Hybrid polysaccharide-silica nanocomposites prepared by the sol-gel technique

New monolithic nanocomposite silica biomaterials were synthesized on the basis of various natural polysaccharides and recently introduced completely water-soluble precursor tetrakis(2-hydroxyethyl) orthosilicate. The sol-gel processes were performed in aqueous solutions without the addition of organic solvents and catalysts. The silica polymerization was promoted by the polysaccharides through acceleration and catalytic effect on the processes. By introducing poly(vinyl alcohol) or poly(ethylene oxide) in the precursor solution, it was shown that the jellification took place in the case of the hydroxyl-containing polymer. Therefore, it was suggested that the catalysis was caused by a formation of hydrogen bonds between hydroxyl groups in macromolecules and products of precursor hydrolysis (silanols). It was also demonstrated that the polysaccharides radically changed the structure of biomaterials. In place of the cross-linked nanoparticles, there was a three-dimensional network from crossed or branched fibers and uncrossed spherical particles that filled the mesh space. The density of network, thickness of fibers, and properties of synthesized biomaterials depended on the polysaccharide type, charged degree of their macromolecule, and concentration. By varying these parameters, it was possible to manipulate the structural organization of hybrid polysaccharide-silica nanocomposites.     

Transition Metal Catalysis Using Functionalized Dendrimers

Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems. These novel polymers have inspired many chemists to develop new materials and several applications have been explored, catalysis being one of them. The recent impressive strides in synthetic procedures increased the accessibility of functionalized dendrimers, resulting in a rapid development of dendrimer chemistry. The position of the catalytic site(s) as well as the spatial separation of the catalysts appears to be of crucial importance. Dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes, their efficient natural counterparts, whereas the surface-functionalized systems have been proposed to fill the gap between homogeneous and heterogeneous catalysis. This might yield superior catalysts with novel properties, that is, special reactivity or stability. Both the core and periphery strategies lead to catalysts that are sufficiently larger than most substrates and products, thus separation by modern membrane separation techniques can be applied. These novel homogeneous catalysts can be used in continuous membrane reactors, which will have major advantages particularly for reactions that benefit from low substrate concentrations or suffer from side reactions of the product. Here we review the recent progress and breakthroughs made with these promising novel transition metal functionalized dendrimers that are used as catalysts, and we will discuss the architectural concepts that have been applied.     

Progress in Synthetic Polymers Based on Natural Amino Acids

α-Amino acids are one type of the main building blocks of living systems, being the primary components of all naturally occurring peptides and proteins. They are the simplest optically active compound in the nature and have multiple functional groups, which enable them to be transformed into a wide variety of optically active substances. The resulting materials show a wide variety of functions such as electron transfer, information transfer, photo reactivity and selective catalytic function, which cannot be imitated by synthetic compounds. Functional macromolecular materials using biological chiral resources such as amino acids have been drawing much interest due to their biocompatibility and biodegradability easing the ecological trouble because amino acid residues can be targeted for cleaving by different enzymes. Also, this type of polymer contains nitrogen, which the organism needs for their growth and shows excellent hydrophilic character, reasonably high melting points and good materials properties even at relatively low molecular weights. However, polymers composed of amino acids alone have limited thermal stability and are insoluble in many common organic solvents, which make these materials difficult to fabricate and utilize. Preparation of hybrid systems between conventional synthetic polymers and linear sequences of amino acids are interesting because amino acid segments possess unique properties, such as directional polarity, chirality and their capability to undergo specific noncovalent interactions. These properties can potentially be used for designing novel hierarchical superstructures with tunable material properties for a wide variety of applications. Herein, the synthesis and properties of synthetic macromolecules having natural amino acids are reviewed in details up to now with excluding polypeptides.     

Biological functions of synthetic polysaccharides

Structurally well-defined polysaccharide derivatives have been synthesized via ring-opening polymerization of bicyclic anhydro sugar derivatives and via enzyme-catalyzed polymerization using phosphorylase. This article describes the following four synthetic polysaccharides, (a) Octadecylated amphiphilic polysaccharides, (b) Regiospecifically fluorinated polysaccharide, (c) Comb-shaped branched polysaccharide, (d) Graft copolymers having pendant polysaccharide chains. None of these polysaccharides could be prepared by direct chemical modifications of natural polysaccharides. The synthetic polysaccharides are useful as tools to elucidate structure-function relationships and exhibit various novel biofunctional properties.     

