ReviewLiposomes in analyses
Section snippets
General background on liposomes
Liposomes are highly versatile structures for research, therapeutic, and analytical applications. They are composed of a lipid bilayer with the hydrophobic chains of the lipids forming the bilayer and the polar headgroups of the lipids oriented towards the extravesicular solution and inner cavity (Fig. 1).
Phospholipids with different polar headgroups functionalized for conjugation or to reduce liposome aggregation and hydrophobic regions of different chain length and saturation are used to
Liposomes in analyses: general background and assay formats
Liposomes offer much utility as analytical reagents due to their high surface area, large internal volume, and ability to conjugate bilayer lipids with a variety of biorecognition elements. Supported planar bilayers formed upon liposome fusion for the study of molecular interactions are beyond the scope of this article, but have been extensively reviewed elsewhere [73], [74]. While excellent reviews of the uses of liposomes in immunoassays are available in the literature [75], [76], [77], this
Assays relying on liposome encapsulation volume and bilayer composition
Labels for nucleic acid diagnostics and immunoassays ideally yield stable, rapid, sensitive and inexpensive analytical assays [82]. They can generally be grouped into three broad categories: individual labels, such as quantum dots, fluorescent or radioactive tags; multiple labels, such as branched DNA, dendrimers, or latex beads; and labels which actively generate signaling molecules, such as enzymes. Liposomes fall into the multiple label category since hundreds to hundreds of thousands of
Assays relying on liposome size and bilayer composition
The following papers describe using liposomes purely for their comparatively large size and bilayer composition to generate analytical signals. Measurements from quartz-crystal microbalance (QCM) are commonly employed. QCMs are piezoelectric quartz-crystal transducers which exhibit a decrease in frequency upon binding of materials onto their surface [136]. The change in frequency is directly related to the mass of the materials bound and can extend into the nanogram range [137]. Surface-plasmon
Comparison of liposomes to other signal enhancement methods
The advantage of liposomes as signal amplification tool has been pointed out by all researchers integrating these multi-label systems into the analytical assay. However, encapsulation efficiency, steric hindrance of the binding events due to the large size of the vesicles and their multivalency make a theoretical calculation of signal amplification in comparison to single labels more difficult. This section will review the available literature on experimental data comparing liposome attached
Future directions
This review was intended to elucidate the variety of ways in which liposomes have been used to date as analytical reagents. These methods included relying on the substantial mass and charge difference that a tagged liposome could provide and the large number of signaling molecules that can be released to provide a signal. While many variations were presented, further study will likely yield even more options for using liposomes in analysis including furthering the use of chemiluminescent
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