Sebastian W. Schmitt

Cell type- and brain region-resolved mouse brain proteome

Brain transcriptome and connectome maps are being generated, but an equivalent effort on the proteome is currently lacking. We performed high-resolution mass spectrometry–based proteomics for in-depth analysis of the mouse brain and its major brain regions and cell types. Comparisons of the 12,934 identified proteins in oligodendrocytes, astrocytes, microglia and cortical neurons with deep sequencing data of the transcriptome indicated deep coverage of the proteome. Cell type–specific proteins defined as tenfold more abundant than average expression represented about a tenth of the proteome, with an overrepresentation of cell surface proteins. To demonstrate the utility of our resource, we focused on this class of proteins and identified Lsamp, an adhesion molecule of the IgLON family, as a negative regulator of myelination. Our findings provide a framework for a system-level understanding of cell-type diversity in the CNS and serves as a rich resource for analyses of brain development and function.

Control of oligodendroglial cell number by the miR-17-92 cluster

The generation of myelinating cells in the central nervous system requires the initiation of specific gene expression programs in oligodendrocytes. We reasoned that microRNAs (miRNAs) could play an important role in this process by regulating crucial developmental genes. Microarray profiling of cultured oligodendrocytes identified the miR-17-92 miRNA cluster as highly enriched in oligodendrocytes. We specifically deleted the miR-17-92 cluster in oligodendrocytes using 2′,3′-cyclic nucleotide 3′ phosphodiesterase (Cnp)-Cre mice. Absence of miR-17-92 leads to a reduction in oligodendrocyte number in vivo and we find that the expression of these miRNAs in primary cultures of oligodendrocyte precursor cells promotes cell proliferation by influencing Akt signaling. Together, these results suggest that the miRNA pathway is essential in determining oligodendroglial cell number and that the miR-17-92 cluster is crucial in this process.

Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A Promising Material for Research and Applications in Nanotechnology

Silicon nanowires (SiNW) were formed on large grained, electron-beam crystallized silicon (Si) thin films of only ∼6 μm thickness on glass using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed outstanding structural properties of this nanomaterial. It could be shown that SiNWs with entirely predetermined shapes including lengths, diameters and spacings and straight side walls form independently of their crystalline orientation and arrange in ordered arrays on glass. Furthermore, for the first time grain boundaries could be observed in individual, straightly etched SiNWs. After heat treatment an electronic grade surface quality of the SiNWs could be shown by X-ray photoelectron spectroscopy (XPS). Integrating sphere measurements show that SiNW patterning of the multicrystalline Si (mc-Si) starting thin film on glass substantially increases absorption and reduces reflection, as being desired for an application in thin film photovoltaics (PV). The multicrystalline SiNWs directly mark a starting point for research not only in PV but also in other areas like nanoelectronics, surface functionalization, and nanomechanics.

Metabolism and functions of lipids in myelin.

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are lipid-rich and multilamellar membrane stacks. The lipid composition of myelin varies significantly from other biological membranes. Studies in mutant mice targeting various lipid biosynthesis pathways have shown that myelinating glia have a remarkable capacity to compensate the lack of individual lipids. However, compensation fails when it comes to maintaining long-term stability of myelin. Here, we summarize how lipids function in myelin biogenesis, axon-glia communication and in supporting long-term maintenance of myelin. We postulate that change in myelin lipid composition might be relevant for our understanding of aging and demyelinating diseases. This article is part of a Special Issue titled Brain Lipids.

Large area fabrication of vertical silicon nanowire arrays by silver-assisted single-step chemical etching and their formation kinetics

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time (up to 15 h), solution temperature (10-80 °C), AgNO3 (5-200 mM) and HF (2-22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10-50 °C). The activation energy for the formation of SiNWs on Si(100) has been found to be equal to ∼0.51 eV . It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance (as low as <3% for Si(100) NWs and <12% for mc-Si NWs) compared to ∼35% for the polished surface in the 350-1000 nm wavelength range. The remarkably low reflection surface of SiNW arrays has great potential for use as an effective light absorber material in novel photovoltaic architectures, and other optoelectronic and photonic devices.

Enhanced photovoltaics inspired by the fovea centralis

The fovea centralis is a closely-packed vertical array of inverted-cone photoreceptor cells located in the retina that is responsible for high acuity binocular vision. The cones are operational in well-lit environments and are responsible for trapping the impinging illumination. We present the vertical light-funnel silicon array as a light-trapping technique for photovoltaic applications that is bio-inspired by the properties of the fovea centralis. We use opto-electronic simulations to evaluate the performance of light-funnel solar cell arrays. Light-funnel arrays present ~65% absorption enhancement compared to a silicon film of identical thickness and exhibit power conversion efficiencies that are 60% higher than those of optimized nanowire arrays of the same thickness although nanowire arrays consist of more than 2.3 times the amount of silicon. We demonstrate the superior absorption of the light-funnel arrays as compared with recent advancements in the field. Fabrication of silicon light-funnel arrays using low-cost processing techniques is demonstrated.

