Surface plasmon resonance (SPR) has been widely used for sensing purposes, particularly in bio-related fields. Information acquired through SPR usually comprises changes in film thickness and dielectric constant caused by substances adsorbed on the surface of the sensor substrate. Therefore, the sensitivity of the measurement might be insufficient or imprecise when sensing substances with a small molecular mass. One of the solutions to this problem is the acquisition of information on changes in the fluorescence intensity in addition to changes in the film thickness and dielectric constant. It is known that induced SPR significantly enhances the incident light energy at the surface of the sensor substrate, which leads to fluorescence from the dye on the surface. This suggests that fluorescent labels can be used for high-sensitivity biosensing.
Long-range surface plasmon (LRSP) resonance is a specific type of SPR, and the LRSP mode leads to larger electric field enhancement at the surface of the sensor substrate than in the case of normal SPR. Figure 1 shows the structure of an LRSP sensor substrate on which light with a wavelength of 375 nm is incident. The sensor substrate consists of a low-refractive-index dielectric layer (SiO2), a thin metal layer (Al), and an anti-quenching dielectric layer (SiO2). The surface of the anti-quenching dielectric layer was modified with an antigenic protein, transferrin, using a technique for fabricating self-assembled monolayers and the antigen-antibody reaction. Subsequently, anti-transferrin antibodies labeled with three types of Q-dots, each Q-dot having a different fluorescence wavelength, were specifically adsorbed on the transferrin surface of the substrate. Figure 2 shows the fluorescence spectrum of the fabricated surface measured with a fiber spectrometer while the LRSP mode was being induced at the surface of the substrate. The figure clearly reveals fluorescence peaks and a shoulder caused by the existence of antibodies labeled with the three types of Q-dots on the surface of the sensor substrate.
After adsorption of the Q-dot-labeled antibodies, the thickness of the organic layer on the surface of the sensor substrate is approximately 20-30 nm. The LRSP mode "amplifies" slight changes on the surface of the sensor substrate through the enhanced electric field. This "amplification" facilitates improvement of the sensitivity of fluorescent measurements, such as those of the fluorescence spectra with a fiber spectrometer and the fluorescent intensities at several specific fluorescent bands with color filters and photodiodes as appropriate according to each band. As an extension of this study, we will develop a system that realizes highly sensitive fluorescent detection of multimarkers related to diseases on a biochip.