Determination of Resonance Raman Cross-Sections for
Use in Biological SERS Sensing with Femtosecond Stimulated Raman Spectroscopy
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Abstract
Surface-enhanced Raman spectroscopy
(SERS) is a promising technique
for <i>in vivo</i> bioanalyte detection, but accurate characterization
of SERS biosensors can be challenging due to difficulties in differentiating
resonance and surface enhancement contributions to the Raman signal.
Here, we quantitate the resonance Raman cross-sections for a commonly
used near-infrared SERS dye, 3,3β²-diethylthiatricarbocyanine
(DTTC). It is typically challenging to measure resonance Raman cross-sections
for fluorescent dye molecules due to the overwhelming isoenergetic
fluorescence signal. To overcome this issue, we used etalon-based
femtosecond stimulated Raman spectroscopy, which is intrinsically
designed to acquire a stimulated Raman signal without strong fluorescence
or interference from signals resulting from other four-wave mixing
pathways. Using this technique, we found that the cross-sections for
most of the resonantly enhanced modes in DTTC exceed 10<sup>β25</sup> cm<sup>2</sup>/molecule. These cross-sections lead to high signal
magnitude SERS signals from even weakly enhancing SERS substrates,
as much of what appears to be a SERS signal is actually coming from
the intrinsically strong resonance Raman signal. Our work will lead
to a more accurate determination of SERS enhancement factors and SERS
substrate characterization in the biologically relevant near-infrared
region, ultimately leading to a more widespread use of SERS for biosensing
and bioimaging applications