Magnetic Nanoparticles: Current Advances in Nanomedicine, Drug Delivery and MRI

Abstract

Magnetic nanoparticles (MNPs) have evolved tremendously during recent years, in part due to the rapid expansion of nanotechnology and to their active magnetic core with a high surface-to-volume ratio, while their surface functionalization opened the door to a plethora of drug, gene and bioactive molecule immobilization. Taming the high reactivity of the magnetic core was achieved by various functionalization techniques, producing MNPs tailored for the diagnosis and treatment of cardiovascular or neurological disease, tumors and cancer. Superparamagnetic iron oxide nanoparticles (SPIONs) are established at the core of drug-delivery systems and could act as efficient agents for MFH (magnetic fluid hyperthermia). Depending on the functionalization molecule and intrinsic morphological features, MNPs now cover a broad scope which the current review aims to overview. Considering the exponential expansion of the field, the current review will be limited to roughly the past three years.

1. Introduction

Ever since nanotechnology grew into a reckoned field of its own, the implications in medicine and pharmacology became obvious, and are today exploited commercially on many drug formulations. A busy lifestyle, erratic work schedule and oxidative stress, together with genetic and other risk factors have contributed to a surge in cancer incidence throughout the globe, and current estimates predict about 1.26 million deaths for 2022 in the EU due to cancer alone. Conventional approaches come with unavoidable side effects due to systemic exposure and response and are less effective than directed therapy—a field where MNPs and SPIONs, in particular, have emerged as suitable candidates with more efficient delivery of the anticancer drugs and limited negative effects on neighboring tissues and organs.

Magnetic nanoparticles (MNPs) are at the very core of magnetic delivery systems and they aim to tackle site-specific tumors while ideally affording a controlled-release profile suitable for disease treatment. Their multifunctional dimensionality makes it possible for MNPs to be used in nanomedicine as elective candidates for drug targeting therapy when using an externally applied magnetic field.

With tunable physico-chemical properties and a very high surface-to-volume ratio typical for nanoparticles, MNPs can be engineered into drug-delivery systems with similar sizes to the organism’s own antibodies or proteins for improved biocompatibility, while incorporating therapeutic agents that would otherwise be difficult to deliver to the cancer cells. When superparamagnetic nanoparticles (SPIONs) are coated with biologically compatible polymers of fatty acids, systems with improved colloidal stability and reduced tendency of aggregation are obtained. MNPs were also used as contrast agents in magnetic resonance imaging (MRI). When functionalized with epithelial growth factor receptor antibodies or aptamers, an efficient diagnosis tool is created for many types of cancer or even detection of brain inflammation. MNPs are being used as alternative contrast agents in MR imaging owing to their superparamagnetic properties and high relaxivity, doubled by high biocompatibility upon surface functionalization and low toxicity, unlike the Gd complexes used traditionally as contract agent in MRI, which could potentially release Gd into the bloodstream.

The promise of viable candidates in cancer treatment made research in the field of MNPs flourish and today hundreds of reports are published annually describing new or improved strategies for using MNP systems in disease treatment. Conjugation of IONs (magnetite and maghemite are generally best tolerated) with drugs yields drug-loaded IONs that can be directed using an external magnetic field to the site where the tumor cells reside, and this drug-delivery variant is termed magnetic drug targeting (MDT). The variety of such approaches would make a comprehensive review quite difficult and lengthy, hence why the current review focuses on the most notable advances recorded with MNPs over the past three years.

2. Justification and Design Strategies for Magnetic Nanoplatforms

The current review strategy entailed an examination of search results on specific research topics such as “magnetic nanoparticle” and/or “drug delivery”, “nanomedicine”, “MR imaging” or “hyperthermia”, but is also based on pertinent examples dealing with the above topic without specifically containing the mentioned keywords (for instance, articles dealing with ferrofluids were also included, provided a tentative use in biomedical applications was provided in the original text). Based on the occurrence of research directions found in both original research and review articles published in the past three years (2020–2022), the current table of content was decided and data curation of the 500+ articles identified was carried out around these main topics. Given the exponential expansion of the field, only the last three years were selected, and the inclusion of each article on the reference list was decided upon by reading the abstract and deciding on the suitability of the article for the included review topics.

There are multiple reasons pleading for the use of MNPs as carriers for drug and gene delivery, making nanoparticle platforms superior to the traditional administration of the drug alone. Moreover, theranostics are able to integrate today’s MNPs in procedures capable of both diagnosis and disease treatment, which is unachievable via traditional drug administration. Using NPs for drug delivery allows modification of key aspects related to drug solubility, diffusivity, penetration and retention, pharmacokinetics, biodistribution, cytotoxicity, and half-life (controlled- and/or on-demand release).

