were mixed, and the assembly was monitored over time: in a first step (40 minutes) the magnetic particles were clustered in dense and regular assemblies of 250 nm. Afterward, the preformed clusters started to overassembly in noncontrolled structures, that the authors defined as coral‐like aggregates, with micrometer‐range size. By repeating the entire experiment in the presence of an external magnetic field, well‐defined and regular 2 μm, cylindrical bundles were obtained. These large aggregates also occurred in this configuration as a second overassembly, since during the first 40 minutes seeding step spherical magnetic cluster were formed (Li et al. 2017).
The preparation of hybrid nanoparticles, composed of a donor–acceptor‐type conjugated polymer (PCPDTBT), hydrophobic magnetite nanoparticles and a phospholipid, was recently described. The nanoparticles were obtained first drying an organic suspension of the three main components, followed by hydration of the obtained film. The resulting particles, with a nonregular shape and a size between 100 and 150 nm, were further functionalized and stabilized with PEG molecules via NHS chemistry. The hybrid composite showed a 22‐fold photoacoustic intensity increase in the optical window (NIR‐I) as well as a shortening of T2 relaxation time, with a r2 relaxivity of 309.3 mM−1 s−1 at 7 T for the best nanocomposite (Pham et al. 2019).
3.3.4 Polysaccharides Coatings
Another class of molecules extensively used for the coating of magnetic nanocluster is represented by polysaccharides. These molecules are highly biocompatible and, in a certain case, of natural derivation. It is noteworthy that the first FDA‐approved formulations based on magnetic nanoparticles were obtained by assisted nucleation of magnetite, or maghemite, in the presence of dextran‐derivative (e.g. ferucarbotran and ferumoxide).
Kim et al. set a method for the preparation of nanoclusters based on the self‐assembly of magnetic nanoparticles in a modified‐dextran. First, the polysaccharide was modified with the introduction of different oleic acid amount; after that, the NPs, dispersed in the organic phase, was mixed with modified‐dextran and a nanoemulsion was inducted by ultrasonication step. After solvent evaporation, the clusters were resuspended in water. The substitution grade of dextran and the polymer amount used during clustering were selected as main parameters to govern the overall size of the nano‐object (below 100 nm) and the T2 relaxivities response. Moreover, the dextran‐modified surface properties exhibited sufficient affinity to macrophages, and therefore, the nanocluster was tested for the diagnosis, by MRI, of atherosclerotic plaques in vitro and in vivo (Kim et al. 2014).
Recently, Tran et al. modified some commercial nanoclusters, based on the assembly of single nanoparticles in a matrix of dense dextran, for a smart lateral flow application. The surface was modified with imidazole groups for the rapid conjugation with fluorescent Quantum Dots and/or receptor for cellular isolation. The purpose of this nanosystem (overall size around 200 nm) was to develop a future point‐of‐need diagnostics device able to magnetically isolate some specific targets (i.e. cells) and to exploit a smartphone camera for the detection of bright spots (Tran et al. 2019).
Park et al. reported the preparation of regular nanoclusters, based on the aggregation of hydrophobic nanoparticles in natural amphiphilic levan polysaccharides. Via an ultrasonication treatment, the nanoparticles were clustered in the polymeric matrix and therefore transferred in the aqueous phase. The size and the shape of the obtained cluster were heavily affected by the nanoparticle concentration: the cluster size increased with nanoparticles amount up to a critical threshold that avoids the formation of three‐dimensional super‐structures, favoring the bidimensional assembly. The authors demonstrated the universal method for assembly of magnetic, gold NPs, and Quantum Dots, as an individual cluster or as hybrid multifunctional systems. Concerning magnetic nanoparticles, the assembly in clusters of 200–300 nm resulted in a transverse relaxivity increase of 45% (from 65 to 95 mM−1 s−1) at 4.7 T (Park et al. 2020).
3.3.5 Lipidic Coatings
Liposomes represent one of the most investigated platforms for drug administration. These lipidic vesicles are stable, biocompatible, and their preparation is very well‐established. In this section, some examples of nanoparticle clustering obtained by the use of different lipids are described.
Martina et al. proposed one of the first examples of the inclusion of magnetic nanoparticles in lipidic vesicles. Aqueous maghemite NPs obtained by coprecipitation (and stabilized with a citrate capping) were mixed with egg‐yolk L‐α‐phosphatidylcholine (EPC) and 1,2‐diacyl‐SN‐glycero‐3‐phosphoethanolamine‐N‐[methoxy(poly(ethylene glycol))‐2000] (DSPE‐PEG2000), and unilamellar magnetic liposomes were prepared by thin‐film hydration method coupled with sequential extrusion. By this method, 200 nm liposomes were obtained, sterically stabilized by PEG chains and containing superparamagnetic maghemite particles whose concentration can be varied. Magnetophoresis confirmed the superparamagnetic profile and the effect of particles confinement into the vesicle core (Martina et al. 2005).
Ménager group reported the preparation of unilamellar magnetic liposomes by a different approach, namely reverse‐phase evaporation method. In this approach, an aqueous suspension of citrate‐coated magnetic nanoparticles and a chloroform solution of phospholipids (DPPC/DSPC/DSPE‐PEG 2000) were mixed and ultrasonicated to induce the formation of a nanoemulsion. Soon after, the organic solvent was removed by rotary evaporation, and the magnetic liposomes were dispersed in the remaining aqueous phase. After filtration (to discard liposomes with a size above 0.45 μm), nonmagnetic liposomes were removed after magnetic separation. By magnetophoresis, the volume fraction corresponding to magnetic nanoparticles (7 nm) into the liposome was estimated as 33% of total volume (Bealle et al. 2012). These magneto‐liposomes have been further developed for a multiple therapeutic application. A photosensitizer used in photodynamic therapy, namely the hydrophobic m‐THPC, was introduced into the lipidic bilayer of the liposomes. Thus, the multifunctional system was tested in vitro and in vivo for the application of a dual‐treatment, combining magnetic hyperthermia and laser‐assisted photodynamic therapy. A small dose of nanoclusters was intratumorally injected, and the mice were exposed to the noninvasive treatments for three consecutive days. The single‐treatment groups showed only a reduction of tumor volume and regrowth after seven days. The synergistic combination of magnetic hyperthermia and photodynamic therapy produced a total regression of tumoral tissue four days after injection, instead (Di Corato et al. 2015).
Amstad et al. suggested a different architecture for magnetic lipidic vesicle. In their work, the authors investigated the effects of iron oxide capping agents on the localization of NPs in the liposome. By using the traditional oleic acid‐capped NPs, an evident agglomeration of nanoparticles was obtained, and a micelle profile was preferred. By functionalizing the magnetic nanoparticles with palmityl‐nitroDOPA, a selective localization was achieved, with confinement 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).
