Ultrafast FRET at fiber tips: Potential applications in sensitive remote sensing of molecular interaction

• A FRET based sensor using a sensitized fiber tip is developed and characterized.
• Picosecond resolved TCSPC validates actual FRET in the sensitized fiber tip.
• Background fluorescence is eliminated in the time domain measurement strategy.
• Molecular picture of DNA adsorption/desorption on the fiber tip is explored.
• Remote sensing and reusability of the sensitized fiber tip have been established.

Förster resonance energy transfer (FRET) strategy is well adopted in fiber-optics for efficient sensor design. However, resonance type energy transfer from one molecule (donor) to other (acceptor) should meet few key properties including donor to acceptor energy migration in non-radiative way, which is hard to conclude from simply emission quenching of the donor, rather needs careful investigation of excited state lifetime of the donor molecule. In the present study, we have shown that the evanescent field of an optical fiber can be coupled to covalently attached donor (dansyl) molecules at the fiber tip. By using picosecond resolved time correlated single photon counting (TCSPC) we have demonstrated that dansyl at the fiber tip transfers energy to a well known DNA-intercalating dye ethidium upon surface adsorption of DNA at the fiber tip. Our ultrafast detection scheme selectively distinguishes the probe (dansyl) emission from the intrinsic emission of the fiber. The validation of the energy transfer mechanism to be of resonance type (FRET), allows us to estimate the distance between the probe dansyl and the surface adsorbed DNA. We have also used the setup for the remote sensing of the dielectric constant (polarity) of an environment as the excited state lifetime of the probe dansyl heavily depends on the polarity of the immediate host environment. FRET signal from a used fiber tip immediately after adsorption of DNA reveals stepwise surface desorption of the biomolecule in saline solution. The reusability of the fiber tip for sensing has also been demonstrated.