structures, reminiscent of the self-assembly of biological filaments within living cells. This work may contribute to the development of biomimetic supramolecular carriers for applications in biomedicine and fundamental research, for instance, the construction of protocells.

Ready, Set, Grow: From Micelles to Giant Vesicles via Biocatalytic Activation

Controlling physicochemical processes that drive changes in supramolecular aggregates is an important objective toward creating artificial soft micro- and nanomachines. Previous research explored the morphology control of membrane-based materials subjected to externally imposed chemical stimuli. Here, we modulate the microscale morphology of pH-responsive assemblies by using biocatalysis to internally generate changes in global pH. Catalytic reactions offer flexibility in the mechanism and rate at which stimuli are introduced to responsive assemblies, ultimately enabling precision and control over size and morphology. We observed, by dynamic light scattering and fluorescence microscopy, substantial microscale differences between assemblies subjected to manually titrated pH changes compared to biocatalytically activated pH changes, including the growth of giant vesicles from micelles. Coarse-grained molecular dynamics simulations of these metastable self-assembled structures provided insight into the thermodynamics and kinetics of the preferred structures. These results demonstrate the feasibility of using biocatalytic reactions to modulate the size and morphology of supramolecular assemblies, from micelles to giant vesicles.

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.

Recent Progress in Peptide-Based Molecular Probes for Disease Bioimaging

Recent strides in molecular pathology have unveiled distinctive alterations at the molecular level throughout the onset and progression of diseases. Enhancing the in vivo visualization of these biomarkers is crucial for advancing disease classification, staging, and treatment strategies. Peptide-based molecular probes (PMPs) have emerged as versatile tools due to their exceptional ability to discern these molecular changes with unparalleled specificity and precision. In this Perspective, we first summarize the methodologies for crafting innovative functional peptides, emphasizing recent advancements in both peptide library technologies and computer-assisted peptide design approaches. Furthermore, we offer an overview of the latest advances in PMPs within the realm of biological imaging, showcasing their varied applications in diagnostic and therapeutic modalities. We also briefly address current challenges and potential future directions in this dynamic field.

Dynamic supramolecular hydrogels mediated by chemical reactions

Supramolecular self-assembly in a biological system is usually dominated by sophisticated metabolic processes (chemical reactions) such as catalysis of enzymes and consumption of high energy chemicals, leading to groups of biomolecules with unique dynamics and functions in an aqueous environment. In recent years, increasing efforts have been made to couple chemical reactions to molecular self-assembly, with the aim of creating supramolecular materials with lifelike properties and functions. In this feature article, after summarising the work of chemical reaction mediated supramolecular hydrogels, we first focus on a typical example where dynamic self-assembly of molecular hydrogels is activated by in situ formation of a hydrazone bond in water. We discuss how the formation of the hydrazone-based supramolecular hydrogels can be controlled in time and space. After that, we describe transient assembly of supramolecular hydrogels powered by out-of-equilibrium chemical reaction networks regulated by chemical fuels, which show unique properties such as finite lifetime, dynamic structures, and regenerative capabilities. Finally, we provide a perspective on the future investigations that need to be done urgently, which range from fundamental research to real-life applications of dynamic supramolecular hydrogels.

Bifunctional Polymer Brush Reactor for In Situ Synthesis of Hollow Silica-Supported Gold Nanocatalysts

