Glutamate Receptor Incorporated in a Mixed Hybrid Bilayer Lipid Membrane Array, as a Sensing Element of a Biosensor Working under Flowing Conditions

Cholesterol-Enriched Hybrid Lipid Bilayer Formation on Inverse Phosphocholine Lipid-Functionalized Titanium Oxide Surfaces

Hybrid lipid bilayers (HLBs) are rugged biomimetic cell membrane interfaces that can form on inorganic surfaces and be designed to contain biologically important components like cholesterol. In general, HLBs are formed by depositing phospholipids on top of a hydrophobic self-assembled monolayer (SAM) composed of one-tail amphiphiles, while recent findings have shown that two-tail amphiphiles such as inverse phosphocholine (CP) lipids can have advantageous properties to promote zwitterionic HLB formation. Herein, we explored the feasibility of fabricating cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces with the solvent exchange and vesicle fusion methods. All stages of the HLB fabrication process were tracked by quartz crystal microbalance-dissipation (QCM-D) measurements and revealed important differences in fabrication outcome depending on the chosen method. With the solvent exchange method, it was possible to fabricate HLBs with well-controlled cholesterol fractions up to ~65 mol% in the upper leaflet as confirmed by a methyl-β-cyclodextrin (MβCD) extraction assay. In marked contrast, the vesicle fusion method was only effective at forming HLBs from precursor vesicles containing up to ~35 mol% cholesterol, but this performance was still superior to past results on hydrophilic SiO2. We discuss the contributing factors to the different efficiencies of the two methods as well as the general utility of two-tail CP SAMs as favorable interfaces to incorporate cholesterol into HLBs. Accordingly, our findings support that the solvent exchange method is a versatile tool to fabricate cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces.

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Electrografting a Hybrid Bilayer Membrane via Diazonium Chemistry for Electrochemical Impedance Spectroscopy of Amyloid-β Aggregation

Herein, a novel hybrid bilayer membrane is introduced as a platform to study the aggregation of amyloid-β_1–42 (Aβ_1–42) peptide on surfaces. The first layer was covalently attached to a glassy carbon electrode (GCE) via diazonium electrodeposition, which provided a highly stable template for the hybrid bilayer formation. To prepare the long-chain hybrid bilayer membrane (lcHBLM)-modified electrodes, GCE surfaces were modified with 4-dodecylbenzenediazonium (DDAN) followed by the modification with dihexadecyl phosphate (DHP) as the second layer. For the preparation of short-chain hybrid bilayer membrane (scHBLM)-modified electrodes, GCE surfaces were modified with 4-ethyldiazonium (EDAN) as the first layer and bis(2-ethylhexyl) phosphate (BEHP) was utilized as the second layer. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to characterize the bilayer formation. Both positively charged [Ru(NH₃)₆]³⁺ and negatively charged ([Fe(CN)₆]^3-/4-) redox probes were used for electrochemical characterization of the modified surfaces using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS results showed a decrease in charge transfer resistance (R_ct) upon incubation of Aβ_1–42 on the hybrid bilayer-modified surfaces. This framework provides a promising electrochemical platform for designing hybrid bilayers with various physicochemical properties to study the interaction of membrane-bound receptors and biomolecules on surfaces.

Revealing local molecular distribution, orientation, phase separation, and formation of domains in artificial lipid layers: Towards comprehensive characterization of biological membranes

