Ga for Zn Cation Exchange Allows for Highly Luminescent and Photostable InZnP-Based Quantum Dots.

In this work, we demonstrate that a preferential Ga-for-Zn cation exchange is responsible for the increase in photoluminescence that is observed when gallium oleate is added to InZnP alloy QDs. By exposing InZnP QDs with varying Zn/In ratios to gallium oleate and monitoring their optical properties, composition, and size, we conclude that Ga preferentially replaces Zn, leading to the formation of InZnP/InGaP core/graded-shell QDs. This cation exchange reaction results in a large increase of the QD photoluminescence, but only for InZnP QDs with Zn/In ≥ 0.

Tuning the Lattice Parameter of InxZnyP for Highly Luminescent Lattice-Matched Core/Shell Quantum Dots.

Colloidal quantum dots (QDs) show great promise as LED phosphors due to their tunable narrow-band emission and ability to produce high-quality white light. Currently, the most suitable QDs for lighting applications are based on cadmium, which presents a toxicity problem for consumer applications. The most promising cadmium-free candidate QDs are based on InP, but their quality lags much behind that of cadmium based QDs.

High-temperature luminescence quenching of colloidal quantum dots.

Thermal quenching of quantum dot (QD) luminescence is important for application in luminescent devices. Systematic studies of the quenching behavior above 300 K are, however, lacking. Here, high-temperature (300-500 K) luminescence studies are reported for highly efficient CdSe core-shell quantum dots (QDs), aimed at obtaining insight into temperature quenching of QD emission.

Loosening quantum confinement: observation of real conductivity caused by hole polarons in semiconductor nanocrystals smaller than the Bohr radius.

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary.

Quantum dot and Cy5.5 labeled nanoparticles to investigate lipoprotein biointeractions via Förster resonance energy transfer.

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.

A fluorescent, paramagnetic and PEGylated gold/silica nanoparticle for MRI, CT and fluorescence imaging.

An important challenge in medical diagnostics is to design all-in-one contrast agents that can be detected with multiple techniques such as magnetic resonance imaging (MRI), X-ray computed tomography (CT), positron emission tomography (PET), single photon emission tomography (SPECT) or fluorescence imaging (FI). Although many dual labeled agents have been proposed, mainly for combined MRI/FI, constructs for three imaging modalities are scarce. Here gold/silica nanoparticles with a poly(ethylene glycol), paramagnetic and fluorescent lipid coating were synthesized, characterized and applied as trimodal contrast agents to allow for nanoparticle-enhanced imaging of macrophage cells in vitro via MRI, CT and FI, and mice livers in vivo via MRI and CT.

Imaging and quantifying the morphology of an organic-inorganic nanoparticle at the sub-nanometre level.

The development of hybrid organic-inorganic nanoparticles is of interest for applications such as drug delivery, DNA and protein recognition, and medical diagnostics. However, the characterization of such nanoparticles remains a significant challenge due to the heterogeneous nature of these particles. Here, we report the direct visualization and quantification of the organic and inorganic components of a lipid-coated silica particle that contains a smaller semiconductor quantum dot.

Magnetic quantum dots for multimodal imaging.

Multimodal contrast agents based on highly luminescent quantum dots (QDs) combined with magnetic nanoparticles (MNPs) or ions form an exciting class of new materials for bioimaging. With two functionalities integrated in a single nanoparticle, a sensitive contrast agent for two very powerful and highly complementary imaging techniques [fluorescence imaging and magnetic resonance imaging (MRI)] is obtained. In this review, the state of the art in this rapidly developing field is given.

Time-dependent photoluminescence spectroscopy as a tool to measure the ligand exchange kinetics on a quantum dot surface.

The exchange kinetics of native ligands that passivate CdSe quantum dots (hexadecylamine (HDA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP)) by thiols is followed in situ. This is realized by measuring, in real-time, the decrease in emission intensity of the QDs upon addition of hexanethiol (HT) which quenches the emission. The effect of adding an excess of native ligands prior to thiol addition on the capping exchange is studied to provide insight in the bond strength and exchange kinetics of the individual surfactants.

In situ observation of rapid ligand exchange in colloidal nanocrystal suspensions using transfer NOE nuclear magnetic resonance spectroscopy.

Recently, solution NMR-based approaches have been developed that represent useful new tools for the in situ characterization of the capping ligand in colloidal nanocrystal dispersions. So far, this development has focused mainly on tightly bound ligands (no exchange) or ligands in slow exchange with the nanocrystal surface. In such systems, the ligand can be identified and its amount and interaction quantified via 1D (1)H NMR, (1)H-(13)C HSQC, and DOSY spectra.

Luminescent Solar Concentrators--a review of recent results.

