Growth mechanism of nanostructured superparamagnetic rods obtained by electrostatic co-assembly

TL;DR

This study investigates the formation of nanostructured superparamagnetic rods via electrostatic co-assembly of iron oxide nanoparticles and polymers, revealing a morphology diagram and proposing a two-step growth mechanism influenced by ionic strength and concentration.

Contribution

First to provide a morphology diagram for nanoparticle-based rods as a function of ionic strength and concentration, and to propose a detailed two-step formation mechanism.

Findings

Existence of critical nanoparticle concentration and ionic strength for rod formation.

Linear, superparamagnetic rods form at specific conditions.

Mechanism involves cluster formation and magnetic alignment.

Abstract

We report on the growth of nanostructured rods fabricated by electrostatic co-assembly between iron oxide nanoparticles and polymers. The nanoparticles put under scrutiny, {\gamma}-Fe2O3 or maghemite, have diameter of 6.7 nm and 8.3 nm and narrow polydispersity. The co-assembly is driven by i) the electrostatic interactions between the polymers and the particles, and by ii) the presence of an externally applied magnetic field. The rods are characterized by large anisotropy factors, with diameter 200 nm and length comprised between 1 and 100 {\mu}m. In the present work, we provide for the first time the morphology diagram for the rods as a function of ionic strength and concentration. We show the existence of a critical nanoparticle concentration and of a critical ionic strength beyond which the rods do not form. In the intermediate regimes, only tortuous and branched aggregates are detected. At higher concentrations and lower ionic strengths, linear and stiff rods with superparamagnetic properties are produced. Based on these data, a mechanism for the rod formation is proposed. The mechanism proceeds in two steps : the formation and growth of spherical clusters of particles, and the alignment of the clusters induced by the magnetic dipolar interactions. As far as the kinetics of these processes is concerned, the clusters growth and their alignment occur concomitantly, leading to a continuous accretion of particles or small clusters, and a welding of the rodlike structure.