Lipids, together with molecules such as DNA and proteins, are one of the most relevant systems responsible for the existence of life. Selected lipids are able to assembly into various organized structures, such as lipid membranes. The unique properties of lipid membranes determine their complex functions, not only to separate biological environments, but also to participate in regulatory functions, absorption of nutrients, cell-cell communication, endocytosis, cell signaling, and many others. Despite numerous scientific efforts, still little is known about the reason underlying the variability within lipid membranes, and its biochemical significance. In this review, we discuss the structural complexity of lipid membranes, as well as the importance to simplify studied systems in order to understand phenomena occurring in natural, complex membranes. Such systems require a model interface to be analyzed. Therefore, here we focused on analytical studies of artificial systems at various interfaces. The molecular structure of lipid membranes, specifically the nanometric thickens of molecular bilayer, limits in a major extent the choice of highly sensitive methods suitable to study such structures. Therefore, we focused on methods that combine high sensitivity, and/or chemical selectivity, and/or nanometric spatial resolution, such as atomic force microscopy, nanospectroscopy (tip-enhanced Raman spectroscopy, infrared nanospectroscopy), phase modulation infrared reflection-absorption spectroscopy, sum-frequency generation spectroscopy. We summarized experimental and theoretical approaches providing information about molecular structure and composition, lipid spatial distribution (phase separation), organization (domain shape, molecular orientation) of lipid membranes, and real-time visualization of the influence of various molecules (proteins, drugs) on their integrity. An integral part of this review discusses the latest achievements in the field of lipid layer-based biosensors.

Application of N-methyl-D-aspartate receptor nanopore in screening ligand molecules

N-methyl-D-aspartate receptors (NMDARs) are crucial for excitatory synaptic transmission in the central nervous system. To study NMDARs more accurately and conveniently, we developed a stable NMDAR nanopore in a planar lipid bilayer. Pharmacological properties were validated using the allosteric modulator Ro 25-6981 and antagonist D-2-amino-5-phosphonopentanoic acid (D-APV). The cyanotoxin β-N-methylamino-L-alanine (BMAA) found in fresh water systems is suspected to be associated with the development of neurodegenerative diseases. Therefore, BMAA and its two isomers L-2, 4-Diaminobutyric acid dihydrochloride (DAB) and N-(2-aminoethyl) glycine (AEG) and an endogenous excitotoxin, quinolinic acid (QA), were studied using the NMDAR nanopores to assess their effects on NMDAR modulation. We demonstrated that the NMDAR nanopore could reliably detect its ligand molecules at the single-channel level. The study also demonstrated the practicability of NMDAR nanopores, and results were validated using two-electrode voltage-clamp (TEVC) recording. Compared with TEVC recording, the NMDAR nanopores conducted ion channel gating at the single-channel level without being affected by other proteins on the cell membrane. The highly sensitive and accurate NMDAR nanopore technique thus has a unique advantage in screening NMDAR ligand molecules that could be associated with neurodegenerative disease.

Enhanced Temporal Resolution with Ion Channel-Functionalized Sensors Using a Conductance-Based Measurement Protocol

The binding of a target analyte to an ion channel (IC), which is readily detected electrochemically in a label-free manner with single-molecule selectivity and specificity, has generated widespread interest in using natural and engineered ICs as transducers in biosensing platforms. To date, the majority of developments in IC-functionalized sensing have focused on IC selectivity or sensitivity or development of suitable membrane environments and aperture geometries. Comparatively little work has addressed analytical performance criteria, particularly criteria required for temporal measurements of dynamic processes. We report a measurement protocol suitable for rapid, time-resolved monitoring (≤30 ms) of IC-modulated membrane conductance. Key features of this protocol include the reduction of membrane area and the use of small voltage steps (10 mV) and short duration voltage pulses (10 ms), which have the net effect of reducing the capacitive charging and decreasing the time required to achieve steady state currents. Application of a conductance protocol employing three sequential, 10 ms voltage steps (-10 mV, -20 mV, -30 mV) in an alternating, pyramid-like arrangement enabled sampling of membrane conductance every 30 ms. Using this protocol, dynamic IC measurements on black lipid membranes (BLMs) functionalized with gramicidin A were conducted using a fast perfusion system. BLM conductance decreased by 76 ± 7.5% within 30 ms of switching from solutions containing 0 to 1 M Ca2+, which demonstrates the feasibility of using this approach to monitor rapid, dynamic chemical processes. Rapid conductance measurements will be broadly applicable to IC-based sensors that undergo analyte-specific gating.

