Vol.2 No.2 2009

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Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−144−Synthesiology - English edition Vol.2 No.2 (2009) its advantages of having no limitations to the reflective substance and of being able to use both s and p waves. The greatest reason is because the absolute value of change of resonance angle at the time of molecule adsorption is small compared to the SPR sensor. However, the EFC-WM sensor has a sharp band of resonance angle, and therefore can sense large changes in reflectance property at a small angle change. Also, compared to the SPR sensor, the preparation of the EFC-WM sensor is complex since the waveguide layer must be created. However, we thought the key to increasing sensitivity was devising this waveguide layer. If the sensitivity of the EFC-WM sensor can be raised double to triple digits by using these features, it will have sufficient sensitivity as a molecule adsorption sensor.Other than sensitivity, stability in the environment in which the sensor is used, particularly in room temperature, is required. Highly sensitive detection method is affected readily by the environment because of its high sensitivity. In general, biomolecules are dissolved in water in some form such as in blood, urine, or buffer solution. Since the permittivity of water changes according to temperature, the sensor that detects the change in permittivity as in the SPR or EFC-WM sensor is extremely unstable against temperature. In developing a highly sensitive sensor, solving the problem of temperature stability is a major issue.When developing the EFC-WM sensor as an effective sensing method usable in the medical field, there are various requirements in its performance. However, unlike the SPR sensor which can be used only with material that produces SPR, the EFC-WM sensor has higher degree of freedom where any material, as long as it reflects light, can be used as the reflective film, and any material can be used as the waveguide layer, as long as it is a transparent film. There are several areas that can be devised to increase sensitivity and performance. Therefore, we drew the following scenario to increase the sensor performance.2.2 Scenario to increase the performance of the EFC-WM sensorFirst we looked at the structure of the waveguide. Figure 3 shows the simulation result of the electric field profile in the waveguide when a waveguide mode is excited therein. Here the incident light was s-polarized light with wavelength of 632.8 nm, the refractive index of substrate glass was 1.769, the reflective film was Au with thickness of 40 nm, and the waveguide layer was silica glass with thickness of 500 nm. The wavelength surface was immersed in water. The incident light was incoming from the left side of the figure. As shown in the figure, the electric field was strong inside the waveguide, while it became weak at the surface. If the molecules could be guided to the area with a strong electric field, they could be detected sensitively. Therefore, we considered forming pores in the waveguide layer and guiding the sample into the waveguide[7][8]. Pore formation will increase the surface area of the waveguide and will increase the number of adsorbed samples. This is also expected to contribute to higher sensitivity. The size of the pore should be sufficiently smaller than the wavelength of the incident light to prevent scattering. Therefore, when visible light is used, the diameter of the pore should be several tens to 100 nm. Since greater increase in surface area can be obtained by deepening the pores, the pores should be deep so they penetrate the waveguide layer. As a method for forming nanopores with such small diameters and high aspect ratio, we used selective etching by hydrofluoric acid (HF) vapor of latent tracks formed by irradiation with swift heavy ions[9]. Using this technology, it was possible to form nanopores with diameters of several 10 nm and aspect ratio of 40 or more[8].Next we reviewed the materials for the sensing plate, particularly the reflective film[5]. In the conventional EFC-WM sensor, there are many reports of using noble metals such as Au and Ag for the reflective film[10][11]. High sensitivity has been obtained by using such materials. However, these metals have extremely poor adhesion with the glass substrate and plastic substrate used in the EFC-WM sensor or the dielectric layer used as waveguide layer, and have problems of easily peeling away. Therefore, it is necessary to introduce an adhesive layer to maintain high reliability for practical use, but addition of an adhesive layer produces problems of reduced sensor sensitivity, increased cost, and increased manufacturing error. Using optical simulation, we investigated what kind of optical properties a material should have for the EFC-WM sensor, and conducted a comparison of sensor performance by fabricating various sensing plates.Based on the results obtained by the above-mentioned development, we found the silica glass created by thermal oxidation of Si was appropriate as waveguide for nanopore formation, and Si was appropriate as a reflective film. Based Fig. 3 Simulation result of the electric field profile in a waveguide in which a waveguide mode is excited.Field intensity (arb. unit)x (µm)z (µm)WaterAuWaveguideSubstrateglass00.050.10.150.20.200.40.60.8-0.2151050

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