Dot-Size Dependent Excitons in Droplet-Etched Cone-Shell GaAs Quantum Dots

Strain-free GaAs quantum dots (QDs) are fabricated by filling droplet-etched nanoholes in AlGaAs. Using a template of nominally identical nanoholes, the QD size is precisely controlled by the thickness of the GaAs filling layer. Atomic force microscopy indicates that the QDs have a cone-shell shape. From single-dot photoluminescence measurements, values of the exciton emission energy (1.58...1.82 eV), the exciton–biexciton splitting (1.8...2.5 meV), the exciton radiative lifetime of bright (0.37...0.58 ns) and dark (3.2...6.7 ns) states, the quantum efficiency (0.89...0.92), and the oscillator strength (11.2...17.1) are determined as a function of the dot size. The experimental data are interpreted by comparison with an atomistic model

Spatial Distribution of Intracellular Ion Concentrations in Aggregate‐Forming HeLa Cells Analyzed by μ‐XRF Imaging

Protein aggregation is a hallmark of several severe neurodegenerative disorders such as Huntington's, Parkinson's, or Alzheimer's disease. Metal ions play a profound role in protein aggregation and altered metal-ion homeostasis is associated with disease progression. Here we utilize μ-X-ray fluorescence imaging in combination with rapid freezing to resolve the elemental distribution of phosphorus, sulfur, potassium, and zinc in huntingtin exon-1-mYFP expressing HeLa cells. Using quantitative XRF analysis, we find a threefold increase in zinc and a 10-fold enrichment of potassium that can be attributed to cellular stress response. While the averaged intracellular ion areal masses are significantly different in aggregate-containing cells, a local intracellular analysis shows no different ion content at the location of intracellular inclusion bodies. The results are compared to corresponding experiments on HeLa cells forming pseudoisocyanine chloride aggregates. As those show similar results, changes in ion concentrations are not exclusively linked to huntingtin exon-1 amyloid formation

X-ray fluorescence analysis of metal distributions in cryogenic biological samples using large-acceptance-angle SDD detection and continuous scanning at the Hard X-ray Micro/Nano-Probe beamline P06 at PETRA III

A new Rococo 2 X-ray fluorescence detector was implemented into the cryogenic sample environment at the Hard X-ray Micro/Nano-Probe beamline P06 at PETRA III, DESY, Hamburg, Germany. A four sensor-field cloverleaf design is optimized for the investigation of planar samples and operates in a backscattering geometry resulting in a large solid angle of up to 1.1 steradian. The detector, coupled with the Xspress 3 pulse processor, enables measurements at high count rates of up to 10$^{6}$ counts per second per sensor. The measured energy resolution of ∼129 eV (Mn $\Kappa\alpha$ at 10000 counts s$^{−1}$) is only minimally impaired at the highest count rates. The resulting high detection sensitivity allows for an accurate determination of trace element distributions such as in thin frozen hydrated biological specimens. First proof-of-principle measurements using continuous-movement 2D scans of frozen hydrated HeLa cells as a model system are reported to demonstrate the potential of the new detection system

Spatial distribution of intracellular ion concentrations in aggregate-forming HeLa cells analyzed by μ\mu-XRF imaging

Protein aggregation is a hallmark of several severe neurodegenerative disorders such as Huntington's, Parkinson's, or Alzheimer's disease. Metal ions play a profound role in protein aggregation and altered metal-ion homeostasis is associated with disease progression. Here we utilize $\mu$-X-ray fluorescence imaging in combination with rapid freezing to resolve the elemental distribution of phosphorus, sulfur, potassium, and zinc in huntingtin exon-1-mYFP expressing HeLa cells. Using quantitative XRF analysis, we find a threefold increase in zinc and a 10-fold enrichment of potassium that can be attributed to cellular stress response. While the averaged intracellular ion areal masses are significantly different in aggregate-containing cells, a local intracellular analysis shows no different ion content at the location of intracellular inclusion bodies. The results are compared to corresponding experiments on HeLa cells forming pseudoisocyanine chloride aggregates. As those show similar results, changes in ion concentrations are not exclusively linked to huntingtin exon-1 amyloid formation

The Niecke Biradicals and Their Congeners - The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties

Schoeller W. The Niecke Biradicals and Their Congeners - The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties. EUROPEAN JOURNAL OF INORGANIC CHEMISTRY. 2019;2019(11-12):1495-1506.Biradicals within organic chemistry are known to be of fleeting existence. They can be traced only by femtosecond spectroscopy and/or corresponding trapping experiments. The conceptual understanding of a biradical originates mainly from gas phase experiments. These shaped the biradicals as highly unstable species, their lifetimes determined by reactions, which are merely entropy controlled. On the other hand, biradicals in pi-systems of non-Kekule type, lead to species, which either have distinct singlet or triplet ground states. Yet they are still species confined to spectroscopic investigations. The matter changed by the introduction of sterically protecting groups into chemistry, in order to stabilize hitherto highly reactive compounds. With the new growing field of phosphorus organic chemistry, both fields merged and the concept of a stable biradical came to light. The concept matured in subsequent years and it is now a flowering field with promising aspects in materials science. The Niecke-type biradicaloid, which is a basic unit in this area, owes its stability by protection of the phosphorus position with sterically demanding substituents, in particular with the "super-mesityl" (Mes*) group. The electron withdrawing TMS groups at the carbon positions enhance the singlet stability. On this basis it was possible to isolate and structurally characterize these non-Kekule type structures. The description of these systems as biradicals or biradicaloids is determined by their high reactivity and they offer versatility for a variety of chemical reactions. From the point of theory this refers to small energy gaps between adiabatic singlet and triplet states. However, the understanding as biradicals is different to the Doering concept of a biradical vs. a biradicaloid, as anticipated in the transition state of the Cope automerization. In the Niecke-type biradicaloid a corresponding biradical structure is promoted with enhanced pyramidalization at the phosphorus atoms or in more general by a decrease of the Lewis basicity of the lone pairs. The Niecke-type biradical can be converted into a Bertrand-type biradical with trans-annular bonding. This requires changing the donor-ability at the phosphorus centers. This prediction could be proven by either addition with Lewis acids or complexation with transition metal fragments. In the last part of this article, it is briefly shown how biradicals can be utilized as units for building extended pi-systems. They feature an interesting target for the design of interesting materials for nonlinear optics