Monitoring and quantifying the passive transport of molecules through patch-clamp suspended real and model cell membranes

Transport of active molecules across biological membranes is a central issue for the success of many pharmaceutical strategies. Herein, we combine the patch-clamp principle with amperometric detection for monitoring fluxes of redox-tagged molecular species across a suspended membrane patched from a macrophage. Solvent- and protein-free lipid bilayers (DPhPC, DOPC, DOPG) patched from single-wall GUV have been thoroughly investigated and the corresponding fluxes measurements quantified. The quality of the patches and their proper sealing were successfully characterized by electrochemical impedance spectroscopy. This procedure appears versatile and perfectly adequate to allow the investigation of transport and quantification of the transport properties through direct measurement of the coefficients of partition and diffusion of the compound in the membrane, thus offering insight on such important biological and pharmacological issues.

Pore-forming compounds as signal transduction elements for highly sensitive biosensing

Pore-forming compounds are attracting much attention due to the signal transduction ability for the development of highly sensitive biosensing. In this review, we describe an overview of the recent advances made by our group in the design of molecular sensing interfaces of spherical and planar lipid bilayers and natural bilayers. The potential uses of pore-forming compounds, such as gramicidin and MCM-41, in lipid bilayers and natural glutamate receptor channels in biomembrane are presented.

Electrochemical impedance spectroscopy and atomic force microscopic studies of electrical and mechanical properties of nano-black lipid membranes and size dependence

We present electrochemical impedance spectroscopic (EIS) and two-chamber AFM investigations of the electrical and mechanical properties of solvent-containing nano-BLMs suspended on chip-based nanopores of diameter of 200, 400, and 700 nm. The chips containing nanoporous silicon nitride membranes are fabricated based on low-cost colloidal lithography with low aspect ratio of the nanopores. BLMs of DPhPC lipid molecules are constructed across the nanopores by the painting method. Two equivalent circuits are compared in view of their adequacy in description of the EIS performances of the nano-BLMs and more importantly the structures associated with the nano-BLMs systems. The BLM resistance and capacitance as well as their size and time dependence are studied by EIS. The breakthrough forces, elasticity in terms of apparent spring constant, and lateral tension of the solvent-containing nano-BLMs are investigated by AFM force measurements. The exact relationship of the breakthrough force of the nano-BLM as a function of pore size is revealed. Both EIS and AFM studies show increasing lifetime and mechanical stability of the nano-BLMs with decreasing pore size. Finally, the robust 200 nm diameter nanopores are used to accommodate functional BLMs containing DPhPC lipid molecules and gramicidins by using a painting method with drop of mixture solutions of DPhPC and gramicidins. EIS investigation of the functional nano-BLMs is also performed.

Reversible redox of NADH and NAD+ at a hybrid lipid bilayer membrane using ubiquinone

Here, we report the reversible interconversion between NADH and NAD(+) at a low overpotential, which is in part mediated by ubiquinone embedded in a biomimetic membrane to mimic the initial stages of respiration. This system can be used as a platform to examine biologically relevant electroactive molecules embedded in a natural membrane environment and provide new insights into the mechanism of biological redox cycling.

Porous Materials to Support Bilayer Lipid Membranes for Ion Channel Biosensors

To identify materials suitable as membrane supports for ion channel biosensors, six filter materials of varying hydrophobicity, tortuosity, and thickness were examined for their ability to support bilayer lipid membranes as determined by electrical impedance spectroscopy. Bilayers supported by hydrophobic materials (PTFE, polycarbonate, nylon, and silanised silver) had optimal resistance (14–19 GΩ) and capacitance (0.8–1.6 μF) values whereas those with low hydrophobicity did not form BLMs (PVDF) or were short-lived (unsilanised silver). The ability of ion channels to function in BLMs was assessed using a method recently reported to improve the efficiency of proteoliposome incorporation into PTFE-supported bilayers. Voltage-gated sodium channel activation by veratridine and inhibition by saxitoxin showed activity for PTFE, nylon, and silanised silver, but not polycarbonate. Bilayers on thicker, more tortuous, and hydrophobic materials produced higher current levels. Bilayers that self-assembled on PTFE filters were the longest lived and produced the most channel activity using this method.