Considering how time-consuming (~12 years on average) and extremely expensive (up to USD 2 billion) the drug development process can be, it may be surprising to note that, even when an efficient active component is identified, basic hiccups still plague the process, such as a lower grade in the biopharmaceutical system, which assesses the solubility and permeability of a drug. In other words, a highly efficient drug whose development took years of research may never reach the patient in need due to low solubility/permeability in the biological system. This severe shortcoming can be modulated by drug immobilization on magnetic nanocarriers, that are able to transport insoluble drugs to the target site by smart surface modification (for instance, with antibodies that would bind to molecules overexpressed at the tumoral site). Drug formulations employing stable nanoparticle dispersion of the drug can increase absorption, even when using much lower dosages—this advantage alone can make a drug with low bioavailability and poor penetration/absorption when used alone become suitably efficient for drug-delivery systems, with real prospects of reaching the market. Various groups reported that drug molecules conjugated to MNPs can exhibit efficiencies many times higher than using the drug by itself (Figure 1).

Conventional drug administration is typically carried out via oral, parental, pulmonary or transdermal routes, and usually requires several doses for maximum efficiency. However, a controlled release system based on drug-loaded NPs can bypass these shortcomings, by reducing drug quantities and related toxicity, and also the doses required for treatment. In a typical, conventional drug administration, the drug concentration in biological fluids can vary greatly, from subtherapeutic (no effect) to toxic levels; by contrast, the release profile of a drug in a controlled release system is rather constant, and within the limits required for maximal therapeutic effect. Such systems can be regarded as highly beneficial for patients using anti-inflammatory drugs (especially the elders), or diabetic patients; the former will not experience pain due to constant drug concentration in the blood, while the latter will have better glycemic control because insulin would be released on-demand contingent with blood sugar levels. The release can be prolonged to multiple weeks from a single administration, which would be unheard of in the case of traditional administration.

Another key parameter is the enhanced permeability and retention achievable due to specific hypervascularization at the tumor level (with the formation of epithelial pores), which translates into increased permeability to therapeutic drugs when conjugated to polymeric-coated NPs. For efficient drug accumulation, however, the nanoparticle-based drug formulation must be stable in biological fluids (it should not agglomerate/ precipitate), should be tailored regarding size and concentration for optimal penetration through cellular membrane and cellular uptake.

2.1. Synthetic Strategies and Feedback-Driven Design

Synthetic methods currently utilized for SPION particle synthesis involve the hydrothermal route (with typically mesoporous NPs as the outcome [1,2,3,4,5,6,7,8,9], as opposed to the classical co-precipitation route, which has a number of disadvantages including reproducibility issues regarding morphological parameters with direct influence over the magnetic properties [10,11,12] or the well-established sol–gel [13] and (auto)combustion [14] methods. Other synthetic routes focus on natural extracts as bio-inspired routes to MNPs based on iron oxides, with encouraging therapeutic potential [15] and other green synthesis strategies [16]. Other nature-inspired compounds such as magnetic zeolites are also under investigation today [17].

2.2. Physical Characterization

Physical characterization methods were employed for particle size determination, including analysis of magnetization curves [18], cation distribution in ferrites by X-ray absorption [19,20] and Mossbauer spectroscopy [21,22].

Magnetic measurements aim at providing a feedback-driven synthesis route for improving effective magnetic moment [23,24,25], especially when referring to ferrofluids since these target actual biologic systems [26,27,28]. Effective quantification of the heating ability of MNPs can be achieved via SAR determination experiments [29] and novel tools such as small-angle scattering can improve the design of functionalized MNPs [30].

2.3. MNPs with Improved Magnetic Properties: Substitution/Doping Effect

Doping strategies have modified the therapeutic potential by altering the paramagnetic behavior of maghemite (γ-Fe₂O₃) [31], metal ferrites [32], as well as Gd³⁺ substitution of magnetite Fe₃O₄ with effect on superparamagnetic NPs [33].

Various strategies for anisotropy enhancement were explored, including layering of MNPs on amorphous substrates with perpendicular anisotropy [34].

Ferrites of spinel structure have been synthesized; BaFe₂O₄ [35], Mn–Zn ferrite [36], CuFe₂O₄ [37,38], Cu–Ni ferrite [39], Zn, Cu and Co ferrites [40,41], or Ni–Zn–Co ferrites with Gd³⁺ substitution [42].

2.4. Flow Characteristics and Simulated Models for MNPs in Biologic Fluids and High-Performance Ferrofluids

The flow parameters (magnetorheology) were reported recently [43,44,45,46,47,48], tackling also convection [49], location [50] and sedimentation processes [51,52] or multi-core MNPs [26], proper distribution in form of appropriate matrices such as gels [53], overall tracking efficiency [54], as well as guidance for cell behavior [55], and actual flow through artificial blood [56].

