Self-assemblies

Self-assembly of molecules and nanoparticles to build well-defined structures, constitutes another approach to make model catalysts [33,34]. Here, nano-structured surfaces are made from nanoscale building blocks that are synthesized from atoms and molecules by chemical means. There has been a tremendous development in this field during the past decade, which includes a number of different strategies, including microemulsions [33], (micellar) block copolymers [35,36], and template CVD growth [37]. Relatively little work has, however, so far been directed toward heterogeneous catalysis in the sense described in this chapter, i.e., to make supported catalysts [38]. There are many reports on preparations but relatively much fewer on evaluations of catal3dic activity, trends, or reactivity versus particle size, etc. A main issue for model catalysts prepared by self-assembly is whether they maintain the well-defined character after, e.g., template removal and calcinations and other pretreatment steps, before they can be used as model catalysts. 

The self-assembly method produces nanofibers by holding small molecules together via intermolecular interactions. Depending on the chemical structure of the small molecules, there are a few possible mechanisms that can be used to assemble nanofibers. One of the commonly used mechanisms is based on the formation of hydrogels, which contain two interpenetrated phases, i.e., the solid phase and the hquid phase. The liquid phase typically is water, and the solid phase is a network of nanofibers formed by the self-assembly of hydrogelator molecules. 

Nonwovens by Ultrasonic Wave , International Journal of Chemistry, 1, 26-33, 2009. 

The term self-assembly has become ubiquitous in materials science over the past few decades, particularly in the field of soft matter and in related fields at the convergence of soft and hard materials. It is important to define this concept here as it will be used frequently throughout this book. Self-assembly may be described as spontaneous molecular ordering resulting from the balance between entropic and intermolecular forces in a material. A self-assembled system or state is one that forms without external mechanical manipulation of the components. Instead, the elements of the material (molecules, particles, etc.) are subject to forces between these elements and thereby adopt a particular configuration by coming to an equilibrium state. 

In self-assembly processes, you will often hear mentioned that when a system reaches an equifibrium state, its free energy is minimized. This term free energy can have a few different meanings but essentially refers to the maximum amount of energy available in the system that can be converted into work. In physics, the Helmholtz free energy f is commonly used. This can be defined thermodynamically as 

The term entropy is also used casually to describe the random thermal motions in a system (as used in the preceding paragraph) because increased temperature increases entropy. The internal energy U can be considered the energy it took to create the system (e.g., the energy stored in the interatomic bonds and molecular motions), and the TS term here represents the energy given to the system by the environment (Q). 

An increase in the entropy of a system and its environment at constant volume, temperature, and number of particles is related to a change in the free energy by 

Therefore, we can see that an increase in entropy corresponds to a decrease in the free energy when entropy is maximized at equilibrium, free energy is minimized. 

Many examples of self-assembly do not involve reactions but involve merely the alignment of molecules among and between phases. Add a detergent to a mixture of oil and water. The detergent migrates to the oil-water interface. When the interface is saturated, the excess detergent forms nanoscale aggregates known as micelles. More complicated self-assembled structures can result when polymer mixtures are 

13 is that the individual tubes grow at approximately the same rate. It appears that the tube growth can be modeled as a living polymerization. See Section 13.4.1. 

More precisely defined self-assembly systems than those described in the previous section can be prepared from block copolymers. For example, an amphiphilic polyphosphazene diblock based on roughly equal proportions of [N=P(0CH2CH20CH2CH20CH3)2] , as a hydrophilic block, and [N=PPh(0CH2CH20CH2CH20CH3)] , showed a critical micelle concentration (CMC) of 80 mg/L in an aqueous solution [60], whilst the lower CMC values of 12.4 and 5.2 mg/L are reported for polyethylene oxide-block-poly[bis(trifluoroethoxy)phosphazene]s depending on the block length. Amphiphilic triblock polymers with PPG of the same polyphosphazene showed similar self-assembly behaviour with CMC values in a comparable range [56]. 

Well-defined polyphosphazene block copolymers with poly(ferrocenylsilane) (PFS) have also been reported, where the combination of the crystallinity of the PFS block and versatility of the polyphosphazene block crystallisation-directed living supramolecular polymerisations lead to spatially defined and controllable nanostructures [59]. Although not designed specifically for medical applications, they show a prime example of how the tunability of polyphosphazenes can be exploited for advanced macromolecular engineering. 

Allcock in Chemistry and Applications of Polyphosphazenes, Wiley, Hoboken, NJ, USA, 2003. 

Carriedo, F.L.G. Alonso, P. Gomez-Elipe, J.I. Fidalgo, J.L.G. Alvarez and A. Presa-Soto, Chemistry - A European Journal, 2003, 9, 16, 3833. 

Blackstone, A.J. Lough, M. Murray and I. Manners, Journal of the American Chemical Society, 2009,131, 10, 3658. 

