Ion gels by self-assembly of a triblock copolymer in an ionic liquid

We report a new way of developing ion gels through the self-assembly of a triblock copolymer in a room-temperature ionic liquid. Transparent ion gels were achieved by gelation of a poly(styrene-block-ethylene oxide-block-styrene) (SOS) triblock copolymer in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) with as low as 5 wt % SOS triblock copolymer. The gelation behavior, ionic conductivity, rheological properties, and microstructure of the ion gels were investigated. The ionic conductivity of the ion gels is only modestly affected by the triblock copolymer network. Its temperature dependence nearly tracks that of the bulk ionic liquid viscosity. The ion gels are thermally stable up to at least 100 degrees C and possess significant mechanical strength. The results presented here suggest that triblock copolymer gelation is a promising way to develop highly conductive ion gels and provides many advantages in terms of variety and processing.     

Natural supramolecular building blocks: from virus coat proteins to viral nanoparticles

Viruses belong to a fascinating class of natural supramolecular structures, composed of multiple copies of coat proteins (CPs) that assemble into different shapes with a variety of sizes from tens to hundreds of nanometres. Because of their advantages including simple/economic production, well-defined structural features, unique shapes and sizes, genetic programmability and robust chemistries, recently viruses and virus-like nanoparticles (VLPs) have been used widely in biomedical applications and materials synthesis. In this critical review, we highlight recent advances in the use of virus coat proteins (VCPs) and viral nanoparticles (VNPs) as building blocks in self-assembly studies and materials development. We first discuss the self-assembly of VCPs into VLPs, which can efficiently incorporate a variety of different materials as cores inside the viral protein shells. Then, the self-assembly of VNPs at surfaces or interfaces is summarized. Finally, we discuss the co-assembly of VNPs with different functional materials (178 references).     

Single-component,low molecular weight organic supergelators based on chiral barbiturate scaffolds

ABSTRACT

We report here the first chiral barbiturate to act as a single-component LMOG capable of gelating a variety of chlorinated and aromatic solvents. Solution-based DOSY NMR experiments, solid-state VP-SEM, and X-ray crystallography techniques were used to characterize chloroform-based gels at a variety of size domains. This scaffold provides a simple system to study the dynamics of gelation and self-assembly.     

The molecular basis of self-assembly of dendron-rod-coils into one-dimensional nanostructures

We describe here a comprehensive study of solution and solid-state properties of self-assembling triblock molecules composed of a hydrophilic dendron covalently linked to an aromatic rigid rod segment, which is in turn connected to a hydrophobic flexible coil. These dendron-rod-coil (DRC) molecules form well-defined supramolecular structures that possess a ribbonlike morphology as revealed by transmission-electron and atomic-force microscopy. In a large variety of aprotic solvents, the DRC ribbons create stable networks that form gels at concentrations as low as 0.2% by weight DRC. The gels are thermally irreversible and do not melt at elevated temperatures, indicating high stability as a result of strong noncovalent interactions among DRC molecules. NMR experiments show that the strong interactions leading to aggregation involve mainly the dendron and rodlike blocks, whereas oligoisoprene coil segments remain solvated after gelation. Small-angle X-ray scattering (SAXS) profiles of different DRC molecules demonstrate an excellent correlation between the degree-of-order in the solid-state and the stability of gels. Studies on two series of analogous molecules suggest that self-assembly is very sensitive to subtle structural changes and requires the presence of at least four hydroxyl groups in the dendron, two biphenyl units in the rod, and a coil segment with a size comparable to that of the rodlike block. A detailed analysis of crystal structures of model compounds revealed the formation of stable one-dimensional structures that involve two types of noncovalent interactions, aromatic pi-pi stacking and hydrogen bonding. Most importantly, the crystal structure of the rod-dendron compound shows that hydrogen bonding not only drives the formation of head-to-head cyclic structures, but also generates multiple linkages between them along the stacking direction. The cyclic structures are tetrameric in nature and stack into ribbonlike objects. We believe that DRC molecules utilize the same arrangement of hydrogen bonds and stacking of aromatic blocks observed in the crystals, explaining the exceptional stability of the nanostructures in extremely dilute solutions as well the thermal stability of the gels they form. This study provides mechanistic insights on self-assembly of triblock molecules, and unveils general strategies to create well-defined one-dimensional supramolecular objects.     

