Hetero-core structured fiber optic chemical sensor based on surface plasmon resonance using Au/lipid films

In this study, a novel chemical sensor is developed based on the surface plasmon resonance (SPR) on hetero-core structured optical fibers with double-layer films of gold and lipid. Two lipids, namely, oleic acid and phosphoric acid di(2-ethylhexyl) ester (PAEE), were applied to the fiber surface. We observed different shifts of the SPR resonant wavelength and changes in the spectral width for different chemical solutions (i.e., quinine, hydroquinidine, gramine, and caffeine) whose refractive indices are close to each other. The shifts in the SPR resonant wavelength of the Au-coated sensor with oleic acid and PAEE were 22.0 and 14.5 nm for quinine, 20.7 and 12.4 nm for hydroquinidine, -3.4 and 9.2 nm for gramine, and 0 and 0.1 nm for caffeine, respectively. The Au-coated SPR sensor without a lipid membrane did not show these differences, implying different chemical interactions between the lipid and chemicals would occur and change the dielectric functions of the coated lipid films. The oleic acid-coated sensor with thin Au films provided a rapid time response within 2 s. In summary, the proposed lipid-coated sensor with thin Au films facilitates chemical sensing in a limited area, such as the rhizosphere.