303 research outputs found
Nano-assemblies of cationic mPEG brush block copolymers with gadolinium polyoxotungstate [Gd(W5O18)2]9â form stable, high relaxivity MRI contrast agents
Polyoxometalates (POMs) incorporating paramagnetic ions, such as gadolinium, show promise as contrast agents for application in magnetic resonance imaging (MRI). Specifically, [Gd(W5O18)2]9â (denoted as GdWO) has been reported to have a higher relaxivity than commercially available contrast agents, but it's clinical utility has been limited by the intrinsic instability of POMs at physiological pH (7.4). In the current report we present a stability study on neat GdWO and nano-assemblies of block copolymers with GdWO in the pH range 5.0â7.4 to assess their suitability as MRI contrast agents. Neat GdWO only maintained structural stability between pH 5.4 and 6.4, and demonstrated poor MRI contrast at pH 7.4. To address this pH instability, GdWO was self-assembled with cationic mPEG brush block copolymers containing 20 or 40 units derived from the cationic monomer, 2-dimethylaminoethyl methacrylate (DMAEMA). Nano-assemblies with different charge ratios were synthesised and characterised according to their size, stability, contrasting properties and toxicity. The longitudinal relaxivity (r1) of the nano-assemblies was found to be dependent on the charge ratio, but not on the length of the cationic polymer block. Further investigation of PDMAEMA20 nano-assemblies demonstrated that they were stable over the pH range 5.0â7.4, exhibiting a higher r1 than either neat GdWO (2.77 sâ1 mMâ1) or clinical MRI contrast agent Gd-DTPA (4.1 sâ1 mMâ1) at pH 7.4. Importantly, the nano-assembly with the lowest charge ratio (0.2), showed the highest r1 (12.1 sâ1 mMâ1) whilst, stabilising GdWO over the pH range studied, eliciting low toxicity with MDA-MB231 cells
Constraining f(R) gravity in the Palatini formalism
Although several models of theories of gravity within the Palatini
approach have been studied already, the interest was concentrated on those that
have an effect on the late-time evolution of the universe, by the inclusion for
example of terms inversely proportional to the scalar curvature in the
gravitational action. However, additional positive powers of the curvature also
provide interesting early-time phenomenology, like inflation, and the presence
of such terms in the action is equally, if not more, probable. In the present
paper models with both additional positive and negative powers of the scalar
curvature are studied. Their effect on the evolution of the universe is
investigated for all cosmological eras, and various constraints are put on the
extra terms in the actions. Additionally, we examine the extent to which the
new terms in positive powers affect the late-time evolution of the universe and
the related observables, which also determines our ability to probe their
presence in the gravitational action.Comment: reference update and minor changes to match published versio
Methods of photoelectrode characterization with high spatial and temporal resolution
Materials and photoelectrode architectures that are highly efficient, extremely stable, and made from low cost materials are required for commercially viable photoelectrochemical (PEC) water-splitting technology. A key challenge is the heterogeneous nature of real-world materials, which often possess spatial variation in their crystal structure, morphology, and/or composition at the nano-, micro-, or macro-scale. Different structures and compositions can have vastly different properties and can therefore strongly influence the overall performance of the photoelectrode through complex structureâproperty relationships. A complete understanding of photoelectrode materials would also involve elucidation of processes such as carrier collection and electrochemical charge transfer that occur at very fast time scales. We present herein an overview of a broad suite of experimental and computational tools that can be used to define the structureâproperty relationships of photoelectrode materials at small dimensions and on fast time scales. A major focus is on in situ scanning-probe measurement (SPM) techniques that possess the ability to measure differences in optical, electronic, catalytic, and physical properties with nano- or micro-scale spatial resolution. In situ ultrafast spectroscopic techniques, used to probe carrier dynamics involved with processes such as carrier generation, recombination, and interfacial charge transport, are also discussed. Complementing all of these experimental techniques are computational atomistic modeling tools, which can be invaluable for interpreting experimental results, aiding in materials discovery, and interrogating PEC processes at length and time scales not currently accessible by experiment. In addition to reviewing the basic capabilities of these experimental and computational techniques, we highlight key opportunities and limitations of applying these tools for the development of PEC materials
A 2.3-Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?
