Photostabilization of fluorescent molecules  attached to nano­structured surfaces

Bleaching is an often unwanted effect in biological imaging involving fluorescent labels. The rate of bleaching is proportional to the excited state lifetime; the more time a fluorescent molecule spends in an excited state, the more susceptible to bleaching. In the presence of metallic nanostructures, a radiationless energy transfer from dye molecules to the metal particles causes a quenching of fluorescence. This quenching competes with a higher excitation rate due to an electromagnetic field enhancement by the excitation of plasmons in the metal structure. A combination of both effects can result in an increase or decrease of the fluorescence.

Altered fluorescence emission efficiencies have been investigated for ensembles of noble-metal particles and for single molecule/single particle pairs. We showed that the fluorescence quenching induced by an array of gold nanoparticles causes a decrease in bleaching rate and an increase in the total photon yield. This increase in photo-stability can be used to generate a contrast between bound and unbound molecules.

Currently we analyze the influence of (nano-)¬structured gold surfaces on the bleaching time of fluorescent molecules in close vicinity. The gold surface are patterned on a micrometer scale by physical vapor deposition, or nanometer-sized gold particles are arranged in an ordered nanopattern by means of self-assembly of diblock copolymer micells. These studies include variation of the diameter of the gold particles in a wide range (5 – 20 nm), particle arrays with different inter-particle distances (30 – 150 nm), different binding lengths between the gold particle and fluorescent molecules (2 – 16 nm), as well as dye molecules with different excitation wavelengths.

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