Abstract
Plasmon-enhanced spectroscopic techniques have expanded single-molecule
detection (SMD) and are revolutionizing areas such as bio-imaging and single-cell
manipulation. Surface-enhanced (resonance) Raman scattering (SERS or SERRS) combines
high sensitivity with molecular-fingerprint information at the single-molecule
level. Spectra originating from single-molecule SERS experiments are rare events,
which occur only if a single molecule is located in a hot-spot zone. In this spot,
the molecule is selectively exposed to a significant enhancement associated with a
high, local electromagnetic field in the plasmonic substrate. Here, we report an SMD
study with an electrostatic approach in which a Langmuir film of a phospholipid with
anionic polar head groups (PO<sub>4</sub><sup>-</sup>) was doped with cationic
methylene blue (MB), creating a homogeneous, two-dimensional distribution of dyes in
the monolayer. The number of dyes in the probed area of the Langmuir-Blodgett (LB)
film coating the Ag nanostructures established a regime in which single-molecule
events were observed, with the identification based on direct matching of the
observed spectrum at each point of the mapping with a reference spectrum for the MB
molecule. In addition, advanced fitting techniques were tested with the data
obtained from micro-Raman mapping, thus achieving real-time processing to extract
the MB single-molecule spectra.
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