Controlled Clustering of Superparamagnetic Nanoparticles using Block Copolymers : Design of New Contrast Agents for Magnetic Resonance Imaging

TL;DR

This study demonstrates the formation of stable, structured complexes of superparamagnetic nanoparticles with block copolymers that enhance MRI contrast, offering potential for improved biomedical imaging agents.

Contribution

It introduces a new method to control clustering of superparamagnetic nanoparticles using block copolymers, improving MRI contrast agent design.

Findings

Complexes have hydrodynamic diameters of 70-150 nm.

Aggregates contain several tens to hundreds of nanoparticles.

Complexes provide better MRI contrast than individual nanoparticles.

Abstract

When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species, stable complexes are found to form spontaneously. The mechanism is based on electrostatics, and on the compensation between the opposite charges. Electrostatic complexes exhibit a core-shell microstructure. In the core, the polyelectrolyte blocks and the oppositely charged species are tightly bound and form a dense coacervate microphase. The shell is made of the neutral chains and surrounds the core. In this paper, we report on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite gamma-Fe2O3) and cationic-neutral copolymers. The copolymers investigated are poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide), with molecular weights 5000-b-30000 gmol-1 and 110000-b-30000 gmol-1. The mixed copolymer-nanoparticle aggregates were characterized by a combination of light scattering and cryo-transmission electron microscopy. Their hydrodynamic diameters were found in the range 70 - 150 nm and their aggregation numbers (number of nanoparticles per aggregate) between several tens to several hundreds. In addition, Magnetic Resonance Spin-Echo measurements show that the complexes have a better contrast in Magnetic Resonance Imaging than single nanoparticles, and that these complexes could be used for biomedical applications.