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SERS Mapping in Langmuir–Blodgett Films and Single-Molecule Detection
Volume 67, Number 5 (May 2013) Page 563-569
PEDRO H.B. AOKI, EDUARDO G.E. CARREON, DIOGO VOLPATI, MILTON H. SHIMABUKURO, CARLOS J.L. CONSTANTINO, RICARDO F. AROCA, OSVALDO N. OLIVEIRA JR,* and FERNANDO V. PAULOVICH
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 (PO4−) 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.
Index Headings: Single-molecule detection (SMD); SERS; Visual data exploration.