Vol.2 No.2 2009

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Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−150−Synthesiology - English edition Vol.2 No.2 (2009) sensing plate so the change in vertical direction would appear more clearly. Specifically, we reduced the thickness of the Si reflective layer.The fabricated sensing plate had a single-crystalline Si layer of a thickness of about 35 nm and a thermally grown SiO2 waveguide layer of a thickness of about 520 nm on a silica glass substrate. After modifying the waveguide surface with biotin, this plate was set in the optical setup shown in Fig. 12 to conduct the detection test using immunogold conjugate as a sample. The immunogold conjugate consists of 4 to 5 streptoavidins attached to a Au nanoparticle with diameters of 20 nm. The Au nanoparticle absorbs the incident light. We used Tris-buffered saline containing 10 pM of this sample. Figure 15 shows the reflectance property before and after the introduction of the sample. White dots show the reflectance property before introduction and black dots show the reflectance property 20 hrs after introduction. Reduction in reflectance of 0.046 was observed, indicating that the sensor successfully detected the sample at such low concentration.Next, to examine improvement in sensitivity by using dyes, we detected the capture of streptavidin by biotin after dying the streptavidin with a blue dye, Coomassie Brilliant Blue G-250. This dye has an optical absorption band at around 600 nm. The sensing plate and the optical setup used were the same as the previous experiment. The detection test was done using a PBS buffer containing 100 pM of the dyed streptavidin. Figure 16 shows the reflectance property before and after introduction of the sample. White dots show the reflectance property before introduction, and black dots show the reflectance 1 hr after introduction. In this case also, sufficiently large change in reflectance was observed. This detection sensitivity was about three orders of magnitude higher than the sensitivity of a conventional EFC-WM sensor.In the above two examples, change in the dip position accompanying the change in refractive index was thought to have occurred by the adsorption of the samples, but the amount of molecule adsorption was small in both cases, and the change in dip position could not be confirmed clearly. In this type of sensor, further improvement of sensitivity could be expected by nanopore formation. This shall be our future subject of study.In this method, when settings are done to measure only the change in depth of the dip, the detection sensitivity will not be affected by temperature change. This is because the change in refractive index of water by temperature difference causes a change only in the position of the dip, and does not accompany any change in the depth of the dip. Also, since detection is achieved by capturing the optical absorption of substances, there is almost no effect on reflectance property even if a substance without optical absorption is adsorbed. That is, even when some foreign substance adheres to the detection surface, as long as the substance does not absorb the incident light, it will not be detected. Therefore, this method is scarcely affected by adhesion of foreign substances. As described, the present method has several advantages over the conventional method.5 Composition of the researchThe flow of the development is summarized in Fig. 17. In the research for increasing the performance of the sensor, we used a synthesis method called strategic selection[15]. First, to increase the sensitivity of the EFC-WM sensor, which is the core technology, based on simulation, we selected nanoprocessing to the waveguide layer. While we were able to improve the sensitivity by conducting the nanoprocessing, new problems arose in the physical stability of the sensor and the evenness of the processed surface. To solve these problems, we returned to material selection, and selected reflective film materials from a different perspective, that is, we set adhesiveness and processing capabilities as well as sensitivity as new standards of selection. We found that Fig.15 Reflectance property before and after adsorption of streptavidin with gold nanoparticles (immunogold conjugate, concentration 10 pM) on biotin, observed using optical absorption detecting monolithic sensing plate.Fig.16 Reflectance property before and after adsorption of streptavidin dyed with Coomassie Brilliant Blue G-250 (concentration 100 pM) on biotin, observed using optical absorption detecting monolithic sensing plate. 0.50.70.60.870.770.670.870.97171.1ReflectanceAngle of incidence(degrees)AfteradsorptionBeforeadsorption 0.30.50.40.6696870717273ReflectanceAngle of incidence(degrees)0.80.7AfteradsorptionBeforeadsorption

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