biosensors Review Application of Biosensors Based on Lipid Membranes for the Rapid Detection of Toxins Georgia-Paraskevi Nikoleli 1, Dimitrios P. Nikolelis 2,* ID , Christina G. Siontorou 3, Stephanos Karapetis 1 ID and Marianna-Thalia Nikolelis 3 1 Laboratory of Inorganic & Analytical Chemistry, School of Chemical Engineering, Department 1, Chemical Sciences, National Technical University of Athens, 9 Iroon Polytechniou St., 15780 Athens, Greece; [email protected] (G.-P.N.); [email protected] (S.K.) 2 Laboratory of Environmental Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis-Kouponia, 15771 Athens, Greece 3 Laboratory of Simulation of Industrial Processes, Department of Industrial Management and Technology, School of Maritime and Industry, University of Piraeus, 18534 Pireus, Greece; [email protected] (C.G.S.); [email protected] (M.-T.N.) * Correspondence: [email protected]; Tel.: +30-210-7274-754 Received: 17 April 2018; Accepted: 21 June 2018; Published: 26 June 2018 Abstract: Lipid assemblies in the form of two dimensional films have been used extensively as biosensing platforms. These films exhibit certain similarities with cell membranes, thus providing a suitable means for the immobilization of proteinaceous moieties and, further, a number of intrinsic signal amplification mechanisms. Their implementation in the detection of toxins yielded reliable and fast detectors for in field analyses of environmental and clinical samples. Some examples are presented herein, including aflatoxin and cholera toxin detection. The conditions and parameters that determine the analytical specifications of the lipid membrane sensors are discussed, advantages and technology bottlenecks are reviewed, and possible further developments are highlighted. Keywords: biosensors; lipid membranes; toxins 1. Introduction Biosensors exploit the interplay between two components in order to provide useful information about a target compound: A biological moiety and a physicochemical transducer. The interaction between the target compound and the biological moiety (receptor) provides a biochemical signal that is readily converted into an electric signal by the transducer (electrochemical, optical, piezoelectric, calorimetric, etc.). Although sophisticated techniques such as liquid chromatography (LC) provide accurate results, biosensor devices offer a much higher throughput of samples at a lower cost and with less training of personnel. In addition, biosensor devices are not bulky and can be used for in the field measurements. The uses of biosensors are extremely varied, with food and environmental analysis as an emerging and growing application. Nanomaterial-based biosensors represent the integration of material science, molecular engineering, chemistry and biotechnology. Nanomaterials can greatly improve the sensitivity and specificity of biomolecule detection and have great potential to constructing devices for molecular recognition, food and environment monitoring, clinical analysis and pathogen diagnosis. Lipid membrane offers a nature-like environment for the immobilization and control of the receptor. At the same time, the physics of the film per se offer a means for signal transduction and amplification: The biochemical interaction affects significantly lipid–protein and lipid–lipid Biosensors 2018, 8, 61; doi:10.3390/bios8030061 www.mdpi.com/journal/biosensors Biosensors 2018, 8, 61 2 of 18 Biosensors 2018, 8, x FOR PEER REVIEW 2 of 17 interactions,transmembrane thus ion disrupting permeability. the continuity Figure 1 ofprov theides assembly a schematic leading of to a detectable lipid membrane-based alterations of transmembranebiosensor. ion permeability. Figure1 provides a schematic of a lipid membrane-based biosensor. Figure 1. Schematic of a lipid membrane-based biosensor. Figure 1. Schematic of a lipid membrane-based biosensor. 1.1. Mechanism of Signal Generation in Lipid Based Biosensors The signal generation mechanismmechanism using lipid membrane based biosensorsbiosensors depends on the type of fabrication fabrication of of the the lipid lipid membrane membrane and and whether whether the membrane the membrane is freely-suspended, is freely-suspended, filter- or filter- metal- or metal-supportedsupported or support or supporteded on a polymer. on a polymer. The mechanism of signal generation for freely-suspendedfreely-suspended or filterfilter supported BLMs has been provided in the literature [[1–3].1–3]. In the case of lipidlipid membranemembrane based biosensors that use an enzyme as a transduction/recognition element, element, the the enzyme enzyme is islocated located in inthe the lipid lipid film film in such in such a way a way that that the thehydrophobic hydrophobic portions portions of the of