Highly branched thermoresponsive polysaccharide derivative in water. Partly substituted highly branched cyclic dextrin ethylcarbamate

A thermoresponsive highly branched polysaccharide derivative was revealed from commercially available highly branched cyclic dextrin (HBCD), originally synthesized from amylopectin. Eight samples of partially substituted ethyl carbamate derivatives of HBCD (HEC) were prepared with a degree of substitution DS ranging from 0.27 to 1.46. Three samples with DS = 0.88, 1.05, and 1.22 showed LCST type phase separation in water. The intrinsic viscosity and form factor in water were typical of the hyperbranched structure. The intermolecular interactions between HEC and iodine or 1-anilinonaphthalene-8-sulfonic acid (ANS) were appreciably different from those of the linear analog (AEC), suggesting that the locally bent helical conformation of highly branched HEC chains has a different interaction with small molecules. The phase diagram of HEC-water systems was accidentally similar to that of the linear chain with the same molar mass and DS, although the one phase region of the branched polymer chain-poor solvent system is usually wider than that of the corresponding linear chain. This is likely due to the lower hydration nature of the polymer segment of HEC chains than that of the corresponding linear chain.

Dual thermoresponsive polysaccharide derivative - water system. Partially substituted amylose butylcarbamate in water

Partially substituted amylose n-butylcarbamate (ABC) samples were synthesized with weight-average molar mass Mw ranging between 40 kg mol-1 and 220 kg mol-1 with different degree of substitution DS. When DS was between 0.17 and 0.33, the ABC samples were soluble in water. Furthermore, both LCST and UCST type phase separations were observed for the ABC samples in water when DS is >0.26. The closed-loop phase diagrams for the dual thermoresponsive ABC samples in water were constructed by turbidity measurement. The UCST ranged from 70 °C to 77 °C and the LCST ranged from 13 °C to 17 °C. SAXS measurements were performed for dilute aqueous ABC solutions to determine the chain conformation of ABC at various temperatures. The resulting form factor at the polymer mass concentration of 3 mg mL-1 indicated that the chain conformation is almost independent of temperature, except for the chain diameter, which is influenced by the temperature-dependent hydration behavior. This result suggests that the attractive interactions between ABC chains are not very significant even between UCST and LCST, where higher concentrated polymer solutions show macroscopic phase separation.

The aqueous supramolecular chemistry of crown ethers

This mini-review summarizes the seminal exploration of aqueous supramolecular chemistry of crown ether macrocycles. In history, most research of crown ethers were focusing on their supramolecular chemistry in organic phase or in gas phase. In sharp contrast, the recent research evidently reveal that crown ethers are very suitable for studying abroad range of the properties and applications of water interactions, from: high water-solubility, control of Hofmeister series, "structural water", and supramolecular adhesives. Key studies revealing more details about the properties of water and aqueous solutions are highlighted.

Self-assembled Fluorescent Nanoparticles with Tunable LCST Behavior in Water

The development of stimuli-responsive fluorescent materials in water based on organic molecule has drawn significant interest. Herein, we designed and synthesized an amphiphilic molecule M containing a fixed tetraphenylethylene moiety (FTPE) as hydrophobic part and tri(ethylene glycol) (TEG) chains as hydrophilic part. Notably, the FTPE moiety is aggregation-induced emission (AIE) active, while the TEG chains are thermo-responsive. M can self-assemble into fluorescent nanoparticles (NPs) in water, which showed lower critical solution temperature (LCST) behavior. Moreover, its clouding point can be reversibly tuned upon the concentration variation. Interestingly, the NPs can be acted as a fluorescence thermometer in aqueous media owing to their unique AIE and LCST behaviors. Our work herein not only provides an integration strategy to construct stimuli-responsive fluorescent materials but also shows great potential in biological applications including bioimaging and biosensors.

