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Keywords:

  • supramolecular materials;
  • adaptive materials;
  • responsive properties;
  • self-assembly;
  • noncovalent interactions

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Supramolecular materials held together by noncovalent interactions, such as hydrogen bonding, host–guest interactions, and electrostatic interactions, have great potential in material science. The unique reversibility and adaptivity of noncovalent intreractions have brought about fascinating new functions that are not available by their covalent counterparts and have greatly enriched the realm of functional materials. This review article aims to highlight the very recent and important progresses in the area of functional supramoleuclar materials, focusing on adaptive mechanical materials, smart sensors with enhanced selectivity, soft luminescent and electronic nanomaterials, and biomimetic and biomedical materials with tailored structures and functions. We cannot write a complete account of all the interesting work in this area in one article, but we hope that it can in a way reflect the current situation and future trends in this prosperously developing area of functional supramolecular materials.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Beyond molecular chemistry based on the covalent bonds, there has been a prosperously developing world of supramolecular chemistry, which aims at developing highly complex chemical systems from components interacting through noncovalent intermolecular forces.[1-5] In contrast to their covalent counterparts, the noncovalent interactions are reversible and sensitive to external stimuli, which have endowed supramolecular materials with unique and fascinating properties.[6-12]

  • (1)
    Facile to fabricate. For the noncovalent synthesis of supramolecular materials, by elaborately tailoring on the noncovalent interacting arrays, multiple noncovalent interactions can act cooperatively with great specificity. This is in great contrast to the “one-bond formation per step” in most of the covalent synthesis, for which the tedious organic synthesis can be greatly simplified in noncovalent synthesis.[12] For example, just by mixing different pre-tailored building blocks in solution at room temperature, the supramolecular materials can be fabricated efficiently. This method is quite facile, inexpensive and environmentally friendly, considering that the multi-step covalent synthesis and purification process have been avoided to a large extent.
  • (2)
    Good reversibility. Noncovalent interactions are generally weaker and more dynamic compared with covalent interactions. Thus, they can be destructed and reconstructed easily without too much energy input. This results in good reversibility of the thus-prepared supramolecular materials, which can be easily recycled and are capable of self-repair after mechanical damage.
  • (3)
    Adaptive property. This relates to properties that can respond to external stimuli by change in structures and functions. On exposure to external stimuli (input of energy), the supramolecular materials can automatically “find” their most stable state with the lowest free energy by adjusting their noncovalent interactions arrays to adapt to the environment. The adaptivity provides a solid basis for the construction of smart functional materials: for example, drug-delivery vehicles and sensors for special analytes.
  • (4)
    Highly ordered at the nanometer scale. The mission of covalent chemistry is to tailor molecules at the atomic scale, whereas bulk material processing and engineering provide a “top-down” method to control the macroscopic order. There was a gap between the microscopic and macroscopic world before the supramolecular chemistry was established. The rational control over molecular order at nanometer scale is crucial for the supramolecular engineering of functional nanomaterials. Moreover, even for the macroscopic materials, the incorporation of microscopic nanostructures can result in some novel and exciting functions.

This review article aims to summarize the very recent important progress in the area of functional supramolecular materials, focusing on their extraordinary functions, ranging from physical to chemical functions, from microscopic to macroscopic scales, which have been generated by the reversible and adaptive noncovalent interactions of the supramolecular materials. This mainly includes adaptive mechanical materials, smart sensors with enhanced selectivity, soft luminescent and electronic nanomaterials, and biomimetic and biomedical materials with tailored structure and functions. Because of the limitation of the space, we cannot conduct a complete account of all the interesting work in this area, but we hope that it can in some way reflect the current situation and future trends in this prosperously developing area.

2 Self-Healing Supramolecular Materials

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Most artificial mechanical polymeric materials are held together by covalent bonds, which exhibit good robustness and hardness. However, they cannot recover after being ruptured because the purely covalent bonds are irreversible. For decades, there has been a prosperous development of self-healing polymers, which are polymers that can spontaneously repair themselves on mechanical damage.[13-18] The key point is to introduce reversible and dynamic noncovalent interactions into the polymers or microdomains of the bulk materials. The material consisting of self-healing polymers may display enhanced lifetime and reliability.[19-23]

Every coin has two sides. For most supramolecular designs, there is an inevitable compromise between mechanical stiffness and dynamic healing ability: strong interactions result in stiff but less dynamic systems, precluding self-healing, and weak interactions afford dynamic healing but yield materials with low toughness. The synthesis of a stiff material with intrinsic self-healing ability remains a key challenge. Zhibin Guan and his coworkers have recently solved this problem by fabricating multiphase supramolecular thermoplastic elastomers that combine high strength and toughness with self-healing capability[24]. This new design was to program dynamic healing motifs in the soft phase of a hard–soft multiphase system (Figure 1), merging the unique mechanical properties of hybrid polymers with those of dynamic supramolecular assemblies. A polystyrene backbone was chosen as the hard phase and polyacrylate amide (PA-amide) brushes as the soft phase. The pendent secondary amide functional group, including both hydrogen bond donor and acceptor functionality, is capable of forming dynamic networks. Processed from a polar solvent, the brush polymer should generate a core–shell nanostructure with a hard polystyrene core and a soft PA-amide shell. Hydrogen-bonding-directed supramolecular assembly of the PA-amide brushes should result in the formation of a dynamic microphase-segregated nanostructure, which can be reversibly broken and reformed, affording self-healing behaviors. In contrast to most of the previous self-healing polymers, this new system spontaneously heals as a single-component solid material at ambient conditions, without the addition of any external stimulus, healing agent, plasticizer, or solvent.

image

Figure 1. Design principle for the multiphase self-healing brush polymer. a) The hydrogen-bonding brush polymer self-assembles into a double-phase nanostructure morphology during processing. b) The supramolecular connections between soft brushes can rupture reversibly under stress because of the hydrogen-bonding network, in spite of its mechanical robustness. Reproduced with permission.[24] Copyright 2012, Macmillan Publishers Ltd.

