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...

ABSTRACT. hcp Co disk-shaped nanocrystals were obtained by rapid decomposition of cobalt carbonyl in presence of linear amines. Other surfactants, in addition to the amines, like phosphine oxides and oleic acid were used to improve size dispersion, shape control and nanocrystal stability. Co disks are ferromagnetic in character and they spontaneously self assemble into long ribbons. X-ray and electron diffraction, electron microscopy and SQUID magnetometry have been employed to characterize this material.

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.

Colloidal nanocrystal/DNA conjugates hold the promise of becoming powerful probes for biological diagnostics as well as versatile building blocks for nanotechnology. To fully realize this potential, it is important to precisely control the number of oligonucleotides bound to the nanocrystal. Here we demonstrate electrophoretic isolation of 5 and 10 nm gold nanocrystals bearing discrete numbers of single-stranded DNA (1−5). The potential use of these discrete conjugates in the fabrication of novel nanostructures is discussed.

Water-soluble, highly fluorescent, silanized semiconductor nanocrystals with different surface charges were synthesized. To covalently attach the nanocrystals to biological macromolecules with a variety of mild coupling chemistries, the outermost siloxane shells were derivatized with thiol, amino, or carboxyl functional groups. Single-or double-stranded DNA was coupled to the nanocrystal surfaces by using commercially available bifunctional cross-linker. Conjugation had little effect on the optical properties of the nanocrystals, and the resulting conjugates were more stable than previously reported systems. By using the strategies developed in this study, most biomolecules can be covalently coupled to semiconductor nanocrystals. These nanocrystal-DNA conjugates promise to be a versatile tool for fluorescence imaging and probing of biological systems.

Discrete Au nanoparticle/DNA conjugates have been isolated by electrophoresis and used to form small groupings of particles, such as dimers and trimers. The use of purified conjugates leads to a higher yield of the target structure, and it has allowed us better control and understanding of the system. Newly accessible questions, such as the electrophoretic mobility of nanoparticle/DNA hybrids and the critical role of particle surface charge on mobility have been studied. Detailed characterization by Transmission Electron Microscopy (TEM) has now been done due to the higher quality of the samples.A computer program to generate pair distribution functions from TEM images was developed, pointing out the dependence of interparticle distance with DNA length on dimers of particles.2

The uptake of colloidal semiconductor nanocrystals by a large range of eukaryotes (see Figure) is directly correlated with the cell motility, as has been shown by comparing the motions of cancerous and healthy human breast cells. The nanocrystals are more photochemically robust than organic dyes and provide a powerful tool for studying the processes of cell motility and migration—behaviors that are responsible for metastases of primary cancers.

Steam reforming of ethanol (SRE) is a strategic reaction for H2 production. However, despite considerable work, several aspects of the mechanism and catalytic system for this reaction are not fully understood. There have been many efforts to improve the understanding of the catalysts’ behavior during SRE, using both theoretical studies and experimental investigations based on operando characterization techniques. Even though cobalt and nickel are considered the most promising catalytically active metals for industrial SRE, acquiring further knowledge on the reaction mechanism, metal–support interactions, and catalyst deactivation (due to carbon accumulation, sintering, or metal oxidation) will enable the successful design of new and stable catalysts. In this review, we analyze the reaction pathways for metal-catalyzed SRE and discuss the available experimental and theoretical data to suggest alternatives to address three major issues: (i) the impact of particle size and metal oxidation state in the SRE performance; (ii) the importance of metal surface electronic properties to obtain a balanced and stable catalyst; and (iii) the influence of support on the catalyst selectivity and stability. Clarification of these issues is a key point for understanding the SRE reaction and for the development of new high performance catalysts.