Fast, Infrared-Active Optical Transistors Based on Dye-Sensitized CdSe Nanocrystals

We report an optically gated transistor composed of CdSe nanocrystals (NCs), sensitized with the dye zinc β-tetraaminophthalocyanine for operation in the first telecom window. This device shows a high ON/OFF ratio of 6 orders of magnitude in the red spectral region and an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this unexpected infrared sensitivity is due to electron transfer from the dye to the CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that this enables fast rise times during near-infrared optical gating of 47 ± 11 ns. Electronic coupling and accelerated nonradiative recombination of charge carriers at the interface between the dye and the CdSe NCs are further corroborated by steady-state and time-resolved photoluminescence measurements. Field-effect transistor measurements indicate that the increase in photocurrent upon laser illumination is mainly due to the increase in the carrier concentration while the mobility remains unchanged. Our results illustrate that organic dyes as ligands for NCs invoke new optoelectronic functionalities, such as fast optical gating at sub-bandgap optical excitation energies.

Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isotopes in groundwater

The La Paz aquifer system (Baja California Sur, Mexico) is under severe anthropogenic pressure because of high groundwater abstraction for urban supply (city of La Paz, around 222,000 inhabitants) and irrigated agriculture (1900ha). In consequence, seawater has infiltrated the aquifer, forcing the abandonment of wells with increased salinity. The objective of this study was to assess seawater intrusion, understand the hydrogeochemical processes involved and estimate the contribution of seawater in the wells tested. The aquifer comprises mainly the alluvial filling and marine sediments of a tectonic graben oriented north-south, in contact with the Gulf of California. Groundwater samples were collected in 47 locations and analyzed for major constituents. A subset of 23 samples was analyzed for strontium and boron concentrations and isotopic signatures (⁸⁷Sr/⁸⁶Sr and δ¹¹B). Results were interpreted using standard hydrochemical plots along with ad hoc plots including isotopic data. Seawater intrusion was confirmed by several hydrogeochemical indicators, such as the high salinity in areas of intense pumping or the Na⁺-Ca²⁺ exchange occurring in sediments that were previously in chemical equilibrium with fresh water. However, seawater contribution was not sufficient to explain the observed concentrations and isotopic signatures of Sr and B. According to the isotopic data, desorption processes triggered by a modification in chemical equilibrium and an increase in ionic strength by seawater intrusion significantly increased Sr and probably B concentrations in groundwater. From a calculation of seawater contribution to the wells, it was estimated that one-third of the sampled abstraction wells were significantly affected by seawater intrusion, reaching concentrations that would limit their use for human supply or even irrigated agriculture. In addition, significant agricultural pollution (nitrates) was detected. Planned management of the aquifer and corrective measures are needed in order to invert the salinization process before it severely affects water resources in the long term.

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Prevention of interferon-stimulated gene expression using microRNA-designed hairpins

RNA interference allows selective gene silencing, and is widely used for functional analysis of individual genes in vertebrate cells and represents an attractive therapeutic option for treating central nervous system diseases. However, growing evidence exists that the expression of short hairpin RNAs (shRNAs) can trigger cellular immune response resulting in unspecific cellular phenotypes and severe side effects. We found that lentiviral vector (LV)-mediated expression of shRNAs in primary cortical cultures resulted in strong expression of the interferon-stimulated gene oligoadenylate synthetase 1 (Oas1), which was accompanied by accelerated apoptosis and substantial net neuron loss. Modification of the shRNA construct by implementing features of the naturally occurring microRNA-30 (miR-30) precursor avoided Oas1 induction in transduced primary cultures, whereby modification of the passenger strand seems to be a crucial feature to circumvent interferon-stimulated gene expression. This work represents the first experimental study showing that an miR-30-based shRNA construct prevents Oas1 pathway associated off-target effects, which we consider as an essential prerequisite for shRNA use in future gene therapeutic approaches.

Imaging of Tautomerism in a Single Molecule

Fluorescence imaging is used to visualize directly the transfer of two inner hydrogen atoms in single porphycene molecules. This reaction leads to a chemically equivalent but differently oriented structure and hence results in a rotation of the transition dipole moments. By probing single immobilized molecules with an azimuthally polarized laser beam in the focal spot of a confocal microscope we observe ring-like emission patterns, possible only for a chromophore with two nearly orthogonal transition dipole moments. Numerical simulations of the observed emission patterns yield a value of 72 degrees for the angle between the S0-S1 transition moments in the two tautomeric forms.

Confocal microscopy with a high numerical aperture parabolic mirror

A novel high-resolution stage scanning confocal microscope for fluorescence microscopy and spatially resolved spectroscopy with a high numerical aperture (NA 1) parabolic mirror objective is investigated. A spatial resolution close to the diffraction limit is achieved. As microscopic fluorescent test objects, dye-loaded zeolite microcrystals (diameter approx. 0.4 microm) and single fluorescent molecules were used. Confocal fluorescence images show a spatial resolution of .x = 0.8 . both at room temperature and at 1.8 K. Imaging of a quasi-point light source and focusing by the parabolic mirror were investigated experimentally and theoretically. Deviations between the theoretical results for a perfect parabolic mirror and the experimental results can be attributed to small deviations of the mirror profile from an ideal parabola.

A high numerical aperture parabolic mirror as imaging device for confocal microscopy

We explore the diffraction limited focusing and confocal imaging properties of a high NA parabolic mirror for confocal imaging and spectroscopy of nanoparticles and single molecules. Vector field calculations of the electric fields near focus for both linear and radially polarized illumination are discussed and show that the optical field can be similar tightly focused as in the case of a high NA objective lens. Furthermore they show that a high NA parabolic mirror allows an easy orientation of the polarization of the illuminating light in all spatial directions. The simulation of confocal imaging of single molecules is discussed and yields, that the use of radially polarized excitation light gives an easy access to their orientations.

Analysis of the mouse MAP1B gene identifies a highly conserved 4.3 kb 3' untranslated region and provides evidence against the proposed structure of DBI-1 cDNA

We determined the previously unknown 3' end of MAP1B mRNA. We found an unusually long and highly conserved 3' untranslated region (3'UTR) of 4.3 kb and detected a polymorphism in the 3' flanking region probably due to the integration of an endogenous retroviral MuERV-L element. Furthermore, we found that MAP1B 3'UTR overlapped with the 5' end of the cDNA encoding DBI-1. However, further analysis suggested that the published structure of DBI-1 cDNA is most likely the result of fortuitous joining of unrelated cDNA fragments during cloning.