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The concept of self-healing polymers has been recently extended to construct autonomous healing biomimetic electronic skins. Zhenan Bao et al.[25] described a composite material consisting of organic supramolecular polymers based on hydrogen bonding with imbedded nickel nanostructured microparticles (Figure 2), which showed mechanical and electrical self-healing behaviour at ambient conditions. The material was pressure- and flexion-sensitive and hence fit to electronic skin applications. By varying the amount of nickel particles, the electrical conductivity could be tuned and reaches as high as 40 S cm−1. On rupture, the initial conductivity was repeatedly restored with ≈90% efficiency after a 15-second healing, and the mechanical properties were completely restored after about 10 minutes. The composite resistance varied inversely with applied flexion and pressure. These results demonstrate that the repeatable self-healing capability of natural skin can be mimicked in conductive and piezo-resistive materials, thus potentially expanding the horizon of applications of current electronic skin systems.

image

Figure 2. a) Proposed interaction of oligomer chains with Ni particles. b) Demonstration of the healing process for a conductive composite with an LED and a self-healing electrical conductor. 1, undamaged conductor; 2, completely severed conductor (open circuit); 3, electrical healing (inset shows conductor being self-supporting); 4, healed film being flexed to show its mechanical strength and flexibility at room temperature (5 min after rupture). Reproduced with permission.[25] Copyright 2012, Macmillan Publishers Ltd.

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3 Recyclable Supramolecular Soft Materials

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Supramolecular materials organized by noncovalent interactions can stand reversible self-assembly, offering advantages in fabrication, processing, and recycling.[26] A recent example related to this aspect was reported by Boris Rybtchinski et al., who presented an entirely nanostructured noncovalent porous membranes prepared from fibrous assemblies in water.[27] The supramolecular ultrafiltration membranes were easily fabricated in a single step by filtering the fibrous assembly of an amphiphilic perylene diimide derivative (Figure 3b) in water over an inexpensive commercial cellulose acetate support, thus forming a layer with a three-dimensional fibrous nanostructure, the thickness of which was readily adjusted by varying the amount of supramolecular solution during preparation. The membranes are robust owing to strong hydrophobic interactions, allowing their application in the size-selective separation of metal nanoparticles or quantum dots. A thin (approximately 12 mm) membrane was used to filter various gold nanoparticles, and the cut-off size consistently remained on the value of 5 nm (Figure 3c–e). A substantially thicker (approximately 45 mm) membrane was used to fractionate 2.5 and 4.0 nm CdTe quantum dots. The membrane stability is determined by hydrophobic interactions. The addition of ethanol to the aqueous solution passing through the membrane could drastically weaken these interactions, leading to an instantaneous disassembly of the supramolecular membrane (Figure 3f). In this way, the membrane material is recycled: it is easily cleaned, reassembled, and deposited again to produce another ultrafiltration membrane. Thus, multiple, consecutive recycling sequences with reproducible membrane performance are realized. Meanwhile, the large nanoparticle stuck in the membrane (that is, the retentate) was released and could be recycled as well (Figure 3f). From the viewpoint of green chemistry, such recycling of nanoparticles is becoming increasingly important.

image

Figure 3. Filtration of gold nanoparticles. a) Photograph of filtration experiment. b) Molecular structure of PP2b. c) Gold nanoparticles size histogram before filtration. d) Size histogram of the filtrate. e) Size histogram of the retentate and corresponding histogram. Red lines in the histograms indicate the cutoff of the filter. f) Scheme of fabrication, use and recycling of the supramolecular membrane. Adapted with permission.[27] Copyright 2011, Macmillan Publishers Ltd.

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4 Stimuli-Responsive Delivery Vehicles

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Stimuli-responsive nanomaterials have attracted great attention because of their potential applications in drug or gene delivery purposes and cancer therapy.[28-45] To construct supramolecular materials on the nanoscale, amphiphiles are preferable choices.[46, 47] Amphiphiles refer to small or polymeric molecules that contain both hydrophilic and hydrophobic parts, and the two parts are linked on the basis of covalent bonds. They are able to self-assemble in water, on the basis weak and dynamic noncovalent interactions, to form various well-defined molecular assemblies, such as micelles and vesicles. However, the build-up of the responsive assemblies generally needs the covalent incorporation of stimuli-responsive functional moieties into the amphiphilic molecules, which requires tedious covalent synthesis and modification, thus raising the cost of preparation. In addition, organic solvents, and toxic reagents used in chemical synthesis may be incorporated into the assembly and reduce its biocompatibility.

In contrast to conventional amphiphiles based on covalent bonds, there is a new field of supra-amphiphiles emerging; these are amphiphiles that are constructed on the basis of noncovalent interactions or dynamic covalent bonds. In supra-amphiphiles, functional moieties can be attached by noncovalent synthesis, greatly reducing the need for tedious chemical synthesis.[47-56]

