The fluorescence of single molecules can be enhanced with a gold nano-antenna
In a metal, the excitation of the collective motion of electrons — plasmons — produces an intense electric field. Sergei Kuhn and co-workers1 at ETH in Zurich, Switzerland, have now used this electron excitation in a 100-nm gold nanoparticle to create a ‘nano-antenna’ capable of enhancing the fluorescence of a single molecule.
Kuhn and co-workers studied high quantum efficiency terrylene molecules that were vertically embedded in a 20-nm-thick matrix film. These molecules have an emission line within the wavelength range of the gold nanoparticle’s plasmon excitation spectrum, which is critical to observing the fluorescence enhancement. A terrylene molecule experienced a highly inhomogeneous electric field when the nanoparticle was brought within 10 nm of it; a distance much less than the wavelength of the radiation. Such a field has a very different effect on optical properties than excitation from a homogeneous plane wave. By scanning the gold particle in 1-nm steps across a single molecule, a 20-fold increase in the fluorescence intensity and decay rate was observed.
The experimental set-up functions as an 'apertureless' near-field optical microscope with an image resolution of 65 nm. Fundamentally, the high spatial resolution of the measurements yields detailed information about molecular optics that can be compared to three-dimensional calculations.
References
Kuhn, S. et al. Phys. Rev. Lett. (2006). 10.1103/PhysRevLett.97.017402
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Thomas, J. Plasmon Picture. Nature Nanotech (2006). https://doi.org/10.1038/nnano.2006.24
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DOI: https://doi.org/10.1038/nnano.2006.24