Liposome and lipid bilayer arrays towards biosensing applications

Sensitive and selective biosensors for high-throughput screening are having an increasing impact in modern medical care. The establishment of robust protein biosensing platforms however remains challenging, especially when membrane proteins are involved. Although this type of proteins is of enormous relevance since they are considered in >60% of the pharmaceutical drug targets, their fragile nature (i.e., the requirement to preserve their natural lipid environment to avoid denaturation and loss of function) puts strong additional prerequisites onto a successful biochip. In this review, the leading approaches to create lipid membrane-based arrays towards the creation of membrane protein biosensing platforms are described. Liposomes assembled in micro- and nanoarrays and the successful set-ups containing functional membrane proteins, as well as the use of liposomes in networks, are discussed in the first part. Then, the complementary approaches to create cell-mimicking supported membrane patches on a substrate in an array format will be addressed. Finally, the progress in assembling free-standing (functional) lipid bilayers over nanopore arrays for ion channel sensing will be reported. This review illustrates the rapid pace by which advances are being made towards the creation of a heterogeneous biochip for the high-throughput screening of membrane proteins for diagnostics, drug screening, or drug discovery purposes.

Applications of biological pores in nanomedicine, sensing, and nanoelectronics

Biological protein pores and pore-forming peptides can generate a pathway for the flux of ions and other charged or polar molecules across cellular membranes. In nature, these nanopores have diverse and essential functions that range from maintaining cell homeostasis and participating in cell signaling to activating or killing cells. The combination of the nanoscale dimensions and sophisticated - often regulated - functionality of these biological pores make them particularly attractive for the growing field of nanobiotechnology. Applications range from single-molecule sensing to drug delivery and targeted killing of malignant cells. Potential future applications may include the use of nanopores for single strand DNA sequencing and for generating bio-inspired, and possibly, biocompatible visual detection systems and batteries. This article reviews the current state of applications of pore-forming peptides and proteins in nanomedicine, sensing, and nanoelectronics.

Neural membrane signaling platforms

Throughout much of the history of biology, the cell membrane was functionally defined as a semi-permeable barrier separating aqueous compartments, and an anchoring site for proteins. Little attention was devoted to its possible regulatory role in intracellular molecular processes and neuron electrical signaling. This article reviews the history of membrane studies and the current state of the art. Emphasis is placed on natural and artificial membrane studies of electric field effects on molecular organization, especially as these may relate to impulse propagation in neurons. Implications of these studies for new designs in artificial intelligence are briefly examined.

Free-standing lipid bilayers in silicon chips-membrane stabilization based on microfabricated apertures with a nanometer-scale smoothness

In the present study, we propose a method for preparing stable free-standing bilayer lipid membranes (BLMs). The BLMs were prepared in a microfabricated aperture with a smoothly tapered edge, which was prepared in a nanometer-thick Si(3)N(4) septum by the wet etching method. Owing to this structure, the stress on lipid bilayers at the contact with the septum was minimized, leading to remarkable membrane stability. The BLMs were not broken by applying a constant voltage of +/-1 V. The membrane lifetime was 15-45 h with and without an incorporated gramicidin channel. Gramicidin single-channel currents were recorded from the same BLM preparation when the aqueous solutions surrounding the BLM were repeatedly exchanged, demonstrating the tolerance of the present BLM to repetitive solution exchanges. Such stable membranes enable analysis of channel functions under various solution conditions from the same BLM, which will open up a variety of applications including a high throughput drug screening for ion channels.

A metal-supported biomimetic micromembrane allowing the recording of single-channel activity and of impedance spectra of membrane proteins

