Gel-sol Transformation Research Articles

Red-light-responsive cucurbit[8]uril based host–guest interaction enabling the construction of a photoswitchable supramolecular polymeric hydrogel

A red/blue light induced photoswitchable host–guest system based on CB[8] and water soluble low cytotoxic azo derivatives is constructed, enabling the fabrication of supramolecular polymeric hydrogel with red light-induced gel–sol transformation.

Thixotropic composite hydrogels based on agarose and inorganic hybrid gellants

Water can be used as oxidant in conjunction with metal particles to form metal–water propellant to increase the energy of propellant.For this application, water needs to be stored in form of solid and capable of becoming liquid when use. Stable and thixotropic hydrogel has good potential as water-retaining material and oxidant of metal-based propellant. In this study, we prepared organic/inorganic composite hydrogels by combining inorganic gellants hectorite and fumed silica with organic gellant agarose, respectively. The total content of the gellants can be reduced to less than 2% by adding agarose. The influence of agarose on water content, phase transition temperature, centrifugal stability and other basic physical properties of composite hydrogels were discussed. The results show that the composite hydrogels have better thixotropy and stability thanpure inorganic hydrogels, and the gel–sol transformation can be realized by applying shear force or heating to the phase transition temperature. The composite hydrogels have good shear thinning ability and improved mechanical stability. Fumed silica/agarose hydrogels have better physical stability, while the thixotropy and shear thinning ability of hectorite/agarose hydrogels are better.

A fluorescent controllable supramolecular crosslinked polymer constructed by complementary metal coordination interaction.

In this work, two different monomers M1 and M2 were designed and synthesized. M1 + M2 + Zn(OTf)2 could self-assemble to form a supramolecular crosslinked polymer (SCP) based on complementary terpyridine-based metal coordination interaction. The self-assembly of M1 + M2 + Zn(OTf)2 was studied by various techniques, such as 1H NMR, 2D COSY NMR, 2D NOESY NMR, UV-Vis analysis, fluorescence analysis, viscosity measurement, and TEM. The experimental result indicated that the molecular weight of the SCP depended on the initial monomer concentration. The SCP could further turn into supramolecular polymer gel at high concentrations, and the reversible gel-sol transformation could be realized by heating/cooling. Moreover, the fluorescence quenching/enhancement of the SCP could be adjusted by adding base/acid.

Self-healing, pH-sensitive and shape memory hydrogels based on acylhydrazone and hydrogen bonds

Smart hydrogels have wide potential in biological field owing to their self-healing property, pH or temperature sensitivity. However, the preparation of smart hydrogels often requires complicated synthesis process, and the single functionality of hydrogels might limit their application. To solve these problems, we developed a self-healing, pH-sensitive and shape memory hydrogel of poly (diacetone acrylamide-co-acrylamide-co-acrylic acid)/alginate (PDAAc-A), taking advantage of the Schiff-base reaction and the hydrogen bonding interaction in a one-step method. The tensile modulus, fracture stress, fracture strain and toughness of the developed PDAAc-A3 hydrogel are 0.021 MPa, 0.013 MPa, 7.5 mm/mm and 0.07 MJ/m2, respectively. The dynamic and reversible acylhydrazone and hydrogen bonds endow hydrogels with self-healing property and pH-sensitive gel-sol transformation ability. The tensile tests demonstrate that the self-healing efficiency of hydrogel could reach up to 85% after about 30 min. Furthermore, PDAAc-A hydrogel exhibits pH-sensitivity during drug release. In PBS solution with pH value of 5, the hydrogel shows the fastest drug release rate and the highest drug release ratio, which is near 100% after about 4 h. Moreover, in 1 M ferric chloride solution, the shape memory time, shape memory ratio and shape recovery ratio of the resultant hydrogel are 2 min, 100% and 100%, respectively.