On the basis of the concept of supra-amphiphiles, we have developed a new way of preparing enzyme-responsive polymeric systems utilizing the electrostatic interactions between a hydrophilic diblock copolymer and a natural enzyme-responsive molecule, as shown in Figure 4.[57] Adenosine 5′-triphosphate (ATP) was used as a highly effective multi-negatively charged and enzyme-responsive building block for fabricating polymeric supra-amphiphiles. We chose the double hydrophilic block copolymer methoxy-poly(ethyleneglycol)114-block-poly(l-lysine hydrochloride)200 (PEG-b-PLKC), in which the PLKC segment is positively charged, for the association with ATP. PEG-b-PLKC and ATP can form a polymeric supra-amphiphile in aqueous solution as a result of electrostatic interaction. ATP molecules noncovalently attached to the positively charged polylysine segments, thus introducing hydrophobic adenine groups and resulting in the formation of self-assembled aggregates. On addition of phosphatase, the multiply negatively charged ATP was hydrolyzed to a singly charged phosphate and a neutral adenine group. Hence, the PEG-b-PLKC-ATP complex dissociated, accompanied by disassembly of the self-assembled aggregates. Compared with conventional enzyme-responsive polymers, this new method, which does not require covalent synthesis, is very simple, highly efficient, and nontoxic. In addition, the amount of enzyme used for the controlled release experiment is about the average concentration of alkaline phosphatase presented in a healthy adult, which emphasizes the highly efficient responsiveness to enzymes. Along the same line of research, we have also reported an extremely facile method for the noncovalent construction of stimuli-responsive multicompartment micelles by the noncovalent complexation of PEG-b-PLKC and pyridoxal phosphate.[58] The multicompartment micelles are responsive to physiological pH stimulus and enzymatic stimuli. A noteworthy aspect of this model is that no organic solvent, but only water is used in the entire process, which is nontoxic and promising for in vivo applications.

image

Figure 4. Building blocks of the supra-amphiphile and the enzyme-responsive property of the self-assembled aggregates. The supra-amphiphile self-assembles into spherical aggregates, which disassemble on addition of enzyme (calf intestinal alkaline phosphatase (CIAP)) as a result of the enzymatic hydrolysis of ATP. Reproduced with permission.[57]

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The concept of supra-amphiphiles has been widely recognized as a novel and efficient way to fabricate enzyme-responsive delivery vehicles. In our recent work, acetylcholinesterase-responsive polymeric supra-amphiphiles were successfully fabricated by mixing a block copolymer of poly(ethylene glycol)-block-poly(acrylic acid) with myristoylcholine chloride in water.[59] It is well-known that many neural transmission barrier-related diseases, such as myasthenia gravis and Alzheimer's disease, are closely related to the accessible amount of acetylcholinesterase in certain parts of the body. Therefore, it is anticipated that the new enzyme responsive polymeric supra-amphiphiles may be explored as a carrier for drug delivery. Meanwhile, Yu Liu et al. successfully constructed an enzyme-responsive supramolecular vesicle as an operational targeting drug delivery system based on the concept of small molecular supra-amphiphiles.[60] As shown in Figure 5, biocompatible p-sulfonatocalix[4]arene (SC4A) and natural myristoylcholine were chosen as building blocks to fabricate the supramolecular vesicle. As to the dynamic equilibrium characteristics of noncovalent interactions, the self-assembled vesicles were successfully disintegrated by the action of the enzyme. The present binary vesicle from SC4A and myristoylcholine exhibited highly specific and efficient responsiveness to cholinesterase. An enzyme-induced cleavage of myristoylcholine triggered a cascade of events, loss of the hydrophilic−hydrophobic balance of the binary supra-amphiphile, disassembly of the vesicle, and finally the tandem release of entrapped drugs. Although the cleavage rate may be restricted, it is certainly advantageous over the covalent modification of substrates, which will not only reduce their biocompatibility but also interfere the enzyme activity and recognition specificity due to suboptimal reactivity of the enzyme to the modified substrates. Further in vivo study is needed before this concept can find practical application in pharmaceutical fields.

image

Figure 5. Schematic illustration of amphiphilic assemblies of myristoylcholine in the absence and presence of SC4A. The myristoylcholine-SC4A supra-amphiphiles self-assemble into vesicles, which can be specifically dissipated by butyrylcholinesterase (BChE) and release of entrapped hydrophilic drugs (purple rods). By contrast, free myristoylcholine self-assembles into micelle 1 at higher concentration and not disassemble but transfer into micelle 2 accompanied by the BChE reaction without the release of drugs. Reproduced with permission.[60] Copyright 2012, American Chemical Society.

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5 Adaptive Supramolecular Luminescent Organic Materials

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Luminescent organic materials have attracted significant attention thanks to their solution processability, which is appropriate for large-area and flexible displays. In particular, supramolecular multi-chromophoric assemblies have been extensively investigated as processable emissive materials, which provide a better structural control on the donor–acceptor organization and the optical output, compared with their macromolecular counterparts.[61-63] The combination of “noncovalent” and “energy transfer” design principles have been elegantly merged to achieve white-light-emitting organic assemblies in solutions, gels, and crystals. In this regard, Liang-Sheng Liao and Shuit-Tong Lee et al.[62] presented a facile and low-cost noncovalent approach to construct white-light-emitting composite organic micro- or nanostructures based on mixed charge transfer complexes. One-dimensional single-crystalline microtubes of donor (naphthalene)–acceptor (1,2,4,5-tetracyanobenzene) charge transfer complexes have been prepared by etching-assisted charge transfer induced self-assembly. Pyrene was mixed with the microtubes as dopant. By tuning the pyrene concentration, the mixed charge transfer complex microtubes can be tailored to exhibit emission in pure white or other desired colors by adjusting the dopant concentration. Interestingly, the white-light-emissive charge transfer complex microtubes are promising as active optical waveguides. This solution-phase noncovalent self-assembly method provides a novel and efficient approach to construct organic micro- or nanostructures for miniaturized luminescent devices.

