Abstract
The combination of magnetic and plasmonic properties using iron oxide/gold nanocomposite particles is crucial for the development of multimodal molecular imaging probes. In this study, iron oxide/gold composite nanoparticles (NanoIOGs) were synthesized via the on-site reduction of an Au precursor salt by polyethyleneimine (PEI) molecules attached to iron oxide nanoparticles (IONPs), and they were employed in magnetic resonance and dark-field microscope imaging. PEI is considered as a polymeric active stabilizer (PAS), acting as a reducing agent for the synthesis of Au and a dispersant for nanoparticles. When the IONPs prepared at the PEI concentration of 0.02 wt. % were used for the NanoIOG synthesis, Au nanoseeds were formed around the IONPs. The alloy clusters of IONPs/Au crystals were produced with further reduction depending on PEI concentration. The NanoIOGs exhibited superparamagnetism in a magnetic field and plasmonic response in a dark-field (DF) microscope. The sizes, morphologies, magnetizations, and r2 relaxivities of NanoIOGs were affected significantly by the amount of PEI added during the NanoIOG synthesis. It is suggested that the PAS-mediated synthesis is simple and effective, and can be applied to various nanostructured Au-metal alloys.
Highlights
Ultrafine metal particles have received considerable attention because they offer promising and novel options for a wide range of applications
The NanoIOGs containing iron oxide/Au alloy clusters were prepared via the direct addition of HAuCl4 to the stable colloidal solution of PEI-capped iron oxide nanoparticles (IONPs), without any other reducing agent
The cloud-like aggregates of Au nanoseeds and IONPs mixture were found in the transmission electron microscope (TEM) images of type 1, and they can be regarded as precursors of the alloy clusters of iron oxide and gold
Summary
Ultrafine metal particles have received considerable attention because they offer promising and novel options for a wide range of applications. Surface-modified superparamagnetic iron oxide nanoparticles (IONPs) have been widely used in novel biomedical applications, such as contrast enhancement in magnetic resonance (MR) imaging [5,6,7,8,9]. Amino groups in PEI molecules function as proton acceptors/electron donors, reducing Au3+ to Au. In most NanoIOG syntheses, chemical modifications, including ligand combination, are required for the selective formation of Au on the surfaces of IONPs. PEI has been used only as a dispersing agent in NanoIOG syntheses, and its reducing effect has been overlooked. We hypothesized that PEI molecules on the surfaces of IONPs could reduce Au ions, resulting in the on-site formation of Au crystals and eventually alloy clusters of iron oxide and Au. In this study, NanoIOGs were synthesized via the simple addition of an Au precursor salt to PEI-capped IONPs without chemical modification of IONP surfaces or addition of a reductant such as NaBH4. We examined the magnetoplasmonic properties of NanoIOGs to verify their feasibility of application as MR/DF dual imaging probes
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