A differential surface plasmon resonance sensor

A novel technique that detects only the difference in refractive index between two areas of a surface plasmon resonance (SPR) sensor is presented. Since the measurement is independent of the common index, errors due to changes in the bulk index or temperature of the sensed medium are largely eliminated. A single, monochromatic beam is twice reflected from the SPR surface, with the p- and s-polarisations interchanged between reflections. A linear response to refractive index difference is obtained by measuring the change in angle of incidence at the second reflection required to maintain minimum transmission. Measurements with salt solutions of known refractive index are used to demonstrate that this instrument can detect refractive index differences while being insensitive to changes in the refractive index common to both areas of the SPR surface. A typical noise of 1.5 × 10−6 RIU with a ±0.005 RIU refractive index difference range was achieved.