A Novel Variable Selection Method Based on Binning-Normalized Mutual Information for Multivariate Calibration.

Variable (wavelength) selection is essential in the multivariate analysis of near-infrared spectra to improve model performance and provide a more straightforward interpretation. This paper proposed a new variable selection method named binning-normalized mutual information (B-NMI) based on information entropy theory. "Data binning" was applied to reduce the effects of minor measurement errors and increase the features of near-infrared spectra. "Normalized mutual information" was employed to calculate the correlation between each wavelength and the reference values. The performance of B-NMI was evaluated by two experimental datasets (ideal ternary solvent mixture dataset, fluidized bed granulation dataset) and two public datasets (gasoline octane dataset, corn protein dataset). Compared with classic methods of backward and interval PLS (BIPLS), variable importance projection (VIP), correlation coefficient (CC), uninformative variables elimination (UVE), and competitive adaptive reweighted sampling (CARS), B-NMI not only selected the most featured wavelengths from the spectra of complex real-world samples but also improved the stability and robustness of variable selection results.

CO2 Induces Symmetry Breaking in Layered Dipeptide Crystals.

Control of symmetry breaking of materials provides large opportunities to regulate their properties and functions. Herein, we report breaking the symmetry of layered dipeptide crystals by utilizing CO2 to induce the adjacent monomolecular layers to stack from the opposite to the same direction. The role of CO2 is to cover the interlayer interaction sites and force the dipeptides to adsorb at asymmetric positions. Further, the dipeptide crystals exhibit far superior piezoelectricity after symmetry breaking and the piezoelectric voltage generated from the dipeptide-based generators becomes more than 500 % higher than before. This work reveals a potential route to engineer structures and properties of layered materials and provides a deep insight into the control of non-covalent interactions.

Tunable Chirality of Self-Assembled Dipeptides Mediated by Bipyridine Derivative.

Supramolecular peptide assemblies have been widely used for the development of biomedical, catalytical, and optical materials with chiral nanostructures in view of the intrinsic chirality of peptides. However, the assembly pathway and chiral transformation behavior of various peptides remain largely elusive especially for the transient assemblies under out-of-equilibrium conditions. Herein, the N-fluorenylmethoxycarbonyl-protected phenylalanine-tyrosine dipeptide (Fmoc-FY) was used as a peptide assembly platform, which showed that the assembly proceeds multistep evolution. The original spheres caused by liquid-liquid phase separation (LLPS) can nucleate and elongate into the formation of right-handed helices which were metastable and easily converted into microribbons. Interestingly, a bipyridine derivative can be introduced to effectively control the assembly pathway and induce the formation of thermodynamically stable right-handed or left-handed helices at different stoichiometric ratios. In addition, the chiral assembly can also be regulated by ultrasound or enzyme catalysis. This minimalistic system not only broadens the nucleation-elongation mechanisms of protein aggregates but also promotes the controllable design and development of chiral biomaterials.

Co-assembled Supramolecular Gel of Dipeptide and Pyridine Derivatives with Controlled Chirality.

It is commonly considered that amyloid-ß (Aß) fibrils are heavily involved in the neurological diseases. Establishing an external model based on the core recognition motif (diphenylalanine, FF) of Aß would be of significance in understanding the assembly and disassembly of Aß fibrils in living system. Herein, supramolecular gels with structure transition from amyloid-like ß-sheet to different supramolecular helices were obtained through the co-assembly of a N-fluorenylmethoxycarbonyl-protected L-FF (L-FmocFF) with achiral pyridine derivatives. It is found that the different stacking modes (H- or J-aggregates) of additives and the microenvironment of chiral carbon play vital roles for the selectively chiral transfer or amplification of L-FmocFF. The dynamic process of helix formation was also captured. This work provides a convenient co-assembly way to explore the structure basis of Aß fibrils with a controlled chirality.

Self-Assembled Vesicles Formed by Positional Isomers of Sodium Dodecyl Benzene Sulfonate-Based Pseudogemini Surfactants.

