Nanosystems Based on Magnetic Nanoparticles and Thermo- or pH-Responsive Polymers: An Update and Future Perspectives

ConspectusCombining hard matter, like inorganic nanocrystals, and soft materials, like polymers, can generate multipurpose materials with a broader range of applications with respect to the individual building blocks. Given their unique properties at the nanoscale, magnetic nanoparticles (MNPs) have drawn a great deal of interest due to their potential use in the biomedical field, targeting several applications such as heat hubs in magnetic hyperthermia (MHT, a heat-damage based therapy), contrast agents in magnetic resonance imaging (MRI), and nanocarriers for targeted drug delivery. At the same time, polymers, with their versatile macromolecular structure, can serve as flexible platforms with regard to constructing advanced functional materials. Advances in the development of novel polymerization techniques has enabled the preparation of a large portfolio of polymers that have intriguing physicochemical properties; in particular, those polymers that can undergo conformational and structural changes in response to their surrounding environmental stimuli. Therefore, merging the unique features of MNPs with polymer responsive properties, such as pH and thermal stimuli activation, enables smart control of polymer properties operated by the MNPs and <i>vice versa</i> at an unprecedented level of sophistication. These magnetic-stimuli-responsive nanosystems will impact the cancer field by combining magnetic hyperthermia with stimuli-dependent controlled drug delivery toward multimodal therapies. In this approach, a malignant tumor may be destroyed by a combination of the synergic effects of thermal energy generated by MNPs and the controlled release of antitumoral agents, activated by means of either heat or pH changes, finally leading to a much more effective cancer treatment than those available today. Also, taking advantage of such a triggered chemotherapy will overcome the notorious drawbacks of classic chemotherapy. Nevertheless, tracking the changes in the magnetic properties of such pH-responsive magnetic nanoparticles, which are provided by changes in relaxation signals of water molecules surrounding the nanoplatform, is a novel approach to the detection of pathological conditions (such as pH-changes at the ischemic and tumor sites). Despite great efforts by chemists to fabricate different featured materials, there have been few successful preclinical studies to date. A clinical translation of magnetic stimuli-responsive systems would require overcoming the actual nanosystem limitations and the joint efforts of an interdisciplinary scientific community.In this Account, we have framed state of the art magnetic stimuli-responsive systems, focusing on thermo- and pH-responsive behavior, following an organization based on the response mechanisms of polymers. By evaluating the features of the most representative and advanced nanosystems that already exist in literature, we present the challenges to overcome, the future directions to undertake for the development of magnetic stimuli-responsive nanoplatforms that will work under clinical operating conditions and have biodegradable and biocompatible features, and a consideration of the technical aspects

Multilayered Magnetic Nanobeads for the Delivery of Peptides Molecules Triggered by Intracellular Proteases

In this work, the versatility of layer-by-layer technology was combined with the magnetic response of iron oxide nanobeads to prepare magnetic mesostructures with a degradable multilayer shell into which a dye quenched ovalbumin conjugate (DQ-OVA) was loaded. The system was specifically designed to prove the protease sensitivity of the hybrid mesoscale system and the easy detection of the ovalbumin released. The uptake of the nanostructures in the breast cancer cells was followed by the effective release of DQ-OVA upon activation via the intracellular proteases degradation of the polymer shells. Monitoring the fluorescence rising due to DQ-OVA digestion and the cellular dye distribution, together with the electron microscopy studying, enabled us to track the shell degradation and the endosomal uptake pathway that resulted in the release of the digested fragments of DQ ovalbumin in the cytosol

Duality of Iron Oxide Nanoparticles in Cancer Therapy: Amplification of Heating Efficiency by Magnetic Hyperthermia and Photothermal Bimodal Treatment

The pursuit of innovative, multifunctional, more efficient, and safer treatments is a major challenge in preclinical nanoparticle-mediated thermotherapeutic research. Here, we report that iron oxide nanoparticles have the dual capacity to act as both magnetic and photothermal agents. We further explore every key aspect of this magnetophotothermal approach, choosing iron oxide nanocubes for their high efficiency for the magnetic hyperthermia modality itself. In aqueous suspension, the nanocubes’ exposure to both: an alternating magnetic field and near-infrared laser irradiation (808 nm), defined as the DUAL-mode, amplifies the heating effect 2- to 5-fold by comparison with magnetic stimulation alone, yielding unprecedented heating powers (specific loss powers) up to 5000 W/g. In cancer cells, the laser excitation restores the optimal efficiency of magnetic hyperthermia, otherwise inhibited by intracellular confinement, resulting in a remarkable heating efficiency in the DUAL-mode (up to 15-fold amplification), with respect to the magnetophotothermal mode. As a consequence, the dual action yielded complete apoptosis-mediated cell death. In solid tumors <i>in vivo</i>, single-mode treatments (magnetic or laser hyperthermia) reduced tumor growth, while DUAL-mode treatment resulted in complete tumor regression, mediated by heat-induced tumoral cell apoptosis and massive denaturation of the collagen fibers, and a long-lasting thermal efficiency over repeated treatments