Salt/current-triggered stabilization of β-cyclodextrins encapsulated host-guest low-molecular-weight gels

Host-guest supramolecular gels were developed via the self-assembly of inclusion complexes(ICs) ofβ-cyclodextrins/phenylboronic acid gelator(PBA).Salts and current were involved in the self-assembly to stabilize the host-guest gels.The stability of the gels was greatly improved after salts were added.The stable time of gels was extended from 2.5 h to 120 h with the addition of NH_4 NO_3 at the concentration of 2.5×10^-2 g/mL.The morphology of the gel was affected by the concentrations of NH_4NO_3.SEM images revealed that the gels were three-dimensional nanofibrous networks,the sizes of fibers decreased with decreasing NH_4NO_3 concentrations,which affected the stability of gels,further proved by the rheological properties of gels.More stable gels were obtained with current stimulation,the stable time of the gel was increased from 2.5 h to 55 h with current by adding NaBF_4.The current also exhibited significant influence on the aggregation as the voltage varied(0-500 mV) with a constant concentration of salts.The result showed the self-assembly process of host-guest gel could be well controlled via the addition of salts and current to desired morphology and stability.     

Novel organogelators based on pyrazine-2,5-dicarboxylic acid derivatives and their mesomorphic behaviors

A series of new low molecular organogelators (LMOGs) with thermotropic mesophase were synthesized via the reaction of 3,6-dimethyl-pyrazine-2,5-dicarboxylic acid with p-alkoxyl anilines. These compounds readily formed stable gels in a variety of organic solvents and their self-assembly behavior, structure–property relationship were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), ¹H nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy (FTIR) and ultra-violet–visible spectroscopy (UV). The results showed a combination of intra-hydrogen bonding, π–π stacking and van der Waals interaction resulted in the aggregation of the organogelators to form three-dimension fibrous networks. The gels formed were multi-responsive to environmental stimuli, such as temperature, fluorinion, and shear stress. More importantly, all the organogelators exhibited thermotropic hexagonal column mesophase as revealed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and variable temperature XRD studies. A control compound was synthesized and its gelling ability was also checked.     

亚苄基山梨醇衍生物在有机溶剂中的自组装及凝胶化研究

从分子结构的差异、亲溶剂作用、分子几何构型、相转变热焓以及溶剂极性等方面研究了三种亚苄基山梨醇衍生物凝胶剂在有机溶剂中的自组装和凝胶化机理. 三种衍生物凝胶剂在结构上的差别仅在于亚苄基上甲基取代基数量不同. 结果表明: 由于亲溶剂作用的增加和分子几何构型的优化, 含甲基多的凝胶剂在有机溶剂中的自组装能力强, 表现在具有低的最低凝胶化浓度和高的相转变温度. 而溶剂极性的增强, 使三种衍生物凝胶剂形成的凝胶相转变温度降低. 偏光显微镜照片表明该凝胶剂在正辛醇凝胶中的聚集体晶型不同. 场发射扫描电镜照片表明三种衍生物凝胶剂自组装形成相互缠绕的纤维束网络结构. 紫外吸收光谱表明, 对比其溶液态, 三种衍生物聚集体苯环的K带发生红移, 表明π-π堆积作用是亚苄基山梨醇衍生物凝胶剂自组装的驱动力之一; 红移的幅度随苯环上甲基数量的增加而增加, 这与三种衍生物形成的分子凝胶的热稳定性相吻合.     

Strength enhancement of nanostructured organogels through inclusion of phthalocyanine-containing complementary organogelator structures and in situ cross-linking by click chemistry

