Combining AFM and FRET for high resolution fluorescence microscopy

Here we demonstrate a new microscopic method that combines atomic force microscopy (AFM) with fluorescence resonance energy transfer (FRET). This method takes advantage of the strong distance dependence in Förster energy transfer between dyes with the appropriate donor/acceptor properties to couple an optical dimension with conventional AFM. This is achieved by attaching an acceptor dye to the end of an AFM tip and exciting a sample bound donor dye through far-field illumination. Energy transfer from the excited donor to the tip immobilized acceptor dye leads to emission in the red whenever there is sufficient overlap between the two dyes. Because of the highly exponential distance dependence in this process, only those dyes located at the apex of the AFM tip, nearest the sample, interact strongly. This limited and highly specific interaction provides a mechanism for obtaining fluorescence contrast with high spatial resolution. Initial results in which 400 nm resolution is obtained through this AFM/FRET imaging technique are reported. Future modifications in the probe design are discussed to further improve both the fluorescence resolution and imaging capabilities of this new technique.