The preparation of solution-phase white-light emission is quite straightforward, but for device fabrication, the solvent has to be removed during the processing. Thus, retaining the pure white light emission on solid substrates has been a challenge. Muthusamy Eswaramoorthy and Subi J. George et al.[64] have recently demonstrated a novel supramolecular strategy for the design of solution-processable white-luminescent hybrids with good color purity, as shown in Figure 6. To obtain an efficient organization of chromophores, they have selected layered magnesium phyllo(organo)silicate as the inorganic component that has a 2:1 trioctahedral smectite-like structure with aminopropyl pendants (aminoclay (AC)). The aminopropyl pendants ensured the complete dispersion of clay layers in water on nanoscale owing to the electrostatic repulsion between protonated amine groups. The organic donor (coronene tetra-carboxylate (CS) with pure blue fluorescence) and acceptor (sulforhodamine G (SRG) with yellow fluorescence) molecular components were modified as water-soluble anions, and hence could interact with the inorganic component via electrostatic interactions (Figure 6). The spectral overlap between the CS emission and SRG absorption implied that they could act as donor–acceptor couple for excitation energy transfer. The noncovalent anchoring of the organic donor and acceptor chromophores to the inorganic component not only afforded a fine control of partial energy transfer to emit pure white light but also maintained the intermolecular structure during the subsequent solution-transfer processes. More importantly, as the consequence of the dynamic nature of the assembly, the mixed CS–SRG–AC hybrids could be conveniently prepared either by the co-assembly of all three components in aqueous solution (pre-synthetic pathway) or by encapsulation of SRG acceptors to the pre-assembled CS–AC hybrids (post-synthetic). This procedure is very simple and characteristic of the soft hybrids in contrast to the labor- and time-consuming procedures often required for the covalent attachment of donors and acceptors in hybrid materials.

image

Figure 6. Design of soft-hybrids: a) Molecular structures of aminoclay (AC), donor (CS) and acceptor (SRG). b) Proposed schematic illustration of the clay-chromophore noncovalent hybrids and the energy-transfer process. Photographs of the white-light-emitting gel and film are also shown. Reproduced with permission.[65]

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6 Sensitive Supramolecular Sensors

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

The highly dynamic and adaptive noncovalent interactions constituting the supramolecular materials can be disrupted in the presence of competitive reagents (analytes). The contaminant change of their physical properties—for example, absorptions and emissions—can serve as the output signals to register the quantity and quality of the analytes. This is the basic principle for the supramolecular sensors. It should be noted that some noncovalent interactions, such as host–guest interactions,[65] are highly specific, and this may facilitate the fabrication of highly sensitive and selective sensors for a special analyte.

A recent interesting example illustrating this point was an ultrasensitive enzyme sensor that used multivalent nanoparticle networks, which were held together through supramolecular multivalent interactions, reported by Roberto de la Rica and Aldrik H. Velders et al.[66] The key step of this method was to bind the responsive ligands (ferrocenyl groups) to the surface of the nanoparticles through host (β-cyclodextrin)–guest (ferrocenyl) interactions. The strong multivalent interactions between nanoparticles then led to the formation of nanoparticle clusters. Once recognized by horseradish peroxidase, the ferrocenyl moieties could be oxidized by the biocatalytic action, then the multivalent network would be triggered to disrupt. Importantly, the competition with a monovalent guest could amplify the dispersion of the nanoparticles as the output signal. By this method, it is possible to tune the relevant parameters of the bioassay, including dynamic range and limit of detection, as needed for particular analysis. It should be noted that the reversibility of the host-guest noncovalent interactions is critical for its ultra-sensitivity. Moreover, the proposed amplification mechanism could be applied to the detection of other enzymes, such as endonucleases and proteases, by simply binding the bioresponsive ligands (oligonucleotides and peptides) to the surface of the nanoparticles through supramolecular host–guest interactions, in contrast to the commonly used covalent linkages or thiolate chemisorption.

Besides quantitative information, the qualitative information of analytes can be registered by modulating the cooperative noncovalent interactions in the supramolecular sensors. Nobuo Kimizuka et al. have developed a novel amphiphilic receptor TbL+ complex that self-assembles in water and forms stable vesicles.[67] The ligand L2− consisted of bis(pyridine) anionic arms and a long alkyl chain (Figure 7). The aqueous vesicles of TbL+ complexes luminated in water as a consequence of the metal–ligand energy transfer. On addition of varied nucleotides, sigmoidal increase in luminescence intensity was observed for ATP, followed by ADP, while enhancement was hardly observed for AMP. Thus, molecular information could be translated by the luminescent nano-interface. The enhanced luminescence intensity was ascribed to the displacement of coordinated water molecules by the phosphate groups. However, such sigmoidal increase of luminescence intensity was not observed for the structurally relevant water-soluble lanthanide complexes. Thus, the two-dimensional self-assembly of receptor complexes play essential roles in the positive cooperative binding for nucleotides with phosphate anions linked by phosphoanhydride bonds. The binding of TbL+ complexes to each phosphate unit linked by phosphoanhydride bonds occurred cooperatively, probably by adaptively changing their molecular orientation in the bilayer. The dynamic noncovalent interactions of the alkyl chains resulted in the adaptive and highly synergistic assembly between highly organized receptors (TbL+ complex) at the membrane surface. Together with adaptive self-assemblies formed in water from nucleotides and lanthanide ions, it is anticipated that these coordination nanointerfaces can provide a wide range of applications including sensing and diagnostics.

image

Figure 7. Molecular structures of the ligand L2− and TbL+ amphiphilic complex and schematic representation for self-assembly of TbL+ in water for the detection of ATP molecules. Reproduced with permission.[67] Copyright 2011, American Chemical Society.

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Multiple analytes can be detected using one supramolecular sensor based on multiple stimuli-responsive noncovalent interactions. A cross-linked network was fabricated recently by Feihe Huang et al. and was demonstrated to act as a multiple fluorescent sensor.[68] As shown in Figure 8, it was constructed from a fluorescent conjugated polymer and a bisammonium salt cross-linker (DBA) driven by the host−guest interactions between dibenzo[24]crown-8 (DB24C8) and secondary ammonium salt groups. Compared with the pure conjugated polymer, the network exhibited weak fluorescence owing to the aggregation of polymer chains. Owing to the multiple stimuli-responsiveness of host−guest noncovalent interactions, the fluorescence intensity of the system could be enhanced by four types of signals, including a potassium cation, a chloride anion, pH, and heating: the DB24C8 unit could generate a more stable 1:1 complex with K+; Cl could form a tight ion pair with DBA; Et3N could convert the secondary ammonium to the corresponding secondary amine; and heating could cause disassembly by reducing the binding constant. Hence, the network could serve as a cation sensor, an anion sensor, a pH sensor, and a temperature sensor. Moreover, the sensor could function in both solution and thin film. Interestingly, enhanced fluorescence intensity can be observed when such thin film is exposed to ammonia, making it a good candidate for gas detection. Similar mechanism has been recently employed to the construction of various other sensors.[69, 70] For example, analyte-induced fluorescence change has been applied to the rapid and naked-eye detection of biological polyamines with high sensitivity and selectivity.[70]

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Figure 8. Schematic representation of the formation of a supramolecular cross-linked network of conjugated polymer. The network can be induced to disassemble by different signals, including potassium cation, chloride anion, pH increase, and heating. Reproduced with permission.[68] Copyright 2013, American Chemical Society.

