Vol.2 No.2 2009

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Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−146−Synthesiology - English edition Vol.2 No.2 (2009) in PBS buffer, and the concentration was set to 100 nM. Figure 8 shows the measurement results. Figures 8(a) and 8(b) show the results of measurement using the sensing plates with nanopores and without nanopores, respectively. White dots show the reflectance property measured with filling the cuvette with PBS buffer without streptavidin, and black dots show the reflectance property after the adsorption of streptavidin on biotin when the cuvette was filled with PBS buffer containing streptavidin. In both cases, the dip caused by excitation of waveguide mode was observed, and it could be seen that the peak position shifted by adsorption of streptavidin. The amount of shift of the peak by the adsorption of streptavidin in the substrate with nanopores was 0.38°. On the other hand, the peak shift was 0.06° in the substrate without pores. It was shown that dramatic improvement in sensitivity could be obtained by forming pores. However, as shown in Fig. 8(a), the width of the peak widened and the depth decreased by the formation of nanopores. This is thought to be due to the roughening of the waveguide surface by the etching. In fact, as shown in Fig. 7, particle-like roughness could be observed on the waveguide surface after the etching. This problem was greatly improved by the development of monolithic sensing plate as explained in section 3.3.3.2 Material of the reflective filmThe sensitivity of EFC-WM sensor is greatly dependent on the optical property of the reflective film. Therefore, we conducted simulations to project sensor sensitivity for various reflective film materials, and also actually fabricated several types of sensors for demonstration.Figures 9(a), 9(b), and 9(c) show the results of the calculation of reflectance property when Au, W, and Si are used as reflective films, respectively. The refractive index of the substrate used for calculation was 1.769; the thickness of the reflective films were 40, 20, and 30 nm, respectively; and the refractive index and thickness of the waveguide layer were 1.485 and 500 nm. The incident light was s-polarized monochromatic light with wavelength of 632.8 nm. The waveguide surface was assumed to be immersed in water. Although the shape of the reflectance property differs by reflective film materials, peaks are observed in the reflectance properties in all cases. The shape of the reflectance property, that is, whether the waveform peaks upwards or downwards, is determined by the intensity of the background reflected light and the condition of resonance. The positions of these peaks shift due to substance adsorption to the waveguide surface.To learn what kind of optical property a material should have Fig. 7 SEM photographs of the surface (top) and cross section (bottom) of the sensing plate with nanopores.Fig. 8 Reflectance property before and after adsorption of streptavidin on biotin observed using the sensing plate with nanopores (a) and without nanopores (b).Fig. 6 SEM photographs of the surface (left) and cross section (right) of nanopores formed on a thermally-grown SiO2 film.200 nm200 nmSilica glass waveguideSubstrate glassAu (b)(a)5857.55756.50.30.40.50.60.70.80.9ReflectanceAngle of incidence (degrees)55.55554.5540.30.40.50.60.70.80.9ReflectanceAfter adsorptionBefore adsorptionAfter adsorptionBefore adsorptionNanopore500 nm1 µm

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