Stable photoactive organogels were successfully prepared by a two-step sequence involving: 1) formation of thermoreversible organogels by use of a combination of low-molecular-weight organogelators (LMOGs) and ZnII-phthalocyanine (ZnII-Pc) moieties containing complementary organogelator structures, and 2) strength enhancement of the gels by in situ cross-linking with the aid of CuI-catalysed azide-alkyne [3+2] cycloadditions (CuAACs). The optimum click reaction was carried out between a flexible C6 aliphatic diazide and a suitable dialkyne (molar ratio 1:1) added in a low proportion relative to the organogelator system [LMOG+ZnIIPc]. The dialkyne unit was incorporated into a molecule resembling the LMOGs structure in such a way that it could also participate in the self-assembly of [LMOG+ZnIIPc]. The significant compatibility of the multicomponent photoactive organogels towards this strengthening through CuAACs allowed their sol-to-gel transition temperatures (Tgel) to be enhanced by up to 15 degrees C. The Tgel values estimated by the "inverse flow method" were in good agreement with the values obtained by differential scanning calorimetry (DSC). Rheological measurements confirmed the viscoelastic, rigid, and brittle natures of all Pc-containing gels. Transmission and scanning electron microscopy (TEM, SEM) and atomic force microscopy (AFM) revealed the fibrilar nature of the gels and the morphological changes upon cross-linking by CuAAC. Emission of a red luminescence from the dry nanoscale fibrous structure-due to the self-assembly of the Pc-containing compounds in the organogel fibres-was directly observed by confocal laser scanning microscopy (CLSM). The optical properties were studied by UV/Vis and fluorescence spectroscopy. Fluorescence, Fourier-transform infrared (FTIR) and circular dichroism (CD) measurements were also carried out to complete the physicochemical characterization of selected gels. As a proof of concept, two different organogelators (cholesterol- and diamide-based LMOGs) were successfully used to validate the general strategy.     

The remarkable role of hydrogen bond,halogen, and solvent effect on self-healing supramolecular gel

Self-healing supramolecular gels of low-molecular-weight (LMW) molecules are smart soft materials; however, the development of self-healing LMW gelator is still a challenging task because of the lack of in-depth studies about self-healing mechanisms of LMW gels and the solvent effect on gel properties. Therefore, herein a different perspective was used to study a family of D-gluconic acetal-based gelators with variable structural fragments in 14 different solvents, and a more detailed understanding of self-assembly and self-healing mechanism of supramolecular gels was attained. Based on the critical gelation concentration, phase transition temperature, and rheological data, A8 bearing an amide group in side chain and two chlorine atoms linked to benzene ring was found to be an outstanding gelator, which could form gels with good self-healing ability in a variety of solvents. Interestingly, A8 gel formed in n-BuOH demonstrates high transparency, good mechanical strength, self-supporting behavior, and great self-healing ability from mechanical damage. Based on the Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and theoretical calculation analysis, the self-assembly and self-healing mechanisms of A8 gel were proposed, indicating that a combination of hydrogen bonding and halogen effect was responsible for the efficient self-healing behavior of supramolecular gel. Furthermore, the analysis of solvent parameters indicated that the dispersion force of solvent favored gelators to self-assemble, hydrogen bonding donor ability of solvent mainly affected the formation of one-dimensional assembly, and hydrogen bonding receptor ability and polarity of solvent mainly influenced the supramolecular interactions among assemblies, significantly intervening the self-healing ability of gels. Overall, this study provides a new perspective to the understanding of gelator structure–property correlation in solvents and sheds light for future development of self-healing supramolecular gels.     

Ordered patterns and structures via interfacial self-assembly: superlattices, honeycomb structures and coffee rings

Self-assembly is now being intensively studied in chemistry, physics, biology, and materials engineering and has become an important "bottom-up" approach to create intriguing structures for different applications. Self-assembly is not only a practical approach for creating a variety of nanostructures, but also shows great superiority in building hierarchical structures with orders on different length scales. The early work in self-assembly focused on molecular self-assembly in bulk solution, including the resultant dye aggregates, liposomes, vesicles, liquid crystals, gels and so on. Interfacial self-assembly has been a great concern over the last two decades, largely because of the unique and ingenious roles of this method for constructing materials at interfaces, such as self-assembled monolayers, Langmuir-Blodgett films, and capsules. Nanocrystal superlattices, honeycomb films and coffee rings are intriguing structural materials with more complex features and can be prepared by interfacial self-assembly on different length scales. In this critical review, we outline the recent development in the preparation and application of colloidal nanocrystal superlattices, honeycomb-patterned macroporous structures by the breath figure method, and coffee-ring-like patterns (247 references).     

Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels

Utilizing metal-ligand binding as the driving force for self-assembly of a ditopic ligand, which consists of a 2,6-bis-(1'-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn(II)) and a lanthanide metal ion (La(III)), we have achieved formation of stimuli-responsive metallo-supramolecular gels. We describe herein a series of experimental studies, including optical and confocal microscopy, dynamic light scattering, wide-angle X-ray diffraction, and rheology, to explore the properties of such gels, as well as the nature of the gelation mechanism. Morphological and X-ray diffraction observations suggest gelation occurs via the flocculation of semicrystalline colloidal particles, which results in the gels exhibiting pronounced yielding and thixotropic behavior. Application of mechanical stress results in a decrease in the particle size, which is accompanied by an increase in gel strength after removal of the stress. Moreover, studies show that the presence of lanthanide(III) perchlorate increases the mechano-responsiveness of the gels, as a consequence of reduced crystallinity of the colloidal particles, presumably due to the different coordination ability of lanthanide(III) and zinc(II), which changes the nature of the self-assembly in these materials.     

