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The knowledge of the quantum dot (QD) concentration in a colloidal suspension and the quantitative understanding of the size-dependence of the band gap of QDs are of crucial importance from both applied and fundamental viewpoints. In this work, we investigate the size-dependence of the optical properties of nearly spherical wurtzite (wz) CuInS 2 (CIS) QDs in the 2.7 to 6.1 nm diameter range (polydispersity ≤10%). The QDs are synthesized by partial Cu + for In 3+ cation exchange in template Cu 2- x S nanocrystals, which yields CIS QDs with very small composition variations (In/Cu = 0.91 ± 0.11), regardless of their sizes. These well-defined QDs are used to investigate the size-dependence of the band gap of wz CIS QDs. A sizing curve is also constructed for chalcopyrite CIS QDs by collecting and reanalyzing literature data. We observe that both sizing curves follow primarily a 1/ d dependence. Moreover, the molar absorption coefficients and the absorption cross-section per CIS formula unit, both at 3.1 eV and at the band gap, are analyzed. The results demonstrate that the molar absorption coefficients of CIS QDs follow a power law at the first exciton transition energy (ε E 1 = 5208 d 2.45 ) and scale with the QD volume at 3.1 eV. This latter observation implies that the absorption cross-section per unit cell at 3.1 eV is size-independent and therefore can be estimated from bulk optical constants. These results also demonstrate that the molar absorption coefficients at 3.1 eV are more reliable for analytical purposes, since they are less sensitive to size and shape dispersion.

Synthesis protocols for anisotropic CuInX 2 (X = S, Se, Te)-based heteronanocrystals (HNCs) are scarce due to the difficulty in balancing the reactivities of multiple precursors and the high solid-state diffusion rates of the cations involved in the CuInX 2 lattice. In this work, we report a multistep seeded growth synthesis protocol that yields colloidal wurtzite CuInS 2 /ZnS dot core/rod shell HNCs with photoluminescence in the NIR (∼800 nm). The wurtzite CuInS 2 NCs used as seeds are obtained by topotactic partial Cu + for In 3+ cation exchange in template Cu 2- x S NCs. The seed NCs are injected in a hot solution of zinc oleate and hexadecylamine in octadecene, 20 s after the injection of sulfur in octadecene. This results in heteroepitaxial growth of wurtzite ZnS primarily on the Sulfur-terminated polar facet of the CuInS 2 seed NCs, the other facets being overcoated only by a thin (∼1 monolayer) shell. The fast (∼21 nm/min) asymmetric axial growth of the nanorod proceeds by addition of [ZnS] monomer units, so that the polarity of the terminal (002) facet is preserved throughout the growth. The delayed injection of the CuInS 2 seed NCs is crucial to allow the concentration of [ZnS] monomers to build up, thereby maximizing the anisotropic heteroepitaxial growth rates while minimizing the rates of competing processes (etching, cation exchange, alloying). Nevertheless, a mild etching still occurred, likely prior to the onset of heteroepitaxial overgrowth, shrinking the core size from 5.5 to ∼4 nm. The insights provided by this work open up new possibilities in designing multifunctional Cu-chalcogenide based colloidal heteronanocrystals.

The localized inner 4f shell transitions of lanthanide ions are largely independent of the local surroundings. The luminescence properties of Ln 3+ ions doped into nanocrystals (NCs) are therefore similar to those in bulk crystals. Quantum size effects, responsible for the unique size-dependent luminescence of semiconductor NCs, are generally assumed not to influence the optical properties of Ln 3+ -doped insulator NCs. However, phonon confinement effects have been reported to hamper relaxation between closely spaced Stark levels in Ln 3+ -doped NCs. At cryogenic temperatures emission and excitation from higher Stark levels was observed for Ln 3+ ions in NCs only and were explained by a cutoff in the acoustic phonon spectrum. Relaxation would be inhibited as no resonant low energy (long wavelength) acoustic phonon modes can exist in nanometer sized crystals, and this prevents relaxation by direct phonon emission between closely spaced Stark levels. This phenomenon is known as a phonon bottleneck. Here, we investigate the role of phonon confinement in Ln-doped NCs. High resolution emission spectra at temperatures down to 2.2 K are reported for various Ln 3+ ions (Er 3+ , Yb 3+ , Eu 3+ ) doped into monodisperse 10 nm NaYF 4 NCs and compared with spectra for bulk (microcrystalline) material. Contrary to previous reports, we find no evidence for phonon bottleneck effects in the emission spectra. Emission from closely spaced higher Stark levels is observed only at high excitation powers and is explained by laser heating. The present results indicate that previously reported effects in NCs may not be caused by phonon confinement., Competing Interests: The authors declare no competing financial interest.