The discovery that members of the resorcin[4]arene family self-assemble to form 1, owing to its classification as an Archimedean solid, prompted us to examine the topologies of related spherical hosts with a view to understanding their structures on the basis of symmetry. In addition to providing grounds for classification, we anticipated that such an approach would allow us to identify similarities at the structural level, which, at the chemical level, may not seem obvious and may be used to design large, spherical host assemblies similar to 1. 

it is herein that we now describe the results of this analysis which we regard as the development of a general strategy for the construction of spherical molecular hosts. [11] We will begin by presenting the idea of self-assembly in the context of spherical hosts and then, after summarizing the Platonic and Archimedean solids, we will provide examples of cubic symmetry-based hosts, from both the laboratory and nature, with structures that conform to these polyhedra. 

In a paper describing the structure of regular viruses, Caspar and Klug [4] have shown that viral capsids use self-assembly to construct spherical shells up to a hundred nanometers in diameter by utilizing identical copies of proteins as chemical 

In the previous chapters we discussed polymers where the monomer units are coimected by covalent bonds. In this chapter we will investigate polymers formed by self-assembly of monomers or larger units. We will also diseuss self-assembled structures in which smaller polymers act as the building blocks for larger biological structures. 

FIGURE 7.1. The anatomy of an egg (Copyright 2006 Horst Frank, Wikimedia Commons). 

The word virus is Latin for toxin or poison. It is essentially a simple parasite, an infectious entity that uses the cell as its means to multiply and spread throughout the host. The host has natural defensive 

FIGURE 7.4. Two protein subunits seif-assembie for a ribosome (E. Rosenthai-Kim). 

Relatively simple molecules with complementary functionalities may, under certain conditions, interact with one another to form significantly more complex supramolecular species, held together only by virtue of non-covalent interactions. Examples of such self-assembly processes are repeatedly found in nature. The DNA double-helix requires two complementary strands to become entwined via hydrogen bonds and tt-it stacking in a self-assembly process (see Chapter 1, Section 1.3). The strands recognise each other and join together to form the most thermodynamically stable assembly product. Protein folding and viral assembly operate in a broadly similar manner. 

Self-Assembly The spontaneous and reversible association of molecular species to form larger, more complex supramolecular entities according to the intrinsic information contained in the components. 

Core Concepts in Supramolecular Chemistry and Nanochemistry Jonathan W. Steed, David R. Turner and Karl J. Wallace 2007 John Wiley Sons, Ltd ISBN 978-0-470-85866-0 (Hardback) 978-0-470-85867-7 (Paperback) 

If suitable molecular analogues for these components can be found, then the spontaneous formation of a square complex can be designed. A 90° angle is frequently created from a square-planar metal ion, such as Pd or Pt, which has two cis coordination sites blocked off. The linear sides require a ligand that has metal binding sites directly opposite each other, such as 4,4 -bipyridine. 

Hierarchical Assembly A self-assembled system that comprises several levels of complexity which are built up over several, successive self-assembly processes. One level cannot exist without the preceding one being in place. 

Of course, the above suggestion ignores the problem of possible supramolecular functionality. Whereas the natural systems are invariably characterised by high functionality in terms of their biochemical roles, in contrast, the functionality of the majority of synthetic assemblies so far investigated has very often been either minimal or, indeed, absent altogether. The incorporation of designed functionality into supramolecular systems will thus undoubtedly continue to attract increased attention in future studies. 

In the present context, self-assembly may be defined as the process by which a supramolecular species forms spontaneously from its components. For the majority of synthetic systems it appears to be a beautifully simple convergent process, giving rise to the assembled target in a straightforward manner.  

It must be emphasised that self-assembly is very far from a unique feature of supramolecular systems - it is ubiquitous throughout life chemistry. Biological systems aside, self-assembly is also commonplace throughout chemistry. The growth of crystals, the formation of liquid crystals, the spontaneous generation of synthetic lipid bilayers, the synthesis of metal co-ordination complexes, and the alignment of molecules on existing surfaces are but a few of the many manifestations of self-assembly in chemical systems. 

The Crystal as a Supramolecular Entity, ed. G.R. Desiraju, 1996, Wiley, Chichester, UK. 

A distinctive feature of using weak, non-covalent forces, or for that matter metal-donor bonds, in molecular assemblies is that such interactions are normally readily reversible so that the final product is in thermodynamic equilibrium with its components (usually via its corresponding partially assembled intermediates). This leads to an additional property of most supramolecular systems they have an in-built capacity for error correction not normally available to fully covalent systems. Such a property is clearly of major importance for natural systems with their multitude of intermolecular contacts. It is a factor that will assume increasing importance for the construction of the new (larger) synthetic systems mentioned previously - as both the number of intermolecular contacts present and overall structural complexity are increased. 

The cyclic octapeptide cycfo-[-(D-Ala-L-Glu-D-Ala-L-Gln)2-] has been designed by Ghadiri and co-workers to generate a hydrogen-bonded organic nanotube having an internal diameter of approximately 0.7-0.8 nm (Fig. 20.1.5). 