Dramatic specific-ion effect in supramolecular hydrogels

We report on a pronounced specific-ion effect on the intermolecular and chiral organization, supramolecular structure formation, and resulting materials properties for a series of low molecular weight peptide-based hydrogelators, observed in the presence of simple inorganic salts. This effect was demonstrated using aromatic short peptide amphiphiles, based on fluorenylmethoxycarbonyl (Fmoc). Gel-phase materials were formed due to molecular self-assembly, driven by a combination of hydrogen bonding and π-stacking interactions. Pronounced morphological changes were observed by atomic force microscopy (AFM) for Fmoc-YL peptide, ranging from dense fibrous networks to spherical aggregates, depending on the type of anions present. The gels formed had variable mechanical properties, with G'?values between 0.8?kPa and 2.4?kPa as determined by rheometry. Spectroscopic analysis provided insights into the differential mode of self-assembly, which was found to be dictated by the hydrophobic interactions of the fluorenyl component, with comparable H-bonding patterns observed in each case. The efficiency of the anions in promoting the hydrophobic interactions and thereby self-assembly was found to be consistent with the Hofmeister anion sequence. Similar effects were observed with other hydrophobic peptides, Fmoc-VL and Fmoc-LL. The effect was found to be less pronounced for a less hydrophobic peptide, Fmoc-AA. To get more insights into the molecular mechanism, the effect of anions on sol-gel equilibrium was investigated, which indicates the observed changes result from the specific-ion effects on gels structure, rather than on the sol-gel equilibrium. Thus, we demonstrate that, by simply changing the ionic environment, structurally diverse materials can be accessed providing an important design consideration in nanofabrication via molecular self-assembly.     

Sequential Assembly of Mutually Interactive Supramolecular Hydrogels and Fabrication of Multi-Domain Materials

A two-component self-sorting hydrogel based on acylhydrazide and carboxylic acid derivatives of 1,3:2,4-dibenzylidene-d -sorbitol (DBS-CONHNH₂ and DBS-COOH) is reported. A heating–cooling cycle induces the self-assembly of DBS-CONHNH₂, followed by the self-assembly of DBS-COOH induced by decreasing pH. Although the networks are formed sequentially, there is spectroscopic evidence of interactions between them, which impact on the mechanical properties and significantly enhance the ability of these low-molecular-weight gelators (LMWGs) to form gels when mixed. The DBS-COOH network can be switched “off” and “on” within the two-component gel through a pH change. By using a photo-acid generator, the two-component gel can be prepared combining the thermal trigger with photo-irradiation. Photo-patterned self-assembly of DBS-COOH within a pre-formed DBS-CONHNH₂ gel under a mask yields spatially controlled multi-domain gels. Different gel domains can have different functions, for example, controlling the rate of release of heparin incorporated into the gel, or directing gold nanoparticle assembly. Such photo-patterned multi-component hydrogels have potential applications in regenerative medicine or bio-nano-electronics.     

Novel dimeric cholesteryl-based A(LS)2 low-molecular-mass gelators with a benzene ring in the linker

Three novel dimeric cholesteryl-based A(LS)(2) low-molecular-mass organic gelators (LMOGs) with phthaloyl, isophthaloyl, or terephthaloyl moieties in the linkers were designed and prepared. According to the linker structures, the compounds are denoted as 1 (o-), 2 (m-), and 3 (p-), respectively. Gelation tests revealed that the difference of relative positions of two cholesterol moieties in the benzene ring can produce a dramatic change in the gelation behaviors of the compounds. Importantly, 2 and 3 are more efficient gelators than 1, and their self-assembly behaviors are also very different from each other as revealed by scanning electron microscopy (SEM) measurements. Very interestingly, 2 gels xylene spontaneously at room temperature, and the sol-gel phase transition of the system is mechanically controllable. FTIR and (1)H NMR spectroscopy studies revealed that hydrogen bonding and pi-pi interactions between the molecules of the gelators play an important role in the formation and maintenance of the gels. The X-ray diffraction (XRD) analysis revealed that in the gel of 2/benzene, 2 aggregated into a layered structure with an interlayer distance of 3.54 nm, which is just the length of 2.     