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp.

Paramagnetic lipid-coated silica nanoparticles with a fluorescent quantum dot core: a new contrast agent platform for multimodality imaging.

Silica particles as a nanoparticulate carrier material for contrast agents have received considerable attention the past few years, since the material holds great promise for biomedical applications. A key feature for successful application of this material in vivo is biocompatibility, which may be significantly improved by appropriate surface modification. In this study, we report a novel strategy to coat silica particles with a dense monolayer of paramagnetic and PEGylated lipids.

Nanocrystal core high-density lipoproteins: a multimodality contrast agent platform.

High density lipoprotein (HDL) is an important natural nanoparticle that may be modified for biomedical imaging purposes. Here we developed a novel technique to create unique multimodality HDL mimicking nanoparticles by incorporation of gold, iron oxide, or quantum dot nanocrystals for computed tomography, magnetic resonance, and fluorescence imaging, respectively. By including additional labels in the corona of the particles, they were made multifunctional.

Improved biocompatibility and pharmacokinetics of silica nanoparticles by means of a lipid coating: a multimodality investigation.

Silica is a promising carrier material for nanoparticle-facilitated drug delivery, gene therapy, and molecular imaging. Understanding of their pharmacokinetics is important to resolve bioapplicability issues. Here we report an extensive study on bare and lipid-coated silica nanoparticles in mice.

Binary superlattices of PbSe and CdSe nanocrystals.

In this paper we show that self-organization of colloidal PbSe and CdSe semiconductor nanocrystals with a size ratio of 0.57 leads to binary structures with a AB2 or a cuboctahedral AB13 lattice. The type of superlattice formed can be regulated by the relative concentration of both nanocrystals in the suspension.

Optical investigation of quantum confinement in PbSe nanocrystals at different points in the Brillouin zone.

We present detailed investigations on the optical properties of PbSe nanocrystals. The absorption spectra of monodisperse, quasispherical nanocrystals exhibit sharp features as a result of distinct optical transitions. To study the size dependence, absorption spectra of nanocrystals ranging from 3.

Dipolar structures in colloidal dispersions of PbSe and CdSe quantum dots.

We show by cryogenic transmission electron microscopy that PbSe and CdSe nanocrystals of various shapes in a liquid colloidal dispersion self-assemble into equilibrium structures that have a pronounced dipolar character, to an extent that depends on particle concentration and size. Analyzing the cluster-size distributions with a one-dimensional (1D) aggregation model yields a dipolar pair attraction of 8-10 kBT at room temperature. This accounts for the long-range alignment of the crystal planes of individual nanocrystals in self-assembled superstructures and for anisotropic nanostructures grown via oriented attachment.

Electronic coupling and exciton energy transfer in CdTe quantum-dot molecules.

Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross-linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling.

Chemisorption determines the photovoltage of a Ti/TiO2/Au/dye internal electron emission photovoltaic cell.

The Ti/TiO2/Au junction forms the basis of a promising new type of photovoltaic cell, provided that a light-harvesting antenna layer can be deposited on the thin gold film. We report that the electrical diode characteristics of the TiO2/Au Schottky barrier deteriorate by deposition of a hydrophobic quantum dot film, Merbromin dye adsorption, or electron-hole photogeneration in TiO2 under inert conditions. In the presence of oxygen and water vapor, the Schottky barrier characteristics and high photovoltage are recovered.

Temperature-dependent energy transfer in cadmium telluride quantum dot solids.

Efficiently luminescing colloidal CdTe quantum dots (QDs) were used for the preparation of monodispersed and mixed size QD solids. Luminescence spectra and decay times of the QD emission were measured as a function of temperature to study energy transfer (ET) processes in the QD solids. In the luminescence decay curves of the emission of the largest QDs (acceptors), a rise time of the luminescence signal is observed due to energy transfer from smaller QDs.

The hidden role of acetate in the PbSe nanocrystal synthesis.

Monodisperse spherical, star-shaped, and octahedral PbSe nanocrystals were synthesized via a hot injection method. We show that the shape and size of the colloidal PbSe nanocrystals are determined by the concentration of acetate and that only acetate-free reaction mixtures result in spherical nanocrystals. The presence of acetate leads to efficient oriented attachment of smaller PbSe nanoparticles along the 100 crystal axis.

Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe.

MRI detectable and targeted quantum dots were developed. To that aim, quantum dots were coated with paramagnetic and pegylated lipids, which resulted in a relaxivity, r(1), of nearly 2000 mM(-1)s(-1) per quantum dot. The quantum dots were functionalized by covalently linking alphavbeta3-specific RGD peptides, and the specificity was assessed and confirmed on cultured endothelial cells.