The underlying mechanism of alkali-induced ovalbumin gel transforms to sol: Physicochemical properties, structure and quantitative protein degradation analysis

Different alkalinity could impart certain desirable properties to ovalbumin (OVA), such as improving gel texture or solidification time. In this study, the physicochemical properties, structural changes, and peptide-based quantitative analysis during degradation of OVA treated with 1.25%, 1.5% and 1.75% NaOH solutions at different time were investigated. With the extension of time, the hardness, water holding capacity, particle size, surface hydrophobicity of OVA gel and the number of peptides increased dramatically, and then decreased gradually regardless of alkalinity. By contrast, the pH and springiness of OVA gel exhibited a steady downward trend. Rheological behavior revealed that the OVA gel network degraded from a high viscosity semi-fluid to an aqueous sol. The ordered gel structure was mainly contributed by β-sheets, accompanied by constantly conversion of α-helices, while the liquefaction was caused by the destruction of those regular secondary structures. LC-MS/MS showed that only a small number of common peptides were obtained at different time of protein degradation, and more similar sequences were observed in peptides generated in one day. In summary, it was concluded that although alkali-induced OVA showed a definite gel-sol transformation, the underlying mechanism could not be precisely expressed by degradation into peptides of specific sequences.

Pyromellitic-Based Low Molecular Weight Gelators and Computational Studies of Intermolecular Interactions: A Potential Additive for Lubricant.

Low molecular weight gelators (LMWG) have been extensively explored in many research fields due to their unique reversible gel-sol transformation. Intermolecular interactions between LMWG are known as the main driving force for self-assembly. During this self-assembly process, individually analyzing the contribution difference between various intermolecular interactions is crucial to understand the gel properties. Herein, we report 2,5-bis(hexadecylcarbamoyl)terephthalic acid (BHTA) as a LMWG, which could efficiently form a stable organogel with n-hexadecane, diesel, liquid paraffin, and base lubricant oil at a relatively low concentration. To investigate the contribution difference of intermolecular interactions, we first finished FT-IR spectroscopy and XRD experiments. On the basis of the d-spacing, a crude simulation model was built and then subjected to molecular dynamics (MD) simulations. Then, we knocked out the energy contribution of the H-bonding interactions and π-π stacking, respectively, to evaluate the intermolecular interactions significantly influencing the stability of the gel system. MD simulations results suggest that the self-assembly of the aggregates was mainly driven by dense H-bonding interactions between carbonyl acid and amide moieties of BHTA, which is consistent with FT-IR data. Moreover, wave function analysis at a quantum level suggested these electrostatic interactions located in the middle of the BHTA molecule were surrounded by strong dispersion attraction originating from a hydrophobic environment. Furthermore, we also confirmed that 2 wt % BHTA was able to form gel lubricant with 150BS. The coefficient of friction (COF) data show that the gel lubricant has a better tribological performance than 150BS base lubricant oil. Finally, XPS was performed and offered valuable information about the lubrication mechanism during the friction.

Coordination-Driven Poly[2]Pseudorotaxanes in Highly Polar Organic Solvent.

Self-assembly of polypseudorotaxanes in high-polar organic solvents is difficult due to remarkably weak interactions between macrocycles and axles. Reported here is a novel metal-coordinated poly[2]pseudorotaxane constructed by pillar[5]arene, 1,4-bis(4-pyridyl pyridinium)butane, and [PdCl2(PhCN)2] in highly polar organic solvent of dimethyl sulfoxide (DMSO). Utilizing a combination of 1H NMR, NOESY, DOSY, DLS, SEM, and viscosity measurements, the formation of polypseudorotaxane was shown to be dependent on the concentration of [2]pseudorotaxanes/[PdCl2(PhCN)2] and temperature. Furthermore, a temperature-responsive supramolecular gel with reversibly gel–sol transformation was obtained via spontaneous assembly of the polypseudorotaxanes at high concentrations.

The sol-gel-sol transformation behavior of egg white proteins induced by alkali

In the current study, we found an interesting phenomenon that fresh egg white (EW) undergo the sol-gel-sol transition with alkali treatment. The transformation behavior at different alkalinity (1.5%, 2.0%, and 2.5%) was investigated. As the gel formed, the hardness, lightness, surface hydrophobicity and the total number of identified peptides increased, and then, remarkable reduction when the gel collapsed. Rheological behavior indicated that the viscosity varied with shear rate. Fourier transform infrared spectroscopy (FTIR) showed that β-sheets gradually decreased as the α-helices increased during gel-sol transformation. The quantification of EW peptides analysis revealed that there was no dramatic correlation between the number of identified peptides and alkalinity. It was concluded that the sol-gel-sol transition was strongly dependent on alkali levels, moreover, high concentration promoted gel formation as well as liquefaction. The EW transformation behavior induced by alkali had a significant effect on protein aggregation and denaturation, and further changed physicochemical properties.

