Monolayer polymerization within

Numerous studies involving 2-D self-assembled entities, such as vesicles and monolayers, can only be discussed if the individual constituents of the entities are covalently stabilized at least to provide crosslinked materials for maintaining the sheet-like shape on a permanent basis. For example, conventional polymerizations within a monolayer of monomers produce single-stranded, Hnear polymers (Figure 28.4a) which are held in two dimensions for only as long as the monolayer stays intact. Upon dissolution, however, these will attain a 3-D coiled shape, as does any other conventional linear polymer [31]. Self-assembled sheet-like entities from small constituents such as peptoids, will not be considered [32]. 

One approach could be the attempt to include the lipids into the stabilization process. Lipid molecules bearing polymerizable groups can actually be arranged as planar monolayers or as spherical vesicles and polymerized by high energy irradiation within these membrane like structures under retention of the orientation of the molecules (8,9,36). 

Similarly, Ingall et al. [263] lithiated monolayers of short 3-bromopropylsilane on planar silica substrates to perform LASIP with AN. They obtained up to 245 nm thick PAN films within eight days of polymerization time. 

Surfactants provide several types of well-organized self-assembhes, which can be used to control the physical parameters of synthesized nanoparticles, such as size, geometry and stability within liquid media. Estabhshed surfactant assembles that are commonly employed for nanoparticie fabrication are aqueous micelles, reversed micelles, microemulsions, vesicles [15,16], polymerized vesicles, monolayers, deposited organized multilayers (Langmuir-Blodgett (LB) films) [17,18] and bilayer Upid membranes [19](Fig. 2). 

This review emphasizes an intriguing and potentially useful aspect of the polymerization of lipid assemblies, i.e. polymerization and domain formation within an ensemble of molecules that is usually composed of more than one amphiphile. General aspects of domain formation in binary lipid mixtures and the polymerization of lipid bilayers are discussed in Sects. 1.1 and 1.2, respectively. More detailed reviews of these topics are available as noted. The mutual interactions of lipid domains and lipid polymerization are described in the subsequent sections. Given the proper circumstances the polymerization of lipid monolayers or bilayers can lock in the phase separation of lipids, i.e. pre-existing lipid domains within the ensemble as described in Sect. 2. Section 3 reviews the evidence for the polymerization-initiated phase separation of polymeric domains from the unpolymerized lipids. 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

Entrapment methods of immobilization of bioreceptors utilized the lattice structure of particular base material. They include such methods as entrapment behind the membrane, covering the active surface of biosensors, entrapment within a self-assembled monolayers on the biosensor surface, as well as on freestanding or supported bilayer lipid membranes, and also entrapment within a polymeric matrix membranes, or within bulk material of sensor. All these mentioned methods are widely employed in design of biosensors. The essential condition of success of these methods of immobilization is preservation of sufficient mobility of substrate or products of biochemical reaction, involved in sensing mechanism, as matrix may act as a barrier to mass transfer with significant implications for... 

Despite the simplicity of DET, current densities are usually limited as only monolayer coverage is possible and enzymes may be electrically insulated due to incorrect orientation on the electrode surface. The simplest way to overcome these limitations is to immobilize the enzyme within a polymeric mediator matrix on the electrode surface. This enables multilayer enzyme coverages and, if the mediator can access the enzyme active site, the orientation of enzyme becomes irrelevant. 

These observations are consistent with the mosaic model of the membrane that was derived from monolayer studies (2, 4, 5, 12, 13). Therein, the structural or bimodal (amphipathic) protein in the membrane (natural or artificial) interacts with the polar peripheries of the polymeric lipid structures alongside the protein. The EPR data of Jost et al. (29) support this concept, i.e., an appreciable portion of the lipid is in lateral hydrophilic bonding with the protein whereas the other lipid is free, probably within the lipid cluster, and preserves the lipid character. 

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