A Surface plasmon resonance (SPR) biosensor constructed with common path, heterodyne inteferometric system has been developed. The sensor ship consists of a BK7 substrate coated with gold film on which the receptor of the specific biomolecular or protein has been immobilized. The light source consisting of the s and p polarizations with heterodyne frequency of 60kHz is used to measure the phase difference between these two polarizations. Because the SPR sensor probes the changes of refractive index near the gold film (i.e. about one wave-length), the more the binding of molecules on the sensing surface results in the less sensitivity of the detection. In order to overcome this shortage, we set two quarter-wave plates before and after the SPR prism to make the sensitivity of measurement to be tunable. This sensor could detect the concentration of antibody of sheep IgG as low as several nanograms per milliliter. The results indicate that this system provides high sensitivity and is capable for detecting biomolecular interactions.
A common-path, heterodyne interferometric system for studying the phase variations under surface plasmon resonance (SPR) is presented. The reflected beam from SPR is further going into a total internal reflection device (TIR) for increasing the sensitivity by the phase shift between TE-wave and TM-wave, as described by the Fresneli's equation. With the combination of a SPR prism and a TIR prism, the system can avoid the change of direction in the output light, which is always happened when only a SPR prism has been used. An unaltered output light is convenient for the detection devices. The system utilizes a pair of orthogonally linearly polarized beams with heterodyne frequency of 60 kHz as the light source. They are perfectly collinear so that the noises resulting from the ambient conditions are greatly reduced. Compared with the technique of reflectivity variation measurement, which is widely used in traditional SPR, the phase variation measurement using common-path, heterodyne techniques is estimated to be higher in sensitivity and thus can be used as a high-sensitivity-demanded biosensor.