Lanthanide-doped nanocrystals (NCs) differ from their bulk counterparts due to their large surface to volume ratio. It is generally assumed that the optical properties are not affected by size effects as electronic transitions occur within the well-shielded 4f shell of the lanthanide dopant ions. However, defects and disorder in the surface layer can affect the luminescence properties. Trivalent europium is a suitable ion to investigate the subtle influence of the surface, because of its characteristic luminescence and high sensitivity to the local environment. Here, we investigate the influence of disorder in NCs on the optical properties of lanthanide dopants by studying the inhomogeneous linewidth, emission intensity ratios, and luminescence decay curves for LaPO 4 :Eu 3+ samples of different sizes (4 nm to bulk) and core-shell configurations (core, core-isocrystalline shell, and core-silica shell). We show that the emission linewidths increase strongly for NCs. The ratio of the intensities of the forced electric dipole (ED) and magnetic dipole (MD) transitions, a measure for the local symmetry distortion around Eu 3+ ions, is higher for samples with a large fraction of Eu 3+ ions close to the surface. Finally, we present luminescence decay curves revealing an increased nonradiative decay rate for Eu 3+ in NCs. The effects are strongest in core and core-silica shell NCs and can be reduced by growth of an isocrystalline LaPO 4 shell. The present systematic study provides quantitative insight into the role of surface disorder on the optical properties of lanthanide-doped NCs. These insights are important in emerging applications of lanthanide-doped nanocrystals.

Low-density lipoprotein (LDL) plays a critical role in cholesterol transport and is closely linked to the progression of several diseases. This motivates the development of methods to study LDL behavior from the microscopic to whole-body level. We have developed an approach to efficiently load LDL with a range of diagnostically active nanocrystals or hydrophobic agents. We performed focused experiments on LDL labeled with gold nanocrystals (Au-LDL). The labeling procedure had minimal effect on LDL size, morphology, or composition. Biological function was found to be maintained from both in vitro and in vivo experiments. Tumor-bearing mice were injected intravenously with LDL, DiR-LDL, Au-LDL, or a gold-loaded nanoemulsion. LDL accumulation in the tumors was detected with whole-body imaging methods, such as computed tomography (CT), spectral CT, and fluorescence imaging. Cellular localization was studied with transmission electron microscopy and fluorescence techniques. This LDL labeling procedure should permit the study of lipoprotein biointeractions in unprecedented detail.

We report the synthesis of ultranarrow (Zn,Cd)Te/CdSe colloidal heteronanowires, using ZnTe magic size clusters as seeds. The wire formation starts with a partial Zn for Cd cation exchange, followed by self-organization into segmented heteronanowires. Further growth occurs by inclusion of CdSe. The heteronanowires emit in the 530 to 760 nm range with high quantum yields. The electron-hole overlap decreases with increasing CdSe volume fraction, allowing the optical properties to be controlled by adjusting the heteronanowire composition., (© 2012 American Chemical Society)

The study of lipoproteins, natural nanoparticles comprised of lipids and apolipoproteins that transport fats throughout the body, is of key importance to better understand, treat, and prevent cardiovascular disease. In the current study, we have developed a lipoprotein-based nanoparticle that consists of a quantum dot (QD) core and Cy5.5 labeled lipidic coating. The methodology allows judicious tuning of the QD/Cy5.5 ratio, which enabled us to optimize Förster resonance energy transfer (FRET) between the QD core and the Cy5.5-labeled coating. This phenomenon allowed us to study lipoprotein-lipoprotein interactions, lipid exchange dynamics, and the influence of apolipoproteins on these processes. Moreover, we were able to study HDL-cell interactions and exploit FRET to visualize HDL association with live macrophage cells.

Fluorescent proteins have become an invaluable tool in cell biology. The green fluorescent protein variant EGFP is especially widely applied. Use of fluorescent proteins, including EGFP, however can be hindered by inefficient protein folding, resulting in protein aggregation and reduced fluorescence. This is especially profound in prokaryotic cells. Furthermore, EBFP, a blue fluorescent variant of EGFP, is rarely used because of its dim fluorescence and fast photobleaching. Thus, efforts to improve properties such as protein folding, fluorescence brightness, and photostability are important. Strongly enhanced green fluorescent (SGFP2) and strongly enhanced blue fluorescent (SBFP2) proteins were created, based on EGFP and EBFP, respectively. We used site-directed mutagenesis to introduce several mutations, which were recently shown to improve the fluorescent proteins EYFP and ECFP. SGFP2 and SBFP2 exhibit faster and more efficient protein folding and accelerated chromophore oxidation in vitro. For both strongly enhanced fluorescent proteins, the photostability was improved 2-fold and the quantum yield of SBFP2 was increased 3-fold. The improved folding efficiency reduced the extent of protein aggregation in Escherichia coli, thereby increasing the brightness of bacteria expressing SGFP2 7-fold compared to the brightness of those expressing EGFP. Bacteria expressing SBFP2 were 16-fold more fluorescent than those expressing EBFP. In mammalian cells, the improvements were less pronounced. Cells expressing SGFP2 were 1.7-fold brighter than those expressing EGFP, which was apparently due to more efficient protein expression and/or chromophore maturation. Mammalian cells expressing SBFP2 were 3.7-fold brighter than cells expressing EBFP. This increase in brightness closely resembled the increase in intrinsic brightness observed for the purified recombinant protein. The increased maturation efficiency and photostability of SGFP2 and SBFP2 facilitate detection and extend the maximum duration of fluorescence imaging.