Development of optical fibre chemical probes by oblique angle deposition
Creators: name: Jayawardhana, Sasani
Publisher: name: Swinburne
Year: 2024

description: An optical fibre chemical sensor based on the technique of surface-enhanced Raman scattering (SERS) has been developed. SERS relies on the close interaction between the target analyte and a nanostructured metal surface. This nanostructure was fabricated using the method of oblique angle deposition (OAD) under thermal evaporation. The deposition was carried out by holding the sample surface at an oblique angle to the metal vapour flux, which results in a columnar morphology of the thin film. The viability of thermal OAD for SERS substrates was initially determined by the use of a planar silicon wafer with a thermal oxide layer. It was found that a vapour deposition angle of 86X provided the optimum nanostructure for SERS performance, using thiophenol (C6H5SH) as a test analyte. Strict control of the evaporation current was required to minimise radiant heating inside the evaporation chamber, as any increase in the temperature resulted in a high variability in the SERS response across different deposition runs. This appeared to be due to increased adatom diffusivity on the nanorod surface triggered by the thermal energy, which in turn resulted in morphological changes of the nanostructure. Controlling the heating by regulating the evaporation current produced a highly reproducible SERS substrate which exhibited a repeatability of ß10% based on relative standard deviation (RSD). This insight was used to extend the technique to the tip of a standard multi-mode optical fibre. The earlier results were verified by further tests, and the SERS activity was found to increase with film thickness, with a maximum activity at a nanorod length of 260 ± 28 nm. Although the OAD nanorod structures fabricated on the fibre tip exhibited excellent SERS activity in direct excitation, the signal collection efficiency was found to be reduced down to between 8% and 1%, in the remote excitation where the substrate was excited by coupling light through the fibre. The main reason for this discrepancy was revealed to be due to an incomplete overlap between the confocal light collection area and the spatial distribution of the scattered light in the fibre core. Other factors such as optical coupling between the microscope objective and optrode sensor, waveguide distribution across the fibre core and transmission losses associated with the OAD layer were also found to contribute to the decrease in signal collection efficiency in the remote interrogation. Nevertheless, the appropriate selection of collection optics and optical fibre could result in a more efficient signal collection in the optrode sensor. The possibility of using the intensity of the Raman band (430 cm-1) of the silica as an internal standard to normalize the collected analyte signal was demonstrated. This provided a mechanism to account for the variability in the analyte signal arising due to the waveguide properties and optical misalignment. The potential for using the OAD substrate as an environmental sensor was investigated in the context of detecting atrazine, which is a common herbicide and water contaminant. In addition, issues relating to surface contamination of OAD substrates due to amorphous carbon were also investigated.