A versatile synthesis method of dendrites-free segmented nanowires with a precise size control

Magnetic Nanomaterials as Contrast Agents for MRI

Magnetic Resonance Imaging (MRI) is a powerful, noninvasive and nondestructive technique, capable of providing three-dimensional (3D) images of living organisms. The use of magnetic contrast agents has allowed clinical researchers and analysts to significantly increase the sensitivity and specificity of MRI, since these agents change the intrinsic properties of the tissues within a living organism, increasing the information present in the images. Advances in nanotechnology and materials science, as well as the research of new magnetic effects, have been the driving forces that are propelling forward the use of magnetic nanostructures as promising alternatives to commercial contrast agents used in MRI. This review discusses the principles associated with the use of contrast agents in MRI, as well as the most recent reports focused on nanostructured contrast agents. The potential applications of gadolinium- (Gd) and manganese- (Mn) based nanomaterials and iron oxide nanoparticles in this imaging technique are discussed as well, from their magnetic behavior to the commonly used materials and nanoarchitectures. Additionally, recent efforts to develop new types of contrast agents based on synthetic antiferromagnetic and high aspect ratio nanostructures are also addressed. Furthermore, the application of these materials in theragnosis, either as contrast agents and controlled drug release systems, contrast agents and thermal therapy materials or contrast agents and radiosensitizers, is also presented.

Maskless Spatioselective Functionalization of Silicon Nanowires

Spatioselective functionalization of silicon nanowires was achieved without using a masking material. The designed process combines metal-assisted chemical etching (MACE) to fabricate silicon nanowires and hydrosilylation to form molecular monolayers. After MACE, a monolayer was formed on the exposed nanowire surfaces. A second MACE step was expected to elongate the nanowires, thus creating two different segments. When monolayers of 1-undecene or 1-tetradecyne were formed on the upper segment, however, the second MACE step did not extend the nanowires. In contrast, nanowires functionalized with 1,8-nonadiyne were elongated, but at an approximately 8 times slower etching rate. The elongation resulted in a contrast difference in high-resolution scanning electron microscopy (HR-SEM) images, which indicated the formation of nanowires that were covered with a monolayer only at the top parts. Click chemistry was successfully used for secondary functionalization of the monolayer with azide-functionalized nanoparticles. The spatioselective presence of 1,8-nonadiyne gave a threefold higher particle density on the upper segment functionalized with 1,8-nonadiyne than on the lower segment without monolayer. These results indicate the successful spatioselective functionalization of silicon nanowires fabricated by MACE.

Tuning the magnetic properties of multisegmented Ni/Cu electrodeposited nanowires with controllable Ni lengths

The fabrication of segmented Ni/Cu nanowires (NWs), with tunable structural and magnetic properties, is reported. A potentiostatic electrodeposition method with a single electrolytic bath has been used to fabricate multisegmented Ni/Cu NWs inside a highly hexagonally ordered anodic nanoporous alumina membrane, with diameters of 50 nm and Ni segment lengths (L Ni) tuned from 10 nm up to 140 nm. The x-ray diffraction results evidenced a strong dependence of the Ni NWs crystallographic face-centered-cubic (fcc) texture along the [220] direction on the aspect ratio of the NWs. The magnetic behavior of the multisegmented Ni/Cu NW arrays, as a function of the magnetic field and temperature, is also studied and correlated with their structural and morphological properties. Micromagnetic simulations, together with the experimental results, showed a dominant antiferromagnetic coupling between Ni segments along the wire length for small low aspect-ratio magnetic segments. When increasing the Ni segments' length, the magnetic interactions between these along the wire became stronger, favouring a ferromagnetic coupling. The Curie temperature of the NWs was also found to strongly depend on the Ni magnetic segment length. Particularly the Curie temperature was found to be reduced 75 K for the 20 nm Ni segments, following the finite-size scaling relation with ξ 0 = 8.1 Å and γ = 0.48. These results emphasize the advantages of using a template assisted method to electrodeposit multilayer NWs, as it allows an easy tailor of the respective morphological, chemical, structural and magnetic properties.

Electrochemical pore filling strategy for controlled growth of magnetic and metallic nanowire arrays with large area uniformity

While a variety of template-based strategies have been developed in the fabrication of nanowires (NWs), a uniform pore filling across the template still poses a major challenge. Here, we present a large area controlled pore filling strategy in the reproducible fabrication of various magnetic and metallic NW arrays, embedded inside anodic aluminum oxide templates. Using a diffusive pulsed electrodeposition (DPED) technique, this versatile strategy relies on the optimized filling of branched nanopores at the bottom of templates with Cu. Serving the Cu filled nanopores as appropriate nucleation sites, the DPED is followed by a uniform and homogeneous deposition of magnetic (Ni and Fe) and metallic (Cu and Zn) NWs at a current density of 50 mA cm^-2 for an optimal thickness of alumina barrier layer (∼18 nm). Our strategy provides large area uniformity (exceeding 400 μm²) in the fabrication of 16 μm long free-standing NW arrays. Using hysteresis loop measurements and scanning electron microscopy images, the electrodeposition efficiency (EE) and pore filling percentage (F _p) are evaluated, leading to maximum EE and F _p values of 91% and 95% for Ni and Zn, respectively. Moreover, the resulting NW arrays are found to be highly crystalline. Accordingly, the DPED technique is capable of cheaply and efficiently controlling NW growth over a large area, providing a tool for various nanoscale applications including biomedical devices, electronics, photonics, magnetic storage medium and nanomagnet computing.