ABSTRACT
The formation of closed structures by aqueous suspensions of phospholipids and impermeability of the resulting structures to encapsulated ions was first reported by Bangham, Standish, and Watkins in 1965.1 Later termed liposomes in 1968,2 the formation of these synthetic structures that comprise lipid bilayers encompassing aqueous spaces was a significant finding that paved the way for numerous applications, including their usage as model cell membranes, drug-delivery agents, and components of cosmetic formulations. The importance of these applications has been well reviewed in many sources3,4 and hence will be touched on briefly within this text. The focus of this book is instead on a well-researched but perhaps less well-publicized area of liposome research, which is that of the utility of liposomes as analytical tools. As will be described within, soon after their discovery, liposomes began to find their place in analytical chemistry and biochemistry beginning in the 1970s.5 Many of the properties
that made them well suited for the above applications, such as the internal space to encapsulate a large payload of hydrophilic molecules and its inherent protection from exterior stresses, could be exploited in unique analytical methodologies, offering advantages over existing technologies and approaches not possible with other systems. For example, the amphiphilic nature of the bilayer could be used to incorporate hydrophobic biorecognition elements such as membrane receptors, which would be otherwise challenging, if not impossible, to retain activity in aqueous systems. This introductory chapter is intended to provide a primer for readers entering the field or a refresher for those with existing background to understand some of the fundamentals and utility of liposomes for analytical purposes. It is by no means an exhaustive review of all publications in their respective fields, but a sampling of some of the more well-researched approaches. The great work of many liposome pioneers in their respective areas is introduced within but is captured in detail in the chapters to follow. 1.1 Introduction to Liposomes
In their most simple form, liposomes are often described as a spherical lipid bilayer surrounding an aqueous core. The lipid bilayer of liposomes is composed of double chain, amphiphilic lipids with their nonpolar, hydrophobic tails aligned with each other and their polar, hydrophilic headgroups oriented toward the exterior aqueous solution or directed toward the inner aqueous core (Fig. 1.1).
