Vol.2 No.2 2009

68/98

Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−148−Synthesiology - English edition Vol.2 No.2 (2009) obtained. Since almost no surface roughening occurred in the thermally grown SiO2 film as shown in Fig. 6, we considered the thermal oxidation process of Si in the waveguide layer formation. When Si is used as the reflective film, and if the Si layer is formed thick and the waveguide is formed by thermal oxidation of the surface, we can fabricate a sensing plate with a thermally grown SiO2 film as a waveguide. In this case, it is necessary to use a glass substrate that can endure high-temperature treatment.To realize this idea, we considered using the SOQ substrate for sensing plate formation. Since thick oxidized layer was necessary for waveguide formation, we incorporated the water vapor oxidation method[14] where the speed of oxidizing the Si layer of the SOQ substrate was fast, and we fabricated a plate having a single-crystalline Si reflective layer and a SiO2 waveguide layer on a silica glass substrate. The fabrication process of the sensing plate is shown in Fig. 11. The thickness of the single-crystalline Si layer before the thermal oxidation was 440 nm. When this layer was oxidized for 1 hr in oxygen atmosphere containing water vapor at 1000 ºC, the Si layer surface was oxidized and a waveguide layer with thickness of 482 nm was formed. The remaining Si layer of a thickness of 220 nm would function as the reflective film. We named this sensing plate ‘monolithic sensing plate’.The fabricated sensing plate was set in the optical setup as shown in Fig. 12 to conduct molecule detection tests. Figures 13(a) and 13(b) show the changes in reflectance properties when biotin-streptavidin adsorption was detected using the monolithic sensing plates with nanopores and without nanopores, respectively. The incident light was s-polarized light of He-Ne laser (632.8 nm wavelength). The diameter of the nanopores was about 50 nm, and there were 5 × 109 pores/cm2. The white dots in the figures show the incident angle dependency of the reflected light intensity before adsorption, while the black dots show the intensity after adsorption. It can be seen that the amount of shift of peak position increased about 10 times by the nanopore formation. By the nanopore formation, the width of the dip increased slightly while the depth hardly changed. This is due to the reduction of damage during etching.To ensure the improvement of sensitivity by nanopore formation theoretically, we conducted a simulation using the Fresnel equation. Figures 14(a), 14(b), and 14(c) are conceptual diagrams used in the simulation. Figure 14(a) shows a conventional SPR sensor, 14(b) shows an EFC-WM sensor using a monolithic plate without nanopores, and 14(c) shows an EFC-WM sensor using monolithic plate with nanopores. The prism of the SPR sensor was a right triangle prism with a refractive index of 1.769 and the sensing plate was assumed to have a gold film with a thickness of 51 nm formed on a substrate with a refractive index of 1.769. The prism of the EFC-WM was an isosceles triangle prism having a vertex angle of 30° with refractive index of 1.456. The substrate of the sensing plate was silica glass (n = 1.456), the thickness of the Si reflective layer was 220 nm, and the thickness of the waveguide layer was 450 nm. As in the experiment condition, the diameter and the number of nanopores were set to be 50 nm and 5 × 109 pores/cm2, respectively. As an imitation of the adsorption of streptavidin, Fig.11 Explanation of the process for fabricating the sensing plate by thermal oxidization of the single-crystalline Si layer of the SOQ substrate.Thermaloxidationwithwater vapor482 nmThermally-grownSiO2 filmSilica glasssubstrate1000 ℃ 1h.Single-crystalline Si layerSilica glasssubstrate220 nmSingle-crystallineSi layer440 nmFig.12 Optical setup used in molecular detection using the monolithic sensing plate. Silica glass substrateSiWaveguide30°2θcuvette632.8 nmHe-Ne laserSolution containingsampleDetectorS-polarized light is selectedPolarizing plate θSilica glass prismFig.13 Reflectance property before and after adsorption of streptavidin on biotin observed using the monolithic sensing plate with nanopores (a) and without nanopores (b). The concentration of streptavidin was 1.5 µM. (b)(a)717069680.20.40.60.81ReflectanceAngle of incidence (degrees)69686766ReflectanceAfteradsorptionBeforeadsorption72646500.20.40.60.810AfteradsorptionBeforeadsorption

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