The construction of pseudogemini surfactants based on noncovalent interactions (such as electrostatic interaction and π-π stacking) was a powerful method to assemble well-defined aggregates in aqueous solution. The mixtures of butane-1,4-bis(methylimidazolium bromide) ([mim-C4-mim]Br2) and positional isomers of sodium dodecyl benzene sulfonate (SDBS-0,11 or SDBS-3,8) in a molar ratio of 1:2 were studied to characterize the effect of straight and branched alkyl chains on the aggregation behavior of pseudogemini surfactants. Spontaneous phase transition from micelles to vesicles was formed by these two kinds of complexes. Interestingly, a densely stacked onion-like structure (multilamellar vesicles) with more than one dozen layers was fabricated. The micelle and vesicle phases were characterized in detail by cryogenic transmission electron microscopy, polarized optical microscopy, dynamic light scattering, and rheological measurements. It can be clearly demonstrated that the structure of alkyl chain can significantly influence the surface adsorption, solution self-assembly, and aqueous two-phase system of pseudogemini surfactants. Our work provided a convenient technique to achieve controlled self-assembly by introducing positional isomers of surfactants.

Photoluminescent polymer hydrogels with stimuli-responsiveness constructed from Eu-containing polyoxometalate and imidazolium zwitterions.

Inorganic-organic co-assembly of anionic polyoxometalates (POMs) with zwitterions provides a facile way to fabricate functional soft materials. In this paper, a translucent, photoluminescent polymer hydrogel was fabricated from Weakley-type POM Na9EuW10O36 (EuW10) and polymerizable imidazole-type zwitterion 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) via a one-step synthesis method. Detailed characterization indicated that the polymerization of double bonds in VIPS and electrostatic interactions between EuW10 and VIPS play important roles in the formation of the hydrogels. Additionally, the introduction of non-polymerizable zwitterions 3-(1-methyl-3-imidazolio)propanesulfonate (MIPS) or 3-(1-decyl-3-imidazolio)propanesulfonate (C10IPS) can improve the mechanical and luminous performances of the hydrogels. Especially, C10IPS with a long alkyl chain would more significantly alter the coordination environment of EuW10, and consequently resulted in a more efficient energy transfer process. Further investigations revealed that the chemical environment around the Eu3+ can be highly influenced by organic solvents with stronger coordination abilities than water molecules, such as acetone. The translucency and luminescence intensity of the hydrogels can be reversibly transformed after alternately immersing in acetone or H2O for several minutes. Our results provided a useful strategy for the fabrication of luminescent hydrogels by regulating the noncovalent interactions between POMs and zwitterions.

Polyoxometalate-Based Photochromic Supramolecular Hydrogels with Highly Ordered Spherical and Cylindrical Micellar Nanostructures.

Polyoxometalates (POMs) have attracted much attention in the field of photochromic materials. However, POM-based photochromic supramolecular hydrogels with high transparency and good photochromic properties are seldom reported. In this work, a homogenous, optically transparent, injectable, and photochromic supramolecular hydrogel was fabricated through the coassembly of ammonium heptamolybdate (Mo7 ) and an imidazolium-based zwitterionic amphiphile (3-(1-hexadecyl-3-imidazolio)propanesulfonate (C16 IPS)). The balance between electrostatic attraction and repulsion of Mo7 clusters and zwitterionic amphiphiles enables them to coassemble into a homogenous and transparent supramolecular hydrogel. By adjusting the molar ratio of C16 IPS/Mo7 , ordered spherical micelle-based hydrogels and aligned wormlike micelle-based hydrogels can be obtained. The incorporation of Mo7 into hydrogels endows these hydrogels with excellent photochromic properties. Specifically, after coassembly with C16 IPS, the photochromic ability of hydrogels is significantly enhanced compared with that of a pure aqueous solution of Mo7 . These hydrogels exhibit great potential applications as photochromic materials for the recording of rewritable information.

Interaction among Worm-like Micelles in Polyoxometalate-Based Supramolecular Hydrogel.

The co-assembly of zwitterionic amphiphile and polyoxometalate is a new and promising technique to construct a hierarchical and multifunctional supramolecular hydrogel. To comprehensively investigate the assemble mechanism, zwitterionic amphiphiles with different cations, namely, 3-(1-hexadecyl-3-imidazolio) propanesulfonate (C16IPS) and 3-(1-hexadecyl-2-methyl-3-imidazolio) propanesulfonate (C16bIPS), were designed to complex with silicotungstic acid (HSiW). Hydrogen bonding between the oxygen atoms of HSiW and the protons on C-2 of the imidazolium rings and the steric effect significantly influence the morphology and rheological property of the hydrogel. Interestingly, cross-linked worm-like micelles in parallel, vertical, and tilted distribution were observed using cryogenic transmission electron microscopy. In addition, these aggregates were further stacked into hexagonal phases on a large scale. Hence, deep insights into the relationship among the structure of zwitterionic amphiphile, self-assembled architecture, and the mechanical property of a polyoxometalate-based hydrogel were disclosed.