The effect of viscosity of the medium as a means to counteract the sedimentation tendency of MNPs was investigated [57], while the dispersibility of NPs was shown to be an efficient way to obtain stable ferrofluids and magnetic therapeutic fluids [58,59].

Simulation models of MNPs in the actual blood flow were reported very recently [60,61,62], on the magnetic susceptibility vs. frequency for MNPs [63,64], suitability of poly(vinyl) alcohol PVA coating of MNPs for drug delivery [65] or on the possible replacement of dopamine, the go-to drug used to treat Parkinson’s disease, by magnetite Fe₃O₄ NPs [66]. Alzheimer’s disease is also among the diseases targeted by functionalized SPIONs when conjugated with NIR dyes [67].

2.5. Morphology—Role of Size and Shape

Size control of MNPs [68] was been recently achieved in a green synthesis, bioinspired strategy by employing lysine in the synthesis of SPION magnetite materials [69]. Studies have shown that, for particle sizes higher than 100 nm, the MNPs are subject to macrophage phagocytosis by the spleen and liver, hence the requirements to design particles below 100 nm.

Size and shape effects on hyperthermia performance were also studied [70,71,72]. Results point to relatively lower toxicity when spherical nanoparticles are used (such as those obtained by the polyol method), rather than irregular/polyhedral-shaped NPs. I. Craciunescu et al. have investigated hydrophobic (oleic acid coating)/hydrophilic (azelaic acid-coated) magnetite (Fe₃O₄) and ferrites of Mn and Zn (MFe₂O₄), produced by the polyol method, of different shapes: spherical, cubic, hexagonal, octahedral and sizes (10–100 nm) with interesting findings linking shape and size of MNPs to the hyperthermia procedure. MFH (magnetic fluid hyperthermia) is a technique currently investigated which allows MNP-mediated conversion of alternating magnetic field energy into heat; moreover, this heat release event can be doubled by drug release at the tumor site which enhances the therapeutic chances of success in tumor treatment and should respect a maximal exposure criterion f × H ≤ 5 × 10⁹ Hz × A/m for applicability in biological systems [70]. The SAR (specific absorption rate) is a key parameter quantifying the energy conversion process and is dependent on AC magnetic field amplitude, frequency and MNPs relaxation mechanisms. The magnetization at saturation increases to 90 emu/g in the case of cubic shapes and MNPs of 100 nm average size (Fe₃O₄ and MnFe₂O₄) and lower values (50–70 emu/g) for zinc ferrite nanoparticles [70]. Large-sized MNPs are expected to transfer heat in hyperthermia applications by means of hysteretic losses due to magnetic wall displacements, hence of prime interest parameters are magnetization at saturation M_S and magnetic susceptibility in low AC magnetic fields—information that can be deduced by analysis of hysteresis loops [70]. Additional information regarding NPs size and size distribution, interparticle interactions and magnetic domain structure can be deduced by analysis of FC–ZFC curves (field-cooled–zero-field-cooled) or Mössbauer spectroscopy (for anisotropy energy determination, KV: K is the magnetic anisotropy constant, and V is the nanoparticle magnetic volume). It is worth noting that among the three types of samples analyzed (Fe₃O₄, MnFe₂O₄ and ZnFe₂O₄), the highest heating efficiency was that of the soft magnetic ZnFe₂O₄@azelaic acid (AZA) with a SAR of 175 W/g, more than double that of Fe₃O₄. AZA (SAR = 85 W/g), although it has the lowest saturation field among them. Increasing the heat efficiency is possible when using MNPs in a magnetically frozen regime at room temperature, which will not allow for the formation of moving magnetic domain walls [70].

Manganese ferrite MnFe₂O₄ was investigated extensively due to tunable magnetic properties, high biocompatibility and chemical stability [70,71]. Besides the essential role of hysteresis losses mentioned above (where it was the dominant heat transfer mechanism), two other mechanisms describe the heat transferred by NPs to the surroundings (SPL, specific power losses): Neel and Brownian relaxation [71]. SPL is also strongly influenced by particle size because this parameter alters the shape anisotropy. Chitosan-coated MnFe₂O₄ were obtained by co-precipitation of FeCl₃ and MnCl₂.4H₂O with NH₄OH, producing stable, functionalized MNPs with potential applications as positive/negative MRI contrast agents in the rat model [71]. A theoretical investigation of various sized Fe₃O₄ NPs (25, 50, 100 and 200 nm) showed that NPs with lower sizes produced a higher heat gradient in the tumor mesh (61, 49, 42 and 41, respectively), while those in the 50–100 nm size ranges were found to be the most promising candidates for hyperthermia and cellular uptake [72]. Considering the heat produced by hysteresis per volume unit P = μ₀ f ∫H dM, we can expect better results when the alternating magnetic field frequency f is increased, within the biologically safe limit [72,73]. Theoretical simulations for correlating size with potential hyperthermia applications were also reported [73].