Gold nanoparticles (AuNPs) on carriers have received wide attention as catalysts as a result of their excellent stability and catalytic performance. Herein, we report the design and synthesis of hollow silica-supported gold nanocatalysts (SNPs@AuNPs) composed of highly dispersed AuNPs with approximately 4.30 nm using an in situ colloidal polyelectrolyte template strategy. The monodisperse polystyrene nanospheres accompanied by poly[(2-methacryloyloxyethyl)trimethylammonium chloride] brushes were first synthesized. Subsequently, the facile polymer-brush-engaged strategy for the synthesis of hollow SNPs@AuNPs involves in situ reduction of AuNPs, hydrolytic condensation of silica, and a chemical etching process. In combination with dynamic light scattering, transmission electron microscopy, small-angle X-ray scattering, X-ray powder diffraction, and Fourier transform infrared spectroscopy, the as-obtained polymer brushes were proven as effective versatile nanoreactors for the synthesis of AuNPs and silica nanoparticles without any catalysts. Benefiting from the structural advantages, the resultant hollow SNPs@AuNPs manifested superior catalytic activity and reusability for the reduction of p-nitrophenol by sodium borohydride in aqueous solution. With a delicate design, we believe that this synthetic strategy can be extended to fabricate multifunctional nanomaterials with diverse compositions, which would be of great interest in catalysis, energy, and many other important domains.

Injectable Tripeptide/Polymer Nanoparticles Supramolecular Hydrogel: A Candidate for the Treatment of Inflammatory Pathologies

Supramolecular peptide-based hydrogels attract great attention in several fields, i.e., biomedicine, catalysis, energy, and materials chemistry, due to the noncovalent nature of the self-assembly and functional tunable properties defined by the amino acid sequence. In this work, we developed an injectable hybrid supramolecular hydrogel whose formation was triggered by electrostatic interactions between a phosphorylated tripeptide, Fmoc-FFpY (F: phenylalanine, pY: phosphorylated tyrosine), and cationic polymer nanoparticles made of vinylimidazole and ketoprofen (poly(HKT-co-VI) NPs). Hydrogel formation was assessed through inverted tube tests, and its fibrillary structure, around polymer NPs, was observed by transmission electron microscopy. Interestingly, peptide self-assembly yields the formation of nontwisted and twisted fibers, which could be attributed to β-sheets and α-helix structures, respectively, as characterized by circular dichroism and infrared spectroscopies. An increase of the elastic modulus of the Fmoc-FFpY/polymer NPs hybrid hydrogels was observed with peptide concentration as well as its injectability property, due to its shear thinning behavior and self-healing ability. After checking their stability under physiological conditions, the cytotoxicity properties of these hybrid hydrogels were evaluated in contact with human dermal fibroblasts (FBH) and murine macrophages (RAW 264.7). Finally, the Fmoc-FFpY/polymer NPs hybrid hydrogels exhibited a great nitric oxide reduction (∼67%) up to basal values of pro-inflammatory RAW 264.7 cells, thus confirming their excellent anti-inflammatory properties for the treatment of localized inflammatory pathologies.

Coacervation of Spherical Polyelectrolyte Brushes with Additional Polyelectrolytes Bearing Positive or Negative Charges

By combining small-angle X-ray scattering, wide-angle X-ray scattering, and rheology, the effect of additional polyelectrolyte chains on interactions among spherical polyelectrolyte brushes (SPB) was systematically investigated both on microscopic and macroscopic levels. The negatively charged poly(acrylic acid) (PAA) chains and positively charged poly(dimethyl diallyl ammonium chloride) (PDDA) chains were used as additional polyelectrolyte chains to investigate the local ordered structure and the "polyelectrolyte peak" among SPB. Interestingly, coacervation appeared in the SPB emulsion while introducing additional free polyelectrolyte chains. The addition of excess positively charged PDDA chains would lead to the transformation of the SPB emulsion from the coacervation to the aggregation, while it has not been observed in the case of PAA chains. Moreover, it was further confirmed that the specific local ordered structure was caused by the electrostatic interaction among polyelectrolyte chains of adjacent SPB. This work could enrich our understanding of polyelectrolyte assembly in concentrated SPB, thereby greatly broadening the application fields of SPB.

