We demonstrate a novel epitaxial layer-by-layer growth on upconverting NaYF(4) nanocrystals (NCs) utilizing Ostwald ripening dynamics tunable both in thickness and composition. Injection of small sacrificial NCs (SNCs) as shell precursors into larger core NCs results in the rapid dissolution of the SNCs and their deposition onto the larger core NCs to yield core-shell structured NCs. Exploiting this NC size dependent dissolution/growth, the shell thickness can be controlled either by manipulating the number of SNCs injected or by successive injection of SNCs. In either of these approaches, the NCs self-focus from an initial bimodal distribution to a unimodal distribution (σ <5%) of core-shell NCs. The successive injection approach facilitates layer-by-layer epitaxial growth without the need for tedious multiple reactions for generating tunable shell thickness, and does not require any control over the injection rate of the SNCs, as is the case for shell growth by precursor injection.

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.

Cryo-electron microscopy in conjunction with advanced image analysis was used to analyze the structure of the 26S proteasome and to elucidate its variable features. We have been able to outline the boundaries of the ATPase module in the ''base'' part of the regulatory complex that can vary in its position and orientation relative to the 20S core particle. This variation is consistent with the ''wobbling'' model that was previously proposed to explain the role of the regulatory complex in opening the gate in the ␣-rings of the core particle. In addition, a variable mass near the mouth of the ATPase ring has been identified as Rpn10, a multiubiquitin receptor, by correlating the electron microscopy data with quantitative mass spectrometry.ATPase ͉ cryo-electron microscopy ͉ mass spectrometry ͉ protein degradation ͉ AAA-ATPase C ellular protein levels are regulated through protein synthesis and degradation. Given its destructive potential, intracellular protein degradation must be subject to rigorous spatial and temporal control. In eukaryotic cells, most proteins in the cytosol and the nucleus are degraded by the ubiquitinproteasome system, and malfunctions of this system have been implicated in a wide variety of diseases (1-4). Unlike constitutively active proteases, the proteasome has the capacity to degrade almost any protein, yet it acts with exquisite specificity. The key stratagem is self-compartmentalization: the active sites of the proteolytic machine are sequestered from the cellular environment in the interior of the barrel-shaped 20S proteasome (5). Proteins destined for degradation are marked by ubiquitin, a degradation signal that is recognized by the regulatory 19S complexes (RPs) that associate with the core 20S proteasome or core particle (CP) to form the holoenzyme called the 26S proteasome. The 26S complex is a multimeric assembly with a mass of approximately 2.5 MDa (6).The 20S core complex, which is highly conserved from archaea to higher eukaryotes, was amenable to structure determination by X-ray crystallography, and the resulting structures have revealed the salient features of this prototypical selfcompartmentalizing protease (7-9). In contrast, the 26S holocomplex with 1 or 2 RPs attached to the barrel-shaped core has so far resisted all crystallization attempts. For protein complexes refractory to crystallization, cryo-electron microscopy (cryo-EM) of ''single particles'' is an alternative approach. The amounts of material required for cryo-EM are minute and, moreover, some degree of heterogeneity is tolerable. Although high resolution is often elusive, the medium resolution (1-2 nm) structures afforded by cryo-EM provide a platform for hybrid approaches which, in turn, can provide useful insights into the structure and mechanism of macromolecular assemblies (10). Results and Discussion MS Analysis of Purified 26S Proteasomes.In the case of the 26S proteasome, progress in elucidating its structure has been hampered by the complexity of the system, its variability, and its fragility. 26S p...

Cation exchange was performed on up-conversion NaYF4:Yb,Tm nanoparticles, resulting in NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles as indicated by electron energy-loss spectroscopy 2D mapping. Results show that core–shell nanoparticles with a thin, tunable, and uniform shell of subnanometer thickness can be made via this cation exchange process. The resulting NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles have an enhanced up-conversion intensity relative to the initial core nanoparticles. As potential magnetic resonance imaging (MRI) contrast agents, they were tested for their proton relaxivities. The r1 relaxivity per Gd3+ ion of the nanoparticles with a thin NaGdF4 shell (ca. 0.6 nm thick) measured at 9.4 T was found to be 2.33 mM–1·s–1. This r1 relaxivity is among the highest in all the reported NaYF4–NaGdF4 core–shell nanoparticles. The r1 relaxivity per nanoparticle is 1.56 × 104 mM–1·s–1, which is over 4000 times higher than commercial Gd3+-complexes. The very high proton relaxivity per nanoparticle is critical for targeted MRI as such nanoparticles provide strong contrast even in low concentrations. The presented cation exchange method is a promising way to manufacture core–shell nanoparticles with up-conversion NaYF4:Yb,Tm core and paramagnetic NaGdF4 shell for bimodal imaging, i.e. MR and optical imaging.