Comparison of the Phase Transitions of High-pressure Phases of Ammonium Fluoride and Ice at Ambient Pressure

The phase diagrams of water and ammonium fluoride (NH4F) display some interesting parallels. Several of the crystalline NH4F phases have isostructural ice counterparts and one of the famous anomalies of water, the fact that the liquid is denser than ice Ih, is also found for NH4F. Here we investigate the phase transitions of the pressure-quenched high-pressure phases of NH4F upon heating at ambient pressure with X-ray diffraction and calorimetry, and we compare the results with the corresponding ices. NH4F II transforms to NH4F Isd which is a stacking-disordered variant of the stable hexagonal NH4F Ih polymorph. Heating NH4F III gives a complex mixture of NH4F II and NH4F Isd while some NH4F III remains initially. Complete conversion to NH4F Isd is achieved above ~220 K. The NH4F II obtained from NH4F III persists to much higher temperatures compared to the corresponding pressure-quenched NH4F II. Quantification of the stacking disorder in NH4F Isd reveals a more sluggish conversion to NH4F Ih for NH4F Isd from NH4F III. In general, the presence of stress and strain in the samples appears to have pronounced effects on the phase transition temperatures. NH4F shows a complete lack of amorphous forms at low temperatures either upon low-temperature compression of NH4F Ih or heating NH4F III. The amorphous forms of ice are often used to explain the anomalies of water. It will therefore be interesting to explore if liquid NH4F displays more water-like anomalies despite the apparent lack of amorphous forms at low temperatures

Highly efficient heavy-metal extraction from water with carboxylated graphene nanoflakes

Heavy metals such a lead or cadmium have a wide range of detrimental and devastating effects on human health. It is therefore of paramount importance to efficiently remove heavy metals from industrial wastewater streams as well as drinking water. Carbon materials, including graphene and graphene oxide (GO), have recently been advocated as efficient sorption materials for heavy metals. We show that highly carboxylated graphene nanoflakes (cx-GNF) outperform nano-graphene oxide (nGO) as well as traditional GO with respect to extracting Fe 2+ , Cu 2+ , Fe 3+ , Cd 2+ and Pb 2+ cations from water. The sorption capacity for Pb 2+ , for example, is more than six times greater for the cx-GNF compared to GO which is attributed to the efficient formation of lead carboxylates as well as strong cation-π interactions. The large numbers of carboxylic acid groups as well as the intact graphenic regions of the cx-GNF are therefore responsible for the strong binding of the heavy metal cations. Remarkably, the performance of the as-made cx-GNF can easily compete with previously reported carbon materials that have undergone additional chemical-functionalisation procedures for the purpose of heavy-metal extraction. Furthermore, the recyclability of the cx-GNF material with respect to Pb 2+ loading is demonstrated as well as the outstanding performance for Pb 2+ extraction in the presence of excess Ca 2+ or Mg 2+ cations which are often present under environmental conditions. Out of all the graphene materials, the cx-GNF therefore show the greatest potential for future application in heavy-metal extraction processes

A simple and mild chemical oxidation route to high-purity nano-graphene oxide

Nano-graphene oxide (nGO) is used in a wide range of applications including cellular imaging, drug delivery, desalination and energy storage. Current preparation protocols are similar as for standard graphene oxide (GO) and typically rely on mixtures of sulfuric acid and potassium permanganate. We present a new route to nGO (∼30 nm diameter) using a quite defective arc-discharge carbon source and only 9 M nitric acid as the oxidising agent. The preparation can be scaled up proportionately with current GO protocols with 50 mL of half-concentrated nitric acid able to process one gram of arc-discharge material. The workup is straight forward and involves neutralization with sodium hydroxide which precipitates the sodium salt of nGO from solution. The only by-product of the new procedure is aqueous sodium nitrate which makes this protocol the cleanest route yet to nGO. The presence and quantities of functional groups in our nGO are determined and compared with standard GO. We anticipate that this new route to nGO will foster a range of new applications. The presence of highly reactive carboxylic anhydride groups on our nGO material in particular offers an excellent opportunity for purpose-specific chemical functionalization

