Onion vesicle

A vesicle is a hollow aggregate with a shell made from one or more amphiphilic bilayers. A vesicle formed from a single bilayer is termed a unilamellar vesicle, while one with a shell of several bilayers is known as a multilayer vesicle, or sometimes an onion vesicle. A unilamellar vesicle is sketched in Fig. 4.31. 

This ascertains that the combustion of the surfactant mainly occurs at 285°C. Below this temperature, the silver nanopaiticles are not exposed to the reactive gas phase because the surfactant still covers them. Actually, Ag Ws proved to be active only from 350°C [4]. Importantly, a fraction of the surfactant is only removed at higher temperature, since a peak is also observed around 415°C. So further cleaning of the surface at higher temperature could likely lead to higher accessibility of the NPs and higher activity. Conversely, it was noticed that the activity of the NPs at 350°C decreased after that the catalyst was brought to 400°C [4]. Sintering is a plausible explanation to this observatiom The control on the size and the shape of the particles -that is one of the main interest of the synthesis via onion vesicles [5] - would then be lost. 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

Keywords Block copolymers Director Hydrodynamics Layer normal Layered systems Liquid crystals Macroscopic behavior Multilamellar vesicles Onions Shear flow Smectic A Smectic cylinders Undulations... 

Regev O, Backov R, Faure C (2004) Gold nanoparticles spontaneously generated in onion-type multilamellar vesicles. Bilayers-particle coupling imaged by cryo-TEM. Chem Mater 16 5280-5285... 

Preparing lipid suspension. Mix the desired hpids in an aqueous medium, typically 50 mM phosphate buffer, and vortex the mixture. This procedure results in a dispersion of mulhlamellar vesicles (MLVs), which are onion-like bilayer structures containing several bilayers separated by water. 

Block copolymer vesicles, or polymersomes, are of continued interest for their ability to encapsulate aqueous compartments within relatively robust polymer bilayer shells (Fig. 7) [66, 67]. Eisenberg and coworkers were the first to report the formation of block copolymer vesicles from the self-assembly of polystyrene-h-poly(acrylic acid) (PS-h-PAA) block copolymers. They also have described the formation of a wide range of vesicle architectures in solution from the self-assembly of five different block copolymers PS-h-PAA. PS-h-PMMA, PB-h-PAA, polystyrene-h-poly(4-vinyIpyridinium methyl iodide), and polystyrene-h-(4-vinylpyridinium decyl iodide) [68]. Small uniform vesicles, large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, onions, and vesicles with hollow tubes in the walls have been observed and the formation mechanism discussed. Since vesicles could be prepared with low glass transition polymers such as PB [69, 70] and PPO [71], it has been established than these structures are thermodynamically stable and not trapped by the glassy nature of the hydrophobic part. 

Fig. 1S.17. TEM picture of the onion-type multiplayer vesicles. (Reprinted with permission from Angeiv. Chem. Int. Ed. Engl., 2000, 39, 3310 [63].)...

Shapes of vesicular aggregates range from tubular to spherical, from more exotic large compound (LCV) and starfish vesicles to simpler extended lamellae. Both unilamellar [75] and multilamellar ( onions ) [47,76] vesicles have been observed. One of the possible morphologies formed in solution are tubular vesicles, also known as tubes (rods) [77,78], Soft, water-filled polymer tubes of nanometer-range diameters and several tens of millimeters in length have been prepared via self-assembly of amphiphilic ABA triblock copolymer in aqueous media (Fig. 4). The tubes were mechanically and chemically stable and could be loaded with water-soluble substances [23],... 

The nomenclature used to describe vesicular structures is sometimes variable, but the following definitions are used here. The term vesicle is restricted to the unilamellar vesicle (ULV) architecture depicted in Fig. la. Liposomes are vesicles formed from biologic surfactants such as lecithin and cholesterol, and were first studied by Bangham et al. over 35 years ago [2]. MultilameUar vesicles (MLVs or onion phases) consist of multiple surfactant bilayers forming concentric shells around an aqueous core (Fig. lb). MLVs are generally much larger than ULVs, and can be as large as several microns. 

Liposomes are vesicles consisting of phospholipid membranes. Among liposomes, one distinguishes multilamellar vesicles (MLVs), small unilamellar vesicles (SUVs), and large unilamellar vesicles (LUVs) (Table 4.1). MLVs consist of many liposomes that are folded around each other like the layers of an onion. They form during contact of phospholipids with watery solutions. 

Fig. 3 I rceze-fraclLire TEM of the onion phase in the system tetradecyllrimethylaminoniurn bromide-tetradocyldiinethyl-araincoxidc-hexanol, showing largo mullilamellar vesicles (bar=l rm). Kcproduced Ifom Ref, fl8] with permission cf Aeadcmic Press.

LMV) is encountered in the so-called onion phase, where each LMV includes tens to himdreds of concentric vesicles and where the LMVs arc in contact, as seen in Fig. The onion phase is used in slow-release for-... 

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