theenzyme enzyme is incorporated is incorporated into intothe thelipid, lipid, leaving leaving the theactive active site siteof the of theenzyme enzyme at the at theaqueous aqueous interface interface [1,4]. [1 A,4]. detailed A detailed comparison comparison of the of theprofiles profiles and andmagnitudes magnitudes of the of thecurrent current transients transients that that were were obtained obtained in inour our previous previous works works [1,5] [1,5 ]using using flow flow injection injection technique technique for acetylcholine determination indicate a number of important similarities and and substantial differences. differences. The most significantsignificant similarity is the appearance of well-definedwell-defined transient currents in the same direction in both experiment as the hydronium ion concentration at the BLM surface is altered during the enzymatic reactions. The quantitative signal of these BLMs biosensors was primarily based on diffusion effects that controlled thethe timetime delaydelay ofof thethe appearanceappearance ofof thethe currentcurrent transients.transients. The transients of the flowflow injectioninjection analysis experiments [[2]2] have a maximummaximum magnitudemagnitude of about 50 pA and duration of about 10 s. The mechanism of signal generation was investigated using differential scanning calorimetric studies (DSC)(DSC) withwith eggegg PC/DPPAPC/DPPA vesicles [[2].2]. These studies have shown that the phase transition temperature for the gel to liquid-crystallineliquid-crystalline phase of these vesicles was pH depended. depended. The The results results indicated indicated that that at pH at pH8.0 and 8.0 andat 25 at °C, 25 a◦ gelC, aand gel liquid-crystalline and liquid-crystalline phase phaseco-exists co-exists and this and is induced this is induced by the presence by the presence of Ca2+. This of Ca phase2+. This separation phase separationresults in an results increase in anof increasethe number of the of numberdefects in of the defects phase in structure the phase of structure a lipid film of a lipidand therefore film and thereforethe ion permeation the ion permeation through throughthese membranes these membranes increases. increases. Each BLM Each surface BLM surface is in equilibrium is in equilibrium with its with adjacent its adjacent bulk bulksolution solution and andany alterations any alterations of either of either of the of membrane the membrane surfaces surfaces will result will in result a reorganization in a reorganization of the BLM of the double BLM doublelayer. In layer. this Incase this there case thereare two are twopossible possible mech mechanismsanisms of ofsignal signal genera generation:tion: The The appearance appearance of structural defectsdefects (i.e.,(i.e., artificial artificial ion-channel ion-channel gating) gating) will will drive drive the the ions io tons diffuse to diffuse through through the defect the defect sites. sites. The number of ions that passes through the BLMs depends on the speed of both of “channel” opening and the diffusion of ions from the surface of the lipid membrane to the bulk solution. In the Biosensors 2018, 8, 61 3 of 18 The number of ions that passes through the BLMs depends on the speed of both of “channel” opening and the diffusion of ions from the surface of the lipid membrane to the bulk solution. In the case that there is a slow “channel” formation, this will result that the majority of ions will be dissipated into the bulk solution; in the case that the “channel” opening is fast, this will result in the passage of an appreciable portion of the ions through the lipid film. The former phenomena were noticed when diffusion controlled changes of the pH at one side of a BLM (hydrolytic enzyme reactions) took place with concurrent observations of a charging current transients [1,5] while the latter phenomena were observed in the flow experiments in which the defects number varies dynamically as the pH of the carrier electrolyte solution alters; these results indicate how amplification of analytical signal of bilayer lipid membrane based transducers may be made [2]. The mechanism of signal generation using metal supported lipid membranes (s-BLMs) has been reported in the literature [6,7]. Previous studies have shown a model of a potential across metal supported (s-BLMs) and evaluated the structure of the inner lipid layer (facing the Ag wire electrode). It has been reported that the lipid head groups bind to the electrode by interactions of oxygen atoms of the phosphate groups of the lipids with silver ions in the metal lattice [6,7]. Furthermore, chloride ions can diffuse through the lipid membrane during the initial BLM process of stabilization. Chloride ions would interact with the Ag metal to form AgCl.