Vol.2 No.2 2009

67/98

Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−147−Synthesiology - English edition Vol.2 No.2 (2009) as a reflective film, we calculated the relationship between the complex refractive index n + ki of the reflective film material and the detection sensitivity. The calculation results are shown in Fig. 10. The figure shows the relationships between the maximum change in reflectance obtained when substance with thickness of 5 nm and refractive index of 1.45 was adsorbed to the waveguide surface, and the n and k of the reflective film. Here, the wavelength of incident light was 632.8 nm, and the refractive index of the substrate glass and prism was 1.769. Also, the thickness and refractive index of the waveguide was 350 nm and 1.485 respectively. The optimal values for incident angle of light, direction of polarization, and thickness of the reflective film were calculated by simulation. The material with a complex refractive index with a large change in reflectance can be the considered material suitable as the reflective film material. In the figure, the complex refractive indices of some materials are shown as black dots. From the calculation result, Au, Ag, and Cu, which are materials that have been conventionally used, showed high sensitivity. It could also be seen that Si and Ge, which are materials with large n and small k, also showed good sensitivity.Sensing plates were fabricated using some of the materials listed in Fig. 10, and observation was done using biotin-streptavidin adsorption as in the previous experiment. Streptavidin had a diameter of about 5 nm and a refractive index of 1.45, so similar results could be expected as previous calculations. Table 1 shows the reflective film materials used in the experiment, the maximum reflectance change |Rex| due to the streptavidin adsorption for each reflective film material, and the maximum reflectance change |Rcal| obtained by the above calculation. When Au was used as the reflective film, the Au layer peeled off after forming the sensing plate without the adhesive layer, and no experimental values could be obtained. The highest value of |Rex| obtained in the experiment was 0.505 when Cu was used. However, Cu, although not as bad as Au, had poor adhesiveness, and some peeling occurred. When using a Au reflective film with Cr adhesive layers with a thickness of 0.8 nm, |Rex| was 0.263. This was a relatively high value among the experimental values obtained, but the sensitivity was way lower than the calculated value |Rcal| = 0.719 when Au only was used as the reflective film. The next highest value was 0.234 when a-Si was used. Large reflectance change was expected in the calculation for Ge, but the actual sensitivity was about one-third of the calculated value. This is thought to be because the calculated value was obtained using the complex permittivity of single-crystalline Ge, while in the experiment, Ge layer was deposited using the sputtering method and the formed Ge layer was amorphous.In this research, it was found that high sensitivity could be obtained in conventionally used materials such as Au, Ag, and Cu, but there was a problem in their stability. Although stability issue could be solved to some degree by introducing the adhesive layers, the sensitivity with the adhesive layers was the same as when Si was used as the reflective film. Also since Si has extremely high adhesiveness with glass materials, Si was suitable as the reflective film material for securing both stability and sensitivity.3.3 Monolithic sensing plateFrom the above approach, it was found that our nanopore formation technology was effective in increasing sensitivity, and the Si reflective film was effective in both sensitivity and durability. However, as shown in the SEM photographs of Fig. 7, the surface of the waveguide layer formed by the sputtering method became roughened by the nanopore formation and the reflectance property deteriorated, and as a result, sufficient improvement of sensitivity could not be Fig. 9 Calculation results of incident angle dependency of reflectance when Au (a), W (b), and Si (c) are used as reflective films. The thicknesses of reflective films are 40, 20, and 30 nm, respectively.Fig.10 Calculation result of relationship between the complex refractive index of the reflective film material and detection sensitivity. The black dots in the graph show the complex refractive index of some materials that may be used as the reflective film.Angle of incidence(degrees)Reflectance57585960616200.40.800.40.800.40.850556065705055606570(a)(b)(c)Angle of incidence(degrees)Angle of incidence(degrees)Amount of change in reflectanceknMnMnTaTaMoMoTiTiPtPtNiNiAgAgGeGea-Sia-SiWWCuCuCrCrAuAu10.90.80.70.60.50.40.30.20.1000.511.522.533.544.555.55.554.543.532.521.510.50AuCr/Au/CrCrCuWa-SiGeReflective film materialNA0.1390.2340.0700.0640.5050.2630.4050.2710.0980.1010.6830.3800.719|Δ ex|R|Δ cal|RTable 1 Relationship among the reflective materials used for the sensing plate fabrication, the values of |Rex|, and the values of |Rcal|.

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