Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

Molecular imprinting technology (MIT), often described as a method of making a molecular lock to match a molecular key, is a technique for the creation of molecularly imprinted polymers (MIPs) with tailor-made binding sites complementary to the template molecules in shape, size and functional groups. Owing to their unique features of structure predictability, recognition specificity and application universality, MIPs have found a wide range of applications in various fields. Herein, we propose to comprehensively review the recent advances in molecular imprinting including versatile perspectives and applications, concerning novel preparation technologies and strategies of MIT, and highlight the applications of MIPs. The fundamentals of MIPs involving essential elements, preparation procedures and characterization methods are briefly outlined. Smart MIT for MIPs is especially highlighted including ingenious MIT (surface imprinting, nanoimprinting, etc.), special strategies of MIT (dummy imprinting, segment imprinting, etc.) and stimuli-responsive MIT (single/dual/multi-responsive technology). By virtue of smart MIT, new formatted MIPs gain popularity for versatile applications, including sample pretreatment/chromatographic separation (solid phase extraction, monolithic column chromatography, etc.) and chemical/biological sensing (electrochemical sensing, fluorescence sensing, etc.). Finally, we propose the remaining challenges and future perspectives to accelerate the development of MIT, and to utilize it for further developing versatile MIPs with a wide range of applications (650 references).

http://ir.yic.ac.cn/bitstream/133337/17045/1/Molecular%20imprinting%20perspectives%20and%20applications.pdf

Molecular imprinting: perspectives and applications

BACKGROUND Hydrogels are formed through the cross-linking of hydrophilic polymer chains within an aqueous microenvironment. The gelation can be achieved through a variety of mechanisms, spanning physical entanglement of polymer chains, electrostatic interactions, and covalent chemical cross-linking. The water-rich nature of hydrogels makes them broadly applicable to many areas, including tissue engineering, drug delivery, soft electronics, and actuators. Conventional hydrogels usually possess limited mechanical strength and are prone to permanent breakage. The lack of desired dynamic cues and structural complexity within the hydrogels has further limited their functions. Broadened applications of hydrogels, however, require advanced engineering of parameters such as mechanics and spatiotemporal presentation of active or bioactive moieties, as well as manipulation of multiscale shape, structure, and architecture. ADVANCES Hydrogels with substantially improved physicochemical properties have been enabled by rational design at the molecular level and control over multiscale architecture. For example, formulations that combine permanent polymer networks with reversibly bonding chains for energy dissipation show strong toughness and stretchability. Similar strategies may also substantially enhance the bonding affinity of hydrogels at interfaces with solids by covalently anchoring the polymer networks of tough hydrogels onto solid surfaces. Shear-thinning hydrogels that feature reversible bonds impart a fluidic nature upon application of shear forces and return back to their gel states once the forces are released. Self-healing hydrogels based on nanomaterial hybridization, electrostatic interactions, and slide-ring configurations exhibit excellent abilities in spontaneously healing themselves after damages. Additionally, harnessing techniques that can dynamically and precisely configure hydrogels have resulted in flexibility to regulate their architecture, activity, and functionality. Dynamic modulations of polymer chain physics and chemistry can lead to temporal alteration of hydrogel structures in a programmed manner. Three-dimensional printing enables architectural control of hydrogels at high precision, with a potential to further integrate elements that enable change of hydrogel configurations along prescribed paths. OUTLOOK We envision the continuation of innovation in new bioorthogonal chemistries for making hydrogels, enabling their fabrication in the presence of biological species without impairing cellular or biomolecule functions. We also foresee opportunities in the further development of more sophisticated fabrication methods that allow better-controlled hydrogel architecture across multiple length scales. In addition, technologies that precisely regulate the physicochemical properties of hydrogels in spatiotemporally controlled manners are crucial in controlling their dynamics, such as degradation and dynamic presentation of biomolecules. We believe that the fabrication of hydrogels should be coupled with end applications in a feedback loop in order to achieve optimal designs through iterations. In the end, it is the combination of multiscale constituents and complementary strategies that will enable new applications of this important class of materials.

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Advances in engineering hydrogels

Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/56662/1/Gong-Manuscript-1.pdf

Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.

Stimuli-responsive supramolecular polymeric materials.

