Faiz, Fairuza (2019) Detection of Perfluoroalkyl Compounds with Polyvinylidene Fluoride Coated Optical Fibre. PhD thesis, Victoria University.

Abstract

Perfluoroalkyl substances (PFAS) are a class of man-made chemicals with many uses from fire-fighting foams to surface coatings and other industrial applications. In recent years, PFAS have gained considerable attention within the scientific community and the global media alike. Due to their strong chemical bonds, PFAS are inherently non-biodegradable and therefore persist in the environment. Listed in the Stockholm Convention of Persistent Organic Pollutants, these chemicals have been linked to various health issues in both humans and animals, lately, which are resulting in millions of dollars’ worth of expenses in health care and compensation for the governments of affected countries every year. In addition, the cost of cleaning up PFAS from the environment along with these human costs exceeds $50 billion in Europe alone. There is an urgent need for a portable sensing system to detect PFAS in the environment, including the most common types, perfluorooctanoic acid (PFOA) and perfluorooctane sulphonate (PFOS). At present, the methods available to accurately measure the concentration of PFAS in contaminated samples involve field sampling followed by laboratory-based, time consuming analytical techniques, such as liquid/gas chromatography and tandem mass spectrometry that are unsuitable for real-time field measurements. Existing portable systems have not yet attained the precision of the analytical methods and face challenges in field tests due to various limitations, including lack of specificity, cross sensitivity to environmental conditions and generation of toxic waste. Therefore, this research has focused on providing a proof of principle of a Fabry- Perot Interferometry (FPI) based optical fibre sensor for in situ detection of PFOA in aqueous solution. It has aimed at characterizing the potential of the envisioned PFAS sensing technique to obtain accurate and real time measurements. The proposed research capitalised on the numerous practical advantages offered by optical fibre sensors and the ability of an integrated polyvinylidene fluoride (PVDF) coating at the fibre end-face to respond to the presence of PFOA. To the best of the author’s knowledge, PVDF was experimentally shown to respond to the emerging contaminants for the first time in this work. A novel approach of forming a thin PVDF film (or Fabry-Perot etalon) on an optical fibre end-face was developed and thoroughly characterized. The thermoplastic polymer PVDF, known for its many useful characteristics such as hydrophobicity, corrosion resistance and ferroelectricity, was considered a robust sensor material based on its performance related to water filtration membranes and other engineering applications. This work has reported the synthesis of optimized coating on optical fibres through the immersion precipitation technique and has discussed subsequent experiments with the fabricated PFAS detector that demonstrated reproducible changes in the FPI spectrum in the presence of PFOA. Successful detection of analytes or their change in concentration was denoted by spectral shifts on the obtained FPI reflection spectra. A variation in the optical path difference (OPD) determined through numerical modelling provided a measure of sensitivity of the FPI based system to the different PFOA solutions. Analytical tests confirmed that PVDF adsorbs PFOA by measuring the concentration of PFOA in solution before and after a PVDF film was immersed for several hours. Visual evidence in the form of scanning electron microscopic images also displayed differences in the surface structures of PVDF thin films that were exposed to PFOA. These results supported the inference that the changes in OPD were due to the adsorption of PFOA on the PVDF coated optical fibre. In addition to the solutions containing known amounts of PFOA, real industrial solutions containing residual fire-fighting foam from fire trucks were tested and showed successful detection at low levels. In this case, the solutions contained a mixture of PFOA and PFOS with a range of other PFAS compounds that are typically used in these foam formulations. Further investigation involving alternative optical methods employing refractive index- based measurement utilizing an etched fibre Bragg grating (EFBG) and also a bare optical fibre, showed spectral response to change in PFOA concentration in solution. A shift in the EFBG spectrum due to change in PFOA concentration in the solution in which PVDF membranes had been soaked indicated that the analyte was adsorbed by the polymer. Similarly, a change in the reflection intensity of the signal obtained by a bare fibre end-face in the PFOA solution due to a change in its refractive index, indicated the adsorption of the fluoro surfactant on the PVDF thin film. Following the confirmation of PFOA uptake by PVDF, functionalization of the polymer was also investigated, which revealed that the incorporation of zeolites into PVDF enables more PFOA from aqueous solution to be adsorbed onto the thin film. Fourier transform infrared spectroscopy was used to highlight structural differences in the doped coatings, whereas energy dispersive x-ray spectroscopy was used to show compositional differences between the doped and non-doped PVDF thin films. Thus, this research contributed to the prospect of developing a functionalized sensor for more efficient detection of PFAs while also creating opportunities for further research in water treatment. A temperature characterization test, which was undertaken to eliminate possible signal cross-sensitivity effects, also indicated that the PVDF coated fibre can be considered for thermometric applications due to its good repeatability and linearity of the measurements over a specified temperature range. More importantly, information gathered from the undertaken characterization test was used to optimize the PFOA sensing protocol in order to obtain reliable results. This research has provided experimental evidence to support that a PVDF coated optical fibre can be used as a potential portable PFAS detector. It has demonstrated a novel and simple thin film fabrication and optimization process for selective detection of the emerging contaminant PFOA. Furthermore, by combining an interferometry type optical fibre sensing technology with aqueous PFAS detection this research has established the foundation for future studies that can lead to commercialisation of a portable PFAS sensor for wide-ranging environmental and engineering applications.

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