Nanogel Multienzyme Mimics Synthesized by Biocatalytic ATRP and Metal Coordination for Bioresponsive Fluorescence Imaging

The design of enzyme mimics from stable and nonprotein systems is especially attractive for applications in highly specific cancer diagnosis and treatment, and it has become an emerging field in recent years. Herein, metal crosslinked polymeric nanogels (MPGs) were prepared using Fe^II ion coordinated biocompatible acryloyl-lysine polymer brushes obtained from an enzyme-catalyzed atomic transfer radical polymerization (ATRPase) method. The monoatomic and highly dispersed Fe ions in the MPGs serve as efficient crosslinkers of the gel network, and also as active centers of multienzyme mimics of superoxide dismutase (SOD) and peroxidase (POD). The catalytic activities were compared to those of conventional Fe-based nanozymes. Studies on both cells and animals verify that efficient reactive oxygen species (ROS) responsive biofluorescence imaging can be successfully realized using the MPGs.

Transient Supramolecular Hydrogels Formed by Aging-Induced Seeded Self-Assembly of Molecular Hydrogelators

Here, transient supramolecular hydrogels that are formed through simple aging-induced seeded self-assembly of molecular gelators are reported. In the involved molecular self-assembly system, multicomponent gelators are formed from a mixture of precursor molecules and, typically, can spontaneously self-assemble into thermodynamically more stable hydrogels through a multilevel self-sorting process. In the present work, it is surprisingly found that one of the precursor molecules is capable of self-assembling into nano-sized aggregates upon a gentle aging treatment. Importantly, these tiny aggregates can serve as seeds to force the self-assembly of gelators along a kinetically controlled pathway, leading to transient hydrogels that eventually spontaneously convert into thermodynamically more stable hydrogels over time. Such an aging-induced seeded self-assembly process is not only a new route toward synthetic out-of-equilibrium supramolecular systems, but also suggests the necessity of reporting the age of self-assembling building block solutions in other self-assembly systems.

Photocontrolled Hierarchical Self-Assembly of Anisotropic Micropatterns of Nanofibers onto Isotropic Surfaces

Hierarchical self-assembly is achieved using a visible light triggered photoreaction. A pro-gelator, α-diketone-2,3-didecyloxyanthracene, is photoconverted into a low molecular weight gelator, 2,3-didecyloxyanthracene (DDOA), that self-assembles into nanofibers. Spatial confinement and patterns of these nanofibers onto a surface are achieved by localizing initial nucleation with a focused laser and photogenerate subsequent fiber growth with the laser or gentler wide-field irradiation. Remarkably, collective growth of nanofibers results in anisotropic micropatterns with orientation factors (OF) reaching 79%, resulting in collective emission of linearly polarized light. The OF, distance of collective growth and fiber density, are controlled by the photoirradiation conditions and the balance of interactions between DDOA aggregates and the glass surface. An unprecedented juxtaposition of orthogonally oriented nanofiber patterns on an isotropic surface is achieved with individual control of the fibers' main direction. In perspective, this photochemical method can be extended to a large variety of self-assembling molecules.

Phase Separation in Supramolecular Hydrogels Based on Peptide Self-Assembly from Enzyme-Coated Nanoparticles

Spatial localization of biocatalysts, such as enzymes, has recently proven to be an effective process to direct supramolecular self-assemblies in a spatiotemporal way. In this work, silica nanoparticles (NPs) functionalized covalently by alkaline phosphatase (NPs@AP) induce the localized growth of self-assembled peptide nanofibers from NPs by dephosphorylation of Fmoc-FFpY peptides (Fmoc: fluorenylmethyloxycarbonyl; F: phenylalanine; Y: tyrosine; p: phosphate group). The fibrillary nanoarchitecture around NPs@AP underpins a homogeneous hydrogel, which unexpectedly undergoes a macroscopic shape change over time. This macroscopic change is due to a phase separation leading to a dense phase (in NPs and nanofibers) in the center of the vial and surrounded by a dilute one, which still contains NPs and peptide self-assemblies. We thus hypothesize that the phase separation is not a syneresis process. Such a change is only observed when the enzymes are localized on the NPs. The dense phase contracts with time until reaching a constant volume after several days. For a given phosphorylated peptide concentration, the dense phase contracts faster when the NPs@AP concentration is increased. For a given NPs@AP concentration, it condenses faster when the peptide concentration increases. We hypothesize that the appearance of a dense phase is not only due to attractive interactions between NPs@AP but also to the strong interactions of self-assembled peptide nanofibers with the enzymes, covalently fixed on the NPs.

