Chemical Sensing Using The Life, Death, And Rebirth Of A Photon

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Authors
Andrews, Nicholas L. P.
Keyword
Fluorescence , Excitation Emission Matrix Spectroscopy , Fibre Optics , Hadamard , Modulation , Cavity Ring-Down , Photonic Crystal Fibre , Refractive index , Sensor , Degradation Kinetics , Interferometer , Liquid Scintillator , Lubrication Oil
Abstract
In this thesis, optical sensing platforms using the “life, death and rebirth” of a photon are utilised to quantify and characterise chemicals in various samples. In this analogy, the “life” of a photon is characterised by its interactions with the propagation media, since they alter its speed and direction through the refractive index. The “death” of the photon can be observed by its absorption into an analyte, and after this process the “rebirth” of the photon can exist in the form of fluorescence. The instruments described here are based on fundamental optical techniques that were adapted for use with optical waveguides, thus allowing for compact inexpensive instrumentation that is able to characterise samples on-line and within small volumes. Characterising liquids in small volumes reduces both cost and waste, possibly increasing a company’s potential profit. This thesis presents a hollow core in-fibre photonic crystal fibre interferometer used to measure the refractive index and polarisability of gas in small volumes down to 1.2 μL. Similarly, a new amplified fibre loop cavity ring-down spectrometer is used to measure liquid alkynes through an overtone absorption band to a detection limit of 19% v/v in volumes of about 1 pL. Another instrument was built based on the well-known fluorescence excitation emission matrix (EEM) spectroscopy method. Fluorescence EEM is shown here to have great versatility in monitoring the kinetics involved in the degradation of complex fluorescent samples such as lubrication oil and liquid scintillators. When EEM spectroscopy is combined with multivariate analysis techniques, including parallel factor analysis, the full advantages of the technique are realised as all the fluorescent chemical components can be measured quantitatively. However, as it takes a long time to acquire a single EEM spectrum using commercial instruments, this method was not capable of conducting real-time on-line measurements. In this thesis we therefore developed a Hadamard-transform EEM spectrometer, which through simultaneous modulation of all excitation wavelengths, was able to acquire full EEM spectra in a fraction of the time. This instrument realises the potential of using EEM spectroscopy in real time — a first step towards the goal of a commercial on-line sensor.
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