The merits/demerits of biochemical reaction measurements by SPR reflectance signal at a fixed angle

A prospect for the observation of biochemical reaction by surface plasmon resonance (SPR) is discussed. It has been known that reactions can be monitored either by observation of resonance minimum incidence angle, or by observation of reflectance intensity change at a fixed angle near the resonance minimum. Minimum angle shift measurement is preferred because of better signal to noise ratio (S/N) achievement due to signal averaging effect, inherent in minimum angle determination. However, SPR system with intensity measurement at a fixed angle could be simpler than that with minimum angle shift measurement. The potential for biochemical measurements by SPR at a fixed angle was explored in this report. Theoretical calculation was performed to estimate the signal level of the intensity change at a fixed angle. An SPR measurement system that measures the signal intensity at eight different angles, 0.5° apart, was fabricated to verify the theoretical prediction. Rabbit IgG immunoassay was performed, and demonstrated that measurement of intensity change at a fixed angle has enough sensitivity for biochemical reaction observation. It was demonstrated that below the resonance angle the intensity change waveform of the biochemical reaction is similar to that of resonance minimum data, whereas the polarity of waveform change was reversed above the resonance angle. Merits and demerits of intensity change monitoring and resonance minimum monitoring are discussed.