Magnetic Nanoparticles in Human Health and Medicine

Polymers, large molecules composed of many repeated subunits, are probably the most studied class of molecules used in the nanoparticle clustering. These compounds have several specific physical–chemical properties that allow to, first, help the nanoparticles assembly and, second, provide some specific functional groups on the surface for the conjugation of ligands.

Di Corato et al. developed a strategy to cluster hydrophobic magnetic nanoparticles within a shell of an amphiphilic polymer, namely poly(maleic anhydride alt‐ 1 octadecene) (Di Corato et al. 2009). The protocol was based on the controlled destabilization of a suspension of nanoparticles and polymer in tetrahydrofuran obtained with slow addition of acetonitrile. In the resulting clusters, nanoparticles were collapsed in the core, whereas the polymer enrolled the structure with a dense shell. The thickness of the coating was proportional to the polymer concentration, ranging from few to 30–40 nm. In this contribution, the polymer was functionalized with an organic dye for cell separation and detection, but actually, the structure offered many possibilities of modification. By the same group, colloidal quantum dots were introduced in the assembly, creating a magnetic‐fluorescent platform based on inorganic nanoparticles (Di Corato et al. 2011). CdSe@ZnS dots were dispersed in the nanoparticles suspension before the clustering, and the protocol was applied with no modification. Interestingly, the fluorescent nanoparticles were not collapsed in the core with the magnetic but were confined in the polymer shell, avoiding the fluorescent quenching of the resulting structure. This phenomenon was explained by a different grade of the insolubility of the nanoparticles in acetonitrile, due to the different surfactants on particles surface (TOPO and TOP on QDs, oleic acid, and oleylamine for magnetic nanoparticles). The functionalization with folic acid and the use of different QDs allowed performing a specific ligand separation and a multiplex analysis of sorted cells. As claimed before, this magnetic nanocluster was further modified with thermo‐responsive polymer (Deka et al. 2011) or with silver nanoparticles nucleated in situ on the polymeric surface (Di Corato et al. 2012). A variant of this clustering method involves the aggregation of the nanoparticles (dispersed in tetrahydrofuran) by adding a volume of acetonitrile in a 1 : 1 ratio. By this approach, very dense and organized, but unstable, clusters were obtained. Thus, immediately after the clustering, a polymeric shell was grafted on the surface of the ordered assemblies of nanoparticles, by condensation of a solution of poly(maleic anhydride alt ‐1 octadecene) (Bigall et al. 2013). The separation of the two phases, clustering and polymer coating, was also investigated in a recent study, in which the hydrophobic nanoparticles were first collapsed in the above‐described tetrahydrofuran/acetonitrile mixture and subsequently coated with a thermo‐responsive hyaluronic acid derivative. By comparison with simultaneous one‐pot clustering, the two‐phases approach was considered more efficient to increase the concentration of nanoparticles in the structure core, with a consequence on the magnetic moment and magnetic responsiveness (Rippe et al. 2020).

In the last decade, magnetic nanocubes have aroused interest in the field of materials science as a heat mediator, due to different crystalline and shape anisotropies, compared to the most common spheres, resulting in higher heat capacity (Guardia et al. 2012; Noh et al. 2012). From clustering point of view, this class of nanomaterial is not straightforward to be managed because of the strong interparticle interaction. Materia et al. modified the previously reported procedure for spherical particles adjusting the solvent mixture and the injection rate of the polar solvent. When clustered, the nanocubes showed a lower SAR value, due to the suppression of Brownian contribution into the blocked polymeric superstructure. On the opposite, the relaxivities analysis resulted in a very low r 1value and a definite increase of the r 2/ r 1ratio, in comparison to the individual nanocubes (Materia et al. 2015). Recently, the same group investigated how the heat performance of the iron oxide nanocubes could be preserved or even enhanced by clustering. A possible answer is represented by nanoparticles assemblies with a 2D arrangement. Very small clusters composed of two and three nanocubes, namely dimers and trimers, showed a SAR valued almost doubled if compared to individual nanoparticles. When the nanocubes number overcame the threshold of four particles per cluster, an impressive fall of SAR value was observed (Niculaes et al. 2017). Nanocubes assembled with a 2D‐arrangement were also obtained by using an esterase‐sensitive biopolymer as an encapsulating agent. The enzyme activity resulted in the disassembly of the multiparticle cluster and, as confirmation of the above‐described study, in an enhanced SAR performance. In fact, the 2D‐clusters were split into smaller clusters composed of very few particles, in a chain‐like configuration (Avugadda et al. 2019).