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostuctures

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/neff)3 < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

Hybrid Silicon Nanowires: From Basic Research to Applied Nanotechnology

Silicon nanowires (SiNWs) have attracted particular attention in nanotechnology applica‐ tions due to their unique advantages with respect to electrical, optical and thermoelec‐ tric properties compared to the planar thin films [1-3]. SiNWs show strong absorption of visible light in thin layers, which makes them attractive for the use in novel thin film concepts as in photovoltaics [4]. Like all the one-dimensional (1D) nanostructures, SiNWs have a large surface to volume ratio and thus surface-dominated properties that can be tuned, e.g. by suitable surface functionalization to be applied in sensitive sensors [5, 6]. Surface functionalization affects the overall electrical properties of SiNWs as dop‐ ants surface concentration, surface recombination rate, density of surface states, etc.[2, 7-10]. Atop Silicon atoms on SiNW surfaces can be terminated with a wide variety of molecules bearing covalent interfacial bonds i.e. Si-C [2, 7-10], Si-N [11] and Si-O [12]. Up to now we essentially exploited the Si-C bonds that permit a thorough control of the SiNW surface yielding a strongly reduced tendency to the undesirable oxidation of the SiNWs [2]. With such surface functionalization methods at hand, control and prediction of the electrical transport properties have become so viable that functionalized SiNWs can be considered as auspicious nanoscale building blocks in future high-performance nano-devices in the areas of electronics, opto-electronics, photovoltaics and sensing [13-18]. In this chapter we present a chemically-based surface functionalization method

Modeling the dielectric function of degenerately doped ZnO:Al thin films grown by ALD using physical parameters

Transparent conductive thin films are a key building block of modern optoelectronic devices. A promising alternative to expensive indium containing oxides is aluminum doped zinc oxide (AZO). By correlating spectroscopic ellipsometry and photoluminescence, we analyzed the contributions of different optical transitions in AZO grown by atomic layer deposition to a model dielectric function (MDF) over a wide range of photon energies. The derived MDF reflects the effects of the actual band structure and therefore describes the optical properties very accurately. The presented MDF is solely based on physically meaningful parameters in contrast to empirical models like e.g. the widely used Sellmeier equation, but nevertheless real and imaginary parts are expressed as closed-form expressions. We analyzed the influence of the position of the Fermi energy and the Fermi-edge singularity to the different parts of the MDF. This information is relevant for design and simulation of optoelectronic devices and can be determined by analyzing the results from spectroscopic ellipsometry.

BCAS1 expression defines a population of early myelinating oligodendrocytes in multiple sclerosis lesions

BCAS1 expression identifies newly formed and actively myelinating oligodendrocytes in development, adulthood, and disease. Mapping active myelination with BCAS1 Neuronal axon demyelination causes motor and cognitive impairments in multiple sclerosis (MS) and other demyelinating disorders. Although remyelinating strategies have been proposed, the lack of markers to detect areas of active myelination hampers the development of effective therapies. Fard et al. show that myelinating oligodendrocytes constitute a unique population expressing breast carcinoma amplified sequence 1 (BCAS1) in rodent and human brain tissue. In brain samples from deceased MS patients, BCAS1+ cells are present around lesions, suggesting that remyelination might occur during MS and that BCAS1 expression could be used to track responses to remyelinating compounds for treating demyelinating disorders. Investigations into brain function and disease depend on the precise classification of neural cell types. Cells of the oligodendrocyte lineage differ greatly in their morphology, but accurate identification has thus far only been possible for oligodendrocyte progenitor cells and mature oligodendrocytes in humans. We find that breast carcinoma amplified sequence 1 (BCAS1) expression identifies an oligodendroglial subpopulation in the mouse and human brain. These cells are newly formed, myelinating oligodendrocytes that segregate from oligodendrocyte progenitor cells and mature oligodendrocytes and mark regions of active myelin formation in development and in the adult. We find that BCAS1+ oligodendrocytes are restricted to the fetal and early postnatal human white matter but remain in the cortical gray matter until old age. BCAS1+ oligodendrocytes are reformed after experimental demyelination and found in a proportion of chronic white matter lesions of patients with multiple sclerosis (MS) even in a subset of patients with advanced disease. Our work identifies a means to map ongoing myelin formation in health and disease and presents a potential cellular target for remyelination therapies in MS.