Smart self-assembled hybrid hydrogel biomaterials

Hybrid biomaterials are systems created from components of at least two distinct classes of molecules, for example, synthetic macromolecules and proteins or peptide domains. The synergistic combination of two types of structures may produce new materials that possess unprecedented levels of structural organization and novel properties. This Review focuses on biorecognition-driven self-assembly of hybrid macromolecules into functional hydrogel biomaterials. First, basic rules that govern the secondary structure of peptides are discussed, and then approaches to the specific design of hybrid systems with tailor-made properties are evaluated, followed by a discussion on the similarity of design principles of biomaterials and macromolecular therapeutics. Finally, the future of the field is briefly outlined.     

Silicification and biosilicification

Biosilicification takes place at or very close pH 7.0 and under ambient conditions of temperature and pressure in vivo. The silicic acid transporters and the proteins facilitating biosilicification in diatoms have been identified. Silica synthesis under mild conditions in vitro has been demonstrated using synthetic polymers with control over the resulting silica morphology. The results presented herein show that the silica synthesis in vitro is not specific to particular enzymes/polypeptides due to their particular chemical structure and activity but that many other synthetic macromolecules are also capable of facilitating silica formation at neutral pH. We also report the synthesis of organic-inorganic hybrid materials that have potential in optoelectronic applications.     

Chemistry and Crystal Growth

Single-crystal materials, along with other forms of condensed matter (ceramics, polymers, liquid crystals, etc.) are fundamental to modern technology. The basic research and production of new materials with “tailored” solid-state physical properties therefore necessitate not only chemical synthesis but also the production of single crystals of a particular morphology (either bulk or thin layer crystals) and well-defined crystal defects (doping). In this review, an attempt is made to broaden the traditional synthetic concept of chemistry to the process of single-crystal synthesis. The methods of the resulting approach, which takes into account the specific properties of solid materials, are discussed and illustrated by experimental set-ups for the solution of a range of problems in chemical crystallization. Also included is recent work on the growing of single crystals of high-temperature superconductors, organic non-linear optical compounds, and proteins.     

Sol–gel synthesis of highly effective catalyst based on cobalt tetrasulfophthalocyanine complex and silicon oxide

New hybrid materials based on silicon oxide and water soluble cobalt phthalocyanine complex were synthesized and characterized. It was shown that synthesized material exhibit pronounced catalytic activity in the R–SH type of organic compounds oxidation reaction in comparison with the individual macroheterocycle. The influence of the synthetic route of the hybrid material preparation on its catalytic efficiency was determined.     

Polysaccharides/silica hybrid materials: New perspectives for sustainable raw materials

Polysaccharides/silica hybrids prepared through sol-gel reaction have been studied as a model for new generation of hybrid materials for a virtually unlimited number of applications. Numerous studies have therefore attempted to increase the homogeneity between the organic and inorganic moieties using a wide variety of silica precursors and crosslinking reagents. The current review summarizes and discusses the attempts for using polysaccharides for producing hybrid materials with collective properties from the two counterparts. It discusses several issues and strategies to tune the homogeneity of polysaccharide/silica hybrids and the potential applicability of these hybrid materials in biomedical and industrial fields.     

Dendritic Encapsulation of Function: Applying Nature's Site Isolation Principle from Biomimetics to Materials Science

The convergence of our understanding of structure-property relationships for selected biological macromolecules and our increased ability to prepare large synthetic macromolecules with a structural precision that approaches that of proteins have spawned a new area of research where chemistry and materials science join with biology. While evolution has enabled nature to perfect processes involving energy transfer or catalysis by incorporating functions such as self-replication and repair, synthetic macromolecules still depend on our synthetic skills and abilities to mesh structure and function in our designs. Clearly, we can take advantage of our understanding of natural systems to mimic the structural features that lead to optimized function. For example, numerous biological systems make use of the concept of site isolation whereby an active center or catalytic site is encapsulated, frequently within a protein, to afford properties that would not be encountered in the bulk state. The ability of the dendritic shell to encapsulate functional core moieties and to create specific site-isolated nanoenvironments, and thereby affect molecular properties, has been explored. By utilizing the distinct properties of the dendrimer architecture active sites that have either photophysical, photochemical, electrochemical, or catalytic functions have been placed at the core. Applying the general concept of site isolation to problems in materials research is likely to prove extremely fruitful in the long term, with short-term applications in areas such as the construction of improved optoelectronic devices. This review focuses on the evolution of a natural design principle that contributes to bridging the gap between biology and materials science. The recent progress in the synthesis of dendrimer-encapsulated molecules and their study by a variety of techniques is discussed. These investigations have implications that range from the preliminary design of artificial enzymes, catalysts, or light-harvesting systems to the construction of insulated molecular wires, light-emitting diodes, and fiber optics.     