ENZYME IMMOBILIZATION FOR BIOLOGICAL FUEL CELL APPLICATIONS 

FIGURE 11.5 (a) Structure of the cross-linker DSP, PBSE, and DDPSE. (b) A size comparison of a 25 nm diameter MWCNT, 7 nm diameter laccase, and 26 A N—N distanced DDPSE. (Reproduced with permission from Ref. [63]. Copyright 2011, Wiley-VCH Verlag GmbH.) 

Cross-linked enzyme aggregates (CLEAs) rely on chemical cross-linking to form a self-assembled enzyme matrix that can be fabricated into, or onto, an electrode material. CLEAs find application in EECs, particularly anodic catalysts [65]. GOx, for example, can be cross-linked in the presence of ammonium sulfate onto the surface of various nanomaterials. The aggregation of the enzyme results in a substantial increase in volumetric protein loading and enhanced stability ( 200 days). The use of GOx as CLEA was used as the anode of a biological fiiel cell and demonstrated reproducible power density and stability up to 50 C. 

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A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

As discussed in Chapter III, the progression in adsoiptivities along a homologous series can be understood in terms of a constant increment of work of adsorption with each additional CH2 group. This is seen in self-assembling monolayers discussed in Section XI-IB. The film pressure r may be calculated from the adsorption isotherm by means of Eq. XI-7 as modified for adsorption from dilute solution ... 

The self-assembly process can be continued to form multilayer films of up to 25 layers [33,48,49]. The reliability of this process is illustrated in Fig. XI-3, where the thickness grows linearly with the number of reacted layers. These thick layers have many interesting applications. 

A. Ulman, An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly, Academic, New York, 1991. 

Deposited Langmuir-Blodgett films take on many of the same stmctures as the Langmuir monolayers discussed in Section IV-4C, and they are often compared to the self-assembling monolayers described in Section XI-IB. The area... 

Investigate the differences between LB films and self-assembled monolayer SAMs (Chapter XI). Which are finding more practical use, and what are the potential applications of each ... 

Marrian C R K, Perkins F K, Brandow S L, Koloski T S, Dobisz E A and Calvert J M 1994 Low voltage electron beam lithography in self-assembled ultrathin films with the scanning tunneling microscope Appi. Rhys. Lett. 64 390... 

Giancarlo L C and Flynn G W 1988 Scanning tunneling and atomic force microscopy probes of self-assembled, physisorbed monolayers A/ / . Rev. Phys. Chem. 49 297... 

Freunscht P, Van Duyne R P and Schneider S 1997 Surface-enhanced Raman spectroscopy of trans-stilbene adsorbed on platinum- or self-assembled monolayer-modified silver film over nanosphere surfaces Chem. Phys. Lett. 281 372-8... 

The self-assembly of alkanethiols on gold has been an important topic in surface chemistry over die last few years [39] and STM has contributed significantly to our understanding of these systems. In particular, the... 

The overwhelming majority of AEM studies on organic surfaces has concerned organic thin films on inorganic snbstrates and, in particvilar, those deposited via Langmnir-Blodgett or self-assembly processes [35]. These films... 

Straniok S J, Parikh A N, Tao Y-T, Allara D L and Weiss P S 1994 Phase separation of mixed-oomposition self-assembled monolayers into nanometer soale moleoular domains J. Phys. Chem. 98 7636... 

Salmeron M, Neubauer G, Folch A, Tomitori M, Ogletree D F and Sautet P 1993 Viscoelastic and electrical properties of self-assembled monolayers on Au(111) films Langmuirs 3600... 

Salmeron M, Liu G-Y and Ogletree D F 1995 Molecular arrangement and mechanical stability of self-assembled monolayers on Au(111) under applied load Force in Scanning Probe Methods ed H-J Guntherodt et al (Amsterdam Kluwer)... 

Lowack K and Helm C A 1998 Molecular mechanisms controlling the self-assembly process of polyelectrolyte multilayers Macromolecules 31 823-33... 

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

An even coarser description is attempted in Ginzburg-Landau-type models. These continuum models describe the system configuration in temis of one or several, continuous order parameter fields. These fields are thought to describe the spatial variation of the composition. Similar to spin models, the amphiphilic properties are incorporated into the Flamiltonian by construction. The Flamiltonians are motivated by fiindamental synnnetry and stability criteria and offer a unified view on the general features of self-assembly. The universal, generic behaviour—tlie possible morphologies and effects of fluctuations, for instance—rather than the description of a specific material is the subject of these models. 

Karaborni S, Esselink K, Hilbers P A J, Smit B, Karthauser J, van Os N M and Zana R 1994 Simulating the self-assembly of Gemini (dimerio) surfaotants Science 266 254... 

Gompper G and Sohiok M 1994 Self-assembling amphiphilio systems Phase Transitions and Critical Phenomena vol 16, ed C Domb and J Lebowitz (New York Aoademio)... 

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