<\/a>Figure 1: Schematic of the scanning near-field optical microscope utilizing FRET and a JCCR. (A) The sample is illuminated from the front of the JCCR through an objective lens. Fluorescence is collected through the same objective and a transparent non-interfering AFM probe tip is used. (B) Magnified cartoon of the tip-sample interaction in (A). The JCCR is frequency-matched to the absorption band of the analyte to be detected (a fluorescing small molecule, nanoparticles, or protein of interest). Light is absorbed in the J-aggregate thin film, and then its energy is transferred to the analyte. A fluorescence moiety is attached to the AFM probe tip. When the tip comes within the F\u00f6rster radius of the analyte (typically 2-5 nm), fluorescence from the probe is detected in the far field.<\/p><\/div>\n
The non-destructive chemical mapping of surfaces with single-molecule sensitivity and sub-5-nm lateral resolution is an extraordinary challenge. Optical techniques such as absorption, fluorescence, or Raman spectroscopy are powerful tools for chemical detection because of the unique optical signature that each molecule possesses. However, the maximum resolution of a classical (far-field) light microscope is limited by the fundamental law of diffraction. Typically, the resolving power of visible light cannot be better than ~200-300 nm in the lateral dimensions (x and y) and ~500-800 nm axially (z) [