Xiaoxiao Chu

Hybrid Gels Assembled from Fmoc–Amino Acid and Graphene Oxide with Controllable Properties

A supramolecular gel is obtained from the self-assembly of an ultralow-molecular-weight gelator (N-fluorenyl-9-methoxycarbonyl glutamic acid) in good and poor solvents. The gelators can self-assemble into a lamellar structure, which can further form twisted fibers and nanotubes in the gel phase. Rheological studies show that the gels are robust and rigid, and are able to rapidly self-recover to a gel after being destroyed by shear force. Fluorescence experiments reveal the aggregation-induced emission effects of the gel system; the fluorescence intensity is significantly enhanced by gel formation. Graphene oxide (GO) is introduced into the system efficiently to give a hybrid material, and the interaction between gelators-GO sheets is studied. Rheological and fluorescent studies imply that the mechanical properties and the fluorescent emission of the hybrid materials can be fine-tuned by controlling the addition of GO.

Melamine as an Effective Supramolecular Modifier and Stabilizer in a Nanotube-Constituted Supergel

Self-assembly of N-fluorenyl-9-methoxycarbonyl glutamic acid (Fmoc-Glu) in water generates metastable single-wall nanotubes. These nanotubes entangle and bundle together to form unstable gels that shrink with time and finally result in lamellar crystalline precipitates. Melamine (Mm) was employed as a supramolecular modifier and stabilizer to improve the stability of the nanotubes. Mm interacts with the carboxyl-rich surfaces of the nanotubes via H-bonds and static electronic forces to diminish the high affinity of individual nanotubes and facilitate Fmoc-Glu supergelation (critical gelation concentration <0.1 wt %). Although the basic process of nanotube formation is not disturbed, Mm inverts the supramolecular helicity of nanotubes from P to M.

Switchable and orthogonal self-assemblies of anisotropic fibers

Anisotropic and well-defined β-cyclodextrin (β-CD) and sodium laurate (SL) gel fibers, with tetragonal and lamellar packing modes respectively, were combined to generate orthogonal and switchable assemblies for the first time. Through controlling the inclusion–exclusion processes between β-CD and SL by changing external stimuli, β-CD gel fibers, SL gel fibers and their coexistence states can be easily tailored. Different assembled states exhibited versatile morphologies and microstructures, which were detected by microscopy and X-ray scattering measurements. Furthermore, controllable assemblies provide facile ways to control the mechanical properties of the hybrid gels. The orthogonal self-assembly with self-sorting properties would enable this system to attract interest in material science.

Controllable self-growth of a hydrogel with multiple membranes

The controllable self-growth of a supramolecular hydrogel of folic acid (FA) was developed based on the conduction of transition metal ions. The growth behavior of the gel could be flexibly controlled by adjusting the ambient environmental factors such as gelator concentration, temperature and external chemical stimuli. The obtained gel was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and rheological measurements. Differential scanning calorimetry (DSC) showed that the gel possessed excellent thermal stability. A mechanism for the fibrous formation of the gel was suggested based on the experimental results of Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR) and UV-Vis spectroscopy. The gel exhibited multiple stimuli-responsive properties to changes in ligand and pH. Furthermore, the gel can be incorporated into multi-layer hydrogels in both artificial and spontaneous ways, showing the advantages of self-growth and flexible control of the gel system. This novel hydrogel and the preparation strategy may provide a new route to rationally design advanced materials for biomedical applications.

Self-recovering β-cyclodextrin gel controlled by good/poor solvent environments

A unique β-cyclodextrin (β-CD) gel with ordered structure was fabricated. The ordered aggregation of β-CD was achieved by controlling the solubility of β-CD in good/poor solvents. Physicochemical properties of the gel were systematically investigated. Mechanical properties were tested by rheological measurements, and the fibrous morphology was observed by confocal laser scanning microscopy. The anisotropic property of gel fibers was detected under polarized optical microscopy. Well-defined tetragonal and channel packed microstructures were formed in gel fibers, which revealed hollow channels of cyclodextrins. By changing physicochemical environments, macroscopic phase transitions were aroused, and self-recovering phenomena occurred when a certain amount of NaCl was added into the gel as a stimulus or the shear rate loop test was performed (thixotropy). By adjusting the concentrations of external stimuli or volume ratios of good/poor solvents, properties of the gel including mechanical strength or thermostability can be easily altered. The ordered empty channels assembled by CDs with the self-recovering properties may shed new light on this traditional host molecule.