Supramolecular Thermotropic Ionic Liquid Crystals Formed via Self-Assembled Zwitterionic Ionic Liquids.

Supramolecular thermotropic ionic liquid crystals (ILCs) with hexagonal and lamellar phases were fabricated by the self-assembly of zwitterionic ionic liquids, which were formed by 3-(1-alkyl-3-imidazolio) propanesulfonate with different alkyl chain lengths C nIPS ( n = 12, 14, 16) and 3,4,5-tris(dodecyloxy)benzoic acid (TDBA) based on intermolecular electrostatic interactions. The phase behaviors of ILCs were investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and polarized optical microscopy (POM). The highly ordered and well-defined microstructure of ILCs can be considered to be proton pathways and to radically improve the ionic conductivity, suggesting the induction of proton conduction through a hopping mechanism.

Responsive Self-Assembly of Supramolecular Hydrogel Based on Zwitterionic Liquid Asymmetric Gemini Guest.

A low-molecular-weight supramolecular hydrogel has been fabricated based on host-guest interactions between ß-cyclodextrin (ß-CD) and an asymmetric gemini zwitterionic liquid (ZIL) containing an azobenzene and bis(trifluoromethanesulfonyl)imide. Reversible sol-gel phase transitions were triggered by light and temperature. The binding stoichiometry, mainly noncovalent interactions in the hydrogel, photo- and thermoresponsive mechanisms, and mode of inclusion in the complex were studied in detail by NMR spectroscopy, UV/Vis spectroscopy, isothermal titration calorimetry, and control experiments. Interestingly, the ß-CD in the complex can be photo-manipulated to shuttle along the guest molecule, accompanied by a change in the circular dichroism signals. Reversible switching of the conductivity accompanies the sol-gel phase transition, which demonstrates the stability of the supramolecular hydrogel and its potential application in multi-stimuli-responsive electric sensor materials.

Coassembly of a Polyoxometalate and a Zwitterionic Amphiphile into a Luminescent Hydrogel with Excellent Stimuli Responsiveness.

The co-existence of electrostatic attraction and repulsion between zwitterionic amphiphile and anionic polyoxometalate (POM) promotes the formation of aqueous inorganic-organic coassemblies. Injectable, luminescent supramolecular hydrogels have been fabricated by using this novel and versatile technique. Hydrogels with different characteristics can be flexibly manipulated by adjusting the molar ratios of compounds. Interestingly, a linear relationship between the luminescence intensity and temperature accompanying the sol-gel transition was observed. In addition, the emission properties of the hydrogels were sensitive to external acid and base gasses. Such multi-stimuli responsive luminescent hydrogels provide an attractive pathway to construct intelligent optical soft materials, especially for biological sensors for which stimuli responsiveness is necessary.

Multiple-Responsive Hierarchical Self-Assemblies of a Smart Supramolecular Complex: Regulation of Noncovalent Interactions.

We herein report a smart amphiphilic supramolecular complex ([MimA-EDA-MimA]@[DBS]2) with stimuli-responsive self-assembly, constructed by 3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium chloride (MimACl), sodium dodecyl benzene sulfonate (SDBS), and ethylenediamine (EDA). The self-assembly of [MimA-EDA-MimA]@[DBS]2 shows triple-sensitivities in response to pH, concentration, and salt. At a low pH, only micelles are formed, which can transform into vesicles spontaneously when the pH increases to 11.8. Vesicles can gradually fuse into vesicle clusters and elongated assemblies with increasing concentration of [MimA-EDA-MimA]@[DBS]2. Chainlike aggregates, ringlike aggregates, or giant vesicles can be formed by adding inorganic salts (i.e., NaCl and NaNO3), which could be derived from the membrane fusion of vesicles. The noncovalent interactions, including π-π stacking, hydrogen bonding, and electrostatic interactions, were found to be responsible for the topology evolution of assemblies. Thus, it provides an opportunity to construct smart materials through the regulation of the role of noncovalent interactions in self-assembly.

Single lithium-ion polymer electrolytes based on poly(ionic liquid)s for lithium-ion batteries.