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7 Smart Materials for Microscopic and Macroscopic Scale Recognition

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

The term of “molecular recognition” relates to the specific interaction between two or more molecules through noncovalent interactions.[71-73] Molecular recognition has an important role in nature, with the best-known example being the complementarity exhibited by base pairs in DNA.[74] The exploration of function supramolecular materials based on molecular recognition has been extensively studied at the molecular level[75-84] and recently has been well extended to the fabrication of macroscopic materials. [85-87]

One dream of chemists is to synthesize molecules with a high precision under a pre-designed order by mimicking the programmed recognition events happened in nature. For example, ribosomes, which can be found in all living cells, builds proteins by joining together amino acids in a specific sequence according to the genetic information encoded in mRNA, during which the key translation step is the reorganization of specific amino acids by tRNA. Very recently, an amazing artificial system mimicking this process has been reported by David A. Leigh.[88] In their study, an artificial small-molecule machine was designed on the basis of a rotaxane: a molecular ring was threaded and could travel along onto a molecular axle. The molecular machine could pick up amino acids that blocked its path and could then synthesize a peptide in a pre-designed sequence-specific manner. As shown in Figure 9, the ring carried a thiolate group that iterative removed amino acids in order on the “track” and transferred them to a peptide-elongation site through native chemical ligation. The synthesis was demonstrated with ≈1018 molecular machines acting in parallel. This process can generate milligram quantities of a peptide with a single sequence. This study is a great advancement in the field of truly functional molecular machines.[89]

image

Figure 9. Proposed mechanism for sequence-specific peptide synthesis by artificial molecular machine. Reproduced with permission.[88] Copyright 2013, American Association for the Adancement of Science.

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Molecular recognition is also essential for the construction of smart surfaces. On the basis of molecular recognition, molecular-imprinting techniques for synthesizing artificial host systems have generated a new dimension to mimic complicated molecular-recognition systems in nature, and their applications are promising in many fields: for example, chemosensors, separation, and catalysis.[90-94] By taking advantage of the unconventional layer-by-layer (LbL) assembly and molecular-imprinting techniques, our group has developed the method of fabricating the surface-imprinted multilayer films by incorporating template molecules into LbL films.[95-97] However, the size and shape of the imprinted sites may change during removal or rebinding of the template molecules owing to the flexibility of the polymer matrix. Recently, we have combined supramolecular chemistry with LbL assembly to develop new methods to overcome this problem and to endow the LbL films with improved properties.[98] Stable multilayer films with cucurbit[8]uril (CB[8]) have been successfully constructed on the basis of the alternating LbL assembly of a novel side-chain pseudopolyrotaxane (PMVCnH2n+1 in Figure 10) and a photoreactive polyanion (PAA-N3). The multilayer films with CB[8] as molecular containers could accommodate suitable electron donors—that is, 1-(anthracen-2-ylmethyl)pyridinium bromide (AnPy)—owing to host-enhanced charge transfer interactions and exhibit good properties as surface-imprinted multilayers. The imprinting sites are not subject to geometrical changes because of the rigid structure of CB[8], and moreover its defined size allows for specific recognition. Moreover, the loading and release of the guest is redox-controllable and reversible. Besides CB[8], cucurbitural with cavities larger or smaller than CB[8] can be included into LbL films in a similar way and this allows for incorporation of different guest molecules.

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Figure 10. Molecular structures of CB[8], the PMVCnH2n+1-CB[8] complex, PAA-N3 and proposed mechanism of the redox-controlled reversible encapsulation of AnPy in CB[8] of the LbL film assembled by the PMVCnH2n+1-CB[8] complex and PAA-N3. Reproduced with permission.[98]

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It will be fascinating if the molecular recognition events could be scaled up. If macroscopic scale objects could recognize each other in a predictable fashion, various new macroscopic architectures and functions can be realized. Akira Harada and colleagues have reported an exciting example in this regard. They demonstrated that well-defined molecular-recognition events could be used to direct the assembly of macroscopic objects into larger ordered structures.[19, 85] In their work, acrylamide-based gels functionalized with either host (cyclodextrin rings) or guest (adamantane group) moieties were synthesized (Figure 11a). Host–guest interactions could be visualized by means of the self-assembly of objects on the scale of millimeters to centimeters: pieces of host and guest gels were shown to adhere to one another through the molecular recognition of the cyclodextrin and adamantane groups on their surfaces (Figure 11b and 11c). Moreover, by changing the size and shape of the host and guest units, different gels could be selectively assembled and recognize each other to form distinct macroscopic structures on the order of millimeters to centimeters in size. It is expected that, macroscopic recognition on the basis of molecular recognition hold great promise in self-assembly in the macroscopic construction of new architectures by selective and reversible binding properties.

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Figure 11. a) Molecular structures of the acrylamide-based gels functionalized with host (cyclodextrin) or guest (adamantine). b,c) Photographs showing macroscopic self-assembly between cyclodextrin host gels and adamantine guest gels. Reproduced with permission.[85] Copyright 2011, Macmillan Publishers Ltd.

