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Single-molecule surface-enhanced Raman scattering (SM-SERS) is one of the vital applications of plasmonic nanoparticles.
Trapping of plasmonic nanoparticles on metal film is extremely sensitive to three parameters: plasmon resonance of the nanoparticles, SPP resonance of the film and excitation wavelength (see Supplementary Figs 2 and 3 for the data on resonances of various metal films and nanoparticles).
For conventional optical imaging, a × 4, 0.13 numerical aperture (NA) lens was used, whereas for the SM-SERS measurements, a water immersion OBL ( × 60, 1.0 NA) was used.
The captured light was routed towards the camera or into a high resolution, confocal Raman spectrometer (Lab Ram HR, 789 mm focal length, pinhole size=400 μm).
We used a Dove prism (N-BK7, refractive index=1.519), on to which a silver-coated glass coverslip was adhered through an optically matched oil. The excitation source was a 532-nm continuous-wave frequency-doubled Nd: YAG laser (power up to 200 m W), which was p-polarized by a λ/2 plate and routed into the prism for TIR by weakly focusing lens L (focal length=150 mm).
The angle of incidence at the interface was 76.7° from the normal to the surface, which was close to the TIR angle of Ag–water interface (see Supplementary Fig. The optical image and the Raman scattered signal from the metal–water interface was collected via an objective lens (OBL).