GRAFT COPOLYMERIZATION OF CELLULOSE,CELLULOSE DERIVATIVES,AND LIGNOCELLULOSE

Abstract

The growth of polymer science has led to the development of new materials in direct competition with natural materials, many of which have been in use since earliest times. This has caused researchers to look more critically at both natural and synthetic macromolecules in order to learn more about their underlying structures and their relation to the properties exhibited by the macromolecules. In this regard, chemical modifications have been devised to impart certain desirable properties of both natural and synthetic macromolecules, and their applications have become an integral part of such chemical modifications. Various chemical modifications (e.g., change of functionality, oxidative degradation, inter- and intramolecular gelation, graft copolymerization), have been practiced to add improved properties to the base polymers. However, among all these methods, modification of polymers via graft copolymerization has been the subject of much interest and has made paramount contribution toward improved industrial and biomedical applications.     

A novel conformationally adaptive macrocyclic tetramaleimide with flipping pyrene sidewalls

The synthesis, structure, and properties of pyrene-based conformationally adaptive macrocycles are described. This new type of conformationally adaptive macrocycle was constructed through Perkin reaction,followed by imidization. By changing the condensation partner as the linking unit, a family of conjugated macrocycles with different sizes of the cavity was synthesized, which provide a simple and modular synthetic strategy towards the conformationally adaptive macrocycles. Furthermore, the macr...     

Fullerenes as Key Components for Low‐Dimensional (Photo)electrocatalytic Nanohybrid Materials

An emerging class of heterostructures with unprecedented (photo)electrocatalytic behavior, involving the combination of fullerenes and low‐dimensional (LD) nanohybrids, is currently expanding the field of energy materials. The unique physical and chemical properties of fullerenes have offered new opportunities to tailor both the electronic structures and the catalytic activities of the nanohybrid structures. Here, we comprehensively review the synthetic approaches to prepare fullerene‐based hybrids with LD (0D, 1D, and 2D) materials in addition to their resulting structural and catalytic properties. Recent advances in the design of fullerene‐based LD nanomaterials for (photo)electrocatalytic applications are emphasized. The fundamental relationship between the electronic structures and the catalytic functions of the heterostructures, including the role of the fullerenes, is addressed to provide an in‐depth understanding of these emerging materials at the molecular level.     

Chemical synthesis of hybrid materials based on PAni and PEDOT with polyoxometalates for electrochemical supercapacitors

Hybrid organic–inorganic materials based on conjugated polymers constitute state-of-the-art compounds with recognized technological implications. In the area of energy conversion, production and storage devices, these materials have been applied as electrodes for batteries, supercapacitors, fuel cells or solar cells, among others. Their importance relies on the wide variety of organic and inorganic counterparts that these hybrids can be made of. The properties from each part can be tailored in order to contribute to a final desired characteristic or the combined properties from both. The unique combination of useful properties found in these materials include electronic conductivity (e⁻ or h⁺), ionic transport, reversible electroactivity, electrooptical properties typical of semiconductors as well as electrochromic, pH- and composition-dependent properties, all of them to add to their polymeric nature. This is an excellent basis for the design of hybrid materials in which either of these properties or their combinations work to enhance or combine with those of a myriad inorganic phases with electronic, magnetic, photochemical, electrochemical, optical or catalytic properties. A large variety of functional hybrid materials can thus be designed and fabricated in which multifunctionality can be easily built to address specific technological needs. In this work we present our most recent results on new synthesis methodology developed for the chemical synthesis of the hybrid PAni/PMo12 and their application as electrochemical supercapacitors. We also report the synthesis of a new hybrid material of PEDOT/PMo12 synthesized for the first time by chemical methods and applied also in electrochemical supercapacitors. Initial results shows capacitance values as high as 168 F/g for the hybrid PAni/PMo12 and about 130 F/g for the hybrid PEDOT/PMo12.     

Modular Toolkit of Multifunctional Block Copoly(2-oxazoline)s for the Synthesis of Nanoparticles

Post-polymerization modification provides an elegant way to introduce chemical functionalities onto macromolecules to produce tailor-made materials with superior properties. This concept was adapted to well-defined block copolymers of the poly(2-oxazoline) family and demonstrated the large potential of these macromolecules as universal toolkit for numerous applications. Triblock copolymers with separated water-soluble, alkyne- and alkene-containing segments were synthesized and orthogonally modified with various low-molecular weight functional molecules by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene (TE) click reactions, respectively. Representative toolkit polymers were used for the synthesis of gold, iron oxide and silica nanoparticles.     

