Low-concentration microsubstance detection technology is essential to ensure that our lives are safe and secure. Clinical consultations require the identification of substances causing a symptom in order to determine the appropriate treatment. It is also necessary that lifestyle diseases be detected before they develop. As is apparent from the recent pandemics of H1N1 influenza and foot-and-mouth disease, it is important to promptly detect viruses, fungi, and pollutants harmful to humans or livestock on site so as to eliminate them and prevent their spread. The sensor used for such detection must be highly sensitive, precise, easily transportable, easy-to-use, and inexpensive.

Figure 1 shows the optical arrangement in our waveguide-mode sensor under development^[1]. The sensor chip has a single-crystalline Si layer with a thickness of several hundred nanometers and a SiO₂ waveguide layer with a thickness of around 400 to 500 nm on a SiO₂ glass substrate^[2]. The equipment irradiates s-polarized visible light toward the sensor chip placed on the bottom face of the prism, as shown in the figure, and detects changes in the intensity of the reflected light. The properties of the reflected light are sensitive to the permittivity of the waveguide surface. Therefore, the reflected light intensity will change significantly if a substance (e.g., an antibody) that has the tendency to adsorb a specific material is introduced on the chip surface and if the specific material (e.g., an antigen) is actually adsorbed by the substance.

We have successfully improved the detection sensitivity by a digit or more by forming 5 × 10⁹ vertical holes, each with a diameter of approximately 50 nm, per square centimeter on the SiO₂ waveguide layer by means of our unique nanohole creation technology^[2]-[4]. The waveguide-mode sensor is also sensitive to colors^[5]. Therefore, high-sensitivity detection is achieved by marking specimens with dyes or metallic nanoparticles. When gold nanoparticles with a diameter of 20 nm were used as the marker, we found that the adsorption of one gold nanoparticle per square micrometer of the chip surface was adequate for detection^[5].

We have significantly downsized our equipment by using the spectral readout method instead of the angle scanning method that changes θ in Fig.1. Figure 2 shows a photograph of a prototype of the equipment. We are attempting to downsize the equipment to the size of a pencil case.

We have successfully detected oil, vitamins, protein, flu viruses, metallic nanoparticles, metallic nano-thin films, and Escherichia using our waveguide-mode sensor. Making use of the expertise that we have gained, we intend to develop a sophisticated marker measurement sensor to prevent lifestyle diseases.