Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation

TL;DR

This study uses simulations to reveal how the transverse relaxation time T2 in NMR scales with nanoparticle aggregation, showing dependence on aggregate structure and orientation, which informs sensor optimization.

Contribution

It provides a detailed analysis of T2 scaling with nanoparticle aggregate size and structure, introducing a model that links fractal dimension to relaxation behavior.

Findings

T2 scales as a power law with the number of nanoparticles in an aggregate.

The scaling exponent depends on the fractal dimension of the aggregate.

T2 is highly sensitive to the orientation of nanoparticle pairs relative to the magnetic field.

Abstract

The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2 of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between T2 and details of the aggregate. We find that in the motional averaging regime T2 scales as a power law with the number N of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension d of the aggregates. We find T2 N^{-0.44} for aggregates with d=2.2, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, T2 is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times.