# Recent Lipid Membrane-Based Biosensing Platforms
## 1 Introduction  
The early 1960's attempts to reconstitute lipid bilayers in vitro gradually established a lipid membrane technology path, which is quite intriguing, in the sense that the dynamic and complex nature of biological membranes could be actually simulated with a lipid solution, some skill and a set of affordable instrumentation, yet highly demanding in expertise when the extremely fragile lipid bilayer produced, freely suspended between two electrolyte interfaces, should remain intact for the duration of an experiment [1]. Nowadays lipid membrane constructs can be shaped in many architectures (bilayers, multilayers, mixed layers, branched, doped, nanodiscs, tethered, functionalized, etc.) to serve, efficiently and effectively, a dual purpose: analytical detection [2] and simulation studies [3]. Both research fields benefit from the capability to reproduce some properties of biological membranes and package them into sensing platforms for biosensor devices or membrane interaction studies, respectively. Interestingly, the former field uses the term artificial lipid membranes [2] while the latter exploits similar principles and methods to yield biomembranes or membrane mimics [3], the different connotations refer to the scopes of research and not the research tool per se. The number of devices based on lipid membranes that were used to monitor food toxicants, environmental pollutants, and compounds of clinical interest has increased tremendously for the last two decades. During the last decade, a number of efforts to prepare stabilized lipid devices were successful and this has given the opportunity to construct biosensors to detect food toxicants and environmental pollutants in real samples and in the field. The advantages of lipid film devices are summarized as follows: The membranes have an appropriate biocompatible structure with fastresponse times, high sensitivity and selectivity, small size, portability, and provide advantages as compared with the bulky liquid chromatographic (LC) instruments. The new generation of stabilized lipid membrane nanosensors has the potentiality to construct site-specific analytical devices with respect to analytical performance, operational stability, and response. Lipid membrane technology reduces the complex membrane properties and processes to well-controlled and defined interactions between biological moieties, lipids, and transducers (Figure 1). Although the very nature of biological membrane still remains elusive [4], it is easy to isolate specific membrane properties for drug permeability [5] or protein-lipid interaction studies [6], or even to control and manipulate nano-processes on the lipid bilayer by changing macro-parameters (pH, temperature, ionic strength, lipid composition, etc.) [7]. Biological moieties (enzymes, antibodies, receptors, ligands, DNA, etc.) can be easily immobilized on the surface of lipid membranes [2] or embedded into the lipid organization [6] using thermodynamically driven self-assembly processes or more precise techniques, such as patterning [8] or surface printing [9]. The lipid micro-environment presents a compatible setting for biological species to retain their full functionality while experimented upon. Further, the physical state of the lipid membrane offers an intrinsic signal transduction and amplification mechanism, perfectly fitted for electrochemical sensing: When the biological moiety, attached on or embedded in the membrane, interacts with the target analyte, lipid-protein and lipid-lipid interactions are affected to a degree sufficient to disrupt the lipid organization [1]; this affects the flux of ions through the membrane and can be readily detected as current alterations. Figure 1: Simplified overview of the basic lipid membrane platform for biosensor development, illustrating, also, the critical physical chemistry that gears the biochemical information produced on the membrane surface for electrochemical transduction and the most common screening strategies employed.