Magnetic Alignment for Plasmonic Control of Gold Nanorods Coated with Iron Oxide Nanoparticles

Abstract

Plasmonic nanoparticles that can be manipulated with magnetic fields are of interest for advanced optical applications, diagnostics, imaging, and therapy. Alignment of gold nanorods yields strong polarization‐dependent extinction, and use of magnetic fields is appealing because they act through space and can be quickly switched. In this work, cationic polyethyleneimine‐functionalized superparamagnetic Fe3O4 nanoparticles (NPs) are deposited on the surface of anionic gold nanorods coated with bovine serum albumin. The magnetic gold nanorods (MagGNRs) obtained through mixing maintain the distinct optical properties of plasmonic gold nanorods that are minimally perturbed by the magnetic overcoating. Magnetic alignment of the MagGNRs arising from magnetic dipolar interactions on the anisotropic gold nanorod core is comprehensively characterized, including structural characterization and enhancement (suppression) of the longitudinal surface plasmon resonance and suppression (enhancement) of the transverse surface plasmon resonance for light polarized parallel (orthogonal) to the magnetic field. The MagGNRs can also be driven in rotating magnetic fields to rotate at frequencies of at least 17 Hz. For suitably large gold nanorods (148 nm long) and Fe3O4 NPs (13.4 nm diameter), significant alignment is possible even in modest (<500 Oe) magnetic fields. An analytical model provides a unified understanding of the magnetic alignment of MagGNRs.

Generation of Circular Dichroism from Superposed Magnetically Oriented Magnetic Nanoparticles

“…Equation may give a useful idea for the design of an effective photo-isolator. Further applications of the present approach of the optical chirality induction in magnetically aligned achiral nanoparticles will be exciting, especially for magnetic nanoparticles and nanorods coated with Ag or Au. − …”

Magnetically Induced Anisotropic Interaction in Colloidal Assembly

“…When made into anisotropic shapes, such nonmagnetic nanostructures often exhibit unique and orientation-dependent physical properties compared to their spherical counterparts, such as localized surface plasma resonance − and polarized light emission. − However, fully exploiting their shape-dependent properties is hampered by the lack of efficient assembly strategies to realize positional and orientational control. As such, incorporating magnetic components into these anisotropic nanostructures is a powerful solution to this challenge, conferring the advantages of the magnetic assembly strategy, ,− including convenience, flexibility, efficiency, remote control, and scalability. It is worth pointing out that, in some cases, the magnetic components may interfere with the desired properties and therefore need to be removed after the assembly process.…”

Messenger Materials Moving Forward: The Role of Functional Polymer Architectures as Enablers for Dynamic Nano‐to‐Macro Messaging

“…For example, decoration of gold nanorods with iron oxide “satellite” nanoparticles, renders the plasmonic gold nanorods responsive to external magnetic fields (Figure 5E); in a homogeneous magnetic field, the nanorods will align parallel to the magnetic field direction (Figure 5F). [ 50 ] This alignment brings the optical anisotropy of gold nanorods into effect at the macroscale, as light absorption and scattering will depend on the orientation of the nanorods with respect to the light propagation direction. This effect can be further enhanced using polarizer filters and enables the visualization and dynamic tracking of external magnetic fields.…”