ZnO nanoparticles (NPs) are prone to dissolution, and uncertainty remains whether biological/cellular responses to ZnO NPs are solely due to the release of Zn(2+) or whether the NPs themselves have additional toxic effects. We address this by establishing ZnO NP solubility in dispersion media (Dulbecco's modified Eagle's medium, DMEM) held under conditions identical to those employed for cell culture (37 °C, 5% CO2, and pH 7.68) and by systematic comparison of cell-NP interaction for three different ZnO NP preparations. For NPs at concentrations up to 5.5 μg ZnO/mL, dissolution is complete (with the majority of the soluble zinc complexed to dissolved ligands in the medium), taking ca. 1 h for uncoated and ca. 6 h for polymer coated ones. Above 5.5 μg/mL, the results are consistent with the formation of zinc carbonate, keeping the solubilized zinc fixed to 67 μM of which only 0.45 μM is as free Zn(2+), i.e., not complexed to dissolved ligands. At these relatively high concentrations, NPs with an aliphatic polyether-coating show slower dissolution (i.e., slower free Zn(2+) release) and reprecipitation kinetics compared to those of uncoated NPs, requiring more than 48 h to reach thermodynamic equilibrium. Cytotoxicity (MTT) and DNA damage (Comet) assay dose-response curves for three epithelial cell lines suggest that dissolution and reprecipitation dominate for uncoated ZnO NPs. Transmission electron microscopy combined with the monitoring of intracellular Zn(2+) concentrations and ZnO-NP interactions with model lipid membranes indicate that an aliphatic polyether coat on ZnO NPs increases cellular uptake, enhancing toxicity by enabling intracellular dissolution and release of Zn(2+). Similarly, we demonstrate that needle-like NP morphologies enhance toxicity by apparently frustrating cellular uptake. To limit toxicity, ZnO NPs with nonacicular morphologies and coatings that only weakly interact with cellular membranes are recommended.

Annealing [FeL2][BF4]2⋅2 H2O (L=2,6‐bis‐[5‐methyl‐1H‐pyrazol‐3‐yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T1/2=205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL2][BF4]2⋅2 H2O and [ZnL2][BF4]2⋅2 H2O are not perfectly isostructural but, unlike the iron compound, they undergo single‐crystal‐to‐single‐crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re‐cooling. The low‐temperature phase of [CuL2][BF4]2 involves ordering of its Jahn–Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low‐temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high‐spin iron compound, before the onset of spin‐crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper‐doped [Fe0.97Cu0.03L2][BF4]2 imply its low‐spin phase contains two distinct cation environments in a 2:1 ratio.

4‐(tert‐Butylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine (L) was obtained in low yield from a one‐pot reaction of 2,4,6‐trifluoropyridine with 2‐methylpropane‐2‐thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL2][BF4]2⋅solv (1[BF4]2⋅solv) and [FeL2][ClO4]2⋅solv (1[ClO4]2⋅solv; solv=MeNO2, MeCN or Me2CO) exhibit a variety of structures and spin‐state behaviors including thermal spin‐crossover (SCO). Solvent loss on heating 1[BF4]2⋅x MeNO2 (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a “reverse‐SCO” step in its warming cycle at a scan rate of 5 K min−1. The reverse‐SCO is not observed in a slower 1 K min−1 measurement, however, confirming its kinetic nature. The final product [FeL2][BF4]2⋅0.75 MeNO2 was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF4]2⋅y Me2CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low‐spin 1[ClO4]2⋅0.5 Me2CO is not isostructural with its BF4− congener, and undergoes single‐crystal‐to‐single‐crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the Ptrue1‾ space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low‐temperature phases when the materials are fully low‐spin.

ABSTRACT:The crystallization of ZnO microrods by hydrothermal treatment of a suspension formed from reaction of zinc acetate and sodium hydroxide has been examined using scanning and transmission electron microscopy as a function of hydrothermal reaction time. Polycrystalline hexagonal ZnO microrods first appeared after 0.5 h reaction time at 120 °C. These early stage rods were composed of stacks of hexagonal layers, each ~ 50 nm in thickness containing closely aligned assemblies of nanocrystallites < 20 nm in size. A further growth stage involving smaller pseudo-hexagonal columns nucleating and growing along <0001> from the (0001) basal layers of preformed polycrystalline hexagonal microrods was identified. For reaction times > 3h, the microrods were single crystals, as identified by TEM lattice imaging and electron diffraction, and many were in the form of double rods.

Improved powders for capturing CO2 at high temperatures are required for H2 production using sorption‐enhanced steam reforming. Here, we examine the relationship between particle structure and carbonation rate for two types of Na2ZrO3 powders. Hollow spray‐dried microgranules with a wall thickness of 100–300 nm corresponding to the dimensions of the primary acetate‐derived particles gave about 75 wt % theoretical CO2 conversion after a process‐relevant 5 min exposure to 15 vol % CO2. A conventional powder prepared by solid‐state reaction carbonated more slowly, achieving only 50 % conversion owing to a greater proportion of the reaction requiring bulk diffusion through the densely agglomerated particles. The hollow granular structure of the spray‐dried powder was retained postcarbonation but chemical segregation resulted in islands of an amorphous Na‐rich phase (Na2CO3) within a crystalline ZrO2 particle matrix. Despite this phase separation, the reverse reaction to re‐form Na2ZrO3 could be achieved by heating each powder to 900 °C in N2 (no dwell time). This resulted in a very stable multicycle performance in 40 cycle tests using thermogravimetric analysis for both powders. Kinetic analysis of thermogravimetric data showed the carbonation process fits an Avrami–Erofeyev 2 D nucleation and nuclei growth model, consistent with microstructural evidence of a surface‐driven transformation. Thus, we demonstrate that spray drying is a viable processing route to enhance the carbon capture performance of Na2ZrO3 powder.