2.6. Intra- and Interparticle Interactions: Colloidal Stability and Size Differentiation

Interparticle interaction has long been seen as a possible cause of further aggregation [74]. R. Das et al. have shown the effect of shape (NRs nanorings of 55 nm length vs. NTs nanotubes of 470 nm length) on the MFH performance corresponding to Fe₃O₄ nanoparticles, while also raising the question of inter- and intraparticle interaction [74]. The morphology of NPs was controlled by the amount of NaH₂PO₄.2H₂O used in the first precipitation step (higher concentration leads to NRs, lower concentration favors NTs). The iron oxide nanotubes NTs showed higher effective anisotropy and M_S, but lower SAR value (80 W/g at 400 Oe and 300 Hz) than nanorings NRs featuring weaker intraparticle interactions (110 W/g). It becomes apparent that MFH is positively influenced by using MNPs of lower volume and weaker intraparticle interactions [74].

Colloidal stability is a key parameter to preventing undesired NP accumulation before ever reaching the target organ [75,76,77,78,79,80]. For instance, optical tracking of iron oxide ferrofluids at 10 T and a 100 T/m gradient has shown that aqueous ferrofluids are best investigated in high fields, which offer a reliable estimation of their behavior under lower, practical fields [75]: 0.25 vol% iron oxide as stabilized dispersion of citrate-coated maghemite nanoparticles (γ-Fe₂O₃), and commercial Fe₄O₄ ferrofluids [75]. Depending on the magnetic field strength (0.3–0.5 T and ~20 T/m for a neodymium magnet, 10 T and 100 T/m for a Bitter magnet), citrate-coated maghemite remains separately dispersed. However, when MNPs of higher polydispersity are used, the largest NPs separate rapidly from the solution while smaller NPs remain dispersed because of their low dipolar coupling energies [75].

V. Pilati et al. synthesized aqueous ferrofluids using the electric double-layer (EDL) strategy to maintain their solution stability [76]. These systems were based on biomagnetic core-shell ZnMn mixed ferrite@ maghemite shell out of which two specific compositions were further investigated, namely Zn_δMn_{1 − δ}Fe₂O₄@γ-Fe₂O₃ (δ = 0.2 and 0.5). The surface was further covered by a maghemite layer by exposing the Zn_δMn_{1 − δ}Fe₂O₄ core (co-precipitation) to HNO₃ washing followed by hydrothermal treatment with Fe(NO₃)₃ 0.5 M. The electrostatically stabilized ferrofluid was achieved by peptization of as-synthesized ferrite NPs in a dialysis bag, using HNO₃ with pH fine tuning (final pH = 2.0) and solution ionic strength adjustment by means of NaNO₃ formation [76]. Interestingly, dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) revealed that changing the NPs concentration from dilute to >25 mg/mL is accompanied by a change in global interaction forces from attractive (diluted) to repulsive (concentrated) [76]. L. L. e Castro et al. extended the applicability of EDL repulsive interactions by using Monte Carlo simulations to surfacted MNPs, where the charge is located typically at the extremities of the surfactant molecule (at the organic functionality, such as amino, carboxyl, etc.). They ran the simulations on spherically shaped magnetic NPs using a model proposed by Schnitzer and Morozov—an improvement over the traditional DLVO model traditionally used for modeling colloid stability [77].

J.C. Riedl et al. used maghemite (γ-Fe₂O₃) NPs dispersed in ionic liquids (ILs) based on ethylmethylimidazolium bistriflimide (EMIM TFSI) in a pursuit to obtain colloids stable from room temperature up to 200 °C; the dispersion of maghemite at concentrations up to 12 vol% was shown to be stable for several days at 200 °C [78]. M. Boskovic et al. synthesized Fe_{3 − x}Gd_xO₄ (x = 0, 0.1, 0.2) NPs of diameter ~8 nm by the coprecipitation method and by coating with citric acid (CA) with improved colloidal stability; the sample Fe₂.₈₀Gd₀.₂₀O₄@CA embedded in human serum albumin afforded magnetic microspheres (MMS) as suitable carriers for drug-delivery applications [79]. Polymeric coatings of iron oxide nanoparticles such as silica-coated Fe₃O₄ NPs (diblock copolymers obtained by living cationic polymerization, PEO-b-PMAA) are oftentimes used because they lower Gibbs free energy of magnetic nanoparticles in solution, hence maintaining colloidal stability and preventing agglomeration [80].

