Optical fibre sensors with surface-immobilised fluoroionophores.

Abstract

Regular monitoring of the concentration of ionic nutrients such as Nitrate (N), Phosphate (P) and Potassium (K) ions in soils is necessary for agricultural management. Optical fibre dip sensors provide sensing platforms that have the potential to be small and flexible that can reach the root zone. This thesis contains studies towards the development of novel optical fibre soil nutrient sensors using suspended core fibres (SCFs) and Photoinduced Electron Transfer (PET) based fluoroionophores. SCFs provide long interaction path length that potentially increase the sensitivity and lower the detection limit. Furthermore it requires only nanoliters for analysis. PET fluoroionophores using 4-amino-1,8-naphthalimide as the common fluorophore can be integrated within a SCF to become optical fibre sensors in two ways. The first approach is to pre-mix the fluoroionophore with the analyte to be sensed; the second approach is to immobilise the fluoroionophore on the internal surface of SCF. Chapter 2 and 5 of this thesis has demonstrated both potential operating scenarios are feasible for cation sensing. Furthermore, both cation and anion sensing are feasible using the first approach. Surface immobilisations of the fluoroionophores on the glass materials are critical for the development of the practical SCF sensors (second approach). In order to achieve this, it is first necessary to develop techniques for the characterisation of the functionalised surfaces. Chapter 3 and 4 of this thesis has demonstrated a versatile approach of using different glass model systems and surface analysis techniques such as X-ray photoelectron spectroscopy, Time of flight secondary ion mass spectrometry (ToF-SIMS), fluorescence imaging, spectroscopic ellipsometry, atomic force microscopy for measuring parameters such as the surface density of amine groups and sensor molecules, coating coverage, surface roughness and coating thickness that represent the surface chemistry of SCF. In addition, ToF-SIMS imaging is demonstrated to reveal that the lead ions distribution could be used as a marker of surface coverage of the coating. The application of using ToF-SIMS for relative coating thickness measurement on is also demonstrated in this thesis.