Fluorescent gold nanoclusters (AuNCs) are novel promising nanomaterials for biomedical applications because of their unique physiochemical properties. We use AuNCs stabilized with mercaptoundecanoic acid (MUA-AuNCs) which show a fluorescence emission that peaks at a wavelength around 525 nm, which is in the same wavelength region as the autofluorescence of a biological cell. However, the fluorescence decay time of the MUA-AuNCs (100 ns) is much longer in comparison to the autofluorescence (3 ns). Fluorescence lifetime imaging microscopy (FLIM) is a powerful method to discriminate between emitters of different fluorescence lifetimes. We biofunctionalize these MUA-AuNCs with biomolecules that specifically bind to the receptor expressed on the cell's membrane. To get an image of the whole cell and the bound and internalized AuNCs we use cross-sectional FLIM scans in axial direction at different heights through the cell. We distinguish between specifically bound and internalized MUA-AuNCs on and in cells by means of different FLIM methods supported by element-specific scanning electron microscopy and semi-empirical simulations.
The electronic interaction of semiconductor nanocrystals with adjacent metallic nanostructures is investigated by single
nanocrystal fluorescence spectroscopy. We synthesized CdSe multishell semiconductor nanocrystals coated with silica
shells and coupled them to self-assembled gold nanoparticle films. The nanocrystals showed an average increase of the
on-time photoluminescence intensity by a factor of up to three and a decreased photoluminescence lifetime by about one
order of magnitude. In addition, we observed photoluminescence from gray states and a strong blinking suppression with
an increase of the on-state fraction from 60% to more than 90%.
The fluorescence blinking of single CdSe/ZnS nanocrystals under different experimental conditions is investigated. We show that the blinking process is very sensitive to the particle environment even if the nanocrystals are covered with a few monolayers of ZnS. Especially the presence of oxygen leads to a shortening of the on-times but leaves the off-times almost unaffected. Therefore, oxygen which is adsorbed to the surface might act as a scavenger for photo generated electrons and leaves the particle in its (positively charged) dark state.
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