Self-assembled Nanohybrid from Opposite Charged Sheets: Alternate Stacking of CoAl LDH and MoS_2

Hybrid materials are attracting intensive attention for their applications in electronics, photoelectronics, LEDs, field-effect transistors, etc. Engineering new hybrid materials and further exploiting their new functions will be significant for future science and technique development. In this work, alternatively stacked self-assembled CoAl LDH/MoS_2 nanohybrid has been successfully synthesized by an exfoliation-flocculation method from positively charged CoAl LDH nanosheets(CoAl-NS) with negatively charged MoS_2 nanosheets(MoS_2-NS). The CoAl LDH/MoS_2 hybrid material exhibits an enhanced catalytic performance for oxygen evolution reaction(OER) compared with original constituents of CoAl LDH nanosheets and MoS_2 nanosheets. The enhanced OER catalytic performance of CoAl LDH/MoS_2 is demonstrated to be due to the improved electron transfer, more exposed catalytic active sites, and accelerated oxygen evolution reaction kinetics.     

Organic–Inorganic Hybrid Saponites Obtained by Intercalation of Titano‐Silsesquioxane

The synthesis and characterization of two bifunctional composite materials based on synthetic saponite clays is here presented. These materials were prepared by intercalation of a Ti‐containing aminopropylisobutyl polyhedral oligomeric silsesquioxane (Ti‐NH₂POSS) in synthetic saponite samples containing interlayer sodium (Na‐SAP) or protons (H‐SAP). Hybrid organic–inorganic materials, Ti‐NHM‐1 and Ti‐NHM‐2, were obtained upon ion exchange. Structural, spectroscopic, and thermal properties of both hybrid materials were investigated in detail along with their catalytic activity in cyclohexene oxidation.     

An investigation of the properties of poly(dimethylsiloxane)-bioinspired silica hybrids

Elastomers typically require the incorporation of reinforcing fillers in order to improve their mechanical properties. For commercial silicone systems silica and titania are typically used as fillers. Fumed and precipitated silica are made on an industrial scale for many applications; however, we have shown recently that biological and synthetic macromolecules can generate new silica structures using a bioinspired route. Herein we have incorporated bioinspired silica fillers into poly(dimethylsiloxane) (PDMS) elastomers and investigated their mechanical, morphological and thermal properties as a function of filler loading. The equilibrium stress-strain characteristics of the PDMS-bioinspired silica hybrids were determined as a function of bioinspired filler loading and the Mooney-Rivlin constants (2C₁ and 2C₂) were calculated. The thermal characteristics, in particular glass transition temperatures (T_g) and melting points (T_m), of the PDMS-bioinspired silica hybrids were characterized using differential scanning calorimetry (DSC). The thermal stability of these hybrid materials were investigated using thermogravimetric analysis (TGA). The morphology of the samples was characterized using scanning electron microscopy (SEM), and the filler dispersion was characterized using ultra small angle X-ray scattering (USAXS) and scanning electron microscopy (SEM). Although spherical silica particles were used here, we have demonstrated elsewhere that this bioinspired synthetic route also enables highly asymmetric silica structures to be prepared such as fibres and sheets. This methodology therefore offers the interesting possibility of preparing new hybrid systems where the properties are highly anisotropic.     

Carbon Nanotube and Gold‐Based Materials: A Symbiosis

Carbon nanotubes constitute a novel class of nanomaterials with potential applications in many areas. The attachment of metal nanoparticles to carbon nanotubes is new way to obtain novel hybrid materials with interesting properties for various applications such as catalysts and gas sensors as well as electronic and magnetic devices. Their unique properties such as excellent electronic properties, a good chemical stability, and a large surface area make carbon nanotubes very useful as a support for gold nanoparticles in many potential applications, ranging from advanced catalytic systems through very sensitive electrochemical sensors and biosensors to highly efficient fuel cells. Here we give an overview on the recent progress in this area by exploring the various synthesis approaches and types of assemblies, in which nanotubes can be decorated with gold nanoparticles and explore the diverse applications of the resulting composites.     

The challenge of assembling molecules into materials

Linear organic chains and their architectural derivatives such as branched and crosslinked macromolecules have been the main focus of polymer science for many decades. A new direction which should be pursued in the field is the exploration of polymers as “nanostructures”. This exploration would study synthetic polymers as shape invariant objects with nanoscale dimensions which resemble the folded proteins of nature. Such synthetic macromolecules would have well defined shapes which may even include topographical features. Ideally, the definition of shape could include well defined dimensions as well, but molecular objects of common shape but varying dimensions could also exhibit interesting properties. One could for example envision polymers as flat plates (two-dimensional polymers), discs, ellipsoids, spheres with cavities, parallelepipeds, among many other possible shapes. Such objects must have fairly high molar masses, possibly in the range of hundreds to thousands of kilodaltons, and would therefore not be accessible as chemical compounds synthesized by conventional methods. At the same time, the prospects of learning “folding rules” for synthetic chemical sequences in a biomimetic approach to molecular nanostructures seems to be an extremely difficult goal at the present time. This is suggested given our contemporary difficulties in understanding the protein folding problem.