Joint Use of Ultrasound and Liposomes An Analytical Tool

Liposomes are spherical vesioles formed by the aggregation of amphiphilio phospholipid moleoules in a bilayer struoture. Liposome formation ooours when phospholipids are dispersed into an aqueous medium — usually water — as a result of interaotions between phospholipids and water. Thus, liposomes encapsulate part of the aqueous medium in whioh they are suspended. The amphiphilic charaoter of phospholipids allows them to form olosed struotures where hydrophobic and (or) hydrophilio moleoules oan be entrapped or anohored. 

Liposomes occur in nature, but can also be easily synthesized in the laboratory. Depending on the preparation method used, whioh influenoes their size — in relation to the number of bilayer shells — and physical properties, liposomes are olassified as small unilamellar vesicles (SUVs, 25-50 nm), large unilamellar vesioles (LUVs, 100 nm to 1 pm), giant unilamellar vesicles (GUVs, 1.0-200 pm) multilamellar vesioles (MLVs, 0.1-15 pm), and multi-vesicular vesicles (MWs, 1.6-10.5 pm) the last consists of several small vesicles. Bicelles, which contain surfactant molecules in the lipid bilayer, constitute a special type of liposome. 

The mechanical and chemical properties of liposomes can be altered by incorporating a great variety of compounds into their structure. Thus, hydrophilic molecules can be entrapped in the aqueous cavity, hydrophobic molecules can be entrapped within the organic chains of the bilayer and amphiphilic compounds can be anchored to the polar head groups or inserted into the organic chains depending on the particular functional group. 

Liposomes have aroused interest in a great variety of areas from biochemistry and molecular biology to cosmetics and food technology. One of the most salient applications of liposomes has been promoted by their high similarity to natural cell membranes, for which they are extensively used as substitutes in medical and pharmaceutical research. Since their inception, liposomes have often been used as models for studying the nature of cell membranes, the structure and functions of which they can mimic quite closely. One example is the determination of membrane distribution coefficients of drugs with a view to estimate their ability to penetrate cells. Interactions between analytes and phospholipid membranes depend on the characteristics of both the analytes and the membrane. 

Liposomes can play two different roles in analytical chemistry [71]  

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