Received October 6, 2019, accepted October 22, 2019, date of publication October 29, 2019, date of current version November 8, 2019. Digital Object Identifier 10.1109/ACCESS.2019.2950012 Bio-Based Polycationic Polyurethane as an Ion-Selective Membrane for Nitrate Tapered Optical Fiber Sensors ATIQAH YUSOFF 1 , NUR HIDAYAH AZEMAN 1 , (Member, IEEE), MUHAMMAD FAIZAL MOHAMED KASSIM 2 , NADHRATUN NAIIM MOBARAK 2 , KHAIRIAH HAJI BADRI 2 , MOHD ADZIR MAHDI 3 , (Senior Member, IEEE), MOHD SUKOR SU’AIT 4 , AND AHMAD ASHRIF A. BAKAR 1 , (Senior Member, IEEE) 1 Photonics Technology Laboratory, Center of Advanced Electronic and Communication Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Malaysia 2 School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Malaysia 3 Wireless and Photonics Network Research Center, Faculty of Engineering, Universiti Putra Malaysia (UPM), Serdang 43400, Malaysia 4 Solar Energy Research Institute, Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Malaysia Corresponding authors: Nur Hidayah Azeman (nhidayah.az@ukm.edu.my); Mohd Sukor Su’ait (mohdsukor@ukm.edu.my); and Ahmad Ashrif A. Bakar (ashrif@ukm.edu.my) This work was supported in part by the Universiti Kebangsaan Malaysia through the Human Capital under Grant MI-2018-018, in part by the Research University under Grant GUP-2017-30, and in part by the Ministry of Education Malaysia under the Fundamental Research Grant Scheme under GrantFRGS/1/2018/TK04/UKM/02/6. ABSTRACT A novel bio-based polycationic polyurethane as an ion-selective membrane for nitrate sensing was successfully developed. In this work, the intermolecular interactions at active polymeric sites play a primary role in selective nitrate-ion detection. From the experiment, FTIR shows a significant shift from 1543 cm −1 to 1548 cm −1 in N-H bending, indicating that intermolecular interactions occur between the poly- cationic polyurethane and nitrate. AFM shows that the surface roughness of the polycationic polyurethane decreases from 95.7 nm to 12.2 nm after immersion in nitrate solution. Meanwhile, FESEM images show that the bright area, which represents the hard segment of polycationic polyurethane, decreases after immersion, indicating that the nitrate is interacting with the hard segment of the polycationic polyurethane via intermolecular interaction. Furthermore, EIS shows that the conductivity increases from 2.84 × 10 −11 to 5.34 × 10 −11 S cm −1 after ion exchange occurs between the iodide and nitrate on the polycationic polyurethane. To assess the sensing performance, the sensor probe is fabricated by coating the polycationic polyurethane thin film on the tapered region of an optical fiber. Rapid detection, good repeatability, and a sensitivity of 5.94 × 10 −2 µW/ppm are obtained for nitrate detection using the above bio-based-sensing material. The selectivity study also shows that the sensing material possesses high affinity toward the nitrate ion. INDEX TERMS Optical fiber sensor, chemical sensor, ion-selective membrane, nitrate sensing, polyurethane. I. INTRODUCTION Nitrate is an ionized form of nitrogen and an important nutri- ent for all photosynthetic organisms. Because of their involve- ment in the biogeochemical cycle in water reservoirs, nitrate sensing in water is vital for monitoring and sustaining water quality conditions [1], [2]. However, the excessive consump- tion of nitrate by humans may lead to methemoglobinemia The associate editor coordinating the review of this manuscript and approving it for publication was Sukhdev Roy. disease. Furthermore, excessive amounts of nitrate can also cause harmful impacts toward the environment, such as eutrophication, algal blooms and the death of aquatic life [3]. Conventionally, nitrate detection is carried out in the lab- oratory, where the water sample is collected and preserved under a specific condition prior to analysis. Under certain conditions, an additional pretreatment process is required before the analysis [4]. The water sample is then ana- lyzed using sophisticated techniques such as atomic absorp- tion/emission spectroscopy (AAS/AES), chromatography, VOLUME 7, 2019 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ 157103