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.
Vol. 8, Iss. 6 Virtual Journal for Biomedical Optics
Pedro H.B. Aoki, Eduardo G.E. Carreon, Diogo Volpati, Milton H. Shimabukuro, Carlos J.L. Constantino, Ricardo F. Aroca, Osvaldo N. Oliveira, and Fernando V. Paulovich, "SERS Mapping in Langmuir-Blodgett Films and Single-Molecule Detection," Appl. Spectrosc. 67, 563-569 (2013)