Comprehensive investigation of evanescent wave optical fiber refractive index sensor coated with ZnO nanoparticles

Combination of optical fiber and semiconductor metal oxide nanostructure provide a label-free refractive index (RI) sensor with an excellent limit of detection. ZnO NPs (nanoparticles) coated evanescent based multimode optical fiber sensor is established to detect different crude oil concentrations blended in diethyl ether. The efficacy of fabricated sensors with respect to wavelength shift and intensity changes is tested with different crude oil concentration from 0% (RI: 1.35) to 100% (RI: 1.47). By increasing the crude oil concentration, the maximum shift of ~10 nm towards lower wavelength is occurred. The signal in visible area originated from the electron–hole recombination at a deep level via singly ionized oxygen vacancies (VO +). Increasing the crude oil concentration leads to decreasing the transmission intensity up to 72% of its maximum value. Changes in refractive index of ZnO nanostructure from 2.671 to 2.726 with the change in oil concentration from 0% to 100% respectively, leads to different quantity of evanescent wave absorption taken placed. Different absorption rate and reflection angle in the cladding-ZnO interface are responsible for intensity modulation. The interaction of oxygen vacancies on ZnO surface, electron migration from donor media to the valance band and lowering the band-gap with decreasing the crude oil concentration are responsible for RI modification of ZnO. Therefore, an economic, high sensitive and multipurpose dual sensing scale has been proposed for crude oil detection.