in the lipidic bilayer, with a concentration of 10 wt %. Alternating magnetic fields were used to control timing and dose of repeatedly released cargo from this pegylated vesicles; the inducted local heating of the membranes caused a transient change of the permeability, without effect on the system structure (Amstad et al. 2011).
Nandwana et al. reported the preparation of lipidic nanocapsules with a peculiar hollow‐core structure. In details, these nanocapsules were obtained by an emulsion process of cationic lipids and water‐dispersed ferrites. As a result, a micellar architecture was obtained, with a hollow hydrophobic core, exploited for drug loading, and a hydrophilic surface, entirely decorated with a very high density of magnetic nanoparticles. Interestingly, the initial Mn–Zn ferrites, synthesized by thermal decomposition method, show a high r2 relaxivity at 3 T (425 mM−1 s−1), and that what resulted even increased when the particles were confined in the nanocapsule structure (680 mM−1 s−1). These results were explained by the synergistic interactive magnetism between adjacent nanoparticles (Nandwana et al. 2018).
Salvatore et al. described the preparation of a sophisticated system based on the assembly of different building blocks: a DPPC‐based liposome, a ds‐DNA conjugated with a cholesteryl unit (that inserts spontaneously into the liposome membrane), hydrophobic iron oxide nanoparticles, and hydrophilic iron oxide@gold core‐shell nanoparticle (transferred in water by functionalization with a methoxy‐PEG and a thiolated oligonucleotide). By the sequential assembly of these blocks, a peculiar architecture was obtained, with hydrophobic particles embedded in the lipidic bilayer. In contrast, the core@shell nanoparticles were grafted on the liposome surface via interaction with ds‐DNA‐cholesteryl and subsequent insertion in the liposome. The liposome core was instead used as a carrier for a test payload. The authors demonstrated that the different confinement of these magnetic nanoparticles could be exploited for a sequential release of payload or oligonucleotide by just tuning into alternate magnetic field (AMF) impulses. In detail, 3.22 kHz AMF for five minutes provoked the release of the hydrophilic drug contained in the aqueous core of magnetoliposomes. Subsequently, the application of a 6.22 kHz AMF for 15 minutes induced the melting of DNA strands and the release of the zipper therapeutic oligonucletotide (Salvatore et al. 2016).
3.3.6 Other Molecules
In this last subsection, some other molecules, not classified in the previous groups, were explored for the clustering of magnetic nanoparticles.
Qiu and coworkers have set a general method for the preparation of nanoparticle clusters in an oil‐in‐water emulsion using cetyltrimethylammonium bromide (CTAB) as an emulsifier (Qiu et al. 2010). To show the general applicability of the method, they prepared clusters of metallic and semiconductor nanocrystals besides magnetic nanoparticles. The obtained clusters were spherical and were composed of densely packed individual nanoparticles, regardless of the type of nanocrystals employed.
Wu et al. started from magneto‐plasmonic nanoparticles (IONPs@Au core‐shell) of 6 nm to prepare nanoclusters, with a diameter of 180 nm, by oil‐in‐water microemulsion method. In detail, hydrophobic nanoparticles obtained by thermal decomposition, in hexane, were mixed with an aqueous solution of sodium dodecyl sulfate. After sonication, the mixture was heated in a water bath at 80 °C for 10 minutes to evaporate the organic solvent. Their superstructure was easily functionalized, by exploiting the gold outer surface of nanoparticles, with thiolated PEG or antibody to ensure a high biocompatibility and a specific target recognition, respectively. The magnetic cluster deserved their superparamagnetic profile, and in addition, the close proximity of the core‐shell particles in the nanocluster led to strong near‐infrared (NIR) plasmon resonances for medical application (Wu et al. 2014).
Smith et al. reported the clustering of 5 nm‐diameter oleic acid‐capped SPIONs in superstructures with very high relaxivity due to control of cluster size coupled with optimization of hydrophilicity at the surface. The authors synthesized different hyperbranched polyglycerol molecules to mimic the properties of glycogen to adsorb water molecules. By emulsification method and subsequent evaporation of the organic phase, regular clusters between 42 and 80 nm were obtained, by tuning the polyglycerol molecular architecture. Interestingly, the r2 relaxivity passed from 122 mM−1 s−1 of the bare unclustered SPIONs to a maximum value of 719 mM−1 s−1, which was close to their theoretical maximal limit. The described effect was due to two factors: the molecular architecture and to the polyglycerol thickness, and consequently to the hydrophilicity of such coating (Smith et al. 2015).
3.4 Theranostic Relevant Examples
The main problem for the use of magnetic nanoparticle in therapeutic treatments is the suitable concentration of nanoparticles at the target. Specifically, in magnetic hyperthermia, the local increase of temperature is mainly due to a mass effect of several nanoparticles that “heat” the whole region. As claimed in a paper from Pellegrino's group, the single‐particle heating capability is limited to 0.5 nm from the nanoparticles surface (Riedinger et al. 2013). Many research groups overcome this limitation injecting, as proof of concept application, magnetic nanoparticles into the solid tumor. This methodology is accepted to validate the efficacy of novel nanomaterial, but it is not useful in clinical. Other groups injected nanoparticle intravenously, but with very high iron concentration, far from values usually approved by FDA. Albarqi et al. started from the synthesis of alternative nanomaterials to exploit the clustering for delivery of a high quantity of material to the tumor mass. The exotic nanomaterials are based on the synthesis of hexagonal iron oxide nanoparticles doped with cobalt and manganese (CoMn‐IONPs). These elements were incorporated in the crystal lattice of iron oxide, leading to an enhancement of the magnetic properties. Moreover, nanoparticles with a different surface magnetic anisotropy, based on cubic or hexagonal shape, showed a superior heating performance recently. The CoMn‐IONPs show a diameter of 14.80 ± 3.52 nm, with a dopant concentration of 8% for cobalt and 5% for manganese. These nanoparticles possessed a saturation magnetization of 93 emu g−1, higher than iron oxide nanoparticles, and a very high SAR of 1718 W g−1, measured in an organic solvent at 420 kHz with an applied field of 26.9 kA m−1. CoMn‐IONP nanoclusters were obtained by a solvent evaporation process, by using a pegylated poly(ε‐caprolactone), with a hydrodynamic size of 80 nm. After water‐transferring in clusters, the SAR value is still suitable for therapeutic approach (1237 W g−1) and higher than the value obtained with a suspension of single nanoparticles in water (997 W g−1), as an effect of strong magnetic dipole−dipole interactions into the cluster core. Concerning the in vivo experiment, CoMn‐IONP nanoclusters were intravenously injected in nude mice grafted with a xenograft subcutaneous tumor. The clusters were detected in the tumor after 10 minutes, with a maximum accumulation after 5 hours and a decrease after 24 hours. The application of an alternating magnetic field (10 minutes), 12 hours after nanocluster injection, induced temperature increase up to 44 °C. The application of four hyperthermia cycles within 30 days inhibited the growth of subcutaneous ovarian tumors in comparison to control groups (Albarqi et al. 2019) (Figure 3.3).
Figure 3.3 Schematic illustration of the nanoclusters prepared by encapsulation of hexagon‐shaped cobalt‐ and manganese‐doped iron oxide nanoparticles (a). NIR fluorescence biodistribution analysis at various time points after i.v. injection (b).
Source: Reprinted with permission from Albarqi et al. (2019). Copyright 2019 American Chemical Society.
In another relevant study, Nie et al. modified one of the synthetic methods described in Section