Another modified‐amphiphilic polymer, poly(isobutylene‐ alt ‐maleic anhydride), was used for the fabrication of small nanocluster based on the assembly of hydrophobic nanoparticles. The maleic anhydrides of the polymer backbone acted as an anchor point for the addition of PEG (10%), dopamine (70%) and cystamine, a disulfide linker (20%). The three ligands ensured, respectively, high biocompatibility to the cluster, a high affinity to the nanoparticles surface and an amine for an additional functionalization. Therefore, chlorin e6, a photosensitive drug, was linked to the cystamine for the preparation of this multifunctional polymer. The cluster was prepared by a solvent exchange strategy, mixing the chloroform‐dispersed nanoparticles with a solution of the polymer in DMSO. After ultrasonication and evaporation of the chloroform, the DMSO was changed with water by dialysis. The resulting redox‐responsive nanocluster was suitable for the detection of the high‐reductive intracellular environment (typical of tumors) because of the cleavage of the disulfide bridge and the subsequent release of the drug, for MR imaging and the photodynamic therapy of solid tumors (Yang et al. 2018).

Paquet et al. in 2010 reported the clustering process of hydrophobic nanoparticles in regular assembly assisted by sodium dodecyl sulfate (SDS). This was one of the first paper in which was obtained a fine control over the nanocluster size, ranging from 40 to 200 nm. The method was based on a microemulsion of two components: the fatty acid‐coated nanoparticles dispersed in toluene and an aqueous solution of SDS. By ultrasonication and subsequent ripening at 90 °C, the organic solvent was entirely evaporated, and dense spherical aggregates were obtained. Some key parameters, as surfactant and nanoparticles concentration, the volume ratio of the emulsion, were monitored to control the clustering process and the final diameter of the magnetic nanospheres. As shown in this contribution and also in more recent ones, the SDS clusters need an additional surface coating to stabilize the structure. In this chapter, a 20 nm‐additional shell of polymethacrylate derivates was grafted on the cluster surface (Paquet et al. 2010). Starting from the SDS‐coated cluster, the research group also investigated as the nature and the thickness of the additional polymer shell could vary the relaxivities of the magnetic clusters. By using a precipitation polymerization method, a pH‐sensitive hydrogel coating composed of acrylic acid, N,N' ‐methylenebis‐acrylamide and N ‐isopropylacrylamide was polymerized onto the clusters. The hydrogel significantly enhances the transverse relaxation rates by lowering the diffusion coefficient of water molecules near the magnetic nanoparticles. By tuning the pH or the initial thickness of the hydrogel, an r 2increase (in comparison to the bare magnetic nanoparticles) was observed from 44% (low pH, the low water content in the thin shell) to 85% (neutral pH, the high water content in the thick shell) (Paquet et al. 2011). Also, Wu et al. in 2015, reported the clustering of hydrophobic magnetic nanoparticles by emulsification method assisted by SDS surfactant. A toluene dispersion of NPs was mixed with a SDS aqueous solution, and the mix was sonicated and kept at 90 °C for two hours. The resulting cluster had a diameter below 200 nm and a fine distribution. To ensure higher nanosystem stability, a shell of polydopamine was polymerized on the cluster surface in alkaline conditions, starting from dopamine monomer. The polydopamine ensured a higher NIR absorption to the cluster, exploitable for photothermal therapy. In a proof of concept magnetophoresis experiment, cancer cells were incubated with polydopamine‐nanocluster with an external magnet and irradiated with a 808 nm laser, achieving a 90% cytotoxicity at the highest concentration (Wu et al. 2015b). Starting from this protocol, Mandriota et al. evaluated many different parameters (choice and concentration of surfactant, size of nanoparticles, choice of organic solvent, oil/water phase ratio, and scalability) to produce clusters with a size around 100 nm. Then, a polydopamine shell was grafted on the cluster (from 4 to 27 nm), and the efficiency of a pH‐sensitive release was assessed, loading a chemotherapy drug as a model, the cisplatin. Below pH 5, an abrupt release of the drug was obtained after 24 hours, with a partial degradation of the nanocluster, and a release of nanoparticles, at pH 3 after 72 hours. In vitro experiments confirmed that nanocluster significantly improved the cellular uptake of the platinum drug, by increasing its cytotoxicity at low dose (Mandriota et al. 2019).