We present the results of an intensive campaign of spectroscopic and
photometric monitoring of the peculiar Wolf-Rayet star WR 134 from 1989 to
1997.
This unprecedentedly large data set allows us to confirm unambiguously the
existence of a coherent 2.25 +/- 0.05 day periodicity in the line-profile
changes of He II 4686, although the global pattern of variability is different
from one epoch to another. This period is only marginally detected in the
photometric data set. Assuming the 2.25 day periodic variability to be induced
by orbital motion of a collapsed companion, we develop a simple model aiming at
investigating (i) the effect of this strongly ionizing, accreting companion on
the Wolf-Rayet wind structure, and (ii) the expected emergent X-ray luminosity.
We argue that the predicted and observed X-ray fluxes can only be matched if
the accretion on the collapsed star is significantly inhibited. Additionally,
we performed simulations of line-profile variations caused by the orbital
revolution of a localized, strongly ionized wind cavity surrounding the X-ray
source. A reasonable fit is achieved between the observed and modeled
phase-dependent line profiles of He II 4686. However, the derived size of the
photoionized zone substantially exceeds our expectations, given the observed
low-level X-ray flux. Alternatively, we explore rotational modulation of a
persistent, largely anisotropic outflow as the origin of the observed cyclical
variability. Although qualitative, this hypothesis leads to greater consistency
with the observations.Comment: 34 pages, 16 figures. Accepted by the Astrophysical Journa
The Running of the Cosmological and the Newton Constant controlled by the Cosmological Event Horizon
We study the renormalisation group running of the cosmological and the Newton
constant, where the renormalisation scale is given by the inverse of the radius
of the cosmological event horizon. In this framework, we discuss the future
evolution of the universe, where we find stable de Sitter solutions, but also
"big crunch"-like and "big rip"-like events, depending on the choice of the
parameters in the model.Comment: 14 pages, 7 figures, minor improvements, references adde
Quantum fields, cosmological constant and symmetry doubling
Energy-parity has been introduced by Kaplan and Sundrum as a protective
symmetry that suppresses matter contributions to the cosmological constant
[KS05]. It is shown here that this symmetry, schematically Energy --> - Energy,
arises in the Hilbert space representation of the classical phase space
dynamics of matter. Consistently with energy-parity and gauge symmetry, we
generalize the Liouville operator and allow a varying gauge coupling, as in
"varying alpha" or dilaton models. In this model, classical matter fields can
dynamically turn into quantum fields (Schroedinger picture), accompanied by a
gauge symmetry change -- presently, U(1) --> U(1) x U(1). The transition
between classical ensemble theory and quantum field theory is governed by the
varying coupling, in terms of a one-parameter deformation of either limit.
These corrections introduce diffusion and dissipation, leading to decoherence.Comment: Replaced by published version, no change in contents - Int. J. Theor.
Phys. (2007
Stratus 9/VOCALS ninth setting of the Stratus Ocean Reference Station & VOCALS Regional Experiment
The Ocean Reference Station at 20°S, 85°W under the stratus clouds west of northern Chile is
being maintained to provide ongoing climate-quality records of surface meteorology; air-sea
fluxes of heat, freshwater, and momentum; and of upper ocean temperature, salinity, and velocity
variability. The Stratus Ocean Reference Station (ORS Stratus) is supported by the National
Oceanic and Atmospheric Administrationâs (NOAA) Climate Observation Program. It is
recovered and redeployed annually, with cruises that have come between October and December.
