Bio-Based Polycationic Polyurethane as an Ion-Selective Membrane for Nitrate Tapered Optical Fiber Sensors

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 polycationic 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.

Polysaccharides as the Sensing Material for Metal Ion Detection-Based Optical Sensor Applications

“…The vast advantages of using an optical fiber sensor technique for chemical and biosensing have been attested for the analysis of different analytes [ 2 , 76 , 77 ]. This technique is cost effective, resistant to electromagnetic interference and more importantly, it can be applied in situ.…”

“…In chemical sensing and biosensing, the use of an ordinary optical fiber limits the interaction of propagating light with the target analyte due to the thick core-cladding diameter of the optical fiber, hence lowering the sensor sensitivity [ 2 , 76 , 77 ]. This limitation can be improved by tapering the optical fiber structure, hence producing a thinner core-cladding diameter of the optical fiber and enabling a high interaction between the propagating light and the chemical analyte, leading to higher sensor sensitivity.…”