The role of liposomes in analytical processes
Introduction
Liposomes are self-assembled structures that occur in nature; they can also be easily synthesized in the laboratory. Since Bangham et al. [1] described for the first time the preparation of liposomes, they have been often used as model membrane systems to reveal the basic nature of cell membranes. The use of liposomes is of current interest in several areas (e.g., biochemistry and molecular biology [2], [3], cosmetics and food technology [4], [5], and as drug delivery systems in in vivo pharmacology [6], [7]).
The role of liposomes in analytical chemistry can be considered from two different points of view:
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as an analyte, when the liposome or its ingredients is the object of the analysis; and,
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as an analytical tool, when liposomes influence some step of the analytical process.
Liposomes and their constituents have been investigated using several analytical techniques, mainly involving separation [8], [9].
Liposomes as analytical tools can play significant roles in the analytical process, mainly to improve analytical properties, particularly sensitivity, because of their usefulness as signal-amplification reagents. Also, liposomes can increase selectivity when they are used as transport devices or active phases in separation techniques.
In both cases, the experience acquired using liposomes in other research areas has been extrapolated to their application as analytical tools.
The development of analytical systems in which liposomes are used as artificial cell membranes to study body distribution mechanisms as efficient drug-delivery systems has also deserved special attention [10].
Interest in the analytical use of liposomes is growing, as shown by the numerous articles recently published using different techniques, such as liquid chromatography, capillary electrophoresis, immunoassays and sensors. This review presents the state-of-the-art of the use of liposomes as versatile analytical reagents in these techniques, exclusively discussing the advantages and the limitations of the most recent applications. We briefly describe technical aspects of liposome preparation and ways to modify their morphological and structural characteristics and thus, their functionality. Because the full potential of liposomes has probably not yet been achieved, we also discuss future perspectives of the analytical use of liposomes.
Section snippets
Features and preparation of liposomes
Liposomes are spherical vesicles integrated by one or more phospholipid bilayers that encapsulate a part of the aqueous media in which they are suspended. The formation of liposomes occurs when phospholipids are dispersed into water as a result of the unfavourable interactions between phospholipids and water. The amphiphilic character of phospholipids and their capability of forming closed structures allow both hydrophobic and hydrophilic molecules to be trapped inside liposomes. The nature of
Characterisation of liposomes
Paternostre et al. [18] have classified the parameters involved in characterising liposomes into three categories:
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morphological (size, shape, stability, size distribution or polydispersity, and electrical liposome charge);
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structure organization (permeability and fluidity);
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functionality (encapsulation efficiency, stability, reactivity and fusion capability).
The study of morphological properties requires
Liquid chromatography
The use of stationary phases containing liposomes has given rise to immobilised liposome chromatography (ILC), which has shown its usefulness in studying solute-membrane interactions and, to a lesser extent, in achieving several solute separations. Methods for liposome immobilization in gel beads involve steric entrapment [36], hydrophobic binding [37], avidin–biotin affinity binding [38], [39], [40], [41], [42] or covalent attachment [43].
Liposomes have frequently been used as model membranes
Liposomes and analytical properties
The use of liposomes in an analytical method entails an additional step for their preparation and characterization that is only justified if some analytical feature can be improved. Immunoassays and sensors have extensively taken advantage of the signal amplification offered by liposomes, as thousands of molecules, such as fluorescent dyes, can be entrapped or surface-attached to improve LODs. Table 3 summarises some examples, and includes the number of marker molecules per liposome.
An
Future trends
Liposomes have been widely used to improve sensitivity, but they possess other features that have been rarely applied for analytical purposes. For example, the special composition of liposomes provides a natural environment to lipophilic receptors, such as glycolipids, to extend the field of applications of these compounds in affinity assays, as shown in the determination of several bacterial toxins [62], [73], [88]. Other affinity reagents with low solubility in water could be used in a
Acknowledgements
The authors gratefully acknowledge financial support from the Spanish MCyT (Ministerio de Ciencia y Tecnologı́a) (Grant No. BQU 2003-03027).
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