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The macroscopic recognition based on host–guest interactions has also been used by Kimoon Kim[86] et al. to fabricate a supramolecular velcro used for reversible underwater adhesion, which is a very challenging task because the adhesion between the adhesive and adherend surfaces is generally rather low in an aqueous environment. The strategy, using a velcro or “hook-and-loop fastener” type mechanism, involves functionalizing two separate silicon surfaces ([Si]): a “loop” surface functionalized with cucurbit[n]uril (CB[7]) hosts, and a “hook” surface with aminomethylferrocene (Fc) guests (Figure 12). The CB[7] loops and Fc hooks formed a strong supramolecular velcro based on the strong host–guest interaction (Ka∼1012), which adhered strongly with each other in water without the need of curing agents. The velcro exhibited excellent holding power and lap shear adhesion strength, which could be tuned by controlling the density of the Fc hooks. Remarkably, their strategy established here offers both mechanical reversibility and chemical switchability at the macroscopic level, benefiting from the strength and reversibility of the noncovalent host-guest interaction. Unlike mussel-mimetic adhesives, this system does not rely on any external curing agents for underwater adhesion, apart from water, and permits potential biological applications.

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Figure 12. a) The proposed mechanism of supramolecular velcro for underwater adhesion based on CB[7]- and Fc-modified surfaces. The supramolecular velcro can be reversibly fastened and unfastened by external stimuli (mechanical force or chemical redox). b) Photograph showing the holding power of supramolecular velcro in water. Reproduced with permission.[86]

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8 Adaptive Mechanical Materials with Improved Mechanical Properties

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

The adaptivity of the noncovalent interactions can be inherited to the supramolecular materials. Taking muscle cells as an example, they are entities of small lipids and functional proteins, such as myosin and actin, organized by noncovalent interactions. Therefore, the mechanical properties of muscle cells are highly adaptive: once “feeling” the changes of Ca2+ concentration in the surrounding microenvironment, the muscle cells can reversibly contract and retract by regulating the noncovalent interactions between myosin and actin[99]. Inspired by nature, recently, much attention has been focused on the development of adaptive mechanical materials by the incorporation of rationally engineered noncovalent arrays into functional materials.[23, 100-108]

Mechanically adaptive nanomaterials can be facilely fabricated using the concept of supra-amphiphiles. Yongfeng Zhou and his co-workers have prepared supramolecular linear hyperbranched block molecules through noncovalent host-guest coupling between the β-cyclodextrin-functionalized hyperbranched polyglycerol (CD-g-HPG) and adamantane functionalized long alkyl chains (AD-C18) (Figure 13a).[109] The obtained supra-amphiphiles further self-assembled into vesicles in water, which could be readily disassembled by the introduction of competitive hosts β-cyclodextrin. Interestingly, the vesicles show great ductility: they can be highly deformed upon being compressed on the surface. For example, in tapping-mode AFM experiments, when the force of the AFM tip was increased, the two vesicles in Figure 13b (white line) showed an expansion of about 300% in radii after deformation (Figure 13c). The vesicle's adaptive deformation was attributed to the sliding or decomplexation of host-guest noncovalent complexes of C18-b-HPGs under external force. The vesicle ductility is a demonstration of the unique dynamic property of supra-amphiphiles.

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Figure 13. a) Schematic illustration of the preparation, self-assembly, and disassembly of C18-b-HPG. b,c) Self-assemblies of C18-b-HPG captured by AFM. The force was increased to 2.1 nN on AFM tip in image c). Reproduced with permission.[109] Copyright 2012, American Chemical Society.

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Both covalent and noncovalent interactions combined in one material may generate exciting new properties. Natural materials such as biominerals exhibit mechanical properties that surpass those of their synthetic counterparts. A key feature of these materials, which sets them apart from synthetic crystals, is their nanocomposite and hierarchical structures. Fiona C. Meldrum et al. demonstrated the production of artificial biominerals where single crystals of calcite occluded a remarkable 13 wt% of 20 nm anionic diblock copolymer micelles acting as ‘pseudo-proteins'.[110] Calcium carbonate was facilely precipitated in the presence of micelles of a pH-responsive diblock copolymer (PSPMA30-PDPA47), which exhibited a coronal layer of anionic carboxylated chains, as shown in Figure 14a. The occlusion of copolymer micelles at high levels facilitates the first truly comprehensive study of the influence of additives on the structure and properties of calcite single crystals. The authors revealed that the particulate occlusions affected the calcite lattice from the atomic to the micrometer length scale. The synthetic crystals exhibited similar texture and defect structures to biogenic calcite crystals and were harder than pure calcite, as derived from the AFM indentation experiments (Figure 14b–e). The incorporation of noncovalent interactions inside the inorganic materials paves a new way for exploring novel and exciting functions, which cannot be achieved with their counterparts.

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Figure 14. a) Schematic illustration of the encapsulation of copolymer micelles within calcite single crystals. b, d) SEM and AFM images and c, e) AFM profiles of nano-indentation marks on a calcite control crystal and a nanocomposite crystal after nano-indentation. b,c) Pure calcite crystal, and d,e) a nanocomposite calcite crystal. Adapted with permission.[110] Copyright 2011, Macmillan Publishers Ltd.

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Another fascinating example in this regard is the biomimic of the Ig domains titin. Different natural materials have evolved adaptive properties to maintain their functionality over a range of stress and strain. This includes changing properties in response to external stimuli such as temperature. Recently, a linear polymer mimicking the titin architecture was exploited by Zhibin Guan.[111] As shown in Figure 15a, the polymer was consisting of a tandem array of biomimetic modules composed by the quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidone (UPy) cyclic motif. The polymer exhibits a rare combination of high modulus, high toughness, and adaptive properties, including self-healing capacity and shape memory. The proposed mechanism for these observed properties is illustrated as follows: on stretching, the modules gradually unfold (Figure 15b), resulting in the absorption of energy. When the polymer is cooled and stress is removed, many of the unfolded UPy units are in close proximity to other opened units on neighboring chains, for which they can dimerize and freeze the new polymer shape. When temperature and time are applied to the system, the interchain UPy dimers become dynamic, and the modules can return to their original more stable cyclic self-dimerized state, regaining their original dimensions and properties. Notably, the noncovalent hydrogen bonding modulars have a critical role for these observed excellent adaptive properties.

