Towards chemical analysis of nanostructures in biofilms II: tip-enhanced Raman spectroscopy of alginates |
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Authors: | Thomas Schmid Andreas Messmer Boon-Siang Yeo Weihua Zhang Renato Zenobi |
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Institution: | (1) Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland |
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Abstract: | This study examines the feasibility of using tip-enhanced Raman spectroscopy (TERS) for label-free chemical characterization
of nanostructures in biological systems. For this purpose, a well-defined model system consisting of calcium alginate fibers
is studied. In a companion paper, calcium alginate fibers and their network structures were shown to be a good model for the
extracellular polysaccharides of biofilms at the nanoscale. TERS analysis of biological macromolecules, such as alginates,
is complicated by heterogeneity in their sequence, molecular weight, and conformations, their small Raman cross-section, and
the large number of functional groups, which can chemically interact with the silver surface of the tip and cause significant
band shifts. Due to these effects, Raman frequencies in TERS spectra of biopolymers do not necessarily resemble band positions
in the normal Raman spectrum of the bulk material, as is the case for less complex samples (e.g., dye molecules) studied so
far. Additionally, analyte decomposition due to laser heating can have a significant influence, and carbon contamination signals
can sometimes even overwhelm the weak analyte signals. Based on the investigation of alginates, strategies for spectra correction,
choice of appropriate reference samples, and data interpretation are presented. With this approach, characteristic frequency
ranges and specific marker bands can be found for biological macromolecules that can be employed for their identification
in complex environments.
Figure TERS spectrum of a calcium alginate fiber bundle |
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Keywords: | Alginate Biological samples Biofilm Tip-enhanced Raman spectroscopy (TERS) Atomic force microscopy (AFM) |
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