Fe₃O₄@organic@Au: core–shell nanocomposites with high saturation magnetisation as magnetoplasmonic MRI contrast agents

Abstract

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 ...

Contrast agents for MRI: 30+ years and where are we going?

“…An increasing number of articles report on contrast agents, particularly nanoparticles, with impact on multiple modalities, although far fewer reports exist on their in vivo application relative to in vitro studies. As for macromolecular agents, care should be taken on how these multimodal probes are designed and synthesized to maximize the efficacy of each component [47]. In this issue, the Louie group presents an example of dual positron emission tomography (PET) and MRI imaging agents specifically designed to target macrophages [48].…”

Iron Oxide Based Nanoparticles for Magnetic Hyperthermia Strategies in Biological Applications

“…However, for the coating of magnetite by gold crystal mismatch is a problem and gives rise to different effects. Smolenski et al65 prepared small Fe 3 O 4 @OAc particles (4.8 nm) by thermal decomposition of [Fe(acac) 3 ] in the presence of OAc and oleylamine and achieved Fe 3 O 4 @Au (80 nm) by replacing OAc with 3‐aminopropylphosphonic acid and sonicating in HAuCl 4 . The hybrid gold‐coated magnetite was reported to show a lower saturation ( M sat = 81 A m 2 kg –1 ) than that of Fe 3 O 4 @OAc ( M sat = 92 A m 2 kg –1 ) due to Au‐ion diffusion into the Fe 3 O 4 lattice causing magnetic disorder.…”

Biomimetic Systems in Nanomedicine

“…431,427 This method allowed a better control over shell thickness 431,427 and av oided the decrease of saturation magnetization that takes place in the magnetic core when gold directly coats the magnetic NP. 432 The unique physical properties of the magnetic NPs and the gold nanoshell were shown to be preserved using a polymer "cushion" layer between the core and the shell. 422 These improved core-shell NPs have clinical potential as multimodal platforms for a range of biomedical applications such as image guided cellular hyperthermia or thermoresponsive drug delivery.…”