This critical review gives a short overview of the widespread use of gold nanoparticles in biology. We have identified four classes of applications in which gold nanoparticles have been used so far: labelling, delivering, heating, and sensing. For each of these applications the underlying mechanisms and concepts, the specific features of the gold nanoparticles needed for this application, as well as several examples are described (142 references).

Synthesis of ultrasmall water-soluble fluorescent gold nanoclusters is reported. The clusters have a decent quantum yield, high colloidal stability, and can be readily conjugated with biological molecules. Specific staining of cells and nonspecific uptake by living cells is demonstrated.

Colloidal semiconductor nanocrystals have been exploited in several applications in which they serve as fluorophores, because of the tunability of the wavelength of the emitted light. [1][2][3] The possibility of exactly controlling the size of nanocrystals is of great importance in the development of these materials, as this will lead to nano-objects with well-defined and reproducible properties. Whereas this goal seems to be hard to achieve with large nanocrystals, it might be viable for clusters consisting of a few tens or hundreds of atoms, as in this size regime a handful of structures can have an exceptionally high stability and therefore would form preferentially over any other combination of atoms. This concept is already well-known for several metal clusters, as for some of them several "magic" structures exist that are formed by closed shells of atoms. [4][5][6][7] Cluster molecules that can be considered as the smallest building units of semiconductors have been investigated in the past.As an example several tetrahedral cluster molecules based on the general formula [z-(where E = S or Se; M = Zn or Cd; and R = alkyl or aryl) or similar were reported some years ago. [8,9] The series was formed only by clusters containing a well-defined number of atoms, and therefore, characterized by particularly stable structures; thus, these structures can also be termed "magic-size clusters" (MSCs). Different families of almost monodisperse CdS clusters of sizes down to 1.3 nm were reported by Vossmeyer et al., [10] whereas CdSe MSCs were observed later in the solution growth of colloidal nanocrystals [11] and the various cluster sizes found were explained as arising from the aggregation of smaller clusters. Soloviev et al. synthesized and crystallized a homologous series of CdSe cluster molecules [12,13] (very similar in structure to those reported earlier [8,9] ) that were capped by selenophenol ligands. Also in many high-temperature organometallic syntheses of colloidal CdSe nanocrystals, either the transient formation of ultrasmall, highly stable CdSe clusters was noticed, [14,15] or these clusters could be isolated using size-selective precipitation. [16,17] Recently, one type of CdSe MSC has been synthesized in a water-in-oil reverse-micelle system.[18]Here, we report a method for controlling the sequential growth in solution of CdSe MSCs of progressively larger sizes. Each of these types of clusters is characterized by a sharp optical-absorption feature at a well-defined energy. During the synthesis, the relative populations of the different families of MSCs varied, as smaller MSCs evolved into larger MSCs. We can model the time evolution of the concentration of the various magic sizes using a modification of a continuous-growth model, by taking into account the much higher stability of the various MSCs over nanocrystals of any intermediate size.For the synthesis of the CdSe MSCs reported here a mixture of dodecylamine and nonanoic acid was used to decompose cadmium oxide at 200°C under an inert atmosphere. Th...

Amphiphilic polymer‐coating for nanoparticles: The facile one‐pot synthesis of a comblike polymer can be tailored in order to vary its hydrophobic side‐chains or to directly incorporate functional molecules without the need for crosslinkers. This leads to a general and robust method of phase‐transfer of hydrophobic nanoparticles to aqueous solution, as well as to their modification with functional molecules.

We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd(2+) ⇒Cu(+) ⇒Zn(2+). When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd(2+) with Cu(+) yields Cu(2)Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu(2)Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase.

Arranging anisotropic nanoparticles into ordered assemblies remains a challenging quest requiring innovative and ingenuous approaches. The variety of interactions present in colloidal solutions of nonspherical inorganic nanocrystals can be exploited for this purpose. By tuning depletion attraction forces between hydrophobic colloidal nanorods of semiconductors, dispersed in an organic solvent, these could be assembled into 2D monolayers of close-packed hexagonally ordered arrays directly in solution. Once formed, these layers could be fished onto a substrate, and sheets of vertically standing rods were fabricated, with no additional external bias applied. Alternatively, the assemblies could be isolated and redispersed in polar solvents, yielding suspensions of micrometer-sized sheets which could be chemically treated directly in solution. Depletion attraction forces were also effective in the shape-selective separation of nanorods from binary mixtures of rods and spheres. The reported procedures have the potential to enable powerful and cost-effective fabrication approaches to materials and devices based on self-organized anisotropic nanoparticles.

Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes has been demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly symmetric crystal structure. We report here a procedure based on a sequence of two cation exchange reactions, namely, Cd(2+)⇒Cu(+) and then Cu(+)⇒Zn(2+), by which we transform colloidal CdSe(core)/CdS(shell) nanorods first into into Cu(2)Se/Cu(2)S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphological and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the second step of the exchange reaction (Cu(+)⇒Zn(2+)), a large excess of Zn(2+) ions added over the Cu(+) ions present in the Cu(2)Se/Cu(2)S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.

AgBiI4 powder, crystals, and polycrystalline films were synthesized by sealed tube solid state reactions, chemical vapor transport (CVT), and solution processing, respectively, and their structural, optical and electronic properties are reported. The structure of AgBiI4 is based unambiguously upon a cubic close packed iodide sublattice, but it presents an unusual crystallographic problem: we show that the reported structure, a cubic defect-spinel, cannot be distinguished from a metrically cubic layered structure analogous to CdCl2 using either powder or single crystal X-ray crystallography. In addition, we demonstrate the existence a noncubic CdCl2-type polymorph by isolation of nontwinned single crystals. The indirect optical band gap of AgBiI4 is measured to be 1.63(1) eV, comparable to the indirect band gap of 1.69(1) eV measured for BiI3 and smaller than that reported for other bismuth halides, suggesting that structures with a close-packed iodide sublattice may give narrower band gaps than those with perovskite structures. Band edge states closely resemble those of BiI3; however, the p-type nature of AgBiI4 with low carrier concentration is more similar to MAPbI3 than the n-type BiI3. AgBiI4 shows good stability toward the AM1.5 solar spectrum when kept in a sealed environment and is thermally stable below 90 °C.