One-Step Nanosurface Self-Assembly of d-Peptides Renders Bubble-Free Ultrasound Theranostics

The surface bioinspired modification of particles and films is a mainstream direction in biomaterial design and application. The interfacial coating of extracellular-matrix-like hydrogel can endow functional inorganic nanoparticles high circulation stability and biocompatibility but remains challenging due to large surface tension difference between organic gelators and solid nanosurfaces. Herein, the supramolecular hydrogel of NapG_dF_dF_dK around gold nanorods (Au NRs-Gel) has been constructed by the amidation-grafting modification and the protonation-induced interface-assistant assembly of peptide precursors. As a proof of concept study, the acoustic cavitation experiments and in vitro ultrasound imaging have proved that the abundant hydrophobic microdomains as well as the water-rich network in the supramolecular hydrogel can serve as valid sites to efficiently generate and stabilize nanobubbles as cavitation seeds to realize bubble-free ultrasound imaging. In vivo augmented ultrasound imaging and imaging-guided high intensity focused ultrasound (HIFU) therapy based on the Balb/c mice bearing HeLa tumor model have been conducted. As the first example of using nanosurface hydrogelation to endow nanoparticles with bubble-free ultrasound theranostic ability, this work offers a simple approach to design multifunctional nanovehicles for ultrasound-guided drug/protein/gene delivery.

Hierarchically Compartmentalized Supramolecular Gels through Multilevel Self-Sorting

Hierarchical compartmentalization through the bottom-up approach is ubiquitous in living cells but remains a formidable task in synthetic systems. Here we report on hierarchically compartmentalized supramolecular gels that are spontaneously formed by multilevel self-sorting. Two types of molecular gelators are formed in situ from nonassembling building blocks and self-assemble into distinct gel fibers through a kinetic self-sorting process; interestingly, these distinct fibers further self-sort into separated microdomains, leading to microscale compartmentalized gel networks. Such spontaneously multilevel self-sorting systems provide a "bottom-up" approach toward hierarchically structured functional materials and may play a role in intracellular organization.

Controlled Fabrication of Micropatterned Supramolecular Gels by Directed Self-Assembly of Small Molecular Gelators

Herein, the micropatterning of supramolecular gels with oriented growth direction and controllable spatial dimensions by directing the self-assembly of small molecular gelators is reported. This process is associated with an acid-catalyzed formation of gelators from two soluble precursor molecules. To control the localized formation and self-assembly of gelators, micropatterned poly(acrylic acid) (PAA) brushes are employed to create a local and controllable acidic environment. The results show that the gel formation can be well confined in the catalytic surface plane with dimensions ranging from micro- to centimeter. Furthermore, the gels show a preferential growth along the normal direction of the catalytic surface, and the thickness of the resultant gel patterns can be easily controlled by tuning the grafting density of PAA brushes. This work shows an effective "bottom-up" strategy toward control over the spatial organization of materials and is expected to find promising applications in, e.g., microelectronics, tissue engineering, and biomedicine.

A novel supramolecular AIE gel acts as a multi-analyte sensor array

Supramolecular gelator WJ could form a stable supramolecular organogel (WJG) with strong aggregation-induced emission (AIE). Utilizing the AIE gel, we successfully developed a multi-analyte sensor array which could accurately identify CN⁻, Al³⁺, and Fe³⁺ and l-Cys.