Featured this week
Plutonium is a transuranic radioactive chemical element with symbol Pu and atomic number 94. It is an actinide metal of silvery-grey appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.  Very small amounts of plutonium occur naturally. Plutonium-239 and plutonium-240 are formed in nuclear power plants when uranium-238 captures neutrons. 
It has taken one of the most powerful lasers on the planet, but scientists have done it. They’ve confirmed the existence of ‘superionic’ hot ice – frozen water that can remain solid at thousands of degrees of heat. This bizarre form of ice is possible because of tremendous pressure, and the findings of the experiment could shed light on the interior structure of giant ice planets such as Uranus and Neptune. On Earth’s surface, the boiling and freezing points of water vary only a little – generally boiling when it’s very hot, and freezing when it’s cold. But both these state changes are at the whim of pressure (that’s why the boiling point of water is lower at higher altitudes). In the vacuum of space, water can’t exist in its liquid form. It immediately boils and vaporises even at -270 degrees Celsius – the average temperature of the Universe – before desublimating into ice crystals. But it’s been theorised that in extremely high-pressure environments, the opposite occurs: the water solidifies, even at extremely high temperatures. Scientists at Lawrence Livermore National Laboratory directly observed this for the first time just recently, detailed in a paper last year. They created Ice VII, which is the crystalline form of ice above 30,000 times Earth’s atmospheric pressure, or 3 gigapascals, and blasted it with lasers. The resulting ice had a conductive flow of ions, rather than electrons, which is why it’s called superionic ice. Now they’ve confirmed it with follow-up experiments. They have proposed the new form be named Ice XVIII. In the previous experiment, the team had only been able to observe general properties, such as energy and temperature; the finer details of the internal structure remained elusive. So, they designed an experiment using laser pulses and X-ray diffraction to reveal the ice’s crystalline structure. “We wanted to determine the atomic structure of superionic water,” said physicist Federica Coppari of the LLNL. “But given the extreme conditions at which this elusive state of matter is predicted to be stable, compressing water to such pressures and temperatures and simultaneously taking snapshots of the atomic structure was an extremely difficult task, which required an innovative experimental design.” Here’s that design. First, a thin layer of water is placed between two diamond anvils. Then six giant lasers are used to generate a series of shockwaves at progressively increasing intensity to compress the water at pressures up to 100-400 gigapascals, or 1 to 4 million times Earth’s atmospheric pressure. At the same time, they produce temperatures between 1,650 and 2,760 degrees Celsius (the surface of the Sun is 5,505 degrees Celsius). This experiment was designed so that the water would freeze when compressed, but since the pressure and temperature conditions could only be maintained for a fraction of a second, the physicists were uncertain that the ice crystals would form and grow. So, they used lasers to blast a tiny piece of iron foil with 16 additional pulses, creating a wave of plasma that generated an X-ray flash at precisely the right time. These flashes diffracted off the crystals inside, showing the compressed water was indeed frozen and stable. “The X-ray diffraction patterns we measured are an unambiguous signature for dense ice crystals forming during the ultrafast shockwave compression demonstrating that nucleation of solid ice from liquid water is fast enough to be observed in the nanosecond timescale of the experiment,” Coppari said. These X-rays showed a never-before-seen structure – cubic crystals with oxygen atoms at each corner, and an oxygen atom in the centre of each face. “Finding direct evidence for the existence of crystalline lattice of oxygen brings the last missing piece to the puzzle regarding the existence of superionic water ice,” said physicist Marius Millot of the LLNL. “This gives additional strength to the evidence for the existence of superionic ice we collected last year.” The result reveals a clue to how ice giants such as Neptune and Uranus could have such strange magnetic fields, tilted at bizarre angles, and with equators that don’t circle the planet. Previously, it was thought that these planets had a fluid ocean of ionic water and ammonia in place of a mantle. But the team’s research shows that these planets could have a solid mantle, like Earth, but made of hot superionic ice instead of hot rock. Because superionic ice is highly conductive, this could be influencing the planets’ magnetic fields. “Because water ice at Uranus and Neptune’s interior conditions has a crystalline lattice, we argue that superionic ice should not flow like a liquid such as the fluid iron outer core of the Earth. Rather, it’s probably better to picture that superionic ice would flow similarly to the Earth’s mantle, which is made of solid rock, yet flows and supports large-scale convective motions on the very long geological timescales,” Millot said. “This can dramatically affect our understanding of the internal structure and the evolution of the icy giant planets, as well as all their numerous extrasolar cousins.” The research has been published in Nature.
Researchers have developed an electrochemistry-based method to detect surfactants, specifically perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), with high sensitivity and specificity (Anal. Chem. 2019, DOI: 10.1021/acs.analchem.9b01060). Perfluorinated surfactants are highly stable due to perfluoroalkyl moieties, and are common in products like non-stick coatings and fire-fighting foam. Chronic exposure to two such perfluoroalkyl substances, PFOS and PFOA, has been linked to health issues in humans. Though these two chemicals are no longer used in industry, they persist in the environment and can contaminate drinking water. Long Luo, an analytical chemist at Wayne State University, began his search for a novel way to detect these harmful chemicals after one such PFOS/PFOA contamination event in a Michigan town during the summer of 2018. The most commonly used detection method uses high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS), which requires complex instrumentation and can cost up to $300 per sample, Luo says. Hoping to develop a simpler, less expensive method, the team turned to electrochemistry. Their method is based on a phenomenon known as electrochemical bubble nucleation. Applying electric potential to an electrode in an aqueous solution splits water into hydrogen gas and oxygen. Ramping up the current, increases gas concentration near the electrode until a bubble forms, blocking the electrode surface and causing the current to drop. Surfactants reduce surface tension and make it easier for such bubbles to form, meaning the amount of current required to form those bubbles is inversely related to surfactant concentration. To test their method, Luo and his collaborators fabricated tiny platinum electrodes less than 100 nm in diameter (smaller electrodes are more sensitive). The team could detect PFOS and PFOA concentrations as low as 80 µg/L and 30 µg/L, respectively. Preconcentrating samples using solid-phase extraction moved the limit of detection below 70 ng/L—the health advisory level for drinking water set by the U.S. Environmental Protection Agency. The method also remained sensitive and selective for surfactant detection even in the presence of a 1,000-fold greater concentration of poly(ethylene glycol), a non-surfactant molecule with a molecular weight similar to that of PFOS. “Electrochemical methods, in general, have great promise for measuring very low concentrations of contaminants in complex matrices,” says Michelle Crimi, an environmental engineer at Clarkson University. “I look forward to hearing more about the future of this technology, including its validation in field-contaminated water samples.” Creating a handheld device for testing water in streams and other field sites—not just drinking water—is the ultimate goal, Luo says. An important step in that process will be developing a pre-treatment phase to eliminate other surfactants that also promote bubble formation at electrodes, like sodium dodecyl sulfate. Such interference would be unlikely in drinking water samples, Luo says, because most compounds are not as stable as perfluoroalkyl substances and are destroyed during water treatment processing.