This thesis is concerned with the label-free detection of small molecules and
macromolecules using enhanced Raman spectroscopic techniques: Surfaceenhanced Raman Spectroscopy (SERS) and photo-induced enhanced Raman
spectroscopy (PIERS). The work focuses on the development of reproducible
and stable plasmonic nanoparticles (NPs) incorporated into flexible substrates
to realise their SERS potential. The materials have been designed to fulfil the
criteria of flexible and sensitive substrates in many applications with an
emphasis in the biomedical field, especially within wound healing. A number of
measurements on varying molecules were conducted to assess the SERS
efficacy of the substrates. Most of the novel research presented in this thesis is
proof-of-concept, laying the framework for further work.
Chapter I highlights the importance of SERS and potential of PIERS with a
thorough evaluation of the literature. Motivations behind the research as well as
the aims of this work are addressed. Theoretical concepts are introduced with a
touch on the mathematical background. Key principles and parameters
influencing SERS and considerations are all covered to give a complete outline
of the topic.
The next few chapters focus on the results of this thesis. Chapter II provides
synthetic routes for gold nanoparticles and includes a comparative study on the
influence on nanoparticle shape on their subsequent SERS function. The
effects of capping agents on the shape, size and stability of the NPs are
examined. These are fully characterised and functional SERS testing is carried
out using a range of molecules to fully evaluate their SERS ability. Surface
modification is also attempted in this work with great concern given to
overpowering solvent effects. Several biomarkers are introduced with qualitative
SERS analysis. Duplex testing was performed to determine the specificity and
sensitivity of the gold NPs.
Chapter III starts investigating flexible materials as SERS substrates. These are
readily available, low cost and biocompatible materials with gold NPs
incorporated into them to become SERS active. Simple methods for synthesis
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and data collection are presented as well as their SERS results. Paper and
PDMS, whilst having some advantages lack clear Raman enhancement with
inherent material signals dominating the spectra. Gelatine based hydrogels are
chosen as an ideal candidate for SERS substrates. Many gelatine/polymer
blends are synthesised and characterised to find the optimum gel regarding
both mechanical and chemical permanence as well as SERS capability. Studies
looking into cross-linking agents to establish the best synthetic protocol are
executed. There is always a trade-off between highest functional efficiency and
a more ‘green’ and sustainable approach. The PVA/hydrogel nanocomposite
material exhibits valuable SERS whilst maintaining a facile methodology for
synthesis and data acquisition.
Chapter IV extends the SERS studies to biological macromolecules such as
proteins and wound biomarkers. Both colloidal NPs and hydrogels are used as
substrates. Concentration studies are conducted and attempts at correlating
intensity with concentration are made, to limited realization. Real samples from
patients are probed with patterns and trends hypothesised. These are
corroborated against photographic evidence of the wound. Preliminary machine
learning is employed to help classify the data with suggestive, positive results.
Chapter V inspects PIERS on biomolecules with varying degrees of
enhancements, but low concentration detection of glucose is reported. Different
TiO2 substrates are investigated for their influence in the PIERS effect and are
characterised appropriately. The preparation method, film thickness and wetting
behaviour of the property are likely parameters that can impact the PIERS
property of the substrate. Comparisons between SERS and PIERS reveal
complex mechanistic considerations between the two phenomena. As a novel
technique the possibilities to fully realise the potential of PIERS is yet to be
explored, but there is a lot more research to be done to fully understand the
effect.
This thesis concludes with a summary of the key findings from the experimental
work and presents possible avenues for further research to consolidate and
advance the current work.