Hydrophobic magnetite nanoparticles synthesized from thermal decomposition of iron salts must be rendered hydrophilic for their application as MRI contrast agents. This process requires refunctionalizing the surface of the nanoparticles with a hydrophilic organic coating such as polyethylene glycol. Two parameters were found to influence the magnetic behavior and relaxivity of the resulting hydrophilic iron oxide nanoparticles: the functionality of the anchoring group and the protocol followed for the functionalization. Nanoparticles coated with PEGs via a catecholate-type anchoring moiety maintain the saturation magnetization and relaxivity of the hydrophobic magnetite precursor. Other anchoring functionalities, such as phosphonate, carboxylate, and dopamine decrease the magnetization and relaxivity of the contrast agent. The protocol for functionalizing the nanoparticles also influences the magnetic behavior of the material. Nanoparticles refunctionalized according to a direct biphasic protocol exhibit higher relaxivity than those refunctionalized according to a two-step procedure which first involves stripping the nanoparticles. This research presents the first systematic study of both the binding moiety and the functionalization protocol on the relaxivity and magnetization of water-soluble coated iron oxide nanoparticles used as MRI contrast agents.

The magnetic properties of iron oxide nanoparticles govern their relaxivities and efficacy as contrast agents for MRI. These properties are in turn determined by their composition, size and morphology. Herein we present a systematic study of the effect of particle size and shape of magnetite nanocrystals synthesized by thermal decompositions of iron salts on both their magnetism and their longitudinal and transverse relaxivities, r1 and r2, respectively. Faceted nanoparticles demonstrate superior magnetism and relaxivities than spherical nanoparticles of similar size. For faceted nanoparticles, but not for spherical ones, r1 and r2 further increase with increasing particle size up to a size of 18 nm. This observation is in accordance with increasing saturation magnetization for nanoparticles increasing in size up to 12 nm, above which a plateau is observed. The NMRD (Nuclear Magnetic Resonance Dispersion) profiles of MIONs (Magnetic Iron Oxide Nanoparticles) display an increase in longitudinal relaxivity with decreasing magnetic field strength with a plateau below 1 MHz. The transverse relaxivity shows no dependence on the magnetic field strength between 20 MHz and 500 MHz. These observations translate to phantom MR images: in T1-weighted SWIFT (SWeep imaging with Fourier Transform) images MIONs have a positive contrast with little dependence on particle size, whereas in T2-weighted gradient-echo images MIONs create a negative contrast which increases in magnitude with increasing particle size. Altogether, these results will enable the development of particulate MRI contrast agents with enhanced efficacy for biomedical and clinical applications.

The synthesis and characterization of core-shell Fe 3 O 4 @organic@Au nanoparticles displaying plasmonic behavior, high magnetism, and high relaxivity is presented. The incorporation of a thin organic layer between the two metals is crucial to maintaining the saturation magnetisation of the superparamagnetic core.Magnetic Iron Oxide Nanoparticles (MIONs) are increasingly used as MRI contrast agents due to their high saturation magnetisation (M S ) which leads to high transverse relaxivity.1 Likewise, gold nanoparticles have become the material of choice in imaging techniques that exploit their plasmonic properties such as dark field spectroscopy and Surface Enhanced Raman Spectroscopy (SERS).2 The combination of these two materials in a single nanocomposite displaying both magnetic and plasmonic properties, a so-called magnetoplasmonic assembly,3 is particularly attractive given the complementarity in terms of resolution and 3D imaging capabilities of the plasmonic and MR imaging techniques.1 , 2 Herein we present the synthesis and characterization of magnetite@organic layer@gold core-shell nanocomposites which display high saturation magnetisation due to the presence of a thin organic layer.Multimodal nanocomposites comprised of both iron oxide and gold components can be synthesized in either of two ways. In the first case, both nanoparticles of gold and iron oxide are embedded in a polymer or silica coating.3 , 4 This approach unfortunately results in assemblies of size typically around 150-200 nm, which are too large for cellular imaging and for most in vivo applications.5 Alternatively, a core-shell structure, typically with an iron oxide core and an outermost gold layer, enables the synthesis of smaller nanoparticles, typically 80 nm in diameter. Advantageously, these assemblies can also be readily functionalized with biomolecules since the chemistry of conjugation of proteins and nucleic acids to gold surfaces is well established.6 -9 The disadvantages of these structures is twofold. First, the coating on the iron oxide core, which includes the gold layer, is typically ca. 35 nm thick or more. Since this thick coating increases the distance separating the magnetic core from the solvent water molecules, both the longitudinal and the transverse relaxivities of the material are significantly reduced.1 Secondly, and importantly, direct coating of gold onto iron oxide nanoparticles severely decreases the saturation † Electronic Supplementary Information (ESI) available: Detailed experimental procedure and characterization of the nanoparticles. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript magnetization (on a per iron basis) of the magnetic core by 78% or more.9 , 10 This further reduces the relaxivities, and hence the potency of the nanocomposites as MR contrast agents.The mechanism by which the gold coating decreases the saturation magnetisation of the magnetite core is presently not well understood. Similar effects have been observed in gold coated cobalt nanoparticles.11 ...

Being positive about anions: Hydrocarbon complexes containing negative‐valent Hf are obtained for the first time as tris(polyarene)hafnates(2−), polyarene=anthracene (An) and naphthalene, where the latter functions as a synthon for the unknown atomic Hf2− (see scheme, cot=1,3,5,7‐cyclooctatetraene). Tris(anthracene)metalates(2−) of Ti and Zr were also accessed, which completes an unprecedented triad of tris(arene)metal complexes.

A responsive MION-based MRI contrast agent for the detection of copper(I) is presented. Induced agglomeration of azide and acetylene-functionalized magnetite nanoparticles via Cu(I) catalysed Huisgen cycloaddition leads to significant decrease in longitudinal relaxivity due to the slow exchange of water molecules trapped within the cluster with bulk solvent. Agglomeration leads to an initial two fold increase followed by a sharp and almost complete loss in transverse relaxivity for clusters larger than 200 nm in size. The decrease in r2 for clusters reaching the static dephasing regime has two significant implications for particulate responsive MRI contrast agents. First, the maximum increase in r2 is barely two fold, second, since r2 does not increase continuously with increasing cluster size, the r1/r2 ratio cannot be used to determine the concentration of an analyte ratiometrically.