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Figure 15. a) Molecular structure of the biomimetic linear modular polymer with UPy motif modules. b) Schematic illustration of the biomimetic modular polymer with and without stress, showing the probable molecular mechanism for its adaptivity. Reproduced with permission.[111] 2009, American Chemical Society.

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Similar strategies can be extended to the design and fabrication of highly stretchable hydrogels. A wonderful example, designed by Zhigang Suo et al., is the synthesis of hydrogels from polymeric composite networks with ionic and covalent crosslinks.[112] The composite system exhibits outstanding mechanical properties, such as high stretchability, even for samples containing notches. As shown in Figure 16a, in their alginate–polyacrylamide hybrid gel, ionic crosslinks and covalent crosslinks are intertwined and joined by covalent crosslinks (blue triangles) between amine groups on polyacrylamide chains and carboxyl groups on alginate chains. Although the resulting gels contain 90% water, they can be stretched beyond 20 times their initial length (Figure 16b) and have fracture energies of as high as 9 000 J m−2. The gels' toughness is attributed to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis provided by unzipping the network of noncovalent ionic crosslinks. These highly stretchable gels may expand the scope of hydrogel practical applications.

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Figure 16. a) Schematic illustration of the alginate–polyacrylamide hybrid gel. The alginate polymer-Ca2+ noncovalent ionic crosslinks and polyacrylamide covalent crosslinks are intertwined, which are then joined by covalent crosslinks (blue triangles). b) The gel can be stretched to 21 times of its initial length. Reproduced with permission.[112] Copyright 2012, Macmillan Publishers Ltd.

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9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Over the past decades, sustained advances in electronics—for example, integrated circuit technologies for memory and processors—has pushed towards the deep sub-100 nm regime, which is not accessible for the conventional “top-down” fabrication method, such as laser lithography. In dealing this problem, the method of molecular self-assembly based on noncovalent interactions, mostly π–π interactions, provides a unique and valuable tool to the construction of electronics at the nanometer scale.[113-118] It has been well-studied that the highly ordered alignment of functional molecules has a critical role for the improvement of opto- or electrical performances.[119-122] The self-organization of functional molecules based on noncovalent interactions is a facile tool to control the molecular order of nanomaterials for their desired functions.

A key challenge is to develop nanowires with high charge-transport properties, as this is crucial for the minimization of many electronical devices. A recent contribution to this area is reported by Lifeng Chi and Frank Würthner et al.[123] As indicated in Figure 17, they have shown that a kind of amphiphilic semisynthetic bacteriochlorophyll derivative self-assembles into highly defined nanotubes reminiscent of the chlorosomal light-harvesting antennae of green bacteria. These nanotubes exhibit exceptional charge-transport properties. Conductive AFM measurements performed for single nanotubes showed conductivity values of approximately 0.48 S m−1, which are unprecedented for natural supramolecular structures and are comparable in magnitude to those of semiconducting supramolecular oligomers and polymers. Considering the excellent exciton-transport capabilities and semiconductivity, the chlorophyll-dye-based bulk-heterojunction materials appear highly promising for organic photovoltaics.

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Figure 17. Chemical structure of zinc chlorin and the corresponding space-filling (CPK) model (left). Schematic model of self-assembled nanotubes formed upon injection of a concentrated solution of chlorin in methanol or THF into water (right). Reproduced with permission.[123]

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Another important optoelectronic device is the organic heterojunction, which is crucial in the development of organic thin-film solar cells. However, most studies regarding this topic are focused on so-called bulk heterojunctions, which are formed only coincidentally from donor/acceptor mixtures on phase separation. In 2006, on the basis self-assembly of functional molecular building blocks, Takuzo Aida et al. fabricated a highly ordered nanotube acting as a supramolecular donor–acceptor heterojunction with an extraordinarily wide interface.[124] Furthermore, they recently reported a semiconducting organic heterojunction at nanoscale.[125, 126] One of the designed building block (HBC 1), a gemini-shaped hexa-perihexabenzocoronene, self-assembles into seed nanotube (Figure 18a), which can be morphologically stabilized by the metal-coordinating of the bipyridine units on the periphery (Figure 18b). The other building block, HBC 2, with four electron-withdrawing fluorine substituents, can adhere electronically to the seed termini and self-assemble selectively from their nanotubular facets. When HBC 1 and HBC 2 coassemble stepwise (Figure 18c), the resultant connecting segments are electronically dissimilar to one another (Figure 18d). The dissimilar nanotubular segments, which were connected noncovalently, were electronically communicable with one another over the heterojunction interface and displayed characteristic excitation energy transfer and charge transport properties, which are not present in a mixture of the corresponding homotropically assembled nanotubes. The elegant way established here to fabricate nanoscale semiconducting organic heterojunctions is crucial for the development of efficient photon-to-electrical energy conversion systems. More importantly, it demonstrates that the rational design and interplay of various noncovalent interactions is a valuable approach to connect two nanodevices at molecular level.

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Figure 18. Molecular structures of HBCs 1 and 2 and a–c) schematic illustrations of the preparation of the semiconducting organic heterojunction at the nanoscale. d) Schematic illustration of an idealized cross section of the heterojunction interface. Reproduced with permission.[126] Copyright 2011, American Association for the Adancement Science.