A monopyrrolotetrathiafulvalene based naphthalimide tailored organogelator with stimuli responsive properties and absorption for rhodamine B

A novel 1,8-naphthalimide functionalized monopyrrolotetrathiafulvalene (MPTTF) derivative had been established as a low molecular weight supramolecular gelator. Supramolecular organogels could be obtained in solvents with different polarity through the heating-cooling process. Further addition of 7,7,8,8-tetracyanoquino-dimethane (TCNQ) to the supramolecular gel systems via the electron donor-acceptor interaction between the MPTTF and TCNQ, especially in polar solvents, could fabricate charge-transfer (CT) complex gels at relatively lower critical gelation concentrations. Interestingly, the self-assembly mode of the CT complexes was closely related to the type of solvents. As expected, due to the dynamic and reversible nature of the non-covalent interactions, the resultant gels exhibited external stimuli-responsiveness to different parameters, including temperature, chemical redox reagent and acid-base. However, the redox stimulus response of the gels depended on the polarity of the solvents, and the reversible gel-sol transformation could be achieved in nonpolar solvent, such as benzene. Moreover, the prepared gels showed irreversible gel-sol transitions on successively triggering with trifluoroacetic acid and triethylamine unless washed with water. Importantly, the original and CT complex gels could effectively remove the cationic rhodamine B dye from water.

Facile Access to Guar Gum Based Supramolecular Hydrogels with Rapid Self-Healing Ability and Multistimuli Responsive Gel-Sol Transitions.

In this work, we prepare guar gum-based supramolecular hydrogel through the formation of borate/didiol bonds. This dynamic and reversible noncovalent borate/didiol interaction is critical for the multifunctional properties of supramolecular hydrogel. The FT-IR and XRD analysis verified the existence of boronate ester interactions between borax and guar gum. Moreover, the viscoelastic and mechanical behaviors of the hydrogels with different guar gum concentrations showed that the storage modulus and compressive stress were highest at guar gum concentration of 2 wt %. Besides, due to the dynamic and reservable properties of boronate ester, these guar gum-based hydrogels had excellent self-healing property, outstanding reformable and injectable capability. In addition, hydrogels also exhibited reversible gel-sol transformations by the application of physicochemical stimuli such as thermal and pH value. The coupling of these multifunctional properties made from low-cost, environment friendly, and readily available materials could have potential applications in many biomedical areas in the future. We expect that this simple strategy of fabricating the self-healing guar gum hydrogels with multistimuli responsive properties may enrich the avenue in the exploration of multifunctional guar gum based hydrogels to expand their potential applications in various fields.

Gel Formed by Self-Assembly of a Urea-Modified Monopyrrolotetrathiafulvalene Derivative Displays Multi-Stimuli Responsiveness and Absorption of Rhodamine B.

A new monopyrrolotetrathiafulvalene-based derivative containing a urea group was designed, synthesized and thoroughly characterized. It proved to be a non-gelator in a single solvent even when heated or sonicated. However, it could self-assemble in a CHCl3 (CH2 Cl2 )/n-hexane mixture to form a thermo-responsive supramolecular organogel. SEM, FT-IR spectroscopy, UV/Vis absorption spectroscopy, and SAXS revealed that in the organogel system, the gelators self-assembled into supramolecular networks with a J-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen-bonding, and van der Waals forces. Interestingly, the gel phase was shown to undergo reversible gel-sol transformation induced by Fe3+ -Vitamin C (Vc), trifluoroacetic acid-triethylamine (TFA-TEA) and picric acid (PA)-NaOH. In particular, in the presence of picric acid, the experimental results proposed that charge transfer occurred from the electron-donor gelator to the electron-acceptor picric acid due to the possibility of complex formation. Furthermore, the formed organogel could behave as the matrix for encapsulating cationic fluorescent dye from wastewater, and the adsorption efficiency was directly proportional to the concentration of the gelator.

