Capsule formation, reviewed

Each polysaccharide structure is synthesized by a distinct set of enzymes, and the biosyntheses of the individual structures will not be discussed here. The biosynthetic gene clusters for many different capsular and extracellular polysaccharides are known, and many of the enzymes involved have been characterized. For a general review of the genetics and biochemistry of CPS production, the reader is referred to a review by Roberts [321]. There are also many excellent reviews discussing capsule formation in individual bacteria, including H. influenzae [322], S. Aureus [323], E. coli [312], and pathogenic streptococci [324,325]. 

The basic principle involved in this method is to form an emulsion and then precipitate components of the continuous phase around the droplets of the discontinuous phase to form a wall (capsule). The mechanism of capsule formation by this process has been reviewed in the literature (e.g., [47]) and numerous patents exist further describing the manufacturing process in detail (e.g., [48,49]). 

Having already examined the use of the LbL method to make various nanocapsules, including polymer nanocapsules, and having already encountered the use of star polymers for catalyst encapsulation, we turn our attention to other methods for the formation of polymeric nanocapsules. Useful reviews of the formation of these capsules using various methods are available [78-84]. 

Heterocycle-forming polymerization reactions have been the most extensively reviewed of the three methods for obtaining heterocyclic polymers. A series of monographs are available that cover the literature through 1971 quite well (B-72MI11109). These monographs have served to provide an excellent foundation for our efforts. Whenever possible, we have cited more recent references, especially any reviews. It is the purpose of this section to depict succinctly the rich variety of chemistry that has been employed to form heterocyclic polymers of this type. This section, then, is a capsule summary of most of the heterocycles that have been generated concomitant with formation of a polymer. 

Physiological studies have centred chiefly on the pseudophyllidean and cyclophyllidean egg (Fig. 7.1). The formation of the capsule or egg shell (p. 171) has been studied extensively and these processes are dealt with in detail below. The field has been the subject of a number of reviews (138,170,451-453, 721, 796, 888, 889, 953). 

The structure, ultrastructure and formation of the hymenolepidid egg has been reviewed in detail by Ubelaker (888). Its general morphology is shown in Fig. 7.14). Although there are only the usual three basic embryonic membranes (p. 179) in the developing egg - shell/capsule, outer envelope, and inner envelope - the fully formed egg often appears to be more complex due to further differentiation of these layers. The following structures can be recognised (Fig. 7.11). 

Molecular capsules are structurally elaborated receptors that completely surround the hosted molecule(s). Encapsulation based on covalent bonds yields permanent arrangements of molecules-within-molecules. Reversible encapsulation, on the other hand, is based on self-assembling through formation of weak supramolecular bonds and offers possibilities for a dynamic in out exchange of encapsulated molecules. Most of the dimeric capsules developed by Rebek and his group are obtained through reversible self-assembly of resorcinarene subunits. When simultaneously encapsulated in the cylindri-cally shaped inner space of these capsules, two reactant molecules are temporarily isolated from others in solution and display reactivity features different from those in bulk solution. The matter has been extensively reviewed,and will not be discussed here. 

Abstract Nanoparticles (NPs, diameter range of 1-100 nm) can have size-dependent physical and electronic properties that are useful in a variety of applications. Arranging them into hollow shells introduces the additional functionalities of encapsulation, storage, and controlled release that the constituent NPs do not have.This chapter examines recent developments in the synthesis routes and properties of hollow spheres formed out of NPs. Synthesis approaches reviewed here are recent developments in the electrostatics-based tandem assembly and interfacial stabilization routes to the formation of NP-shelled structures. Distinct from the well-established layer-by-layer (LBL) synthesis approach, the former route leads to NP/polymer composite hollow spheres that are potentially useful in medical therapy, catalysis, and encapsulation applications. The latter route is based on interfacial activity and stabilization by NPs with amphiphilic properties, to generate materials like colloidosomes, Pickering emulsions, and foams. The varied types of NP shells can have unique materials properties that are not found in the NP building blocks, or in polymer-based, surfactant-based, or LBL-assembled capsules. 

In the remaining sections of this chapter, the physical and host properties of CB[n] as they relate to their apphcations as nanoreactors will be discussed in detail, and a detailed representative survey of the specific types of reactions which have been templated or catalyzed by them will be presented. This is not intended to be a comprehensive review, as the number of publications on CB[n] both as hosts and as nanoreactors is extensive. Furthermore, the many other various uses of CB[ ] (briefly described previously) for the formation of rotaxanes and other interlocked species, self-assembled capsules and adducts, and in drug dehvery, as inhibitors of reactions, and as components of nanomachines and nanostructures, will not be covered. This chapter will, in a nutshell, explore the uses of CB[ ] as nanosize reaction flasks. 

Abstract In this review, novel hierarchical self-assembled structures based on reversible organo-metaUic supramolecular polymers are discussed. Firstly, we discuss recent advances in the field of coordination polymers, considering cases in which transition metal ions and bis- or multiligands are used to build up organo-metallic supramolecular polymers. Secondly, we review hierarchical self-assembled structures based on these coordination polymers, such as polyelectrolyte layer-by-layer films, capsules, complex coacervate core micelles and microemulsions, and nanoribbons. Finally, we give a short perspective on the formation of coordinationpolymeric hierarchical self-assembled structures. The implications of fundamental and applied research, as well as aspects of new technologies are also discussed. 

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