We describe the synthesis of water-soluble semiconductor nanoparticles, and discuss and characterize their properties. Hydrophobic CdSe/ZnS core/shell nanocrystals with a core size between 2 and 5 nm are embedded in a siloxane shell and functionalized with thiol and/or amine groups. Structural characterization by AFM indicates that the siloxane shell is 1-5 nm thick yielding final particle sizes of 6-17 nm, depending on the initial CdSe core size. The silica coating does not significantly modify the optical properties of the nanocrystals. Their fluorescence emission is about 32-35 nm FWHM, and can be tuned from blue to red with quantum yields up to 18%, mainly determined by the quantum yield of the underlying CdSe/ZnS nanocrystals. Silanized nanocrystals exhibit enhanced photochemical stability over organic fluorophores. They also display high stability in buffers at physiological conditions (>150 mM NaCl). The introduction of functionalized groups onto the siloxane surface would permit the conjugation of the nanocrystals to biological entities. 2 IntroductionRecent progress in wet chemistry has established a robust route towards the synthesis of highly luminescent semiconductor nanocrystals having sizes ranging from 1.5 to 8 nm. 1,2 By judiciously controlling the growth conditions, the size, and even the shape of II-VI nanocrystals can be accurately tailored. 3,4 The ability to control these parameters has a profound impact in materials science since it can be harnessed for engineering assemblies of nanometer scale units with novel characteristics. [5][6][7][8] At this point, the prominent focus has been the optical properties of these semiconductor nanocrystals. They are governed by strong quantum confinement effects and, therefore, are size dependent. 2,9,10 The absorption onset and fluorescence emission shift to larger energy with decreasing size.Moreover, while the absorption spectrum is a continuum from the bandgap into the UV, the emission pattern is narrow, symmetric, and does not depend on the excitation frequency. Several different sizes of nanocrystals can thus be excited simultaneously with a single excitation source, resulting in well-resolved colors of emission. Further passivation of the nanocrystal surface by a thin shell of a higher band gap material does not significantly modify the absorption and emission features but increases the nanoparticle quantum yield up to 50-70%. [11][12][13] The passivation shell also imparts an efficient photochemical stability, so that the photobleaching is reduced, and the number of photons a single nanocrystal can emit dramatically increases. The ability to discriminate many different colors simultaneously under long-term excitation holds great promise for fluorescent labeling technologies, especially in biology. 14,15 In this respect, organic dye molecules suffer from several limiting factors.First, their narrow absorption bands make it difficult to excite several colors with a single 3 excitation source. In addition, due to the large spectral overlaps between t...

Due to their interesting properties, research on colloidal nanocrystals has moved in the last few years from fundamental research to first applications in materials science and life sciences. In this review some recent biological applications of colloidal nanocrystals are discussed, without going into biological or chemical details. First, the properties of colloidal nanocrystals and how they can be synthesized are described. Second, the conjugation of nanocrystals with biological molecules is discussed. And third, three different biological applications are introduced: (i) the arrangement of nanocrystal-oligonucleotide conjugates using molecular scaffolds such as single-stranded DNA, (ii) the use of nanocrystal-protein conjugates as fluorescent probes for cellular imaging, and (iii) a motility assay based on the uptake of nanocrystals by living cells.

Thiol-modified single stranded oligonucleotides of different lengths (8 to 135 bases) were attached to the surface of 10 nm diameter Au nanocrystals with different DNA/Au ratios (1, 2, ..., saturation). The electrophoretic mobility of these conjugates was determined on 2% agarose gels, and the effective diameter of the DNA/Au conjugates was determined. This diameter depends on the conformation of the oligonucleotides adsorbed on the Au surface. For low surface coverage, nonspecific wrapping of the DNA around the nanoparticles was observed. For high surface coverage, short oligonucleotides were shown to be oriented perpendicular to the surface and fully stretched. For high surface coverage and long oligonucleotides, the inner part close to the Au surface was determined to be fully stretched and pointed perpendicular to the surface, whereas the outer part adopts random coil shape.