Modeling and analysis of fiber optic sensors

Fiber optic-based sensors have many advantages over the traditional ion-selective electrodes. They are robust and show negligible drift. In addition, they are capable of detecting changes with high precision. These features make fiber optic sensors especially suitable for on-line and remote industrial applications. A fiber optic pH sensor consists of an indicator dye immobilized in a polymer matrix. The pH of the solution is related to the relative fraction of the protonated and the dissociated species of the dye, whose absorption spectrum changes according to pH of the surroundings. The response of the sensor is determined by the dissociation equilibrium of the immobilized dye and its interaction with the polymer matrix and the solvent. In order to have meaningful pH measurements it is important to understand the sensing mechanisms and the corresponding absorption spectra.; In this dissertation, we investigate and model the physico-chemical phenomena taking place in the pH sensitive polymer membranes during the pH measurements. The modeling effort identifies the key parameters that affect the pH measurements and provides guidelines for the design of optical sensors. A kinetic model is presented to account for the effects of environmental factors (i.e. ionic strength, temperature, solvent composition, dye buffering capacity) on the {dollar}pKsb{lcub}a{rcub}{dollar} of indicator dyes. It is demonstrated throughout that the {dollar}pKsb{lcub}a{rcub}{dollar} differences observed between sensors can be correlated to the immobilized dye concentration. A calibration and interpretation model is thus formulated such that the titration behavior of any sensor can be predicted based on the {dollar}pKsb{lcub}a{rcub}{dollar} of a reference sensor. Our proposed measurement model, which is based on analyzing the complete recorded spectrum, includes corrections for other optical effects, such as scattering and changes in the optical path due to swelling of the polymer matrix. The combined interpretation and measurement models proposed in this dissertation enable optical sensors to produce highly reliable and quantitative pH measurements.; With the understanding of the sensing mechanisms and the interactions of the reagent with the polymer matrix and solvent, it is expected that optical sensors for detecting other chemical species can also be designed and optimized for on-line, non-invasive measurements.