Background-One of the features of high-risk atherosclerotic plaques is a preponderance of macrophages. Experimental studies with hyperlipidemic rabbits have shown that ultrasmall superparamagnetic particles of iron oxide (USPIOs) accumulate in plaques with a high macrophage content and that this induces magnetic resonance (MR) signal changes. The purpose of our study was to investigate whether USPIO-enhanced MRI can also be used for in vivo detection of macrophages in human plaques. Methods and Results-MRI was performed on 11 symptomatic patients scheduled for carotid endarterectomy before and 24 (nϭ11) and 72 (nϭ5) hours after administration of USPIOs (Sinerem) at a dose of 2.6 mg Fe/kg. Histological and electron microscopical analyses of the plaques showed USPIOs primarily in macrophages within the plaques in 10 of 11 patients. Histological analysis showed USPIOs in 27 of 36 (75%) of the ruptured and rupture-prone lesions and 1 of 14 (7%) of the stable lesions. Of the patients with USPIO uptake, signal changes in the post-USPIO MRI were observed by 2 observers in the vessel wall in 67 of 123 (54%) and 19 of 55 (35%) quadrants of the T2*-weighted MR images acquired after 24 and 72 hours, respectively. For those quadrants with changes, there was a significant signal decrease of 24% (95% CI, 33% to 15%) in regions of interest in the images acquired after 24 hours, whereas no significant signal change was found after 72 hours. Conclusions-Accumulation

A key issue in research on ferrofluids (dispersions of magnetic colloids) is the effect of dipolar interactions on their structure and phase behaviour, which is not only important for practical applications but gives fundamental insight in dipolar fluids in general. In 1970, de Gennes and Pincus predicted a Van der Waals-like phase diagram and the presence of linear chains of particles in ferrofluids in zero magnetic field. Despite many experimental studies, no direct evidence of the existence of linear chains of dipoles has been reported in the absence of magnetic field, although simulations clearly show the presence of chain-like structures. Here, we show in situ linear dipolar structures in ferrofluids in zero field, visualized on the particle level by electron cryo-microscopy on thin, vitrified films of organic dispersions of monodisperse metallic iron particles. On systematically increasing the particle size, we find an abrupt transition from separate particles to randomly oriented linear aggregates and branched chains or networks. When vitrified in a permanent magnetic field, these chains align and form thick elongated structures, indicating lateral attraction between parallel dipole chains. These findings show that the experimental model used is well suited to study the structural properties of dipolar particle systems.

The investigation of solution‐borne nanostructures by transmission electron microscopy (TEM) is a frequently used analytical method in materials chemistry. In many cases, the preparation of the TEM sample involves drying and staining steps, and the collection of images leads to the interaction of the specimen with the electron beam. Both aspects call for cautious interpretation of the resulting electron micrographs. Alternatively, a near‐native solvated state can be preserved by cryogenic vitrification and subsequent imaging by low‐dose cryogenic TEM. In this Minireview, we provide a critical analysis of sample preparation, and more importantly, of the acquisition and interpretation of electron micrographs. This overview should provide a framework for the application of (cryo)‐TEM as a powerful and reliable tool for the analysis of colloidal and self‐assembled structures with nanoscopic dimensions.

Controlled modification of horseradish peroxidase with an apolar polymer chain (see figure) by cofactor reconstitution leads to giant amphiphiles, which form vesicular aggregates in aqueous solution.

Inspired by the remarkable shapes and properties of CaCO(3) biominerals, many studies have investigated biomimetic routes aiming at synthetic equivalents with similar morphological and structural complexity. Control over the morphology of CaCO(3) crystals has been demonstrated, among other methods, by the use of additives that selectively allow the development of specific crystal faces, while inhibiting others. Both for biogenic and biomimetic CaCO(3), the crystalline state is often preceded by an amorphous precursor phase, but still limited information is available on the details of the amorphous-to-crystalline transition. By using a combination of cryoTEM techniques (bright field imaging, cryo-tomography, low dose electron diffraction and cryo-darkfield imaging), we show for the first time the details of this transition during the formation of hexagonal vaterite crystals grown in the presence of NH(4)(+) ions. The formation of hexagonal plate-like vaterite occurs via an amorphous precursor phase. This amorphous phase converts into the crystalline state through a solid state transformation in which order and morphology develop simultaneously. The mineral initially develops as polycrystalline vaterite which transforms into a single crystal directed by an NH(4)(+)-induced crystal plane that acts as a templating surface.