A novel tethered bilayer lipid micromembrane (tBLmicroM) was prepared and characterized. It consists of a mercury cap electrodeposited on a platinum microelectrode, about 20 microm in diameter. The micromembrane was prepared by tethering to the mercury cap a thiolipid monolayer and by then self-assembling a lipid monolayer on top of it. The thiolipid consisted of a disulfidated tetraoxyethylene hydrophilic spacer covalently linked to two phytanyl chains. Upon incorporating OmpF porin in the tBLmicroM, its single-channel activity was recorded by the patch-clamp technique, and its particular features described. An electrochemical impedance spectrum of the tBLmicroM incorporating OmpF porin is also reported. To the best of our knowledge, this tBLmicroM is the first metal-supported biomimetic micromembrane capable of incorporating non-engineered channel proteins in a functionally active state from their detergent solutions, and of allowing the recording of single-channel activity and of impedance spectra of these proteins via ion translocation into the hydrophilic spacer. The limited spaciousness of the spacer prevents a statistical analysis based on current-amplitude or blockage-time histograms. Nonetheless, the robustness, stability, ease of preparation and disposability of the present tBLmicroM may open the way to the realization of a channel-protein microarray platform allowing a high throughput drug screening.

Impedance analysis of valinomycin activity in nano-BLMs

Nano-black lipid membranes (nano-BLMs) were obtained by functionalization of highly ordered porous alumina substrates with an average pore diameter of 60nm based on a self-assembled alkanethiol submonolayer followed by spreading of 1,2-diphytanoyl-sn-glycero-3-phosphocholine dissolved in n-decane on the hydrophobic substrate. By means of impedance spectroscopy, we analyzed the influence of the self-assembled alkanethiol submonolayer on the electrical properties of the nano-BLMs as well as their long-term stability. We were able to stably integrate nano-BLMs into a flow through system, which allowed us to readily exchange buffer solutions several times and accounts for mass transport phenomena. The ionophore valinomycin was successfully inserted into nano-BLMs and its transport activity monitored as a function of different potassium and sodium ion concentrations reflecting the specificity of valinomycin for potassium ions.

Highly efficient biocompatible single silicon nanowire electrodes with functional biological pore channels

Nanoscale electrodes based on one-dimensional inorganic conductors could possess significant advantages for electrochemical measurements over their macroscopic counterparts in a variety of electrochemical applications. We show that the efficiency of the electrodes constructed of individual highly doped silicon nanowires greatly exceeds the efficiency of flat Si electrodes. Modification of the surfaces of the nanowire electrodes with phospholipid bilayers produces an efficient biocompatible barrier to transport of the solution redox species to the nanoelectrode surface. Incorporating functional alpha-hemolysin protein pores in the lipid bilayer results in a partial recovery of the Faradic current due to the specific transport through the protein pore. These assemblies represent a robust and versatile platform for building a new generation of highly specific biosensors and nano/bioelectronic devices.

Artificial receptors

Herein I will provide a brief overview of artificial receptors with emphasis on molecularly imprinted polymers (MIPs) and their applications. Alternative techniques to produce artificial receptors such as in silico designed and modelled polymers as well as different receptors designed using libraries of more or less natural composition will also be mentioned. Examples of these include aptamers and bio-nanocomposites. The physical presentation of the receptors is important and may depend on the application. Block polymerization of MIPs and grinding to particles of suitable size used to be the preferred technique, but today beaded materials can be produced in sizes down to nanobeads and also nanofibers can be used to increase available surface area and thereby capacity. For sensor applications it may be attractive to include the artificial receptors in surface coatings or in membrane structures. Different composite designs can be used to provide additional desirable properties. MIPs and other artificial receptors are gaining rapidly increasing attention in very shifting application areas and an attempt to provide a systematic account for current applications has been made with examples from separation, solid-phase extraction, analysis, carbohydrate specific experiments, and MIPs-directed synthesis.

Biomimetic Interactions of Proteins with Functionalized Nanoparticles: A Thermodynamic Study

Gold nanoparticles (NPs) functionalized with L-amino acid-terminated monolayers provide an effective platform for the recognition of protein surfaces. Isothermal titration calorimetry (ITC) was used to quantify the binding thermodynamics of these functional NPs with alpha-chymotrypsin (ChT), histone, and cytochrome c (CytC). The enthalpy and entropy changes for the complex formation depend upon the nanoparticle structure and the surface characteristics of the proteins, e.g., distributions of charged and hydrophobic residues on the surface. Enthalpy-entropy compensation studies on these NP-protein systems indicate an excellent linear correlation between DeltaH and TDeltaS with a slope (alpha) of 1.07 and an intercept (TDeltaS0) of 35.2 kJ mol(-1). This behavior is closer to those of native protein-protein systems (alpha = 0.92 and TDeltaS0 = 41.1 kJ mol(-1)) than other protein-ligand and synthetic host-guest systems.

