Optical Microscopy Using the Faraday Effect Reveals in Situ Magnetization Dynamics of Magnetic Nanoparticles in Biological Samples.

The study of exogenous and endogenous nanoscale magnetic material in biology is important for developing biomedical nanotechnology as well as for understanding fundamental biological processes such as iron metabolism and biomineralization. Here, we exploit the magneto-optical Faraday effect to probe intracellular magnetic properties and perform magnetic imaging, revealing the location-specific magnetization dynamics of exogenous magnetic nanoparticles within cells. The opportunities enabled by this method are shown in the context of magnetic hyperthermia; an effect where local heating is generated in magnetic nanoparticles exposed to high-frequency AC magnetic fields. Magnetic hyperthermia has the potential to be used as a cellular-level thermotherapy for cancer, as well as for other biomedical applications that target heat-sensitive cellular function. However, previous experiments have suggested that the cellular environment modifies the magnetization dynamics of nanoparticles, thus dramatically altering their heating efficiency. By combining magneto-optical and fluorescence measurements, we demonstrate a form of biological microscopy that we used here to study the magnetization dynamics of nanoparticles in situ, in both histological samples and living cancer cells. Correlative magnetic and fluorescence imaging identified aggregated magnetic nanoparticles colocalized with cellular lysosomes. Nanoparticles aggregated within these lysosomes displayed reduced AC magnetic coercivity compared to the same particles measured in an aqueous suspension or aggregated in other areas of the cells. Such measurements reveal the power of this approach, enabling investigations of how cellular location, nanoparticle aggregation, and interparticle magnetic interactions affect the magnetization dynamics and consequently the heating response of nanoparticles in the biological milieu.

Clot-targeted magnetic hyperthermia permeabilizes blood clots to make them more susceptible to thrombolysis.

BACKGROUND: Thrombolysis is a frontline treatment for stroke, which involves the application of tissue plasminogen activator (tPA) to trigger endogenous clot-degradation pathways. However, it is only effective within 4.5 h of symptom onset because of clot contraction preventing tPA permeation into the clot. Magnetic hyperthermia (MH) mediated by tumor-targeted magnetic nanoparticles is used to treat cancer by using local heat generation to trigger apoptosis of cancer cells. OBJECTIVES: To develop clot-targeting magnetic nanoparticles to deliver MH to the surface of human blood clots, and to assess whether this can improve the efficacy of thrombolysis of contracted blood clots. METHODS: Clot-targeting magnetic nanoparticles were developed by functionalizing iron oxide nanoparticles with an antibody recognizing activated integrin αIIbß3 (PAC-1). The magnetic properties of the PAC-1-tagged magnetic nanoparticles were characterized and optimized to deliver clot-targeted MH. RESULTS: Clot-targeted MH increases the efficacy of tPA-mediated thrombolysis in contracted human blood clots, leading to a reduction in clot weight. MH increases the permeability of the clots to tPA, facilitating their breakdown. Scanning electron microscopy reveals that this effect is elicited through enhanced fibrin breakdown and triggering the disruption of red blood cells on the surface of the clot. Importantly, endothelial cells viability in a three-dimensional blood vessel model is unaffected by exposure to MH. CONCLUSIONS: This study demonstrates that clot-targeted MH can enhance the thrombolysis of contracted human blood clots and can be safely applied to enhance the timeframe in which thrombolysis is effective.

Fe2+ Deficiencies, FeO Subdomains, and Structural Defects Favor Magnetic Hyperthermia Performance of Iron Oxide Nanocubes into Intracellular Environment.

Herein, by studying a stepwise phase transformation of 23 nm FeO-Fe3O4 core-shell nanocubes into Fe3O4, we identify a composition at which the magnetic heating performance of the nanocubes is not affected by the medium viscosity and aggregation. Structural and magnetic characterizations reveal the transformation of the FeO-Fe3O4 nanocubes from having stoichiometric phase compositions into Fe2+-deficient Fe3O4 phases. The resultant nanocubes contain tiny compressed and randomly distributed FeO subdomains as well as structural defects. This phase transformation causes a 10-fold increase in the magnetic losses of the nanocubes, which remain exceptionally insensitive to the medium viscosity as well as aggregation unlike similarly sized single-phase magnetite nanocubes. We observe that the dominant relaxation mechanism switches from Néel in fresh core-shell nanocubes to Brownian in partially oxidized nanocubes and once again to Néel in completely treated nanocubes. The Fe2+ deficiencies and structural defects appear to reduce the magnetic energy barrier and anisotropy field, thereby driving the overall relaxation into Néel process. The magnetic losses of these nanoparticles remain unchanged through a progressive internalization/association to ovarian cancer cells. Moreover, the particles induce a significant cell death after being exposed to hyperthermia treatment. Here, we present the largest heating performance that has been reported to date for 23 nm iron oxide nanoparticles under intracellular conditions. Our findings clearly demonstrate the positive impacts of the Fe2+ deficiencies and structural defects in the Fe3O4 structure on the heating performance into intracellular environment.

Dynamical Magnetic Response of Iron Oxide Nanoparticles Inside Live Cells.

Magnetic nanoparticles exposed to alternating magnetic fields have shown a great potential acting as magnetic hyperthermia mediators for cancer treatment. However, a dramatic and unexplained reduction of the nanoparticle magnetic heating efficiency has been evidenced when nanoparticles are located inside cells or tissues. Recent studies suggest the enhancement of nanoparticle clustering and/or immobilization after interaction with cells as possible causes, although a quantitative description of the influence of biological matrices on the magnetic response of magnetic nanoparticles under AC magnetic fields is still lacking. Here, we studied the effect of cell internalization on the dynamical magnetic response of iron oxide nanoparticles (IONPs). AC magnetometry and magnetic susceptibility measurements of two magnetic core sizes (11 and 21 nm) underscored differences in the dynamical magnetic response following cell uptake with effects more pronounced for larger sizes. Two methodologies have been employed for experimentally determining the magnetic heat losses of magnetic nanoparticles inside live cells without risking their viability as well as the suitability of magnetic nanostructures for in vitro hyperthermia studies. Our experimental results-supported by theoretical calculations-reveal that the enhancement of intracellular IONP clustering mainly drives the cell internalization effects rather than intracellular IONP immobilization. Understanding the effects related to the nanoparticle transit into live cells on their magnetic response will allow the design of nanostructures containing magnetic nanoparticles whose dynamical magnetic response will remain invariable in any biological environments, allowing sustained and predictable in vivo heating efficiency.