3. Coating and Surface Functionalization of MNPs: Polymers, Acids, Amines, Siloxanes, Other Coatings

Synthetic routes were implemented using flow reactor design, including surface-coated MNPs with, for instance, PEG [81,82,83,84,85,86,87,88,89], citric acid functionalization [90], aspartic acid [91], latex [92], pectin [93,94], polycations for A549 cells [95], other acid formulations [96] or polymers [97,98,99], biopolymers [100,101], PLGA poly(lactic-co-glycolic acid) copolymer [102,103], or poly(L-lactide-co-glycolide) copolymer [104]. Size differentiation was made possible by using chromatography in a simulated bed configuration [105]. The antibacterial properties of various MNP coatings were reported, and even carbohydrates were literally “sugar-coated” on Cu-doped Fe₃O₄ NPs [106,107].

These surface-modified versions of NPs can exhibit enhanced saturation magnetization, offer a better anchor for drug molecules to bind and can sometimes even serve as active catalysts for aromatic compounds synthesis including N-containing heterocycles [108,109,110,111,112,113], photocatalysis [114,115,116,117] and biocatalysis for tumor therapy [118,119,120]. M. Rajabzadeh et al. reported the innovative use of CuI Immobilized on Tricationic Ionic Liquid Anchored on Functionalized Magnetic Hydrotalcite (Fe₃O₄/HT-TIL-CuI) for Ullman-type C–N coupling reactions between aryl halides and N(H)-heterocycles (benzimidazoles, pyrazoles and triazoles) (no additives, under air atmosphere) in the presence of 2.5 mol% of nanocatalyst [108]. The inorganic–organic hybrid catalyst Fe₃O₄@SiO₂-L-tryptophan (L-tryptophan functionalized silica-coated MNPs) based on Fe₃O₄ synthesized by co-precipitation with NH₄OH from ferric and ferrous chloride salt sources was synthesized and evaluated as a recyclable magnetic nanocatalyst for the synthesis of spiro[indene-2,2′-naphthalene]-4′-carbonitrile derivatives [109]. The choice of L-tryptophan, a chiral α-amino acid was motivated by the presence of both amino (-NH₂) and carboxylic (-COOH) moieties, through which it can partake in various catalytic transformations [109]. Another very recent report focuses on AlCl₃@nano Fe₃O₄–SiO₂ multi-layer magnetite nanocatalyst for the one-pot synthesis of spiro[benzochromeno [2,3-d]pyrimidin-indolines] by a three-component condensation in refluxing C₂H₅OH of different naphthols, isatin derivatives, and barbituric acids [110]. Other research efforts made use of coordination, i.e., the binding affinity of polymers containing P-derived functional groups—phosphonic acids R–H₂PO₃, known for strong affinity to metals and multidentate binding ability [111], or participation of MNPs’ polymeric coating in further functionalization by activation of esters under mild conditions to form amide bonds, click chemistry consisting of Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC process), or amine addition to isocyanates, among others [112]. In fact, CoFe₂O₄ MNPs recently synthesized by D. Aurélio et al. made use of a hydrothermal process producing cobalt ferrite NPs capped by oleic acid, which was further exchanged with N-containing organic acids such as 11-maleimidoundecanoic acid [113].

The surface functionalization and conjugation with various drugs lead, broadly speaking, to core-shell structures [121,122], including siloxanic coatings of MNPs which are environmentally benign and can be used for biomedical approaches [123,124,125,126,127,128,129,130]. Surface functionalization of SPIONs for biomedical applications by Ar plasma was very recently reported by Asghari et al. [131], or by other chemical methods [132,133,134,135,136,137,138]. Detailed experimental parameters are included in Section 5.

4. MR Imaging

MRI is a non-invasive imaging technique that exploits the ability of protons to align and process around B₀ (an applied magnetic field) and to relax when perturbed from B₀ by the application of a transverse radiofrequency. This relaxation process comprises two distinct terms: T1-recovery or longitudinal relaxation (positive contrast enhancement, sensitive to MNPs thickness, hence effective only for thin coatings) and T2-decay or transverse relaxation (favored by MNPs high susceptibility, negative contrast-enhanced, most used for SPIONs). This research direction is also motivated by the need to replace current contrast agents based on Gd³⁺ complexes, which pose worrying health issues and undesired side effects. As valuable MRI contrast agents, SPION nanoparticles have gained increased attention and popularity, especially iron oxide-based [139,140,141,142,143]. Relaxivity is a direct and quantifiable measure of a contrast agent’s efficiency: R1 (1/T1) and R2 (1/T2), and depends on the type of MNPs used, applied field and temperature.