During the 2008 cruise on the NOAA ship Ronald H. Brown to the ORS Stratus site, the primary
activities were recovery of the Stratus 8 WHOI surface mooring that had been deployed in
October 2007, deployment of a new (Stratus 9) WHOI surface mooring at that site; in-situ
calibration of the buoy meteorological sensors by comparison with instrumentation put on board
by staff of the NOAA Earth System Research Laboratory (ESRL); and observations of the stratus
clouds and lower atmosphere by NOAA ESRL. A buoy for the Pacific tsunami warning system
was also serviced in collaboration with the Hydrographic and Oceanographic Service of the
Chilean Navy (SHOA). The DART (Deep-Ocean Assessment and Reporting of Tsunami) carries
IMET sensors and subsurface oceanographic instruments. A DART II buoy was deployed north
of the STRATUS buoy, by personnel from the National Data Buoy Center (NDBC)
Argo floats and drifters were launched, and CTD casts carried out during the cruise.
The ORS Stratus buoys are equipped with two Improved Meteorological (IMET) systems, which
provide surface wind speed and direction, air temperature, relative humidity, barometric
pressure, incoming shortwave radiation, incoming longwave radiation, precipitation rate, and sea
surface temperature. Additionally, the Stratus 8 buoy received a partial CO2 detector from the
Pacific Marine Environmental Laboratory (PMEL). IMET data are made available in near real
time using satellite telemetry. The mooring line carries instruments to measure ocean salinity,
temperature, and currents.
The ESRL instrumentation used during the 2008 cruise included cloud radar, radiosonde
balloons, and sensors for mean and turbulent surface meteorology.
Finally, the cruise hosted a teacher participating in NOAAâs Teacher at Sea Program.Funding was provided by the National Oceanic and Atmospheric Administration
under Grant No. NA17RJ1223 for the Cooperative Institute for Climate and Ocean Research (CICOR)
Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'
Spatially averaged inhomogeneous cosmologies in classical general relativity
can be written in the form of effective Friedmann equations with sources that
include backreaction terms. In this paper we propose to describe these
backreaction terms with the help of a homogeneous scalar field evolving in a
potential; we call it the `morphon field'. This new field links classical
inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret,
e.g., quintessence scenarios by routing the physical origin of the scalar field
source to inhomogeneities in the Universe. We investigate a one-parameter
family of scaling solutions to the backreaction problem. Subcases of these
solutions (all without an assumed cosmological constant) include
scale-dependent models with Friedmannian kinematics that can mimic the presence
of a cosmological constant or a time-dependent cosmological term. We explicitly
reconstruct the scalar field potential for the scaling solutions, and discuss
those cases that provide a solution to the Dark Energy and coincidence
problems. In this approach, Dark Energy emerges from morphon fields, a
mechanism that can be understood through the proposed correspondence: the
averaged cosmology is characterized by a weak decay (quintessence) or growth
(phantom quintessence) of kinematical fluctuations, fed by `curvature energy'
that is stored in the averaged 3-Ricci curvature. We find that the late-time
trajectories of those models approach attractors that lie in the future of a
state that is predicted by observational constraints.Comment: 36 pages and 6 Figures, matches published version in Class.Quant.Gra
NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage
We report high-precision X-ray monitoring observations in the 0.4-10 keV band
of the luminous, long-period colliding-wind binary Eta Carinae up to and
through its most recent X-ray minimum/periastron passage in February 2020. Eta
Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux
level of ergs s cm, followed by a rapid
plunge to its observed minimum flux, ergs s
cm near 17 February 2020. The NICER observations show an X-ray recovery
from minimum of only 16 days, the shortest X-ray minimum observed so far.
We provide new constraints of the "deep" and "shallow" minimum intervals.
Variations in the characteristic X-ray temperature of the hottest observed
X-ray emission indicate that the apex of the wind-wind "bow shock" enters the
companion's wind acceleration zone about 81 days before the start of the X-ray
minimum. There is a step-like increase in column density just before the X-ray
minimum, probably associated with the presence of dense clumps near the shock
apex. During recovery and after, the column density shows a smooth decline,
which agrees with previous measurements made by SWIFT at the same
orbital phase, indicating that changes in mass-loss rate are only a few percent
over the two cycles. Finally, we use the variations in the X-ray flux of the
outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta
of ergs s near the time of the "Great Eruption'
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