Direct electrochemical transduction of an immunological reaction by bilayer lipid membranes

Immunoelectrodes in protein detection: comparison between glassy carbon and a semimetallic Ni/P thin film as binding support. Biological applications

Though immunoelectrodes can allow direct detection of very low protein amounts (about 0.1 pmol) in vitro and in vivo, they are not yet widely used because they need quality improvement. Based on a few works devoted to the basic electrochemical phenomenon occurring when antibodies are linked onto a solid support and during antigen/antibody complex formation, we have coated two different supports with antibodies: the classical glassy carbon fiber or an epoxy plate covered with an amorphous semimetallic (nickel/phosphorus) thin film obtained by means of an electrochemical deposit. The antibody/antigen complex formation induces direct and/or indirect ionic movements and a current flow through the conductive support toward a very low-noise and high-sensitivity preamplifier stage in an I/V configuration. The proposed electrochemical treatment (hydrophilization), applied to both carbon and Ni/P electrodes, improves antibody binding and reliability of the response to antigens. The Ni/P probes present several advantages when compared to carbon fiber: better conductivity, possibility of surface quality control, and semimetallic nature, making them unbreakable. Several applications were proposed: somatostatin-14 detection with both carbon fiber and Ni/P plate electrodes, and histamine detection in simple and complex fluid media. Dose-response curves and analysis of the results lead us to conclude that the obtained currents are directly related to the quantity of antigen.

Ammonium ion minisensors form self-assembled bilayer lipid membranes using gramicidin as an ionophore. Modulation of ammonium selectivity by platelet-activating factor

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.

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.