CO₂ ice structure and density under Martian atmospheric conditions

Clouds composed of CO2 ice form throughout the Martian atmosphere. In the mesosphere, CO2 ice clouds are thought to form via heterogeneous ice nucleation on nanoparticles of meteoric origin at temperatures often below 100 K. Lower altitude CO2 ice clouds in the wintertime polar regions form up to around 145 K and lead to the build-up of the polar ice caps. However, the crystal structure and related fundamental properties of CO2 ice under Martian conditions are poorly characterised. Here we present X-ray diffraction (XRD) measurements of CO2 ice, grown via deposition from the vapour phase under temperature and pressure conditions analogous to the Martian mesosphere. A crystalline cubic structure was determined, consistent with the low-pressure polymorph (CO2-I, space group Pa-3 (No. 205)). CO2 deposited at temperatures of 80 - 130 K and pressures of 0.01 – 1 mbar was consistent with dry ice and previous literature measurements, thus removing the possibility of a more complicated phase diagram for CO2 in this region. At 80 K, a lattice parameter of 5.578 ± 0.002 Å, cell volume of 173.554 ± 0.19 Å3 and density of 1.684 ± 0.002 g cm−3 was determined. Using these measurements, we determined the thermal expansion of CO2 across 80 – 130 K that allowed for a fit of CO2 ice density measurements across a larger temperature range (80 – 195 K) when combined with literature data (CO2 density = 1.72391 - 2.53×10−4 T - 2.87×10−6 T2). Temperature-dependent CO2 density values are used to estimate sedimentation velocities and heterogeneous ice nucleation rates, showing an increase in nucleation rate of up to a factor of 1000 when compared to commonly used literature values. This temperature-dependent equation of state is therefore suggested for use in future studies of Martian mesospheric CO2 clouds. Finally, we discuss the possible shapes of crystals of CO2 ice in the Martian atmosphere and show that a range of shapes including cubes and octahedra as well as a combination of the two in the form of cubo-octahedra are likely

Structure and nature of ice XIX

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a Pbcn-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials

Graphene Nanoflake Antibody Conjugates for Multimodal Imaging of Tumors

Graphene-based materials are promising scaffolds for use in the design of tailored-made nanomedicines. Herein, the synthesis and characterization of a series of multifunctional carboxylated graphene nanoflakes (GNFs) conjugated to monoclonal antibodies (mAbs) for tumor-specific binding and modulation of pharmacokinetics is presented. GNF–mAb constructs are coupled to a fluorophore (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene [BODIPY]) for applications in optical imaging, a paramagnetic Gd3+ complex, [GdDOTAGA(H2O)]−, and the hexadentate chelate desferrioxamine B (DFO) for radiolabeling with 89Zr4+ (t1/2 = 78.41 h) ions and applications in dual-modality positron emission tomography/magnetic resonance imaging (PET/MRI). Experimental properties of [89Zr]GdDOTAGA–ZrDFO–GNF–trastuzumab are tested in extensive chemical, spectroscopic, radiochemical, and cellular assays in vitro, and assessment of the pharmacokinetics by PET imaging in mice bearing a human ovarian cancer model illustrates the potential of using GNF–mAbs to develop multifunctional PET/MRI probes

Origin of the low-temperature endotherm of acid-doped ice VI: new hydrogen-ordered phase of ice or deep glassy states?

On the basis of a low-temperature endotherm, it has recently been argued that cooling acid-doped ice VI at high pressures leads to a new hydrogen-ordered phase. We show that the endotherms are in fact caused by the glass transitions of deep glassy states related to ice VI. As expected for such endothermic overshoot effects, they display a characteristic dependence on pressure and cooling rate, they can be produced by sub-T_{g} annealing at ambient pressure, and they can be made to appear or disappear depending on the heating rate and the initial extent of relaxation. It is stressed that the existence of a new crystalline phase, as it has been suggested, cannot depend on the heating rate at which it is heated. X-ray diffraction shows that samples for which the low-temperature endotherm is present, weak or absent, as observed at a heating rate of 5 K min⁻¹, are structurally very similar. Furthermore, we show that the reported shifts of the (102) Bragg peak upon heating are fully consistent with our scenario and also with our earlier neutron diffraction study. Deuterated acid-doped ice VI cooled at high pressure also displays a low-temperature endotherm and its neutron diffraction pattern is consistent with deep glassy ice VI. Accessing deep glassy states of ice with the help of acid doping opens up a fascinating new chapter in ice research. Compared to pure ice VI, the glass transition temperature is lowered by more than 30 K by the acid dopant. Future work should focus on the deep glassy states related to all the other hydrogen-disordered ices including the ‘ordinary’ ice Ih