Using a domestic microwave oven and new, inexpensive precursors, a rapid and reliable synthesis of highly luminescent CdSe/ZnS NPs was developed. To evaluate the quality of our core/shell particles for varying shell thickness in comparison to that of CdSe/ZnS nanoparticles obtained commercially, the parameter fluorescence quantum yield is been used as well as a new, straightforward, thiophenol-based shell-quality test as a tool to ensure a dense ZnS shell without holes and cracks, which is a prerequisite for high luminescence and stability.

We investigated the correlation between the thickness of the ZnS shell of CdSe-ZnS quantum dots (QDs), the stability of the particles, and the fluorescence quantum yield. As a measure for stability, a new shell quality test was developed. This test is based on the reaction of the QDs with photochemically formed thiophenol radicals and communicates an imperfect ZnS shell by a rapid and complete loss of fluorescence. The quantum yield increases from less than 5% for unshelled CdSe up to 50%, with an increase in ZnS shell thickness up to 0.6-0.8 nm. At the same time, the particles become significantly more stable, as revealed by the shell test.

Low toxic InP/ZnS quantum dots (QDs), ZnS:Mn(2+)/ZnS nanocrystals and CdSe/ZnS nanoparticles were rendered water-dispersible by different ligand-exchange methods. Eventually, they were coated with bovine serum albumin (BSA) as a model protein. All particles were characterised by isotachophoresis (ITP), laser Doppler velocimetry (LDV) and agarose gel electrophoresis. It was found that the electrophoretic mobility and colloidal stability of ZnS:Mn(2+)/ZnS and CdSe/ZnS nanoparticles, which bore short-chain surface ligands, was primarily governed by charges on the nanoparticles, whereas InP/ZnS nanocrystals were not charged per se. BSA-coated nanoparticles showed lower electrophoretic mobility, which was attributed to their larger size and smaller overall charge. However, these particles were colloidally stable. This stability was probably caused by steric stabilisation of the BSA coating.

Fluorescence microscopy in combination with multiple, simultaneous labeling of biomolecules has been a key breakthrough in cell biology. However, the spatiotemporal resolution of this approach is limited by bleaching of the fluorescence label and illegitimate cross-reference of the label. CdSe-based semiconductor nanocrystals with their excellent bleaching stability would be an alternative to overcome this limitation. We therefore explored direct immunofluorescence based on nanocrystal-conjugated antibodies using plant microtubules as model. We compared two strategies of bioconjugation, covalent coupling of antitubulin antibodies to BSA-coated nanocrystals and covalent coupling to nanocrystals that were surrounded by functionalized silica shells. Both nanoparticle-antibody conjugates were used to follow the dynamic reorganization of microtubules through the cell cycle of a tobacco cell culture in double and triple staining with FITC as conventional fluorochrome and Hoechst 33258 as marker for mitotic duplication of DNA. BSA-coated nanocrystals visualized fluorescent dots that decorated the various arrays of microtubules. The specificity of the antibody was maintained after conjugation with the nanocrystals, and the antibodies correctly represented the dynamics of cell-cycle-dependent microtubular reorganization. However, this approach did not yield a contiguous signal. In contrast, silica-shelled nanocrystals visualized contiguous microtubules in the same pattern as found for the conventional fluorochrome FITC and thus can be used as labels for direct immunofluorescence in plant cells.