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•Review of the state-of-the-art of U spectrochemistry in understanding gas-phase oxidation and molecular formation.•Emission, absorption and fluorescence spectroscopy of U atoms and ...molecules.•Modeling for predicting spatio-temporal evolution of U oxide formation.
Studies related to U gas-phase oxidation through plasma- and thermo-chemistry are important for many fields, including environmental monitoring, forensic analysis, debris analysis in a weapon detonation event, and nucleation physics. Recently, significant efforts have been made to understand the chemical pathways involved in the progression from U atoms to diatoms (UO) and polyatomic molecules (UxOy), employing optical spectroscopy tools and computational modeling. In many studies, laser ablation of U or a U-containing flow reactor are used as a highly resource-efficient, repeatable, tunable, and lab-scale testbed for studying gas-phase oxidation in U plasmas. The spectroscopic analysis of high-temperature gas-phase oxidation of U is challenging due to the congested U spectra, resolution limitations of instrumentation, and the numerous chemical reaction pathways possible. This article focuses on the current understanding and challenges related to studying U plasma chemistry, specifically U gas-phase oxidation and molecular formation, via optical spectroscopy of plasmas and associated computational and spectral modeling. The physical and chemical processes involved in the evolution from U atoms to U oxide molecules to nanoparticles and agglomerates (i.e., debris) are discussed in the context of optical spectroscopic studies. The article concludes by highlighting opportunities for future research efforts based on existing knowledge published in the literature.
Vertebrate genomes are partitioned into contact domains defined by enhanced internal contact frequency and formed by two principal mechanisms: compartmentalization of transcriptionally active and ...inactive domains, and stalling of chromosomal loop-extruding cohesin by CTCF bound at domain boundaries. While Drosophila has widespread contact domains and CTCF, it is currently unclear whether CTCF-dependent domains exist in flies. We genetically ablate CTCF in Drosophila and examine impacts on genome folding and transcriptional regulation in the central nervous system. We find that CTCF is required to form a small fraction of all domain boundaries, while critically controlling expression patterns of certain genes and supporting nervous system function. We also find that CTCF recruits the pervasive boundary-associated factor Cp190 to CTCF-occupied boundaries and co-regulates a subset of genes near boundaries together with Cp190. These results highlight a profound difference in CTCF-requirement for genome folding in flies and vertebrates, in which a large fraction of boundaries are CTCF-dependent and suggest that CTCF has played mutable roles in genome architecture and direct gene expression control during metazoan evolution.
Abstract
The formation of Ce
2
O
3
from cerium metal requires complex reducing conditions. Alternatively, we show that different stoichiometries and structures of cerium oxides can be generated in ...high‐temperature laser experiments by varying the availability of oxygen and the use of either pulsed laser ablation (PLA) or continuous wave (CW) laser heating of cerium metal. X‐ray diffraction and Raman spectroscopy suggest that a low structural order Ce
2
O
3
/CeO
2
mixed structure is formed in a 1% O
2
atmosphere with PLA. For CW heating in a 1% O
2
environment, Raman measurements show the formation of crystalline A‐type Ce
2
O
3
in proximity to the point of laser impingement. These experiments were repeated in
18
O
2
and allowed us to report for the first time the corresponding Raman spectrum of Ce
2
18
O
3
.
The early-time kinetic behavior of species in a plume produced from laser ablation of U3Si2 in an environment containing 2% O2 is characterized using time-resolved absorption spectroscopy. The UO ...band around 593.55 nm and the SiO band around 230 nm is observed as well as various atomic and ionic uranium transitions. Temperatures and concentrations of these species are tracked and reported as well.
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•Time-resolved absorption spectroscopy of laser ablated U3Si2 in oxygen environment.•Atomic and ionic species form initially followed by monoxides and then higher oxides.•Species development governed by oxygen handoff between uranium and silicon products.•Results can inform nuclear fireball post-detonation and fractionation models.
