Development of graphene oxide coated long period grating based fibre optic sensors

Graphene, a “wonder material”, is the world’s thinnest, strongest, and stiffest material, as well as being an excellent conductor of heat and electricity. Graphene oxide (GO), a derivative of graphene with oxygen containing groups at its basal plan and its edges, has gained significant attention as a sensing material in chemical and biochemical sensing due to the extremely rich surface chemistry that it possess in comparison with other graphene nanomaterials. On the other hand, optical fibres have emerged as a major sensing mechanism due to their attractive features such as, ability to provide remote sensing, multiplexing capability, resistant to harsh environments, small size and light weight, biocompatibility and immune to electromagnetic interference. Thus, there lies an opportunity to combine the advantages of fibre optic sensors with unique characteristics of GO to develop novel sensor systems. In this regard, if a thin film overlay of GO is deposited on a Long Period Gratings (LPGs) surface, the Refractive Index (RI) of the coated GO overlay will change with perturbations in the surrounding medium, which will be reflected in the transmission spectrum of the GO coated LPG, forming the basis of a graphene nanomaterial-based fibre optic sensor system. This thesis reports the design and development of a suite of GO coated LPG based external RI sensors in the fields of sodium chloride (NaCl) salinity measurement in water, Relative humidity (RH) measurements in structural health monitoring and Bovine Serum Albumin (BSA) concentration measurement in biosensing, all of which illustrate the versatility of the GO coated LPG sensor approach. An external RI sensor system which contained GO coated LPG as the basis was developed and its performance was analysed with varying overlay thicknesses of GO to optimise the sensor design. The developed sensor system increased the external RI sensitivity of an uncoated LPG by 83%, which highlighted the enhanced RI sensitivity achieved by GO overlays on fibre gratings. Surrounding RI variations were achieved with the use of NaCl solutions with varying concentrations, which successfully established the NaCl salinity measuring capability of the developed GO coated LPG sensor probes. These promising results indicated that GO coated LPGs provide an efficient and stable basis for the development of more species-specific, highly sensitive chemical and biochemical sensor systems. A satisfactory linear response of a GO coated LPG based RH sensor probe was achieved in the RH region of 60%RH - 95%RH, reporting a sensitivity of 0.15 dB/%RH at room temperature. Simultaneous measurement of temperature and RH levels was achieved and by this, the GO-coated LPG sensor probe has shown that it has the potential to be developed into a temperature compensated fibre optic humidity probe by optimising its temperature response in the operational RH range of 60%RH – 95%RH. A novel but simple method of measuring BSA protein concentration in a label-free manner has been introduced by depositing a thin layer of GO on a Dual Resonance Peak LPG (DLPG) surface. Strong binding affinity of GO with protein molecules was successfully established by analysing the BSA concentration detection performance of the fabricated GO coated DLPG sensor probe. A limit of detection (LOD) of 0.9 μg/mL was achieved, which proved to be 11 times better in sensitivity than that of a GO-coated LPG sensor probe. A highest sensitivity of 22 nm/(g/mL) was achieved by the dual peak separation of the GO coated DLPG sensor probe in the BSA concentration region from 0.2 g/mL to 0.8 g/mL. These promising results indicated that the GO coated DLPG sensor probe developed in this work has the ability to be used as highly sensitive, label-free biosensing platform, which could also be made species-specific by modifying the GO surface with other species-specific functional groups. The research concludes by considering future directions for the work conducted in this thesis