Subnanometer Local Temperature Probing and Remotely Controlled Drug Release Based on Azo-Functionalized Iron Oxide Nanoparticles

Local heating can be produced by iron oxide nanoparticles (IONPs) when exposed to an alternating magnetic field (AMF). To measure the temperature profile at the nanoparticle surface with a subnanometer resolution, here we present a molecular temperature probe based on the thermal decomposition of a thermo-sensitive molecule, namely, azobis­[<i>N</i>-(2-carboxyethyl)-2-methylpropionamidine]. Fluoresceineamine (FA) was bound to the azo molecule at the IONP surface functionalized with poly­(ethylene glycol) (PEG) spacers of different molecular weights. Significant local heating, with a temperature increase up to 45 °C, was found at distances below 0.5 nm from the surface of the nanoparticle, which decays exponentially with increasing distance. Furthermore, the temperature increase was found to scale linearly with the applied field at all distances. We implemented these findings in an AMF-triggered drug release system in which doxorubicin was covalently linked at different distances from the IONP surface bearing the same thermo-labile azo molecule. We demonstrated the AMF triggered distance-dependent release of the drug in a cytotoxicity assay on KB cancer cells

Mesoscale Assemblies of Iron Oxide Nanocubes as Heat Mediators and Image Contrast Agents

Iron oxide nanocubes (IONCs) represent one of the most promising iron-based nanoparticles for both magnetic resonance image (MRI) and magnetically mediated hyperthermia (MMH). Here, we have set a protocol to control the aggregation of magnetically interacting IONCs within a polymeric matrix in a so-called magnetic nanobead (MNB) having mesoscale size (200 nm). By the comparison with individual coated nanocubes, we elucidate the effect of the aggregation on the specific adsorption rates (SAR) and on the <i>T</i>₁ and <i>T</i>₂ relaxation times. We found that while SAR values decrease as IONCs are aggregated into MNBs but still keeping significant SAR values (200 W/g at 300 kHz), relaxation times show very interesting properties with outstanding values of <i>r</i>₂/<i>r</i>₁ ratio for the MNBs with respect to single IONCs

Co_<i>x</i>Fe_3–<i>x</i>O₄ Nanocubes for Theranostic Applications: Effect of Cobalt Content and Particle Size

We report a facile synthesis of cube-shaped Co_<i>x</i>Fe_3–<i>x</i>O₄ nanocrystals (NCs), which could be finely tuned in terms of NC size (from 15 to 27 nm) and cobalt stoichiometry (from 0.1 to 0.7). These particles exhibited high specific absorption rate (SAR) values, relevant for magnetic hyperthermia, and high relaxivity values, significant for magnetic resonance imaging applications. The peculiarity of these NCs is that already at low frequencies (such as 105 kHz, a working frequency used on human patients), they display SAR values that are three-times as large as those of iron oxide nanocubes of comparable sizes (and which were already considered outstanding). The highest SAR value recorded on the NCs reported here (915 ± 10 W/g_(Co+Fe) at 105 kHz and 32 kAm^–1) refers to particles with cubic shape, 20 ± 2 nm edge size, and Co stoichiometry between 0.6 and 0.7. The highest <i>r</i>₂ value (958 mM^–1 s^–1) was instead recorded on nanocubes with Co stoichiometry around 0.5/0.6 and size of 20 ± 2 nm. Remarkably, only at this specific size and Co stoichiometry were the NCs not perfect cubes but had a slightly concave shape, which together with their core–shell structure and magnetic parameters might account for the higher <i>r</i>₂ values recorded. NCs reported here, with optimized SAR and <i>r</i>₂ values, are promising tools for theranostic applications

Magnetic-Field-Induced Formation of Superparamagnetic Microwires in Suspension

We demonstrate the formation of stable magnetic microwires (MWs) in solution starting from a highly diluted solution of monomer–thermal initiator–superparamagnetic nanoparticles (SMNPs). Under an external magnetic field (MF) the SMNPs get closely packed into wire-like assemblies that become permanently linked due to simultaneous thermal polymerization of the monomer. As the SMNPs assemble in the form of wires under MF, the concentration of the monomer chains adsorbed onto them increases in the near proximity of these assemblies, promoting the polymerization process during heating. This combined process causes the permanent bonding among the SMNPs, forming smooth MWs with metallic character. Detailed microscopic and spectroscopic studies reveal the mechanism of the process and designate the importance of the external MF, the thermal polymerization, and the high dilution factor of the reaction solution for the formation of free-standing uniform wires with controlled size. This method leads to a novel approach to form long magnetic wires with smooth contour and regular shape, which can be used in various fields of applications like in biomedicine, chemistry, fluidics, etc

