Thomas Nann

Quantum dots versus organic dyes as fluorescent labels

Suitable labels are at the core of luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)—inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry—as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.

Plasmon-Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals

In this Letter we report the plasmon-enhanced upconversion in single NaYF(4) nanocrystals codoped with Yb(3+)/Er(3+). Single nanocrystals and gold nanospheres are investigated and assembled in a combined confocal and atomic force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-infrared at 973 nm. By the use of the atomic force microscope, we couple single nanocrystals with gold spheres (30 and 60 nm in diameter) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal-gold sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.

A four-color colloidal multiplexing nanoparticle system.

Any labeling with multicolor markers can be affected by the autofluorescence of biological tissue due to the UV or blue light excitation sources, or the results are affected by fluorescence resonance energy transfer. In this work, we present novel IR-to-vis upconverting nanoparticles of different rare earth metal dopants. With a single excitation source of 980 nm, four different colors of nanocrystals can be spectrally separated under multiplexing conditions. The particles were phase transferred into polar solvents by means of silica encapsulation and were characterized by transmission electron microscopy, X-ray diffraction, and photoluminescence spectroscopy.

Determination of quantum confinement in CdSe nanocrystals by cyclic voltammetry

Ionization potentials Ip, electron affinities E.A., and the quantum confinement in CdSe nanocrystals were determined by means of cyclic voltammetry. The results were compared to values obtained from spectroscopic measurements, especially UV/vis absorption and photoluminescence emission spectra. Absolute band gap positions were obtained from the electrochemical measurements and discussed with regard to vacuum level values. The results are in good agreement with theoretical expectations and spectroscopic data.

Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production

Efficient production of solar fuels is an imperative for meeting future fossil-fuel-free energy demands. Hydrogen that is derived from the splitting of water by solar energy is clearly attractive as a clean energy vector, and there have been many attempts to construct viable molecular and biomolecular devices for photohydrogen production. A common approach in the construction of such devices is the utilization of tris(bipyridine)ruthenium, zinc porphyrin, or related molecular materials as photosensitizers in conjunction with a tethered or free electrocatalyst or enzymic system. Apart from cost, such systems suffer from having limited lifetimes, which may be attributed at least in part to the intrinsic reactivity of the organic N-donor ligands in the radical anion form of the photoexcited state and photodegradation pathways.

Monodisperse upconverting nanocrystals by microwave-assisted synthesis.

Upconverting nanocrystals have a tremendous potential for applications in fields such as bioanalysis, medical therapy, or display technologies. However, a prerequisite for many applications is the availability of small, monodisperse, and highly luminescent nanocrystals. Here we show, that a microwave-assisted synthesis approach allows for the synthesis of such monodisperse and luminescent upconverting nanocrystals within 5 min in a closed reaction vessel. Even though the same reactants and solvents as with classical conductive heating reactions were used, microwave-assisted synthesis resulted in differently sized and shaped particles and provided superior reaction control. The nucleation and growth mechanism follows a La Mer scheme and can be controlled extremely accurately. It is expected that the fundamental principles of this synthesis approach can be applied to many other types of nanocrystals as well.

Rapid synthesis of highly luminescent InP and InP/ZnS nanocrystals

We report a simple method for the fast synthesis of highly photoluminescent InP and InP/ZnS core–shell nanocrystals (NCs) covering a wide range of emissions from blue to the near infrared. Both InP and InP/ZnS NCs were prepared in one-step, in a one-pot reaction within 20 min using an InCl3 complex as a precursor and zinc carboxylate as an initiator and stabilizer. The quantum yields of the InP and InP/ZnS NCs were 30% and 60%, respectively. This new synthetic method allows fast and reproducible preparation of InP and InP/ZnS NCs with a quality comparable to that of the frequently used CdSe-based ones. It is anticipated that these particles can be used to replace CdSe for many applications.

First solar cells based on CdTe nanoparticle/MEH-PPV composites

Hybrid bulk heterojunction composites are promising material for low-cost organic solar cells. Fundamental measurements with CdTe nanocrystal/MEH-PPV poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] composites and the first realization of a solar cell based on this material are presented. Optical and electrochemical properties are discussed as well as the current voltage characteristic of the resulting cell. It was found, that CdTe nanocrystal/MEH-PPV composites are well suited for an organic solar cell, even though the technological realization needs to be improved.

Catalytically Active Bimetallic Nanoparticles Supported on Porous Carbon Capsules Derived From Metal–Organic Framework Composites

We report a new methodology for producing monometallic or bimetallic nanoparticles confined within hollow nitrogen-doped porous carbon capsules. The capsules are derived from metal-organic framework (MOF) crystals that are coated with a shell of a secondary material comprising either a metal-tannic acid coordination polymer or a resorcinol-formaldehyde polymer. Platinum nanoparticles are optionally sandwiched between the MOF core and the shell. Pyrolysis of the MOF-shell composites produces hollow capsules of porous nitrogen-doped carbon that bear either monometallic (Pt, Co, and Ni) or alloyed (PtCo and PtNi) metal nanoparticles. The Co and Ni components of the bimetallic nanoparticles are derived from the shell surrounding the MOF crystals. The hollow capsules prevent sintering and detachment of the nanoparticles, and their porous walls allow for efficient mass transport. Alloyed PtCo nanoparticles embedded in the capsule walls are highly active, selective, and recyclable catalysts for the hydrogenation of nitroarenes to anilines.

Quantum Dots for Electro-Optic Devices

We discuss the use of quantum dots (QDs) in electro-optic devices. The focus of the article is on key problems in this area: the electronic and optical properties of QDs, dispersing QDs in polymers and the requirements for both, and QDs and polymer composites in devices such as solar cells, light-emitting devices, and detectors. The accompanying article by Geyer et al. in this issue of ACS Nano demonstrates an excellent example of a detector device, which also points to future directions for research in this field.