Silver nanoparticles and thin films for strained-silicon surface enhanced raman spectroscopy and capacitively-coupled radiofrequency heating
Corr, Stuart James (2011) Silver nanoparticles and thin films for strained-silicon surface enhanced raman spectroscopy and capacitively-coupled radiofrequency heating. PhD thesis, Dublin City University.
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The unique properties of silver nanoparticles (Ag NP) have been exploited within the fields of photonics and biotechnology over the last decade due to their wide range of novel uses and applications. The surface plasmon polariton response of Ag NPs and thin films dispersed onto nm-thin layers of strained silicon (ɛ-Si) grown on Si1-xGex virtual substrates can be utilised to provide surface enhanced Raman spectroscopy (SERS) of the ɛ-Si layer compared to the bulk Si1-xGex virtual substrate – overcoming the need to use low resolution, expensive ultra-violet Raman excitation sources. For biological applications it is thought (based on recent work on Au NPs) that Ag NPs can also be applied as effective thermal agents for cancer hyperthermia due to their ability to heat
under the influence of a high-power, radiofrequency (RF) electric field operating at 13.56 MHz – potentially allowing for single-cellular cancer treatment, eliminating the current trends of invasive and life-threatening cancer treatments such as radiation, proton, gene and immunotherapy, and generic surgical resections.
In the first instance, thin films of Ag NPs were deposited onto a variety of ɛ-Si samples (9 nm [Ge 30 %], 17.5 nm [Ge 20 %], and 42 nm [Ge 20 %]) via high pressure, direct
thermal evaporation of Ag nanopowder (source-to-substrate distance of 21 cm).
Alternatively, Ag NPs synthesised via a chemical reduction technique (polyol process) using poly(vinylpyrrolidone) as the capping agent, as well as citrate stabalised NPs,
were also dispersed onto identical samples. Both techniques were analysed for their SERS viability via micro-Raman spectroscopy (488 nm Ar+ excitation source). Where
SERS evidence is shown, enhancement factors and strain values are calculated.
Evidence is also given to shown that small ɛ-Si peak shifts may be due to point-to-point ɛ-Si strain fluctuations. Where appropriate, the plasmonic origin of the SERS effects are highlighted using SEM and UV-VIS data.
For the cell hyperthermia studies the heating efficacies of commercially available Ag and Au NPs, synthesied via citrate reduction techniques, were also investigated by
means of a capacitively-coupled RF heating system with a thermal imaging camera.
Although there was rapid heat production from the as-purchased Ag NP/buffer solutions, NPs dispersed in ultra-pure high pressure liquid chromatography water were
indeed shown to exhibit heat production, albeit on a lesser scale. The fact that buffer heating rates are greater than NP heating rates contradicts recent work as these
researchers did not take these heating attributes into account. Theoretical analysis using a metallic volumetric heating equation as a function of NP size and solution volume, combined with a modified metallic conductivity due to a reduction in the electron mean free path, allowed for close similarities between experimental and theoretical values to be drawn. These results have ultimately shown the potential for Ag NPs as single cellular cancer hyperthermic treatment agents – a technique unrivalled by any current
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