Controlled Release of Doxorubicin Loaded within Magnetic Thermo-responsive Nanocarriers under Magnetic and Thermal Actuation in a Microfluidic Channel

We report a procedure to grow thermo-responsive polymer shells at the surface of magnetic nanocarriers made of multiple iron oxide superparamagnetic nanoparticles embedded in poly(maleic anhydride-<i>alt</i>-1-ocatadecene) polymer nanobeads. Depending on the comonomers and on their relative composition, tunable phase transition temperatures in the range between 26 and 47 °C under physiological conditions could be achieved. Using a suitable microfluidic platform combining magnetic nanostructures and channels mimicking capillaries of the circulatory system, we demonstrate that thermo-responsive nanobeads are suitable for localized drug delivery with combined thermal and magnetic activation. Below the critical temperature nanobeads are stable in suspension, retain their cargo, and cannot be easily trapped by magnetic fields. Increasing the temperature above the critical temperature causes the aggregation of nanobeads, forming clusters with a magnetic moment high enough to permit their capture by suitable magnetic gradients in close proximity to the targeted zone. At the same time the polymer swelling activates drug release, with characteristic times on the order of one hour for flow rates of the same order as those of blood in capillaries

Maghemite Nanoparticles with Enhanced Magnetic Properties: One-Pot Preparation and Ultrastable Dextran Shell

In the field of nanomedicine, superparamagnetic nanoparticles are one of the most studied nanomaterials for theranostics. In this study, a one-pot synthesis of magnetic nanoparticles is presented, with an increased control on particle size from 10 to 40 nm. Monitoring of vacuum level is introduced here as a crucial parameter for achieving a fine particle morphology. The magnetic properties of these nanoparticles are highly affected by disorders or mismatches in crystal structure. A prolonged oxidation step is applied to the obtained nanoparticles to transform the magnetic phases into a pure maghemite one, confirmed by high-resolution X-ray photoelectron spectroscopy analysis, by Mössbauer spectrometry and, indirectly, by increased performances in magnetization curves and in relaxation times. Afterward, the attained nanoparticles are transferred into water by a nonderivatized dextran coating. Thermogravimetric analysis confirms that polysaccharide molecules replace oleic acid on the surface by stabilizing the particles in the aqueous phase and culture media. Preliminary in vitro test reveals that the dextran-coated nanoparticles are not passively internalized from the cells. As a proof of concept, a secondary layer of chitosan assures a positive charge to the nanoparticle surface, thus enhancing cellular internalization

Synthesis of Highly Fluorescent Copper Clusters Using Living Polymer Chains as Combined Reducing Agents and Ligands

We present the synthesis of colloidally stable ultrasmall (diameter of 1.5 ± 0.6 nm) and fluorescent copper clusters (Cu-clusters) exhibiting outstanding quantum efficiencies (up to 67% in THF and approximately 30% in water). For this purpose, an amphiphilic block copolymer poly(ethylene glycol)-<i>block</i>-poly(propylene sulfide) (MPEG-<i>b</i>-PPS) was synthesized by living anionic ring-opening polymerization. When CuBr is mixed with the living polymer chains in THF, the formation of Cu-clusters is detected by the appearance of the fluorescence. The cluster growth is quenched by the addition of water, followed by THF removal. The structural features of the MPEG-<i>b</i>-PPS copolymer control the cluster formation and the stabilization: the poly(propylene sulfide) segment acts as coordinating and reducing agent for the copper ions in THF, and imparts a hydrophobic character. This hydrophobic block protects the Cu-clusters from water exposure, thus allowing to obtain a stable emission in water. The PEG segment instead provides the hydrophilicity, rendering the Cu-clusters water-soluble. To obtain fluorescent and stable Cu-clusters exhibiting outstanding quantum efficiencies, the removal of the excess of free polymer and copper salt was crucial. The Cu-clusters are also colloidally and optically stable in physiological media and showed bright fluorescence even when taken up by HeLa cells, being noncytotoxic when administered at a Cu dose between 10 nM and 1.6 μM. Given the very small size of the Cu-clusters, localization and fluorescent staining of cell nucleus is achieved, as demonstrated by confocal cell imaging performed at different Cu-cluster doses and at different incubation temperatures