Temporal changes in signal intensity of Surface Enhanced Raman Scattering (SERS) upon laser excitation is an
interesting phenomenon in plasmonics. In-depth understanding of the phenomena is highly important especially when
developing a SERS sensor based on the intensity variation of particular Raman peak/band. One of the main challenges in
such a technique is the intensity reduction at a given location upon consecutive measurements. Previously, signal loss in
SERS measurement was attributed to the electric-field induced roughness relaxations in the SERS active surface. In such
cases, as the surface is smoothened out, signals are completely lost. In our observation, the reduction in the spectral
intensity is irreversible but never completely lost and a major part of it can be attributed to the plasmon induced heating
effect. Here, we experimentally demonstrate this effect by studying the SERS signal from four different Raman active
molecules adsorbed onto substrates that contain uniform nano-roughened bi-metallic silver/gold coating. Possible
mechanism that leads to irreversible signal loss is explained. Moreover, solutions for minimising such plasmonic heating
when developing a biosensor are also discussed.
A successful detection of inherently weak Raman signal from molecules is possible with giant enhancement of signal by the process of surface-enhanced Raman scattering (SERS). The SERS-induced enhancement is typically achieved when the molecules adsorbed onto the surface of a noble-metal substrate with nanometric roughness. Such SERS-substrate could be economically fabricated by convective assembly of polystyrene beads followed by metal deposition. The
characterization of mono-metallic substrate showed that the SERS enhancement factor increases with increasing
thickness of Ag or Au, with Ag-substrate giving the greatest SERS enhancement. However, the formation of silver oxide
layer could reduce the shelf-life of the Ag-substrate. Alternatively, Au is also used as the coating material owing to its
chemical inertness and biocompatibility. Despite the decent enhancement of the Au-substrate, Au-layer was found to be
unstable after prolonged incubation in crystal violet solution. The inherent deficiency in adhesiveness of Au to the glass
limits its use as a reliable and cost-effective substrate. In an attempt to improve the SERS-substrate, bimetallic substrate
was fabricated by depositing the Au-film, as a protective layer, on the Ag-substrate. In this case, the top layer of Au of
the bimetallic substrate remained intact after chemical treatment. Furthermore, the bimetallic substrate was shown to
give comparable level of enhancement as an Ag-substrate by choosing a proper thickness ratio of the bimetallic layers.
The result suggests that the design of bimetallic substrate could be optimized to maximize the SERS enhancement while
retaining a decent stability after laser illumination and chemical treatment. Our findings suggest that bimetallic substrates
are potentially useful for a reliable SERS-based biosensing.
Surface Enhanced Raman Scattering (SERS) technique is used as an indispensable and sensitive modality for bio-sensing
due to its ability to distinguish the analyte molecules based on their distinct 'fingerprint' spectra. One of the most
promising SERS substrates for biosensing was fabricated by coating noble metal film over orderly packed nanospheres.
However, the major challenge in developing such a sensor is to achieve reproducible SERS substrate. Here, we report a
new class of SERS substrate with ordered 3D nanostructures fabricated on silicon wafer by deep UV lithography
technique followed by bi-metallic coating of silver and gold. Compared to the substrate fabricated by conventional
nanosphere lithography, this approach allows better control of the nanostructures, which in turn gives uniform surface
roughness for the metal film to provide adequate SERS enhancement with high reproducibility. Significance of this
substrate for biomedical application was demonstrated by glucose sensing under physiologically relevant conditions.
Partitioning and localization of glucose molecules within the first few nanometers of active SERS substrate was achieved
by a self assembled monolayer (SAM) on the surface of substrate.
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