Low-molecular-weight photoresponsive supramulecular hydrogel based on a dicationic azobenzene-bridged pyridinium hydrogelator

Low-molecular-weight supramolecular hydrogels are of significant attractive soft materials as particular functions can be facilely introduced by the straightforward fabrication of such self-assembled systems. In this study, an azobenzene-bridged dicationic pyridinium salt was synthesized, from which photoresponsive supramolecular hydrogel could be fabricated through the π-π stacking and hydrophobic interactions in the aqueous solution. By taking advantages of the UV–vis light induced E/Z photoisomerization behaviors of the incorporated azobenzene photochromophore, reversible gel-sol transformation of such supramolecular hydrogel could be achieved under the alternating UV–vis irradiation conditions. We believed that this photoresponsive supramolecular hydrogel will be a good supplementary in the creation of intelligent soft material.

A Multifunctional Metallohydrogel with Injectability, Self‐Healing, and Multistimulus‐Responsiveness for Bioadhesives

AbstractMultifunctional hydrogel bioadhesives have great prospects in biomedical applications, but their design still faces great challenges, such as multiple and tedious chemical modifications. However, it is difficult to integrate injectable, self‐healing, and stimulus‐responsive properties together. A facile approach based on dynamic metal‐ligand coordination chemistry between chondroitin sulfate (CS) and Fe3+ in the design and synthesis of novel multifunctional metallohydrogel bioadhesives is reported. This CS‐based hydrogel not only has strong tissue adhesion superior to that of commercial fibrin glue, but also exhibits an excellent self‐healing ability and injectability, which are beneficial in the field of bioadhesives. Moreover, the hydrogels undergo a rapid gel–sol transformation in response to multiple external stimuli, including pH, ions, neutral molecules, and chemical redox reactions enabling the rapid removal of the bioadhesive. In addition, metallohydrogels are rapidly formed within 10 s, quick enough to promptly seal the tissue. Importantly, the multifunctional CS‐based bioadhesives are shown to exhibit good biocompatibility, thus allowing the developed materials to meet key requirements for next‐generation tissue adhesives.

UV gelation of single-component polyacrylates bearing dinitrobenzoate side groups

Polyacrylates bearing dinitrobenzoate side groups undergo sol-gel-sol transformations in DMF or THF solutions regulated by alternating UV light and dark conditions. The formation and recombination of radical ionic species via photoinduced electron transfer may be responsible.

Methyl Cinnamate-Derived Fluorescent Rigid Organogels Based on Cooperative π-π Stacking and C═O···π Interactions Instead of H-Bonding and Alkyl Chains.

A new class of rigid low-molecular-mass organic gelators (LMOGs) was synthesized by McMurry and Heck reactions, and their gels and photophysical properties were investigated. The LMOGs lacked alkyl chain and H-bonding units and produced good gelation ability in selected mixed organic solvents facilitated by cooperative π-π stacking and C═O···π interactions. Sensitive gel-sol transformation by molecular aggregation and disaggregation was easily achieved upon heating and cooling. H-H 2D NOESY and X-ray diffraction (XRD) patterns showed the π-π stacking and C═O···π interactions between tiny methyl acrylate groups as "tails". Importantly, this soft interaction model offers a useful tool for the future design and construction of supramolecular structures. At present, the LMOGs reported herein offer a sensitive gel-formation ability and aggregation-induced emission (AIE) property and thus have promising application potentials as functional soft matter in amorphous materials, photoelectric materials, and so on.

Stimuli-responsive bile acid-based metallogels forming in aqueous media

The synthesis and gelation properties of a picolinic acid conjugated bile acid derivative in the presence of metal salts along with the stimuli-responsiveness of the systems are reported. The gels are formed in the presence of Cu2+ ions in the solvent systems composed of 30–50% of organic solvent (MeOH, acetonitrile, or acetone) in water. The gels respond to various stimuli: they can be formed upon sonication or shaking, and their gel–sol transformation can be triggered by a variety of chemical species. NMR, MS, and SEM techniques are exploited in order to gain a deeper insight on the self-assembled systems.

Fluorescent hydrogels with tunable nanostructure and viscoelasticity for formaldehyde removal.

