Optical sensitivity of a microelectrode in contact with an electrolyte

Abstract

A microelectrode sensor mounted on a chip was investigated regarding its properties for detecting optical intensity. Comb-like shaped microelectrodes were constructed by a photolithographic method and consisted of a Cu film of 10 μm thickness on a resin chip. Three comb-like electrodes with varying comb numbers were fabricated; lengths were: a = 10 mm (8 combs), a = 5 mm (4 combs), and a = 2.5 mm (2 combs). NaCl solutions of 1.0 × 10 –4 to 1.0 mol l –1 (2–10 μL) were applied on the microelectrode under white light irradiation of 0–1.5 × 10 3 lx emitted from an LED. Optical sensitivity was then evaluated by measuring the electrical resistance. The effects of electrode size and sensor material on the detection limits for optical sensitivity were investigated. The sensor responded to changes in light intensity when an electrolyte solution contacted the surface of a pair of comb-like electrodes. When a strong light was irradiated onto the microelectrode in contact with an NaCl solution, the electrical resistance markedly reduced. By contrast, when a weak light was irradiated on the microelectrode in contact with an NaCl solution, the electrical resistance increased. When the sensor size or the number of combs was increased, electrical resistance showed a marked decrease with increased light intensity. In particular, an eight-comb microsensor exhibited high responsiveness under weak light intensity within the range of 0–1.0 × 10 3 lx. When the electrolyte concentration is constant, electrical resistance also converges to a constant value. In all the various upper boards tested (polyimide, silicon rubber, glass, and PET film), the resistance value depended on the thickness regardless of the intensity of light irradiation. The various materials showed no effect on light intensity. However, the electrical resistance of the sensor increased for upper boards made of Teflon, polypropylene, and polychlorovinylidene for which electrical resistance was reduced when light intensity was high. It is suggested that polarization occurs when light is irradiated onto a metal sensor surface and the boundary surface of an electrolyte droplet. In the presence of a constant electrolyte concentration, the charge distribution on the surface of the sensor was found to change in accordance with the intensity of light irradiation. These results clearly demonstrate that photosensitivity and ion sensitivity, as well as the formation of the ion layer, are influenced by the choice of the cover material. We applied a microelectrode as an optical detector for a microarray to measure the concentration of a dye such as methylene blue. We measured the electrical resistance values, as well as those of light intensity, of four microelectrodes located on the reverse side of a microarray. Methylene blue solution was prepared at concentrations of 0, 1.0 × 10 –3, 1.0 × 10 –2, and 1.0 × 10 –1 wt% and 50 μL of each solution was applied to each of the microarrays. The upper board material was translucent and irradiated light could pass through the microarrays including the dye solution. The light that penetrated was received by each microelectrode, as well as a photosensor located under microarray. In each of the microarrays, the detection voltage of the microelectrode depended on the depth of the sample color.