Parnell, Harriet
(2018)
Towards a Ge-Sb-Se/S hyperspectral imaging probe for early cancer diagnosis.
PhD thesis, University of Nottingham.
Abstract
Owing to their vitreous nature and mid-infrared (MIR) transparency, chalcogenide glasses are a promising material for remote hyperspectral imaging. For medical applications, such as in-vivo cancer diagnosis, Ge-Sb-Se glasses are a particularly interesting material since, it is believed that Sb-containing chalcogenides are less toxic than their As-containing equivalents. For passive optical fibres which aim to deliver and collect MIR light to and from tissue samples, the main challenge which faces their performance is the removal of extrinsic optical losses. Hence, this Project explores and develops high purity Ge-Sb-Se/S bulk glasses and optical fibres.
Focussing on the GexSb10Se90-x atomic % (at. %) glass series, bulk samples are initially characterised before the fibre-drawing capability of each composition is assessed. Although stoichiometric Ge25Sb10Se65 at. % and non-stoichiometric Ge20Sb10Se70 at. % glasses both exist within the same two-dimensional, overconstrained network, results from their fibre-drawing investigations reveal a significant difference in their resistance against crystallisation. Whereas, non-stoichiometric Ge20Sb10Se70 at. % is shown to produce stable optical fibres with promising low losses, it is found that stoichiometric Ge25Sb10Se65 at. % repeatedly crystallises into a single phase of monoclinic GeSe2.
To produce a low numerical aperture (NA) step-index fibre (SIF), it is suggested that a Ge20Sb10Se70 at. % core glass is paired with a Ge20Sb10Se67S3 at. % cladding glass. Substituting 3 at. % Se for 3 at. % S in the Ge20Sb10Se70-xSx at. % series, is found to increase the glass transition temperature (Tg) by 10 °C and decrease the refractive index by 0.01. It is calculated, that for a SIF consisting of a Ge20Sb10Se70 at. % core and a Ge20Sb10Se67S3 at. % cladding, the NA would be 0.25 at 3.1 µm wavelength. Co-extruded at 267±0.1 °C, and then subsequently drawn into 200 ±5 µm diameter SIF, optical loss measurements demonstrate that MIR light can be successfully guided through a large, circular Ge20Sb10Se70 at. % core. The core-cladding ratio is found to be 95 %.
Calculations using the Antoine equation are used to investigate the optimal conditions required for the bake-out of Se, S and Sb impurities prior to batching. For a high-purity Ge20Sb10Se70 at. % core glass, a distillation technique is developed using 1000 ppm wt. TeCl4 as a hydrogen getter [H] and 700 ppm wt. Al as an oxygen getter [O]. It is shown, that to successfully distil Ge-Sb-Se glass, with [H] and [O] getters, two primary challenges must be overcome. The first suggests that there must be sufficient removal of HCl(g), prior to the start of distillation, in order to avoid a vapour barrier once the silica glass distillation rig is sealed. The second advises precise temperature control, with necessary monitoring, so that there is no separation of Ge-Sb-Se material, either before or after it has distilled.
Successful distillation is eventually achieved in an open system i.e. under flowing vacuum, at a temperature close to 693 °C. Optical fibre loss measurements, conducted on 18 m length of 200 ±10 µm diameter fibre, reveal that the distillation of Ge20Sb10Se70 at. % core glass with 1000 ppm wt. TeCl4 and 700 ppm wt. Al, removes the Ge-O absorption peak at 7.9 µm and significantly reduces, if not removes, all of the Se-H peaks as well. The lowest background loss is also found as 0.44 dB/m at 6.4 µm wavelength.
As a preliminary investigation into the biocompatibility of Ge-Sb-Se glasses for medical applications, two in-vitro cytotoxicity test are explored viz.: a direct contact protocol with an alamarBlue® assay and an elution protocol with a neutral red assay. Due to contradictory results between Trial 1 and Trial 2, it is suggested that further work is required to confirm the cytotoxicity of etched vs. non-etched Ge-Sb-Se fibres.
Overall, there has been significant progress made during this Project, towards the fabrication of high purity Ge-Sb-Se/S SIFs for use in a MIR imaging probe for early cancer diagnosis.
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