Micro-BLMs on highly ordered porous silicon substrates: rupture process and lateral mobility

In a recent paper, we hypothesized that the continuous increase in membrane conductance observed for nano-BLMs is the result of an independent rupturing of single membranes or membrane patches covering the pores of the porous material. To prove this hypothesis, we prepared micro-BLMs on porous silicon substrates with a pore size of 7 mum. The upper surface of the silicon substrate was coated with a gold layer, followed by the chemisorption of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) and subsequent addition of a droplet of 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) dissolved in n-decane. The lipid membranes were fluorescently labeled and investigated by means of fluorescence microscopy and impedance spectroscopy. Impedance spectroscopy revealed the formation of pore-suspending bilayers with high membrane resistance. Increases in membrane capacitance and membrane conductance were observed. This increase in membrane conductance could be unambiguously related to the individual rupturing of membranes suspending the pores of the porous material as visualized by means of fluorescence microscopy. Moreover, by fluorescence recovery after photobleaching experiments, we investigated the lateral mobility of the lipids within the micro-BLMs leading to a mean effective diffusion coefficient of Deff = (14 +/- 1) microm2/s.

Construction of hybrid bilayer membrane (HBM) Biochips and characterization of the cooperative binding between cytochrome-c and HBM

We constructed multi-channel hybrid bilayer membrane (HBM) biochips and characterized them by surface plasmon resonance imaging. Each channel in the biochip was prepared using vesicles with different proportions of negative, neutral, and positive lipids. The HBM surfaces were tested by interaction with two globular proteins that recognize surfaces covered with opposite charges. Spots modified with the same HBM show responses within a relative standard deviation of 10% or smaller. These devices were also used to study in detail the interaction between cytochrome-c (cyt-c) and HBMs. Cooperative binding between cyt-c and negative HBMs was demonstrated. Using an adaptation of the Hill model, we calculated a Hill coefficient of 5 and a 10-fold increase in the binding constant with the increase in cyt-c concentration. We propose that this treatment can be used to evaluate the cooperative binding of surface proteins to membranes.

Channel activity of OmpF monitored in nano-BLMs

Free-standing lipid bilayer membranes can be formed on small apertures (60 nm diameter) on highly ordered porous alumina substrates. The formation process of the membranes on a 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol submonolayer was followed by impedance spectroscopy. After lipid bilayers had thinned, the reconstitution and ionic conducting properties of the outer membrane protein OmpF of E. coli were monitored using single-channel recordings. The characteristic conductance states of the three monomers, fast kinetics, and subconductance states were observed. Blockade of the ion flow as a result of interaction of the antibiotic ampicillin with the protein was verified, indicating the full functionality of the protein channel in nanometer-scale bilayer membranes.

Transport across artificial membranes–an analytical perspective

Biosensors that make use of transport processes across lipid membranes are very rare even though a stimulus, the binding of a single analyte molecule, can enhance the sensor response manifold if the analyte leads to the transport of more than one ion or molecule across the membrane. Prerequisite for a proper function of such membrane based biosensors is the formation of lipid bilayers attached to a support that allow for the insertion of membrane peptides and proteins in a functional manner. In this review, the current state of the art technologies to obtain lipid membranes on various supports are described. Solid supported membranes on transparent and electrically conducting surfaces, lipid bilayers on micromachined apertures and on porous materials are discussed. The focus lies on the applicability of such membranes for the investigation of transport phenomena across lipid bilayers facilitated by membrane embedded peptides, channel proteins and transporters. Carriers and channel forming peptides, which are easy to handle and rather robust, are used frequently to build up membrane based biosensors. However, channel forming proteins and transporters are more difficult to insert functionally and thus, there are yet only few examples that demonstrate the applicability of such systems as biosensor devices.