Direct observation of NMR transverse relaxation in nanopatterned clusters of iron oxide particles

We aim to verify predictions showing T₂ relaxation rate of nanoparticle clusters and its dependence on spacing, size, geometry, and pulse sequence.

We performed a laboratory validation study using nanopatterned arrays of iron oxide nanoparticles to precisely control cluster geometry and image diverse samples using a 4.7T MRI scanner with a T₂ -weighted fast spin-echo multislice sequence. We applied denoising and normalization to regions of interest and estimated relative R₂ for each relevant nanoparticle array or nanocluster array. We determined significance using an unpaired two-tailed t-test or one-way analysis of variance and performed curve fitting.

We measured a density-dependent T₂ effect (p = 8.9976 × 10^-20 , one-way analysis of variance) and insignificant effect of cluster anisotropy (p = 0.5924, unpaired t-test) on T₂ relaxation. We found negative quadratic relationships (-0.0045[log τ_D ]² -0.0655[log τ_D ]-2.7800) for single nanoparticles of varying sizes and for clusters (-0.0045[log τ_D ]² -0.0827[log τ_D ]-2.3249) for diffusional correlation time τ_D  = r_p ² /D. Clusters show positive quadratic relationships for large (3.8615 × 10^-6 [d_pp /r_p ]² -9.3853 × 10^-5 [d_pp /r_p ]-2.0393) and exponential relationships for small (-2.0050[d_pp /r_p ]^0.0010 ) clusters. Calculated R₂ peak values also align well with in silico predictions (7.85 × 10^-4  ms compared with 1.47 × 10^-4 , 4.23 × 10^-4 , and 5.02 × 10^-4  ms for single iron oxide nanoparticles, 7.88 × 10^-4  ms compared with 5.24 × 10^-4  ms for nanoparticle clusters).

Our verification affirms longstanding in silico predictions and demonstrates aggregation-dependent behavior in agreement with previous Monte Carlo simulation studies.