The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assemblies are magnetically soft, and an evolution of the magnetic microstructure is observed with consecutive scans of the microscope tip.

Rarely observed nanoparticle dissolution rate data have been collected and explained for an environmentally and industrially relevant nanomaterial (PbS, the mineral galena) as a function of its particle size and aggregation state using high-resolution transmission electron microscopy (HRTEM) and solution analysis. Under identical anoxic acidic conditions (pH 3 HCl), it has been determined that the dissolution rate of PbS galena varies by at least 1 order of magnitude simply as a function of particle size, and also due to the aggregation state of the particles (dissolution rates measured are 4.4 x 10(-9) mol m(-2) s(-1) for dispersed 14 nm nanocrystals; 7.7 x 10(-10) mol m(-2) s(-1) for dispersed 3.1 microm microcrystals; and 4.7 x 10(-10) mol m(-2) s(-1) for aggregated 14 nm nanocrystals). The dissolution rate difference between galena microparticles and nanoparticles is due to differences in nanotopography and the crystallographic faces present. Aggregate vs. dispersed dissolution rates are related to transport inhibition in the observed highly confined spaces between densely packed, aggregated nanocrystals, where self-diffusion coefficients of water and ions decrease dramatically. This study shows that factors at the nanometer scale significantly influence the release rate of aqueous, highly toxic and bioavailable Pb in natural or industrial environments during galena dissolution.

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The development of antibiotics revolutionized human health, providing a simple cure for once dreaded diseases such as tuberculosis. However, widespread production, use, and mis-use of antibiotics have contributed to the next-generation concern for global public health: the emergence of multiple drug-resistant (MDR) infectious organisms (a.k.a. “superbugs”). Recently, nanotechnology, specifically the use of nanomaterials (NMs) with antimicrobial activity, has been presented as a new defense against MDR infectious organisms. We discuss the potential for NMs to either circumvent microbial resistance or induce its development in light of our current state of knowledge, finding that this question points to a need for fundamental research targeting the molecular mechanisms causing antimicrobial activity in NMs. In the context of current microbial nanotoxicology studies, particularly reductionist laboratory studies, we offer suggestions and considerations for future research, using an illustrative example from our work with silver nanoparticles.

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When deposited from an evaporating solution onto a substrate, even nondescript nanoparticles can organize into intricate spatial patterns. Here we show that a simple but long-ranged anisotropy in nanoparticles' interactions can greatly enrich this scenario. In experiments with colloidal Co nanocrystals, which bear a substantial magnetic dipole, we observe assemblies quite distinct from those formed by nonmagnetic particles. Reflecting the strongly nonequilibrium nature of this process, nanocrystal aggregates also differ substantially from expected low-energy arrangements. Using coarse-grained computer simulations of dipolar nanoparticles, we have identified several dynamical mechanisms from which such unusual morphologies can arise. For particles with modest dipole moments, transient connections between growing domains frustrate phase separation into sparse and dense regions on the substrate. Characteristic length scales of the resulting cellular networks depend non-monotonically on the depth of quenches we use to mimic the effects of solvent evaporation. For particles with strong dipole moments, chain-like aggregates formed at early times serve as the agents of assembly at larger scales. Their effective interactions drive the formation of layered loop structures similar to those observed in experiments.

Environmental context. The advent of nanotechnology means that the release of nanomaterials into the environment is very likely, if not inevitable, and knowing the environmental impact of such nanomaterials is important. A key aspect of understanding this impact is to learn how nanomaterials affect microorganisms, a critical part of the environment; this topic is addressed in this review, which specifically concerns nanoparticle–bacteria interactions. Current studies show that nanoparticles have the potential to impact bacterial viability, although a great deal remains to be understood concerning nanoparticle–bacteria interactions. Abstract. Part of the responsible use of nanotechnology will be to better delineate the potential impact of nanomaterials released into the environment. A key aspect of understanding this impact is to examine the interaction between nanomaterials and microorganisms, which are not only highly abundant in nature but critical for global environmental processes. In this Highlight, current knowledge about the interaction between bacteria and industrially-relevant nanoparticles is reviewed. Important areas for further study are discussed.

We report the first characterization of the internal structural features within rhombohedral nanocrystals of hematite (α-Fe 2 O 3 ), specifically nanoscale pores (nanopores) within these crystals observed by high-angle annular dark-field scanning transmission electron microscopy tomography. Threedimensional observations of the internal structure of hematite nanocrystals suggest that the nanopores are formed due to a large reduction in solid volume during the transformation of a poorly crystalline precursor [aggregates of ferrihydrite: Fe 8.2 O 8.5 (OH) 7.4 ⋅3H 2 O], which results in the formation of pores between grain boundaries. This formation mechanism is different from those previously reported, such as hollow cores originating from screw dislocations. We also discuss dissolution experiments of the hematite nanocrystals in ascorbic acid solution, in which we demonstrated that the nanopores are reactive sites for dissolution and enlarged by preferential etching. Our findings are of fundamental importance to understanding how certain crystal morphologies, internal structures, defects, and reactive sites occur in nanocrystals formed from a poorly crystalline precursor.