A promising method of synthesizing a lithium-containing room temperature polymeric ionic liquid (PIL) by utilizing the zwitterionic effect to turn a lithium salt into a liquid for single lithium-ion conduction was proposed. In this work, the room temperature PIL was constructed by the equimolar monomer mixture of zwitterion 3-(1-vinyl-3-imidazolic)propanesulfonate (VIPS) and 4-styrenesulfnny(trifluoromethylsulfony)imde (LiSTFSI) based on intermolecular electrostatic interactions. In situ photopolymerization with flexible chain poly(ethylene glycol)methyl ether methacrylate (PEGM) and cross-linker poly(ethylene glycol)dimethacrylate (PEGDM) in the presence of propylene carbonate (PC) generated free-standing PIL electrolyte films. The PIL electrolytes with Li+-rich channels have an excellent lithium transference number approaching unity (0.93 at room temperature) while attaining comparable ionic conductivity (σ ≈ 1.31 × 10-4 S cm-1 at 30 °C). The charge-discharge performance of a Li/LiFePO4 half-cell equipped with this PIL electrolyte would provide a promising model system for novel single lithium ion conductors.

Smart low molecular weight hydrogels with dynamic covalent skeletons.

We report a new strategy for fabricating a smart low molecular weight hydrogel based on dynamic covalent chemistry from a bola-type supra-gelator, which was facilely fabricated in situ from two non-assembling building blocks, (3-(2-(4-formylphenoxy) ethyl)-1-methyl imidazolium bromide, MA) and (3,3'-dithiobis (propionohydrazide), DSPDZ), through dynamic acylhydrazone bonding. The obtained low molecular weight hydrogels exhibited redox-responsive and controllable self-healing properties. The role of dynamic covalent bonding in the formation of smart hydrogels is revealed in this study, which provides a simple and bottom-up method for constructing smart low molecular weight hydrogels.

Facile fabrication of thermo/redox responsive hydrogels based on a dual crosslinked matrix for a smart on-off switch.

Stimuli-responsive or "smart" soft materials have raised considerable attention due to their ability to spontaneously respond to external environmental variations and have a great potential for wide applications. Herein, a thermo/redox responsive hydrogel is facilely constructed based on a dual crosslinked matrix: the primary chemical crosslinked copolymer is composed of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) and poly(ionic liquid), and the secondary physical crosslinking component is generated by the ionic coordination between iron ions and carboxyl groups in the poly(ionic liquid). The non-covalent ion coordination crosslinking is introduced into a covalently crosslinked network, which further strengthens the soft PNIPAM matrix and enhances the mechanical performances of the hydrogels. The excellent thermosensitivity of PNIPAM and the good conductive property of poly(ionic liquid) provide the hydrogel with an attractive performance as a thermo-responsive switch. Moreover, the trapped iron ions in the network endow the hydrogels with redox-responsiveness, which could be reversibly chemically oxidized and reduced. The mechanical strength of hydrogels could also be tuned by the crosslinked capacity of iron ions within the gel matrix between the strong binding of the oxidized state (Fe3+) and poor coordination of the reduced state (Fe2+). These stimuli-responsive hydrogels have the potential to be used as smart materials for stimuli-responsive devices.

Co-assembly of Polyoxometalates and Zwitterionic Amphiphiles into Supramolecular Hydrogels: From Crystalline Fibrillar to Amorphous Micellar Networks.

Gelation mechanism is of utmost importance to the rational design of supramolecular hydrogelators. Although both kinetic and thermodynamic controlled self-assembly processes have been widely studied in hydrogels, the formation relationship between crystalline and amorphous gel networks still remains ambiguous. Herein, a gelation transformation from a kinetic to a thermodynamic process was achieved by balancing the rigidity and flexibility of the inorganic-organic co-assemblies. By using polyoxometalates and zwitterionic amphiphiles, the transition morphologies between crystalline and amorphous hydrogel networks were evidenced for the first time, as ordered wormlike micelles. Given the versatile applications of hydrogels in biological systems and materials science, these findings may highlight the potential of inorganic-organic binary supramolecular hydrogelators and fill in the blank between kinetic and thermodynamic controlled gelation processes.

Low-Molecular-Weight Supramolecular Ionogel Based on Host-Guest Interaction.

