Chemical transduction with fluorescent lipid membranes using selective interactions of acetylcholine receptor with agonist/antagonist and acetylcholine with substrate

A triazine herbicide minisensor based on surface-stabilized bilayer lipid membranes

This work describes an electrochemical technique that is suitable for rapid and sensitive screening of the triazine herbicides simazine, atrazine, and propazine. Egg phosphatidylcholine and dipalmitoylphosphatidic acid (DPPA) were used for the formation of self-assembled bilayer lipid membranes supported on silver wire (s-BLMs). Evidence that BLMs could form on silver wires was collected by means of ellipsometry which was done to investigate samples consisting of lipids deposited on planar reflective silver films. The interactions of triazines with s-BLMs produced electrochemical ion current increases which reproducibly appeared within ∼10 s after exposure of the lipid membranes to the herbicides. The sensitivity of the response was maximized by use of BLMs composed of 35% (w/w) DPPA and by alteration of the phase distribution within membranes by the introduction of 1.0 mM calcium ions in bulk solution. The mechanism of signal generation could be a result of rapid adsorption of the triazine on the surface of s-BLMs with a consequent rapid reorganization of the electrostatics of the membrane. The magnitude of the current signal was linearly related to the herbicide concentration, which could be determined at the nanomolar level. The present triazine minisensor exhibited good mechanical stability and longevity (routinely over 48 h), reproducible response characteristics (i.e., sensitivity and response to a given concentration of triazine in solution), fast response times, and low detection limits. The sensor can be simply and reliably fabricated at low cost. Studies have shown high selectivity for triazines in the presence of insecticides and pesticides.

1994 McBryde Medal Award Lecture Investigations of organized monolayer films for biosensor development

Our interests have focused on the investigation and development of biosensors that use chemically selective membranes to measure the concentration of specific species in complex media. One fundamental idea is that protein, which can bind selectively to a specific organic or biochemical species, can be incorporated into an ordered lipid or surfactant membrane assembly such that selective binding events lead to changes in the structure of the membrane (transduction) that can be measured quantitatively. The primary advantage of this method of detection is that it is applicable to interactions of enzymes, antibodies, receptors, and lectins, and it may be extended to investigations of DNA/RNA hybridization. This detection method therefore provides a sensitive generic strategy for sensor applications. The central problem to be solved is how the alteration of the structure of a membrane that is caused by binding events of protein or genetic material can give rise to an analytical signal. We have been focusing our efforts in the areas of fluorescence spectroscopy and electrochemical methods. The electrochemical methods rely on detection in changes of the permeability of membranes to ions and provide systems with very low background signal, leading to the possibility of detection of single molecular-binding events. Fluorescent systems operate on the basis that a chemically selective membrane containing a fluorescent indicator can provide an analytical signal caused by the change of the structure of the membrane due to the binding events. Keywords: fluorescence, lipid membrane, electrochemistry, receptor, biosensor.

Assembly of antibodies in lipid membranes for biosensor development

An investigation of the incorporation of antibody in lipid films of a composition that has been used for biosensor preparation is reported. IgG that is incorporated into lipid monolayers prepared from 7:3 mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidic acid is edge-active, and enters and penetrates the fluid region of the mixed-phase system when monolayers are held at low pressure (< 20 mN/m). It was found that there is an "exclusion pressure" observed in pressure-area (pi-A) curves that are collected for monolayers that contain antibody. This term refers to a specific threshold of lateral pressure (which is reached by monolayer compression) that can cause explusion of antibody from the interior of a membrane. Microscopic images of monolayers containing the fluorescent phospholipid nitrobenzoxadiazole dipalmitoyl phosphatidylethanolamine (NBD-PE), or antibody labeled with tetramethylrhodamine isothiocyanate (TRITC), were used to determine the structure of membranes, and the location of effects on structure caused by IgG. Ellipsometric measurements of lipid monolayers that were cast onto silicon wafers by the Langmuir-Blodgett method were used to study the thickness of monolayers and to investigate the structural changes that occurred at the "exclusion pressure." Both the use of fluorescent antigen and ellipsometry indicated that antibody binding activity was present and was dependent on compression pressure. The effects of pH and ionic strength of subphase, antibody concentration, incubation time, and lateral pressure have been examined. The results may indicate the conditions that can be used to improve the incorporation of active IgG for preparation of biosensors that are based on lipid membranes.

The bilayer lipid membrane as a generic electrochemical transducer of hydrolytic enzyme reactions

Bilayer lipid membranes (BLMs) can be used as generic transducers to monitor hydrolytic enzyme reactions occurring at the membrane surface. The representative enzymatic reactions presented herein were between membrane associated urease and penicillinase with urea and penicillin, respectively. Transient electrochemical signals from BLMs which contained enzyme were obtained by proper selection of the lipid composition of membranes. Negatively charged lipid membranes composed of egg phosphatidylcholine (PC) and 35% dipalmitoylphosphatidic acid were used for this purpose. The results were consistent with an electrostatic mechanism of perturbation of the surface structure of the BLMs, where changes of local hydronium ion activity associated with the enzymatic reaction altered the extent of ionization of the headgroups of the acidic constituent of the membranes, thereby providing a transient charging current which lasted for a period on the order of seconds. The delay time for observation of the transient was directly and reproducibly related to the concentration of the substrate which could be determined over a range of microM to mM levels. The results indicate that BLMs can be used as generic selective electrochemical transducers and as switchable biosensors to monitor rapid enzymatic reactions which alter pH.