Reversible pH-responsive helical nanoribbons formed using camptothecin

The natural antitumor drug camptothecin was found to self-assemble into helical nanoribbons in aqueous solution. The helical nanoribbons showed good reversible pH-responsiveness. The formation and disappearance of the helical nanoribbons can be tuned reversibly through changing the pH value of the solution.

A multistimuli-responsive supramolecular vesicle constructed by cyclodextrins and tyrosine

Stimuli-responsive vesicle is a significant self-assembly. The supramolecular amphiphile strategy provides a simple way to construct the vesicles. Here, we report a β-cyclodextrin/tyrosine supramolecular vesicle based on host-guest recognition. Tyrosine, which is a nontoxic amino acid, is used as the vesicles building blocks. The β-cyclodextrin/tyrosine vesicles were identified by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scattering (DLS). X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) spectrum, hydrogen-1 nuclear magnetic resonance (1H NMR), and two-dimensional nuclear magnetic resonance spectroscopy rotating frame nuclear Overhauser effect spectroscopy (2D NMR ROESY) were further employed to elucidate the vesicle formation mechanism. Furthermore, different host molecules, including α-cyclodextrin and γ-cyclodextrin, can all form vesicles with tyrosine. Finally, the vesicles can respond to multiple external stimuli. Competitive guest molecules and copper ions can all disrupt the vesicles’ architectures.

A supramolecular vesicle of camptothecin for its water dispersion and controllable releasing.

Camptothecin, as an antitumor drug, has shown significant antitumor activity against various cancers through the inhibition of topoisomerase I. However, its poor solubility severely limits the clinical applications. Here, we report a camptothecin supramolecular vesicle based on the host-guest interactions, which can uniformly disperse camptothecin into water and greatly enhance camptothecin aqueous solubility. The camptothecin vesicles were identified by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scattering (DLS). X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-vis spectrum, 1H NMR and 2D NMR ROESY were further employed to study the formation mechanism of the vesicles. Furthermore, camptothecin could be controllably released when the competitive guests were added into the vesicles system. Finally, the camptothecin vesicles in aqueous solution exhibited comparable antitumor activity in vitro as natural camptothecin in DMSO to HeLa cells under the same conditions.

Utilizing dual responsive supramolecular gel to stabilize graphene oxide in apolar solvents

Here, we report a dual stimuli-responsive organogel which can stabilize graphene oxide (GO) in apolar solvents. The gelator 1-octadecyl-ureido-naphthalene (OUN) was synthesized, and it could form organogel in toluene and xylene. The resultant gels can respond to thermal and anion stimuli, bringing about fluorescent changes. Thin nanoribbons are entangled together to form three dimensional networks that can immobilize solvents in gel, providing large superficial area to interact with GO sheets. The addition of GO influences the gel properties, which are studied through rheological, fluorescent, and DSC measurements.

Dual-tuning multidimensional superstructures based on a T-shaped molecule: vesicle, helix, membrane and nanofiber-constructed gel

As a small molecule, N-(9-fluorenylmethoxy carbonyl)-glycine (FG) possesses a hydrophobic plane domain (π-conjugated section) and a hydrophilic domain (amino acid section), which can be designed into multi-dimensional self-assembly structures under dual tuning. Through controlling the concentration of T-shaped FG, the transformation between various morphologies has been achieved: vesicles are obtained at low concentrations (0.0025–0.005 wt%); helical fibers can be found at the concentration of 0.2 wt% though Gly has no stereocenter, resulting in the birth of chiral organization; fibrous bundles can accumulate into a three dimensional network to finally form a supramolecular gel. Taking gel as an example, we devised a variety of nanostructures including nanoparticles, microparticles, nanoribbons and membranes obtained by adding base. The mechanism of self-assembly formation has been investigated and this system is hoped to enrich the category of nanomaterials from amino acid or short peptides.