Influence of ions on the surface plasmon resonance spectrum of a fiber optic refractive index sensor

Surface plasmon resonance (SPR) spectra of a fiber optic SPR probe coated with thin gold layer have been studied experimentally for four ionic (NaCl, KCl, Na2SO4 and MgCl2) and two non-ionic (sucrose and urea) liquid samples of different refractive indices. For the same value of refractive index, the response curves for ionic and non-ionic liquid samples have been found to differ, implying the influence of ions on the resonance wavelength of the SPR spectrum. It is observed that the resonance wavelength, for a given refractive index of the liquid, is higher for ionic sample than that in the case of non-ionic one. This extra increase in resonance wavelength is attributed to the interaction of ions with the free electrons of the metal film. The study finds importance in situations where ions get generated during the processes. In such studies, the effect of ions on SPR spectrum must be incorporated otherwise the SPR spectrum may get wrongly interpreted.