Characterization of a Zinc Oxide Coated Polymer Optical Fiber for Limonene Concentration Measurement

This study evaluates the potential of evanescent wave sensing as a non-invasive technique for limonene concentration characterization, which is one of the biomarkers for liver disease detection. The project proposes the development of a fiber optic sensor as an alternative to the bio-sensing applications for limonene characterization. Chemical etching was employed to remove the cladding from the plastic optical fiber (POF) for the sensor probe design, and zinc oxide (ZnO) was coated onto the POF's core as an enhanced sensing material. An experimental setup was built to measure the output voltage from the POF sensor when the limonene concentration and refractive index were altered. The diameter of the POF sensing probe was fixed at 0, 0.45, 0.50, 0.55, 0.60, 0.65, and 0.70 mm, with a fiber length of 10 cm and an unclad length for the sensing probe of 2 cm. The results show that the output voltage decreases as the limonene concentration increases, and a smaller tapered waist diameter of the POF produces higher voltages. The coated POF sensor also has higher sensitivity than an uncoated sensor. Therefore, the ZnO coated POF sensor can be a potential low-cost and simple sensing device for limonene concentration measurement. The highest sensitivity was recorded at 0.45 mm tapered POF which is 4.026 V/RIU with a linearity of 98.76% and the slope shows good linearity of more than 99%. Additionally, it was shown that coating the sensing probe with ZnO increased the sensor sensitivity by more than 50% in comparison to the uncoated POF.