Hydrogels with ultrahigh water content, ∼99 wt %, and highly excellent mechanical strength were prepared by 4'-para-phenylcarboxyl-2,2':6',2″-terpyridine (PPCT) in KOH aqueous solution. The self-assembled structure, rheological properties, and the gel-sol transformation temperature (Tgel-sol) of PPCT/KOH hydrogels that depend on PPCT and KOH concentrations were studied, indicating easily controllable conditions for producing hydrogels in PPCT and KOH mixtures. An important finding was that the hydration radius (Rh) of cations (M(+) = Li(+), Na(+), K(+), Cs(+), NH4(+), (CH3)4N(+), (CH3CH2)4N(+), (CH3CH2CH2)4N(+), (CH3CH2CH2CH2)4N(+)) plays a vital role in gelation of PPCT/MOH systems. To produce hydrogels in PPCT/MOH systems, the Rh of M(+) must be in a suitable region of 3.29 to 3.58 Å, e.g., K(+), Na(+), Cs(+), and the capability of M(+) for inducing PPCT to form hydrogels is K(+) > Na(+) > Li(+), which is followed by the Hofmeister series. The hydrogels of PPCT and KOH mixtures are responsive to external stimuli including temperature and shearing force, and present gelation-induced enhanced fluorescence emission property. The states of being sensitive to the stimuli can readily recover to the original hydrogels, which are envisaged to be an attracting candidate to produce self-healing materials. A typical function of the hydrogels of PPCT and KOH mixtures is that formaldehyde (HCHO) can speedily be adsorbed via electrostatic interaction and converted into nontoxic salts (HCOOK and CH3OK), making it a promising candidate material for HCHO removal in home furnishings to reduce indoor environmental pollutants.

Supramolecular hydrogels from inclusion complexation of α-cyclodextrin with densely grafted chains in micelles for controlled drug and protein release.

An amphiphilic Py-PCL-b-POEGMA copolymer was prepared by the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Based on the host-guest inclusion complexation of α-CD with densely grafted chains of the POEGMA shell in Py-PCL-b-POEGMA micelles, supramolecular micellar hydrogels were obtained by adding α-CD into Py-PCL-b-POEGMA micellar solutions. Resulting from the dissociation of α-CD from the ICs upon heating, the Py-PCL-b-POEGMA IC-based supramolecular hydrogels presented thermo-responsive properties. As they were heated, the IC-based hydrogels underwent gel-sol transformation because of the breakage of the physical cross-links. Benefiting from the fluorescent pyrene group in the Py-PCL-b-POEGMA copolymer, the system showed fluorescent properties. And the fluorescent intensity decreased obviously and regularly during the sol-gel transformation process, which makes it possible to detect the sol-gel transformation through sensing the fluorescent intensity of the system. Due to the biodegradable and biocompatible properties, the IC-based hydrogels were used as carriers for delivery systems. The hydrogels showed good properties for controlled release, and the release rate and level can be controlled by temperature.

A hexaazatriphenylene-based organogel that responds to silver(I) with high selectivity under aqueous condition

A hexaazatriphenylene (HAT)-based gelator was developed. It exhibits excellent gelation ability. The resulting organogel shows very high selectivity toward Ag+ ion under aqueous condition in the form of gel–sol transformation. The mechanism of such transformation was also investigated through UV–vis spectroscopy and powder X-ray diffraction.

Thixotropic phenomenon in flocculated aqueous dispersions of acrylate methacrylate copolymers

Aqueous gels of two analogous, water-insoluble copolymers A and B have been formed by addition of excess water to concentrated ethanol solutions of the polymers. A and B differed only in their content of cationic groups in a ratio 2:1 (A:B). The gels were converted permanently to fluids (i.e. gel-sol transformation) at high shear rates. Stability of the resulting polymer dispersions depended on the presence and mutual repulsion of the polymer cations. Polymer B dispersions were less stable to the flocculating effect of an electrolyte (sodium chloride). At certain critical concentrations, 0.2 M NaCl (for polymer A), or 0.1 M NaCl (for polymer B) the electrolyte flocculated the otherwise stable dispersions to a gel structure. The electrolyte-flocculated gels were readily redispersed to fluids by shaking but reverted to gels on standing (thixotropic). In contrast the original coacervated gels (without electrolyte) could not be redispersed easily with manual shaking. Lower polymer-polymer interaction in the thixotropic system relates possibly to increases in particle size and irregularity of particle shape during flocculation.