Single Molecule Detection

Fluorescence images of single molecules (Figure 3) show a resolution of 100 nm (FWHM).  Scans show digital bleaching of the molecules which is consistent with single molecule detection.

fig 3a: Single DiI molecules fig 3b: Single rhodamine molecules

        (a)                                                                (b)

Figure 3. (a)  False color fluorescence NSOM image of single DiIC12(3) molecules on a glass surface.  (b)  Single molecule fluorescence excited by linearly polarized light.

The electric field at the tip consists of a propagating field and a non-propagating evanescent field ringing the aperture (at the metal boundary).  The ring like single molecule fluorescence patterns in Figure 3(b) result from excitation by the evanescent field but not from the propagating field which is polarized perpendicular to the molecule’s absorption dipole.  Molecular orientation can be calculated from the shape of the image.

Near-Field Spectroscopy