Jürgen Riegler

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.

Integrated Sample Preparation, Reaction, and Detection on a High-Frequency Centrifugal Microfluidic Platform

We extend the toolbox of lab procedures in life sciences by development of centrifugal microfluidics for high-level process integration. This is accomplished by implementing novel functional principles for sedimentation, batch-mode mixing, frequency-dependent online flow control, and optical read-out, which can be integrated into a process chain. The modular centrifugal setup comprises a microstructured disposable polymer disk as well as a reusable spinning and detection unit. We successfully developed centrifugal microfluidic technologies, which are suitable for sample preparation, process engineering, personalized diagnostics, and hematology, on this platform.

Monodisperse CdSe Nanorods at Low Temperatures

A new synthetic method is presented that allows the preparation of highly monodisperse CdSe nanorods (so called quantum rods) at relatively low temperatures (160 degrees C). This method is characterized by a high aspect ratio of the particles and affords good reproducibility. The morphology of the resulting nanorods was examined by means of transmission electron microscopy (TEM) and the electro-optic properties by means of fluorescence spectroscopy. The conditions of the reaction of nanoparticle growth were examined by varying the concentration of the organometallic precursors, the growth temperature, and the growth time. The experimental findings correspond well with previously published semiempirical pseudo-potential calculations.

Charge transfer mechanism in hybrid bulk heterojunction composites

Charge transfer mechanisms of several conducting polymers and polymer/CdSe nanocrystal composites (hybrid bulk heterojunction composites) were investigated by means of cyclic voltammetry. Potential application of these composites in hybrid light-emitting diodes was discussed. It was found that charge transfer is observed in most of the composites, used so far, but was relatively slow or incomplete. The PVPy/CdSe nanocrystal composite showed promising results, and is favorable for use in electroluminescent devices.

Blue shift of CdSe/ZnS nanocrystal-labels upon DNA-hybridization.

Luminescence color multiplexing is one of the most intriguing benefits, which might occur by using semiconductor Quantum Dots (QDs) as labels for biomolecules. It was found, that the luminescence of QDs can be quenched, and replaced by a luminescence peak at approximately 460 nm on hybridization with certain regions of Arabidopsis thaliana tissue. This effect is site selective, and it is unclear whether it occurs due to an energy transfer process, or due to quenching and scattering of the excitation light. The article describes methods for phase-transfer of differently coloured, hydrophobically ligated QDs, coupling of DNA strands to the QDs surface, and hybridization of the labelled DNA to different cell types of Arabidopsis thaliana. The reason for the luminescence blue-shift was studied systematically, and narrowed down to the above mentioned causes.

Parallelization of chip-based fluorescence immuno-assays with quantum-dot labeled beads

This paper presents an optical concept for the read-out of a parallel, bead-based fluorescence immunoassay conducted on a lab-on-a-disk platform. The reusable part of the modular setup comprises a detection unit featuring a single LED as light source, two emission-filters, and a color CCD-camera as standard components together with a spinning drive as actuation unit. The miniaturized lab-on-a-disk is devised as a disposable. In the read-out process of the parallel assay, beads are first identified by the color of incorporated quantum dots (QDs). Next, the reaction-specific fluorescence signal is quantified with FluoSpheres-labeled detection anti-bodies. To enable a fast and automated read-out, suitable algorithms have been implemented in this work. Based on this concept, we successfully demonstrated a Hepatitis-A assay on our disk-based lab-on-a-chip.

ISOTACHOPHORETIC MEASUREMENTS OF LUMINESCENT SEMICONDUCTOR NANOCRYSTALS

Luminescent semiconductor nanocrystals of CdTe and CdSe were derivatised by mercaptocarboxylic acids. A direct synthesis in the solution of the ligand and a post-synthesis ligand exchange method was used. The two methods were compared and evaluated by isotachophoresis. The electrophoretic mobility as well as the colloidal stability and the coverage with the ligand, were determined. An alternative method for the concentration estimation of nanocrystals by means of isotachophoresis was introduced.

Quantum dots with silica shells

The present paper introduces to the problems related with the application of silica shells to luminescent semiconductor nanocrystals, especially for bioanalytical applications. Two examples for the preparation of silica shells are presented: First, preparation of a very thin silica layer with simultaneous functionalisation and biological application. Second, a new preparation method for silica shells based on a sol-gel approach. In summary, it is shown, that --- despite of all problems --- high-quality silica coated nanocrystals can be prepared and are well suited for bioanalytical applications.