Sensitivity Improvement of Biosensors Using Si Ring Optical Resonators

We have been demonstrating label-free detection of a variety of antigen--antibody reactions using Si ring optical resonators. Although the detection of biomarkers for the diagnosis of diseases generally requires high sensitivity of the order of $10^{-9}$ g/ml, the detection sensitivity of our device is currently of the order of $10^{-6}$ g/ml. In this paper, we show that the sensitivity of $10^{-9}$ g/ml will be possible by adopting the following four strategies: (1) use of slot-type waveguides with light wavelength of 1.3 μm, (2) improvement of quality factor $Q$ of the ring resonator by smoothing the surface roughness, (3) specific adsorption of the bioreceptor protein to the resonator surface, and (4) maintaining temperature within $\pm 0.005$ °C. We have also proposed the on-chip temperature compensation method without the need for temperature control of the sample. By combining the proposed approaches, the sensitivity of the biosensor will be improved by a factor of ${>}100$, thus realizing practical application of our Si ring biosensor.