In order to model the fate and transport of particles following a nuclear explosion, there must first be an understanding of individual physical and chemical processes that affect particle formation. ...One interaction pertinent to fireball chemistry and resultant debris formation is that between uranium and oxygen. In this study, we use laser ablation of uranium metal in different concentrations of oxygen gas, either
O
or
O
, to determine the influence of oxygen on rapidly cooling uranium. Analysis of recovered particulates using infrared absorption and Raman spectroscopies indicate that the micrometer-sized particulates are predominantly amorphous UO
(am-UO
, where 3 ≤ x ≤ 4) and UO
after ablation in 1 atm of pure O
and a 1% O
/Ar mixture, respectively. Energy dispersive X-ray spectroscopy (EDS) of particulates formed in pure O
suggest an O/U ratio of ~ 3.7, consistent with the vibrational spectroscopy analysis. Both am-UO
and UO
particulates convert to α-U
O
when heated. Lastly, experiments performed in
O
environments show the formation of
O-substituted uranium oxides; vibrational frequencies for am-U
O
are reported for the first time. When compared to literature, this work shows that cooling timescales can affect the structural composition of uranium oxides (i.e., crystalline vs. amorphous). This indicator can be used in current models of nuclear explosions to improve our predicative capabilities of chemical speciation.
High-temperature chemistry in laser ablation plumes leads to vapor-phase speciation, which can induce chemical fractionation during condensation. Using emission spectroscopy acquired after ablation ...of a SrZrO3 target, we have experimentally observed the formation of multiple molecular species (ZrO and SrO) as a function of time as the laser ablation plume evolves. Although the stable oxides SrO and ZrO2 are both refractory, we observed emission from the ZrO intermediate at earlier times than SrO. We deduced the time-scale of oxygen entrainment into the laser ablation plume using an 18O2 environment by observing the in-growth of Zr18O in the emission spectra relative to Zr16O, which was formed by reaction of Zr with 16O from the target itself. Using temporally resolved plume-imaging, we determined that ZrO formed more readily at early times, volumetrically in the plume, while SrO formed later in time, around the periphery. Using a simple temperature-dependent reaction model, we have illustrated that the formation sequence of these oxides subsequent to ablation is predictable to first order.
The predictive models that describe the fate and transport of radioactive materials in the atmosphere following a nuclear incident (explosion or reactor accident) assume that uranium-bearing ...particulates would attain chemical equilibrium during vapor condensation. In this study, we show that kinetically driven processes in a system of rapidly decreasing temperature can result in substantial deviations from chemical equilibrium. This can cause uranium to condense out in oxidation states (e.g., UO3 vs UO2) that have different vapor pressures, significantly affecting uranium transport. To demonstrate this, we synthesized uranium oxide nanoparticles using a flow reactor under controlled conditions of temperature, pressure, and oxygen concentration. The atomized chemical reactants passing through an inductively coupled plasma cool from ∼5000 to 1000 K within milliseconds and form nanoparticles inside a flow reactor. The ex situ analysis of particulates by transmission electron microscopy revealed 2–10 nm crystallites of fcc-UO2 or α-UO3 depending on the amount of oxygen in the system. α-UO3 is the least thermodynamically preferred polymorph of UO3. The absence of stable uranium oxides with intermediate stoichiometries (e.g., U3O8) and sensitivity of the uranium oxidation states to local redox conditions highlight the importance of in situ measurements at high temperatures. Therefore, we developed a laser-based diagnostic to detect uranium oxide particles as they are formed inside the flow reactor. Our in situ measurements allowed us to quantify the changes in the number densities of the uranium oxide nanoparticles (e.g., UO3) as a function of oxygen gas concentration. Our results indicate that uranium can prefer to be in metastable crystal forms (i.e., α-UO3) that have higher vapor pressures than the refractory form (i.e., UO2) depending on the oxygen abundance in the surrounding environment. This demonstrates that the equilibrium processes may not dominate during rapid condensation processes, and thus kinetic models are required to fully describe uranium transport subsequent to nuclear incidents.
Early detection of SARS-CoV-2 infection is key to managing the current global pandemic, as evidence shows the virus is most contagious on or before symptom onset. Here, we introduce a low-cost, ...high-throughput method for diagnosing and studying SARS-CoV-2 infection. Dubbed Pathogen-Oriented Low-Cost Assembly & Re-Sequencing (POLAR), this method amplifies the entirety of the SARS-CoV-2 genome. This contrasts with typical RT-PCR-based diagnostic tests, which amplify only a few loci. To achieve this goal, we combine a SARS-CoV-2 enrichment method developed by the ARTIC Network (
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