Perovskite-type materials LaCoO3 and LaCoFeO3 were prepared from their component oxides using mechano-synthesis, by a new method of preparation designated as reactive grinding. When grinding additives are used, the perovskites generated via this process are of unprecedently high specific surface area (>100 m2/g) and their surface chemical properties are of particular interest. The perovskite surface shows a large concentration of surface OHs (due to the low grinding temperature) which upon calcination leads to reformation of the materials lattice. The presence of oxygen during calcination initiates a competitive process, which stabilizes sites with an excess of electronic charge, yielding reduced Co sites upon oxygen desorption (Co2+). XPS analysis illustrates an increase in these reduced Co2+ ion concentration with calcination temperature. At high calcination temperature (>673 K), TPD studies show the release of β-oxygen originating from the bulk.

Perovskite-type oxides synthesized by reactive grinding: Part I. Preparation and characterization

Antimicrobial sensitivity of Escherichia coli to alumina nanoparticles

Metal oxide nanoparticles (NPs) are known to possess strong antimicrobial properties. Aluminum oxide NPs have wide-range applications in industrial as well as personal care products. In the absence of prior reports on the antimicrobial properties of alumina NPs for a wide concentration range, the principal objective of the present work was to study the growth-inhibitory effect of alumina NPs over a wide concentration range (10–1000 μg/mL) on an environmentally relevant gram-negative model microorganism, Escherichia coli. The mean diameter of the NPs was determined to be 179 nm in aqueous dispersion used for this study, and surface area was determined to be 21.23 m 2 /g. The concentration of 1000 μg/mL was found to be moderately inhibitory for bacteria. Almost negligible dependence of growth rate on the concentration of the NPs was observed. The extracellular protein content was found to be slightly lower in case of cells interacting with 1000 μg/mL alumina than the uninteracted control cells. Fourier transform–infrared studies established differences in structure between interacted and uninteracted cells. Alumina NPs showed a mild growth-inhibitory effect, only at very high concentrations, which might be due to surface charge interactions between the particles and cells. Free-radical scavenging properties of the particles might have prevented cell wall disruption and drastic antimicrobial action. This laboratory-scale study suggests that alumina NPs may only exhibit mild toxicity toward microorganisms in the environment. From the Clinical Editor: Metal oxide NPs, inluding aluminum oxide NPs, are known to possess strong antimicrobial properties. The study demonstrated a mild to moderate growth-inhibitory effect of alumina NPs over a wide concentration range (10-1000 μg/mL) on Escherichia coli. Almost negligible dependence on the concentration was observed.

Original Article: Engineering Nanomedicine, Antimicrobial Nanoparticles Antimicrobial sensitivity of Escherichia coli to alumina nanoparticles

This paper, by no means exhaustive, focuses on high-energy ball-milling of oxides, on their mechanically induced changes and on the consequences of such changes on their physical and chemical properties. High-energy ball-milling offers a fortunate combination of technical simplicity and of complexity both of physical mechanisms which act during milling and of mechanosynthesized materials. Its basic interest, which stems from the large diversity of routes it offers to prepare oxides either directly or indirectly, is illustrated with various families of oxides. The direct path is to be favoured when as-milled oxides are of interest per se because of their nanocrystalline characteristics, their defects or their modified structures which result from mechanically driven phase transformations. The indirect path consists of a sequence of steps starting with mechanically activated oxides which may be subsequently just annealed or submitted to a combination of thermal treatments, with the possible occurrence of various chemical reactions, to prepare the sought-after materials with potential gains in processing temperatures and times. High energy ball-milling of oxides is more and more currently used to activate powders and to prepare nano-oxides at moderate temperatures. The interest of an activation step is well illustrated by the broad development of doped titania powders, synthesized by heat treatment of pre-ground reactants, for photocatalytic applications or to develop antibacterial materials. Another important class of applications of high-energy ball-milling is the formation of composites. It is exemplified here with the case of oxide-dispersed strengthened alloys whose properties are considerably improved by a dispersion of ultra-stable nanosized oxides whose formation mechanisms were recently described. The basic understanding of the mechanisms by which oxides or oxide mixtures evolve by high-energy ball-milling appears to be less advanced than it is for metallic materials essentially because of the overall complexity of the oxide structures, of their surfaces, of their defects and of their mechanical behavior.

Transformations in oxides induced by high-energy ball-milling

Aims
The aim of this study is to investigate the antibacterial activity of aluminium oxide nanoparticles (Al2O3 NPs) against multidrug-resistant clinical isolates of Escherichia coli and their interaction with cell envelope biomolecules.

Methods and Results
Al2O3 NPs were characterized by scanning electron microscope (SEM), high-resolution transmission electron microscope (HR-TEM) and X-ray diffraction (XRD) analyses. Antibacterial activity and interaction of Al2O3 NPs with E. coli and its surface biomolecules were assessed by spectrophotometry, SEM, HR-TEM and attenuated total reflectance/Fourier transform infrared (ATR-FTIR). Of the 80 isolates tested, about 64 (80%) were found to be extended spectrum β-lactamase (ESBL) positive and 16 (20%) were non-ESBL producers. Al2O3 NPs at 1000 μg ml−1 significantly inhibited the bacterial growth. SEM and HR-TEM analyses revealed the attachment of NPs to the surface of cell membrane and also their presence inside the cells due to formation of irregular-shaped pits and perforation on the surfaces of bacterial cells. The intracellular Al2O3 NPs might have interacted with cellular biomolecules and caused adverse effects eventually triggering the cell death. ATR-FTIR studies suggested the interaction of lipopolysaccharide (LPS) and L-α-Phosphatidyl-ethanolamine (PE) with Al2O3 NPs. Infrared (IR) spectral changes revealed that the LPS could bind to Al2O3 NPs through hydrogen binding and ligand exchange. The Al2O3 NPs-induced structural changes in phospholipids may lead to the loss of amphiphilic properties, destruction of the membrane and cell leaking.

Conclusions
The penetration and accumulation of NPs inside the bacterial cell cause pit formation, perforation and disorganization and thus drastically disturb its proper function. The cell surface biomolecular changes revealed by ATR-FTIR spectra provide a better understanding of the cytotoxicity of Al2O3 NPs.

Significance and Impact of the Study
Al2O3 NPs may serve as broad-spectrum bactericidal agents to control the emergent pathogens regardless of their drug-resistance mechanisms.

Interaction of Al2O3 nanoparticles with Escherichia coli and their cell envelope biomolecules

Room temperature, high energy ball milling was applied to various transition aluminas (γ, K, χ), producing thermodynamically stable α-alumina–a phenomenon that could otherwise be achieved only by high temperature (1100–1200 °C) heat treatment. The transformation proceeds in two steps. The first one consists of rapid microstructural rearrangements with continuously increasing α-transformation rate. In the second step (1–2 h from the start), only relatively small changes in morphology are observed with a constant α-transformation rate. The rate is influenced only by the milling intensity. The presence or the absence of oxygen in the milling atmosphere has a large influence on the final surface area of α-alumina.

P.A. ZielińAski

Papers

Structural transformations of alumina by high energy ball milling