Supramolecular ionogels were prepared by self-assembly of small molecules through host-guest interaction between ß-cyclodextrin (ß-CD) and a room-temperature ionic liquid (IL) 3-(1-methyl-3-imidazolio)propanesulfonate-lithium bis(trifluoromethanesulfonyl)imide (MIPS-LiTFSI) which contains zwitterion MIPS. 19F NMR and 2D ROESY 1H NMR have been used to prove that only TFSI- is involved in the complexation. 1H NMR, FT-IR, and comparative tests indicated that the electrostatic interaction between imidazole cation and TFSI- anion and intermolecular hydrogen bonding between three compounds also contribute to the formation of supramolecular ionogel. Ionogels with different gel-sol phase transition temperatures can be obtained by adjusting the molar ratio between ß-CD and MIPS-LiTFSI. In addition, the supramolecular ionogels composed of "channel type" structural ß-CD have been constructed. The ionogel with high conductivity and low activation energy open a door to new fields for special applications.

Spontaneous wormlike micelles formed in a single-tailed zwitterionic surface-active ionic liquid aqueous solution.

Wormlike micelles were successfully fabricated by the self-assembly of a single-tailed zwitterionic surface-active ionic liquid (SAIL), 3-(1-hexadecyl-3-imidazolio) propanesulfonate ß-naphthalene sulfonate (C16IPS-Nsa), in aqueous solutions without any additives. With increasing zwitterionic SAIL concentration, spontaneous transition from micelles to wormlike micelles and then a hexagonal phase occurs. Interestingly, the wormlike micelles are closed, stretched and directionally arranged, which was rarely found in previous research studies. This kind of wormlike micelle is useful to serve as novel soft template in the synthesis of functional materials. 1H NMR illustrated that the introduced π-π stacking and hydrophobic interactions originated from the hydrophobic aromatic counterions are responsible for the structural transformation. Density functional theory (DFT) simulated the optimum configuration of C16IPS-Nsa and calculated the interaction energy of C16IPS-Nsa-H2O is 39.6 kJ mol-1. This work paves the way for regulating the self-assembly structures of amphiphiles through changing specific weak interactions.

Photoresponsive Self-Assembly of Surface Active Ionic Liquid.

A novel photoresponsive surface active ionic liquid (SAIL) 1-(4-methyl azobenzene)-3-tetradecylimidazolium bromide ([C14mimAzo]Br) with azobenzene located in the headgroup was designed. Reversible vesicle formation and rupture can be finely controlled by photostimuli without any additives in the aqueous solution of the single-tailed ionic liquid. The photoisomerization of the azobenzene derivative was investigated by (1)H NMR and UV-vis spectroscopy. Density functional theory (DFT) calculations further demonstrate that trans-[C14mimAzo]Br has less negative interaction energy, which is beneficial to aggregate formation in water. The incorporation of trans-azobenzene group increases the hydrophobicity of the headgroup and reduces the electrostatic repulsion by delocalization of charge, which are beneficial to the formation of vesicles. However, the bend of cis-azobenzene makes the cis-isomers have no ability to accumulate tightly, which induces the rupture of vesicles. Our work paves a convenient way to achieve controlled topologies and self-assembly of single SAIL.

Three Stages of Dynamic Assembly Process of Dipeptide-Based Supramolecular Gel Revealed by In Situ Infrared Spectroscopy.

The exploration of short peptide-based assembly is vital for understanding protein-misfolding-associated diseases and seeking strategies to attenuate aggregate formation. While, the molecular mechanism of their structural evolution remains poorly studied in view of the dynamic and unpredictable assembly process. Herein, infrared (IR) spectroscopy, which serves as an in situ and real-time analytical technique, was intelligently employed to investigate the mechanism of phase transition and aggregate formation during the dynamic assembly process of diphenylalanine. Combined with other spectroscopy and electron microscopy technologies, three stages of gel formation and the main driving forces in different stages were revealed. A variety of stoichiometric methods such as continuous wavelet transform, principal component analysis, and two-dimensional correlation spectroscopy techniques were conducted to analyze the original time-dependent IR spectra to obtain detailed information on the changes in the amide bands and hydration layer. The microenvironment of hydrogen bonding among amide bands was significantly changed with the addition of pyridine derivatives, resulting in great differences in the properties of co-assembled gels. This work not only provides a universal analytical way to reveal the dynamic assembly process of dipeptide-based supramolecular gel but also expands their applications in supramolecular regulation and high-throughput screens in situ.