Pseudo Polyampholytes with Sensitively Ion-Responsive Conformational Transition Based on Positively Charged Host-Guest Complexes

Biological polyampholytes are ubiquitous in living organisms with primary functions including that serving as transporters for moving chemical molecular species across the cell membranes. Synthetic amphoteric macromolecules that can change their phase states depending on the environment to simulate some properties of natural polyampholytes are of great interests. Here, we explore implementation of synthetic pseudo polymeric ampholytes with ion-recognition-triggered conformational change. The phase transition behaviors of the ion-recognition-creative polyampholytes that containing deprotonated carboxylic acid groups as negative charges and 18-crown-6 units for forming positively charged host-guest complexes are systematically investigated. The ion-recognition-triggered phase transition behaviors of pseudo polyampholytes are significantly dependent on cation species and concentrations. Only those specific ions like K⁺ , Ba²⁺ , Sr²⁺ and Pb²⁺ ions that can form 1:1 host-guest complexes with 18-crown-6 units in polymers enable to control over the conformational change like that of the traditional pH-dependent polyampholytes. By regulating the content of the carboxylic acid groups to match the content of the ion-recognized positive charges provided by the host-guest complexes, the pseudo polyampholytes are more sensitive to the recognizable cations. Such ion-recognition-triggered amphoteric characteristics make the pseudo polyampholytes acting like biological proteins, nucleic acids and enzymes as molecular transporters, genetic code storage and biocatalysts in artificial systems. This article is protected by copyright. All rights reserved.

Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters

Highly selective separation materials that recover total ammonia nitrogen (i.e., ammonia plus ammonium, or TAN) from wastewaters as a pure product can supplement energy-intensive ammonia production and incentivize pollution mitigation. We recently demonstrated that commercial acrylate cation exchange polymer resins loaded with transition metal cations, or metal-loaded ligand exchangers, can recover TAN from wastewater with high selectivity (TAN/K+ equilibrium selectivity of 10.1) via metal-ammine bond formation. However, the TAN adsorption efficiency required further improvement (35%), and the optimal concentration and pH ranges were limited by both low ammonia fractions and an insufficiently strong resin carboxylate-metal bond that caused metal elution. To overcome these deficiencies, we used a zinc-acrylate ligand exchange resin and a tertiary amine acrylic weak base resin (pH buffer resin) together to achieve resin-mediated pH control for optimal adsorption conditions. The high buffer capacity around pH 9 facilitated gains in the adsorbed TAN per ligand resin mass that enhanced the TAN adsorption efficiency to greater than 90%, and constrained zinc elution (below 0.01% up to 1 M TAN) because of decreased ammonia competition for zinc-carboxylate bonds. During TAN recovery, resin-mediated pH buffering facilitated recovery of greater than 99% of adsorbed TAN with 0.2% zinc elution, holding the pH low enough to favor ammonium but high enough to prevent carboxylate protonation. For selective ion separation, solid phase buffers outperform aqueous buffers because the initial solution pH, the buffering capacity, and the ion purity can be independently controlled. Finally, because preserving the resin-zinc bond is crucial to sustained ligand exchange performance, the properties of an ideal ligand resin functional group were investigated to improve the properties beyond those of carboxylate. Ultimately, ligand exchange adsorbents combined with solid pH buffers can advance the selective recovery of nitrogen and potentially other solutes from wastewaters.

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

Thermoresponsive supramolecular assemblies have been extensively explored in diverse formats, from injectable hydrogels to nanoscale carriers, for a variety of applications including drug delivery, tissue engineering and thermo-controlled catalysis. Understanding the molecular bases behind thermal sensitivity of materials is fundamentally important for the rational design of assemblies with optimal combination of properties and predictable tunability for specific applications. In this review, we summarize the recent advances in this area with a specific focus on the parameters and factors that influence thermoresponsive properties of soft materials. We summarize and analyze the effects of structures and architectures of molecules, hydrophilic and lipophilic balance, concentration, components and external additives upon the thermoresponsiveness of the corresponding molecular assemblies.

