Ultrasound-assisted size tuning of polyacrylic acid coated magnetic nanoparticle clusters for biomedical applications

The hydrodynamic size of magnetic nanoparticle clusters is a critical determinant of their in vivo behaviour and therapeutic efficacy. While alkaline co-precipitation offers a scalable route for polyacrylic acid (PAA) coated superparamagnetic iron oxide nanoparticles (SPIONs), it typically yields polydisperse agglomerates. This work establishes a predictive engineering process using controlled, post-synthesis ultrasound treatment to precisely tune SPION cluster size. Utilising a D-optimal Design of Experiments (DoE) approach, we modelled the influences of sonication parameters on the hydrodynamic diameter, identifying specific energy input as the governing factor for de-agglomeration. The resulting verified regression model (adj. R²=0.9986) enables predictable laboratory scale-up across varying volumes (1-10 ml) and concentrations (1-10 mg/ml) while maintaining material integrity. Quantitative magnetic characterisation revealed that ultrasound-induced fragmentation increases the mass-specific susceptibility, which is attributed to the magnetic de-locking of frustrated cores as inter-cluster spacing increases. Crucially, biological evaluations in B16-F10 melanoma cells demonstrate that this ultrasound-assisted size tuning directly influences cellular loading. Cellular iron mass post SPION incubation was found to follow a dual-variable dependency: while iron loading increases with cluster diameter for a fixed core size, it is significantly impacted by the primary core dimensions. SPION clusters with 12 nm cores exhibited a two-fold higher iron loading (8.23 pg Fe/cell) compared to those with 8 nm cores at equivalent hydrodynamic sizes, highlighting the importance of the magnetic payload per cluster. These findings establish a robust framework for engineering SPIONs with tailored dimensions to maximise and predict the magnetic responsiveness of loaded cells, providing a reliable foundation for future applications such as cell tracking, magnetic drug targeting, and hyperthermia.