Automatic repair: a polymer with pendent dibenzo[24]crown-8 units (purple in picture) was cross-linked by two bisammonium salts (green) to form two supramolecular gels based on host-guest interactions. These two gels are stimuli-responsive materials that respond to changes of the pH value and are also self-healing materials, as can be seen by eye and as evidenced by rheological data.

Self‐Healing Supramolecular Gels Formed by Crown Ether Based Host–Guest Interactions

Supramolecular polymers, polymeric systems beyond the molecule, have attracted more and more attention from scientists due to their applications in various fields, including stimuli-responsive materials, healable materials, and drug delivery. Due to their good selectivity and convenient enviro-responsiveness, crown ether-based molecular recognition motifs have been actively employed to fabricate supramolecular polymers with interesting properties and novel applications in recent years. In this tutorial review, we classify supramolecular polymers based on their differences in topology and cover recent advances in the marriage between crown ether-based molecular recognition and polymer science.

Supramolecular polymers constructed by crown ether-based molecular recognition

CONSPECTUS: Supramolecular polymers, fabricated via the combination of supramolecular chemistry and polymer science, are polymeric arrays of repeating units held together by reversible, relatively weak noncovalent interactions. The introduction of noncovalent interactions, such as hydrogen bonding, aromatic stacking interactions, metal coordination, and host-guest interactions, endows supramolecular polymers with unique stimuli responsiveness and self-adjusting abilities. As a result, diverse monomer structures have been designed and synthesized to construct various types of supramolecular polymers. By changing the noncovalent interaction types, numbers, or chemical structures of functional groups in these monomers, supramolecular polymeric materials can be prepared with tailored chemical and physical properties. In recent years, the interest in supramolecular polymers has been extended from the preparation of intriguing topological structures to the discoveries of potential applications as functional materials. Compared with traditional polymers, supramolecular polymers show some advantages in the fabrication of reversible or responsive materials. The development of supramolecular polymers also offers a platform to construct complex and sophisticated materials with a bottom-up approach. Macrocylic hosts, including crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, are the most commonly used building blocks in the fabrication of host-guest interaction-based supramolecular polymers. With the introduction of complementary guest molecules, macrocylic hosts demonstrate selective and stimuli-responsive host-guest complexation behaviors. By elaborate molecular design, the resultant supramolecular polymers can exhibit diverse structures based on the self-selectivity of host-guest interactions. The introduction of reversible host-guest interactions can further endow these supramolecular polymers with interesting and fascinating chemical/physical properties, including stimuli responsiveness, self-healing, and environmental adaptation. It has been reported that macrocycle-based supramolecular polymers can respond to pH change, photoirradition, anions, cations, temperature, and solvent. Macrocycle-based supramolecular polymers have been prepared in solution, in gel, and in the solid state. Furthermore, the solvent has a very important influence on the formation of these supramolecular polymers. Crown ether- and pillararene-based supramolecular polymers have mainly formed in organic solvents, such as chloroform, acetone, and acetonitrile, while cyclodextrin- and cucurbituril-based supramolecular polymerizations have been usually observed in aqueous solutions. For calixarenes, both organic solvents and water have been used as suitable media for supramolecular polymerization. With the development of supramolecular chemistry and polymer science, various methods, such as nuclear magnetic resonance spectroscopy, X-ray techniques, electron microscopies, and theoretical calculation and computer simulation, have been applied for characterizing supramolecular polymers. The fabrication of macrocycle-based supramolecular polymers has become a currently hot research topic. In this Account, we summarize recent results in the investigation of supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs. These supramolecular polymers are classified based on the different macrocycles used in them. Their monomer design, structure control, stimuli-responsiveness, and applications in various areas are discussed, and future research directions are proposed. It is expected that the development of supramolecular polymers will not only change the way we live and work but also exert significant influence on scientific research.

Bo Zheng

Papers

Stimuli-responsive supramolecular polymeric materials.

Self‐Healing Supramolecular Gels Formed by Crown Ether Based Host–Guest Interactions

Supramolecular polymers constructed by crown ether-based molecular recognition

Supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs.

A Dual-Responsive Supramolecular Polymer Gel Formed by Crown Ether Based Molecular Recognition