The detection of antigen-antibody reactions in normal saline solution, measured using a chip-mounted source-drain electrode coated with an insulation layer with immobilized antibody or antigen molecules, was investigated as a function of concentration. Five pairs of source-drain electrodes (×8 arrays) were fabricated on a glass substrate of 20 mm × 30 mm size, in which the individual Au/Cr (1.0/0.1 μm thickness) sensors had widths of 50 μm, and an inter-electrode interval of 100 μm. The fabricated source-drain electrodes were further coated with an insulation layer comprising various porous materials for use as an adsorbent for receiving antibody molecules. The sensor chip was equipped with a handy-type sensor signal analyzer that comprised either an amplifier circuit with a Miniship™, or a system in a packaged LSI device. The weak current received by each sensor electrode was amplified and modulated with a sensor signal analyzer to give a digital signal, which was then wirelessly transmitted to a PC connected to a receiver device. The effects of the immobilized antibody molecules on the detection selectivity and detection limits were examined. The sensing functions were determined by measurement of the source-drain electrode voltage when the surface of the insulator was contacted with antigen solutions of various concentrations. The sensor ability was also evaluated by comparing the relative selectivity of each sensor. When a sensor surface with immobilized biotin was used for avidin sensing, or immobilized with leukotriene B4 for anti-leukotriene sensing, the sensor showed the largest effect for the selective detection of all sensor testing conducted. In contrast, when the sensor surface was immobilized with avidin for biotin sensing, or immobilized with anti-leukotriene for leukotriene sensing, the sensor exhibited low selectivity. Based on the results obtained, an adsorbed antibody on an adsorption particle is effectively expressed as the selective detection ability for the sensor-array chips. When liquid is deposited on the surface of an insulator, an electric double layer arises at the boundary surface. In this respect, the insulator molecules become polarized, resulting in the formation of opposing charges on both the boundary surface between the liquid and solid, and on the reverse side of the insulator. In the same way, when a solution mixture containing antigens is deposited on the surface of an insulator with antibodies immobilized to its surface, specific antigens and antibody reactions occur and the charge on the reverse side of the insulator is increased. The relationship between the source-drain electrode current and the sensor voltage follows Ohm's law. Based on this principle, a source-drain electrode produces a signal in response to changes in current originating from antigen-antibody reactions at the boundary surface on the insulator layer. The avidin molecule is composed of four domain structures, such that, four acceptors per molecule exist on the surface of the adsorbent particle. Biotin preferably interacts with this acceptor, because the molecular size of biotin is smaller that that of avidin. The sensor voltage changes do not suffer in the presence of cation contamination. In contrast, the molecular size of avidin is much larger than that of the cations in solution. In this respect, the cations are considered to interact with the sensor surface in preference to the biotin antibodies, thus promoting polarization of the insulator layer. As a result, the detection voltages increased, while the detection selectivity decreased.
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