Fluorescent Supramolecular Polymers Formed by Crown Ether-Based Host-Guest Interaction

Inspired by the vast array of assemblies present in nature, supramolecular chemistry has attracted significant attention on account of its diverse supra-structures, which include micelles, vesicles, and fibers, in addition to its extensive applications in luminescent materials, sensors, bioimaging, and drug delivery over the past decades. Supramolecular polymers, which represent a combination of supramolecular chemistry and polymer science, are constructed by non-covalent interactions, such as host-guest interactions, hydrogen bonding, hydrophobic or hydrophilic interactions, metal-ligand interactions, π-π stacking, and electrostatic interactions. To date, numerous host-guest recognition systems have been reported, including crown ethers, cyclodextrins, calixarenes, cucurbituril, pillararenes, and other macrocyclic hosts. Among them, crown ethers, as the first generation of macrocyclic hosts, provide a promising and facile alternative route to supramolecular polymers. In addition, the incorporation of fluorophores into supramolecular polymers could endow them with multiple properties and functions, thereby presenting potential advantages in the context of smart materials. Thus, this review focuses on the fabrication strategies, interesting properties, and potential applications of fluorescent supramolecular polymers based on crown ethers. Typical examples are presented and discussed in terms of three different types of building blocks, namely covalently bonded low-molecular-weight compounds, polymers modified by hosts or guests, and supramolecular coordination complexes.

Self-assembly behaviors of perylene- and naphthalene-crown macrocycle conjugates in aqueous medium

The synthesis of conjugates of perylene diimide (PDI) and naphthalene diimide (NDI) modified with two benzo-21-crown-7 ethers (B21C7) are herein described. Their self-assembly behavior in various solvents was investigated particularly in aqueous medium, due to the recently discovered hydrophilic properties of B21C7 crown macrocycle. An unexpected fluorescence quenching phenomenon was observed in the PDI-B21C7 macrocycle conjugate in chloroform. The detailed UV-vis absorption and fluorescence spectra of these PDI/NDI derivatives in different solvents as well as their morphologies were investigated.

LCST phase behavior of benzo-21-crown-7 with different alkyl chains

The introduction of hydrophobic units into crown ethers can dramatically decrease the critical transition temperature of LCST and realize macroscopic phase separation at low to moderate temperature and concentration. Minor modifications in the chemical structure of crown ethers (benzo-21-crown-7, B21C7s) can effectively control the thermo-responsive properties.

A degradable low molecular weight monomer system with lower critical solution temperature behaviour in water

A degradable thermo-responsive system was prepared and investigated. The degradation behaviour induced by the cleavage process of the thermo-sensitive crown ethers effectively altered the thermo-responsiveness.

LCST behavior controlled by size-matching selectivity from low molecular weight monomer systems

LCST behavior was controlled by crown ether–cation recognition motifs via size-matching selectivity.

Soluble-Insoluble-Soluble Transitions of Thermoresponsive Cryptand-Containing Graft Copolymers

Cryptand-containing alternative copolymers were first made from copolymerization of styrenic derivatives and maleic anhydride and then chemically modified in this work by grafting methoxy poly(ethylene glycol) (MPEG) onto the maleic functional groups. These graft copolymers show interesting multistep soluble-insoluble-soluble (S-I-S) transitions in acidic aqueous media at a cloud point (T cp) and a subsequent mixing temperature (T mix). Turbidity measurements and dynamic light scattering studies indicate that such complex transitions may be attributed to the entropic contribution associated with the dehydration and aggregation of the MPEG groups and then the enthalpic contribution associated with the hydrogen bonding between ethylene glycol and carboxylic acid groups. More importantly, the phase transition temperatures and insoluble temperature ranges are very sensitive to changes in subtle hydrophobic-hydrophilic balance of the copolymers, such as the variation of pH, the cryptand size, and the length of the MPEG graft. The understanding of the S-I-S transition in relation to the structure of the copolymers and the external conditions may be useful in the design of smart materials and sensors.