Rolf Koole

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. This paper reviews results mainly obtained within the framework of the Full-spectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

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. The characteristics of these nanoparticles, as well as their in vitro and in vivo behavior, revealed that they closely mimic native HDL.

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. Results obtained by use of a wide variety of techniques (fluorescence imaging, inductively coupled plasma mass spectrometry, magnetic resonance imaging, confocal laser scanning microscopy, and transmission electron microscopy) showed that the lipid coating, which enables straightforward functionalization and introduction of multiple properties, increases bioapplicability and improves pharmacokinetics.

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. The silica nanoparticles carry a quantum dot in their center and are made target-specific by the conjugation of multiple alphavbeta3-integrin-specific RGD-peptides. We demonstrate their specific uptake by endothelial cells in vitro using fluorescence microscopy, quantitative fluorescence imaging, and magnetic resonance imaging. The lipid-coated silica particles introduced here represent a new platform for nanoparticulate multimodality contrast agents.

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. Through thermal cycling (yoyo) experiments for QDs in polymer matrices, reversible and irreversible luminescence quenching processes can be distinguished. For a variety of core-shell systems, reversible quenching is observed in a similar temperature range, between 100 and 180 °C. The irreversible quenching behavior varies between different systems. Mechanisms for thermal quenching are discussed.

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. Here, we explore the case where capping ligands are in fast exchange with the nanocrystal surface. Using dodecylamine-stabilized CdTe (Q-CdTe|DDA) and octylamine-stabilized ZnO (Q-ZnO|OctA) nanoparticles, we first show that the NMR methods developed so far fail to evidence the bound ligand when the effect of the latter on the exchange-averaged parameters is marginalized by an excess of free ligand. Next, transfer NOE spectroscopy, a well-established technique in biomolecular NMR, is introduced to demonstrate and characterize the interaction of a ligand with the nanocrystal surface. Using Q-PbSe nanocrystals capped with oleic acids as a reference system, we show that bound and free ligands have strongly different NOE spectra wherein only bound ligands develop strong and negative NOEs. For the Q-CdTe|DDA system, transfer NOE spectra show a similar rapid appearance of strong, negative NOEs, thereby unambiguously demonstrating that DDA molecules spend time at the nanocrystal surface. In the case of Q-ZnO|OctA, where a more complex mixture is analyzed, transfer NOE spectroscopy allows distinguishing capping from noncapping molecules, thereby demonstrating the screening potential offered by this technique for colloidal quantum dot dispersions.

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. This agent can be a useful tool in a multitude of applications, including cell tracking and target-specific molecular imaging, and is a step in the direction of truly multi-modal imaging.

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. Temperature-dependent measurements reveal faster kinetics with increasing temperature. A kinetic model to describe the time-dependent measurements is introduced, taking into account the equilibrium between native ligands before thiol addition and describing the evolution of surface coverage by all ligands over time. The model allows us to extract the quenching rate for a single thiol ligand (0.004 ns(-1)) as well as exchange rates, equilibrium constants, activation energies, and changes in Gibbs free energy for replacement of the different native surfactants by HT. The analysis reveals that the substitution half-time of HDA by HT (72 s) is much shorter than for TOP (5 h) or TOPO (2.5 h) under the same conditions. The temperature dependence of the kinetics shows that the activation energy for exchange of HDA/TOPO by hexanethiol (1.6 kJ/mol) is much smaller than for TOP (20.9 kJ/mol).

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. This is done by describing the developments for four different approaches to integrate the fluorescence and magnetic properties in a single nanoparticle. The first type of particles is created by the growth of heterostructures in which a QD is either overgrown with a layer of a magnetic material or linked to a (superpara, or ferro) MNP. The second approach involves doping of paramagnetic ions into QDs. A third option is to use silica or polymer nanoparticles as a matrix for the incorporation of both QDs and MNPs. Finally, it is possible to introduce chelating molecules with paramagnetic ions (e.g., Gd-DTPA) into the coordination shell of the QDs. All different approaches have resulted in recent breakthroughs and the demonstration of the capability of bioimaging using both functionalities. In addition to giving an overview of the most exciting recent developments, the pros and cons of the four different classes of bimodal contrast agents are discussed, ending with an outlook on the future of this emerging new field.

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. High-angle annular dark-field scanning transmission electron microscopy combined with electron energy loss spectroscopy was used to determine the thickness and chemical signature of molecular coating layers, the element atomic ratios, and the exact positions of different elements in single nanoparticles. Moreover, the lipid ratio and lipid phase segregation were also quantified.