Besenhard et al. have demonstrated a reproducible SPION synthesis in a flow reactor using a co-precipitation method using dextran as the surface coverage agent, followed by quenching (by timely 2–100 s addition of 0.32 M citric acid solution-stops nucleation due to chelation of iron ions) after the formation of the desired iron oxide core, achieving nanoparticles of less than 5 nm (Figure 2) [139].

The longitudinal relaxivity (r₁ = 10.7–12.4 mM⁻¹ s⁻¹) and transversal relaxivity achieved (r₂ = 20.5–57.2 mM⁻¹ s⁻¹) recommend this synthetic procedure to produce inexpensive SPIONs as efficient MRI T1 contrast agents and replacements for Gd-based ones (of smaller r₁ in commercial Dotarem^TM or Gadovist^TM, 4.2–5.3 mM⁻¹ s⁻¹) [139].

Some general introductory reviews covering MRI imaging have emerged in the literature [144,145]. Running the reaction at 60 °C, the salt co-precipitation to form spinel phases (magnetite/maghemite) confirmed by real-time XRD data that intermediate ferrihydrite species transform swiftly into the final spinel. Key aspects that influence the efficiency of iron oxide NPs in MRI include magnetization, size, effective radius, inhomogeneity of surrounding generated magnetic field, crystal phase, coordination number of water, electronic relaxation time, and surface modification [145]. T2 relaxivity for instance can be increased by synthesizing SPIONs with improved M_S and effective radius [145]. However, recently another iron-based compound was investigated as a new agent for enhanced hyperthermia therapy and a T₂ contrast agent for MRI application: iron nitride γ′-Fe₄N nanoparticles, which exhibit three times higher saturation magnetization and could also be properly covered by an oleic acid layer for further functionalization [146].

SPIONs are particularly efficient at allowing visualization of the cell line uptake (head and neck, for instance) [147], and their use became widespread in both MRI and MPI (magnetic particle imaging). MPI shares many similarities to MRI and is a tracer-based modality providing convenient diagnostic and therapeutic tools featuring important advantages: high sensitivity (0.1 µm), good spatial resolution (<1 mm) and temporal resolution (<1 s) with medium cost associated [141]. It does come with some drawbacks; typically, SPIONs are used as T2 contrast agents, as demonstrated above [139], and in that respect, they obscure adjacent tissue, while also making the resulted contrast unreliable in some particular cases—air-tissue interfaces or hemorrhagic tissue would behave similarly; additionally, there is the potential risk of heating and peripheral nerve stimulation for patients undergoing the procedure [141]. While some coatings such as Au coatings provide extra stability and corrosion resistance, some critical aspects of Fe₃O₄/Au magnetite/gold core-shell nanostructures pinpoint some clear disadvantages and may be a reason why their current development came to a halt [148], although some scattered reports exist dealing with magnetic plasmonic Co@Au/Ag/Au–Ag core-shell nanoparticles for their biological imaging potential [149], with Fe–Pt@Au core-shell NPs [150] or custom designed for MRI and drug delivery [151,152,153,154,155,156,157,158,159]. For instance, Iancu et al. have shown that gold-coated magnetite Fe₃O₄@Au NPs can act be used as biocompatible drug carriers (at concentrations <2 × 10⁻⁸ mg/cell), while in vivo tests in rats revealed a negative T2 signal at a concentration of 6 mg/100 g body would suffice for obtaining high-quality MRI images [153].

Chelating ligands such as DoS (diblock polymer PDOPA-b-PSar) were shown to bind to Mn²⁺ centers to form novel, uniform micelles Mn²⁺@PDOPA-b-PSar of 73.4 nm size (low polydispersity index PDI = 0.159) that were investigated as MRI contrast agents with good contrast features in imaging owing to the magnetic manganese core [159]. These micelles showed good results in MRI tests as T1-weighted contrast agents with relaxivity r₁ = 27.7 mM⁻¹s⁻¹, and showed promising results for other biomedical applications such as drug release systems, while in vivo tests performed on rats showed cell survival rates higher than 70% [159].

The opportunity of using MNPs as contrast enhancements in MR imaging proves a current topic of interest, as the results are very detailed and the irradiation impact is reduced to a minimum [76,160,161,162,163]. Specific effects of coatings (with amine-carrying molecules) on MNPs’ performance in MRI revealed interesting enhancement effects [164], and examples include coating with sodium oleate [165], chitosan [166,167,168,169,170,171,172,173,174,175] or organic acids [176], as well as amino moieties-poly(acrylamide) coatings [177].

4.1. Radiolabeling

Radiolabeling strategy (¹⁸F, ⁶⁴Cu, etc) is an excellent tool for tumor imaging [178,179], one that is constantly improving owing, in part, to complementary theoretical computations [180,181].