Influence of calcium on the self-assembly of partially hydrolyzed alpha-lactalbumin

Self-assembly of alpha-lactalbumin after partial hydrolysis by a protease from Bacillus licheniformis can result in nanotubular structures, which show many similarities to microtubules. Calcium plays a crucial role in this process. The objective of this investigation was to study the role of calcium in more detail. The kinetics of the hydrolysis step and the self-assembly step were monitored by respectively liquid chromatography-mass spectrometry and dynamic light scattering. The microstructure of the gels finally formed was investigated by transmission electron microscopy. This investigation demonstrates that calcium accelerated the kinetics of the self-assembly, but it had no effect on the hydrolysis kinetics. As a result of the accelerated self-assembly kinetics at a high calcium concentration, the time of gelation decreased as well. A minimum concentration of calcium needed to obtain the tubular alpha-lactalbumin structures was determined. Below R = 1.5 (mole calcium/mole alpha-lactalbumin), turbid gels with randomlike structure were obtained. Between R = 1.5 and R = 6, translucent gels with a fine stranded network of tubules were formed, while higher calcium concentrations had a negative effect on the tubule formation, resulting in amorphous structures. The optimum calcium concentration for alpha-lactalbumin nanotube formation seemed to be around R = 3.     

超分子凝胶在催化有机反应中的应用

超分子凝胶是有机小分子通过分子间非共价作用形成的使溶剂固定的三维网络结构胶体。 综述关注了近年来超分子凝胶在催化有机反应中一个新的应用方向,依据凝胶剂结构特征和催化反应的类型对迄今报道的小分子凝胶催化剂进行了深入的总结与归类,揭示凝胶催化的独特性,并对其存在的问题和发展趋势进行了讨论。     

Robust gels created using a self-assembly and covalent capture strategy

The self-assembly of dendritic building blocks containing multiple terminal alkenes on their surfaces yields soft gel-phase materials--subsequent Grubbs' metathesis leads to covalent cross-linking between the alkenes and the formation of robust swellable gels.     

Amphiphilic ABC Triblock Terpolymer Templating for Mesoporous Silica

Self-assembled triblock terpolymers have attracted intense attention in recent years because of their abundant variety of assembly mesostructure. We reported the synthesis of mesoporous silica materials with the am- phiphilic ABC triblock terpolymer polyethylene-poly（ethylene oxide）-polycaprolactone（PE-PEO-PCL） as a template and tetraethoxysilane（TEOS） as a silica source. Increasing the hydrophilic head group（PEO） led to the decrease of packing parameter g, which gave rise to the mesophase transformation from a cylindrical two-dimensional hexagonal P6mm structure to a cage-type face centered cubic closed-packing mesostructure. This new templating route provided a new insight into the template factor governed inorganic-organic self-assembly mesophases.     

Effect of ionic strength on the self-assembly, morphology and gelation of pH responsive β-sheet tape-forming peptides

Molecular self-assembly is an intrinsic property of proteins central to their biological functionality. One important industrially interesting property is the ability to control and switch on and off self-assembly using a variety of external chemical and physical triggers. Model peptides have been developed with significantly reduced chemical and structural complexity compared to biological proteins. These are ideal systems for exposing the fundamental principles that drive protein-like self-assembly, as well as for establishing in a quantitative manner their structure-function relationship. We investigate simple, short model peptides that adopt a purely β-strand conformation, align in an antiparallel manner and self-assemble in one dimension in solution into long β-sheet nanotapes and higher order aggregates with no other conformation (i.e., helices, turns or random coils) present in the aggregates. These micrometre-long nanostructures gel in solutions at concentrations as low as 0.2% v/v. Their gel-fluid transition has been previously shown to be controlled by pH, temperature, or by mixing with complementary peptides. Here we show the dramatic effect of another chemical trigger, that of physiological-like salt concentration, on the self-assembly, morphology and gelation of a series of systematically designed charged self-assembling tape-forming peptides, each 11 amino acid residues in length, in the pH range of 2-14. This study provides a detailed understanding of the self-assembly of this class of peptides in aqueous solutions of biologically relevant pH and ionic strength. This insight has led to the development of injectable self-assembling peptide lubricants as potential therapeutics for the treatment of early stage knee joint osteoarthritis.