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.