A method for detecting surface plasmon resonance with high resolution (∼ 10-5 degrees or ∼ 10-8 refractive index units) and fast response time (1 μs) is described. In the method, light is focused through a prism onto a metal film on which molecules to be detected are adsorbed. The total internal reflection of the incident light is collected with a bicell photodetector instead of a single cell or an array of photodetectors that are widely used in previous works. The ratio of the differential signal to the sum signal of the bicell photodetector provides an accurate measurement of shift in surface plasmon resonance angle caused by the adsorption of molecules onto the metal films or by conformational changes in the adsorbed molecules. Using the method, we have studied subtle conformational changes in redox protein, cytochrome c, due to an electron transfer reaction.
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