Metal oxides have made important strides as T1 and/or T2 MRI contrast agents [182,183,184,185], either in the form of dysprosium oxide NPs coated with polyacrylic acid [186], gadolinium oxide NPs coated with poly(methyl vinyl ether-alt-maleic acid) [187], paramagnetic gadolinium oxide NPs coated by polyaspartic acid [188], gadolinium NPs coated with sericin—a protein created by silkworms (Bombyx mori) in the production of silk [189], iron oxide-magnetite Fe₃O₄ coated with folic acid [190] or other polymeric coatings [191,192,193,194,195], and colloidally stable Fe NPs [196,197].

4.2. SARS-CoV-2 and MRI with SPIONs

SPIONs’ advances were also stimulated by the world pandemic that burst in late 2019, which urged scientists to find new tools to identify and cure the SARS-CoV-2 virus. In this respect, magnetic NPs have shown real promise not only for detection [143,198,199,200] but also for targeted pulmonary drug release [201,202]. Magnetic particle spectroscopy (MPS) was employed as a viable tool to detect target nucleic acids down to concentrations of 500 pM, without special sample preparation; Bionized NanoFerrite particles with a mean diameter of 80 nm (BNF80), coated with streptavidin were used for this goal, with a Brownian-dominated relaxation mechanism [143]. Most patients infected with the SARS-CoV-2 virus developed respiratory syndromes, therefore the development of microcarriers for targeted drug delivery at the bronchi level became of high interest to the scientific community [201]. Microcarriers of ~2 μm diameter (silica, iron oxide, nickel oxide) were released at the lung level and their adherence to the inner walls of lung branches was studied; interestingly, changing the inlet velocity from constant to pulsatile increased the drug delivery performance to the lungs by ~31% when 10 nm Fe₃O₄ MNPs coated microcarriers were studied in conjunction with a permanent magnet (~4 × 4 × 2 cm), a strategy inspired by previous reports by Cai et al. who observed increased drug release efficiency when utilizing cobalt ferrite NPs [201,202].

4.3. Functionalization Agents and Strategies

Additional improvements can be obtained by experimental advances recorded in synthetic methods [203,204]. Surface functionalization [205,206,207] and polymeric coatings [208,209] are a prerequisite for using MNPs at the core of drug-delivery biocompatible systems. Even the known family of non-ionic block copolymers known under the trade brand Pluronic^TM (the surfactant typically used in many 2D and 3D mesoporous silica synthesis)), were used for anti-cancer formulations [210,211]. Polymer coatings have evolved to the usage of nanopolymers [212].

Along with drugs targeting specific diseases, genes can also be loaded onto functionalized MNPs, with PDMAEMA—a water-soluble cationic polymer capable of DNA electrostatic interaction [213] or coated by hyaluronic acid [214].

5. Therapeutic Features

Despite impressive achievements during the 21st century, life expectancy is still low in many countries, and a common and rising leading cause of serious health problems and ultimately death is the occurrence of cancer, which is believed to eventually be responsible for roughly 1.3 million deaths in 2022 in the European Union alone [215]. Various tumors and cancers were triggered by MNP formulations [216,217,218,219], and a breakthrough was the transition of MNPs from fundamental research to viable options in oncologic treatment [220], theranostic applications [221,222] and drug-delivery systems [98,223,224] (Figure 3).

Antibody immobilization on MNPs can yield powerful sensing platforms [225]. Many biomedical applications including cancer and tumor cell treatment have been reported [226,227,228,229,230,231]. Applications of MNPs in medicine and particularly nanomedicine have been reviewed recently in the literature [232,233,234,235].

5.1. Drug Delivery

Drug-delivery systems have diversified immensely and today they cover a broad spectrum of both MNPs and targeting drug loadings [236,237,238,239,240,241], including intraocular delivery via smart microrobot technology [242], or delivery of erythropoietin-hybridized MNPs for treatment of central nervous system injury [243,244]. Magnetic nanoplatforms for delivery of classical platinum-based anticancer medicine (i.e., cis-platin) were investigated [245] and their toxicity in vitro was evaluated [246]. It was shown that shape contribution to cytotoxicity is very important, with spherically-shaped NPs being comparable less toxic than cylindrical or oval-shaped homologous, especially with increased reactive oxygen species (ROS) concentration.

Efficient delivery of drugs by MNP-coated nanocomposites requires a thorough knowledge of the proteic behavior of the drug molecule [247,248,249]. Pharmacokinetic behavior is of utmost importance since it can lead to better drug administration and by this, to more efficient disease treatment and management [250,251]. Drugs, genes and other biologically relevant molecules can be conjugated with NPs for efficient delivery at the targeted site (MDT, magnetic drug targeting). Drug-delivery applications have gained momentum and many reports have surfaced, dealing with a variety of problematic-to-target-and-cure types of cancer or disease [96,239,252,253,254,255,256,257,258,259,260,261,262]. A summary of the most commonly used anticancer drugs is depicted in Figure 4, with their chemical formulas (where space allowed it) and associated brand-name; the following organs were chosen—brain, stomach, pancreas, liver, breast and lung (Figure 4).