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Conjugated molecules with exceptional optoelectronic properties tend to stack in solution based on noncovalent interactions. However, the diversity of their assembling behavior makes their optoelectronic properties hard to control.[127] Albertus P. H. J. Schenning and Bert Meijer et al. reported the full control of chirality in one dimensional self-assembled stacks by using a supramolecular noncovalent chiral auxiliary approach.[128] The achiral molecules (AOPV3, in Figure 19a) self-assemble in solution to form a racemic mixture of helical stacks, which is then converted into homochiral assemblies with different optical properties by coassembling with a chiral auxiliary (D-TA) (Figure 19b). After removal of the auxiliary, the helicity in these stacks remains intact. Even after partial disassembly, the helicity is recovered on reassembly. In general, this approach represents a new tool to create homochiral one-dimensional systems self-assembled from optoelectronic functional molecules.

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Figure 19. a) Molecular structures of the achiral OPV (AOPV3), the enantiomeric dibenzoyl tartaric acid (D-TA and L-TA) derivative chiral auxiliaries. b) Schematic representation of the self-assembly of AOPV3 to racemic helical stacks and the asymmetric synthesis of AOPV stacks having a P-helicity using a supramolecular chiral auxiliary approach. Reproduced with permission.[128] Copyright 2012, American Chemical Society.

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“Light-harvesting nanofibers” have recently been reported by H. Christopher Fry and Samuel I. Stupp et al., as shown in Figure 20.[129] Peptide amphiphiles, which have been well studied as an important candidate for the construction of one-dimensional nanostructures[130, 131], were used as the structural architecture to guide the formation of fibrous assemblies while ordering metalloporphyrins in a well-organized manner. Rational peptide design was applied to generate a peptide, c16-AHL3K3-CO2H, capable of forming a β-sheet structure that propagates into larger fibrous structures. A porphyrin-binding site, a single histidine, was engineered into the peptide sequence in order to bind the metalloporphyrin—that is, zinc protoporphyrin—to provide photophysical functionality. Further studies indicate strong exciton interactions between neighboring chromophores, suggesting tight packing and close proximity. The beauty of this system lies in the single histidine being available for noncovalent binding allows for the facile fabrication of nanomaterials with desired functions, simply by incorporating the metalloporphyrins with different physical properties, in this case zinc protoporphyrin for photoexcitation or hemin for oxygen activation, highlighting the material's versatility and tenability.

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Figure 20. a) Peptide design: c16-AHL3K3-CO2H lysine (blue), leucine (green), histidine (red), alanine, and c16 (yellow). b) Zinc protoporphyrin. c) β-Sheet organization with zinc protoporphyrin (purple). d) Cross-section of self-assembled fiber. Reproduced with permission.[129] Copyright 2012, American Chemical Society.

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10 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

Supramolecular materials held together by noncovalent interactions have great potential in material science. The unique reversibility and adaptivity of noncovalent intreractions have brought about fascinating new functions that are not available by their covalent counterparts and have greatly enriched the realm of functional supramolecular materials. We have tried to highlight some of the recent progress and hope it can reflect in a way the current situation and future trends in this prosperous developing area. It should be noted that this field is still developing, and many issues need to be addressed before rational design of functional supramolecular materials can reach a high level.

For further advancing this field of functional supramolecular materials, controlled self-assembly is desirable, and it has to be scaled up. A great many of fancy ideas and observations have been developed in the microscopic world, but the time has come to scale them up. One critical issue is how to control and manipulate of the noncovalent interactions at a larger scale for control self-assembly. Although the criterions to control molecular reorganization and self-assembly have been well explored at the microscopic scale, but it can make a difference only when the microscopic world meets the macroscopic one. In this regard, it is really a challenge task to achieve and control long-range supramolecular ordering.

How to control noncovalent bonding in a much more precise and programmable way? Life is a highly ordered entity operated by a series of programmable noncovalent interactions, which should be instructive for the further development of functional supramolecular materials. Although much effort has been devoted to this line of research, this area appears to be still in its infancy.

Generally, dynamic supramolecular materials are unstable and easily broken or dissembled, leading to a series of obstacles regarding practical applications. It is highly desirable to combine the properties of “dynamic” and “robustness/durability” in one material. Considering the mentioned demands, some dynamic covalent bonds show an encouraging outlook.

Supramolecular composite materials have a high potential of providing specifically tailored and further enhanced multiple functions. The advantages of the supramolecular materials, such as facile preparation and adjustment, enhance the development of supramolecular composite materials, including organic–inorganic, covalent–noncovalent bonds, crystalline–uncrystalline structures, and so on. The combination and interplay between these components in one material can lead to fascinating physical and chemical properties.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography

This article is part of an ongoing series celebrating the 25th anniversary of Advanced Materials. The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2013CB834502) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21121004). Many thanks to Miss Myriam Meineck for her great help in polishing the English.

Biography

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Self-Healing Supramolecular Materials
  5. 3 Recyclable Supramolecular Soft Materials
  6. 4 Stimuli-Responsive Delivery Vehicles
  7. 5 Adaptive Supramolecular Luminescent Organic Materials
  8. 6 Sensitive Supramolecular Sensors
  9. 7 Smart Materials for Microscopic and Macroscopic Scale Recognition
  10. 8 Adaptive Mechanical Materials with Improved Mechanical Properties
  11. 9 Highly Ordered Nanomaterials for the Development of Supramolecular Nanoelectronics
  12. 10 Conclusions
  13. Acknowledgements
  14. Biography
  • Image of creator

    Professor Xi Zhang is a professor of the Department of Chemistry, Tsinghua University, Beijing, China. His research interests are focused on supra-amphiphiles, supramolecular polymers, selenium-containing polymers, layer-by-layer assembly, and single-molecule force spectroscopy. He serves as a senior editor of Langmuir and is a member of the advisory board of several journals, including Accounts of Chemical Research and Small, among others. In 2007, he was selected as Member of Chinese Academy of Sciences. Currently, he is the Vice President of the Chinese Chemical Society.