The multifaceted role of biological membranes prompted early the development of artificial lipid-based models with a primary view of reconstituting the natural functions in vitro so as to study and exploit chemoreception for sensor engineering. Over the years, a fair amount of knowledge on the artificial lipid membranes, as both, suspended or supported lipid films and liposomes, has been disseminated and has helped to diversify and expand initial scopes. Artificial lipid membranes can be constructed by several methods, stabilized by various means, functionalized in a variety of ways, experimented upon intensively, and broadly utilized in sensor development, drug testing, drug discovery or as molecular tools and research probes for elucidating the mechanics and the mechanisms of biological membranes. This paper reviews the state-of-the-art, discusses the diversity of applications, and presents future perspectives. The newly-introduced field of artificial cells further broadens the applicability of artificial membranes in studying the evolution of life.

The present paper reports the electrochemical investigation of ion transport through self-assembled bilayer lipid membranes on a metal support and represents a technology suitable for development of an ammonium ion minisensor. Gramicidin D was used as a channel-forming ionophore for selective conduction of ammonium ion through lipid bilayers composed of egg phosphatidylycholine. The ammonium ion sensor exhibited good mechanical stability and longevity (routinely over 48 h) and constant sensitivity and response to a given concentration of ammonium ion in solution. The use of stabilized metal-supported BLMs has allowed the electrochemical investigation of the reversibility of response to ammonium ions and of ionophore binding to lipid membranes. The effects of pH and some possible interferents were examined. Semisynthetic platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, AGEPC) was found to enhance transport of ammonium ions by gramicidin and to reduce potassium interference, whereas the non-acetylated derivative of AGEPC did not exert any alterations in transport ammonium ions. The present microfabricated ammonium sensor based on thin lipid film technology provides advantages of extremely fast response times (to millisecond speeds) to alterations of ammonium ion concentration (0.02-5 mmol L-1), high selectivity to ammonium ions in the presence of volatile amines, and the capability of analyzing small volumes of samples. A detection limit of ammonium ions of approximately 1 x 10(-6) M was attained using BLMs modified with PAF. Furthermore, a device can now simply and reliably be fabricated at low cost and therefore can be used as a disposable sensor.

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This work reports a technique for the stabilization of solventless bilayer lipid membranes (BLMs), and the use of stabilized BLMs as flow detectors. Microporous filters composed of glass fibers, polytetrafluoroethylene (PTFE) and polycarbonate (nominal pore sizes from 1 to 5 pm) can serve as interfaces that separate two solution compartments. The micropores in the filter media can act as supports for formation and stabilization of BLMs. One of the solution compartments is used to cast lipid films on the filters, while a carrier electrolyte solution concurrently flows through the opposing compartment. Optimization of the flow cell design, and the chemical composition and methods for preparation of stabilized BLMs, are described. Lipid membranes composed of mixtures of phosphatidyl choline and phosphatidic acid could respond rapidly to pH alterations of the carrier electrolyte solution. Signals would reproducibly appear within a few seconds following the injection of an electrolyte of different pH than the carrier. Signals took the form of a single ion current transient with magnitude of tens of picoamperes (PA) and a duration of seconds. The mechanism of signal generation is explored by differential scanning calorimetry. The results show that a phase transition within a lipid membrane can be triggered by pH alterations of the electrolyte solution. Stabilized BLMs which provide artificial ion gating events hold prospects for chemical sensing of process streams.

This work describes a technique for the rapid and sensitive determination of acetylcholine, urea, and penicillin in flowing solution streams using stabilized systems of solventless bilayer lipid membranes (BLMs). This method of monitoring substrates of hydrolytic enzyme reactions made use of BLMs which were supported on ultrafiltration membranes such as polycarbonate and glass microfiber; these filter membranes were found to enhance the stability of BLMs for uses in flow injection experiments. The enzymes were immobilized on BLMs by incorporating the protein solution into the lipid matrix at the air/electrolyte interface before the BLM formation, followed by injections of the substrates into flowing streams of a carrier electrolyte solution. Hydronium ions produced by the enzymatic reaction at the BLM surface caused dynamic alterations of the electrostatic fields and phase structure of BLMs, and as a result ion current transients were obtained; the magnitude of these signals was correlated to the substrate concentration, which could be determined at the micromolar level. The response times were ca. 10 s, and acetylcholine, urea, and penicillin could be determined in continuous flowing systems with a maximum rate of 220 samples/h. It is expected that this analytical utility of stabilized BLMs for flow stream uses will provide new opportunities in this strategy of chemical sensing.