Fiber optics chemical sensors based on responsive polymers
Tou, Zhi Qiang
Date of Issue2015
School of Chemical and Biomedical Engineering
Fiber optic chemical sensors (FOCS) have made tremendous progress since developments began in the 1960s with continuous reports of new configurations, materials and applications even today. Techniques like interferometry and plasmonic resonance are set to form the next generation of FOCS, representing a deviation from conventional luminescence based techniques. The thesis presents an optic fiber sensor platform for localized surface plasmon resonance (LSPR) by using a speciality photonic crystal fiber (PCF) to excite LSPR via cladding modes. The sensor offers facile fabrication and achieves high refractive index sensitivity of -731 % transmittance/RIU with limit of detection (LOD) 1.76 x10-5 refractive index unit (RIU). Responsive polymers, exhibiting physio-chemical changes when exposed to specific stimuli are investigated for feasibility in FOCS based on LSPR and interferometry. A pH sensor is proposed by formation of a gold nanoparticles (AuNPs) embedded polyelectrolyte multilayer (PEM) consisting of Chitosan and poly(styrene sulfonate) (PSS) onto the forementioned sensor. The sensor exhibits pH responsiveness between physiological range of pH 6.5 to 8 and its behavior can be modeled by the Henderson-Hasselbach equation. The biocompatibility of the materials enables in-vivo application of the sensor. Interferometry FOCS using hydrogel as sensing material are investigated. Mach Zehnder interferometers (MZI) can be constructed from PCF. A proposed doublepass MZI shows higher Q factor and higher resolution than a single-pass MZI. By coating a thin poly(HEMA-co-DMAEM) hydrogel film, the MZI pH sensor shows a linear pH response with pH 6.75 to 8.25 (R2 = 0.986) with LOD pH 0.004. The sensor is successfully applied to the monitoring of cell culture media. A generalized approach in fabricating hydrogel based interferometric sensors is demonstrated through use of poly(vinyl alcohol) (PVA)-co- poly(acrylic acid) (PAA) hydrogel. The PVA/PAA offers tremendous ease in obtaining thin films on optical fiber via simple dip coating without need for additional cross linkers and polymerisation initiators. The carboxyl groups on PAA offer efficient coupling to a variety of aminated receptors using carbodiimide coupling to realise a myriad of sensing possibilities. The biocompatibility of both PVA and PAA also makes the proposed sensing film suitable for in-vivo applications. The sensor is demonstrated for sensing of small chemical species like Ni2+. It was found that sensitivity of the PVA/PAA is affected by the PVA/PAA ratio and duration of heat induced esterification which controls the degree of cross linking and the maximum amount of receptors that can be immobilised. The optimal PVA/PAA hydrogel is fabrication from a 12:6 wt % ratio and cross linked at 130 oC for 30 min. By modifying the PVA/PAA hydrogel with hydroxyquinoline, the sensor is able to detect Ni2+ with good sensitivity 0.214 nm/μM and LOD 1nM adequate for continuous monitoring of drinking water.