Various types of cancers are currently investigated by means of MNP drug conjugation, targeting lung cancer [263,264,265,266,267], gastric cancer [268,269,270,271], pancreatic cancer [272], hepatocellular carcinoma [273], bone cancer—osteosarcoma [274], blood cancer—leukemia [275,276] anemia—decreased number of red blood cells or hemoglobin [277], breast cancer [278] or liver fibrosis [279] either as a theoretical model [280] or actively conveying the chemotherapeutic medication Docetaxel (DTX or DXL, commercially available under the brand name Taxotere) [281,282], Artemisinin (initially anti-malaria drug in 1972)/Tannic acid [283], or folic acid [284]. Various types of tumors have been investigated, including grade IV astrocytoma—Glioblastoma Multiforme (GBM), an aggressive and rapid-growing brain tumor, whose management requires the MNPs to be able to bypass the feared BBB (blood–brain barrier) [285,286,287,288,289,290,291,292,293]. Brain tumors continue to be a demise sentence, with a 5% patient survival rate 5 years after the first glioblastoma (GBM) diagnosis, as statistics show. One of the many added benefits of utilizing MNPs in a variety of cancer types is the ability to diagnose that specific form of cancer with ease and detect it even in its early stages, which greatly expands the life expectancy of the patients. Smart surface modifications, i.e., coating with a biocompatible layer of tumor/cancer membrane have led for example to important advances in bone cancer treatment. However, MNPs can now address not only cancers but also abnormal levels of sugar or glucose in the blood, hence being potential treatment agents in glycemia; as such, magnetic nanocomposites of α-amylase inhibitors have been designed to this end [294]. Targeted delivery across specific organs was proven feasible by Zhou et al. who used an MNP-robotic capsule to deliver specific drugs at the gastrointestinal level [295] or at the cardiovascular system level [296].

Extending fundamental research to in vitro studies utilizing cell lines revealed important features that iron oxide and other MNPs might present [297,298,299,300,301,302,303,304,305,306,307,308,309,310]. Bionanomaterials are now at the convergence of materials science/nanotechnology and biomedical applications, and the investigation methods have reached a level to match theoretical predictions to in vitro and in vivo behavior of many drug formulations, many of which have become FDA-approved and commercially available to patients in need [311].

Magnetosomes have become important tools to manage cancer growth, and their bacterial biosynthesis is under active development [312,313,314,315,316]. In the final stages of cancer (metastasis), isolation of exosome-active participants in cancer progression and metastasis was shown to be possible when using Fe/Au nanowires [317]. Some of the many important advances recorded in drug delivery over the past three years are summarized in

Table 1. An overview of recent drug-delivery parameters for drug-delivery therapies.

Type of NP	Synthesis of MNPs/Coating	Surface Functionalization	Type of Drug/Molecule	Targeted Disease/Application	Ref.
Fe3O4	Tripolyphosphate (TPP) and glutaraldehyde stabilizers	polyvinyl alcohol/collagen	BSA protein	drug release system	[65]
NixCu1−x-silica nanoparticles (x = 0.675)	sol–gel method	SiO2 silica from tetraethyl-orthosilicate (TEOS, Si(OC2H5)4) hydrolysis and condensation	PTX (C13H18N4O3), BPC (C18H28N2O), PCM (C8H9NO2)	Skin cancer-Humanskin fibroblasts (ATCC-CCL-110, Detroit 551)	[126]
Multifunctional Fe3O4@SiO2-APTES-DOTA	sol–gel	SiO2 with amino-functionality by aminopropyltriethoxysilane (APTES) usage; and 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)	Teniposide, anticancer drug	brain tumors, acute lymphocytic leukemia (ALL)	[258]
MnFe2O4 and Cr2Fe6O12 nanocarriers	combustion/calcination of PVP stabilized metal salt complexes	N/A; curcumin loading by precipitation	curcumin (CUR) release to MCF-7 cells; anti-inflammatory, anti-oxidant, antimicrobial, antispasmodic and antiproliferative activity; (release was pH-dependent)	photosensitizer for photodynamic therapy (PDT); drug delivery	[259]
superparamagnetic iron oxide (Fe3O4) nanoparticles (SPIONs)	co-precipitation; Dextran (DEX) stabilization	Folate (FA)-modification by conjugation to MNPs	camptothecin (CPT) action on AT3B-1 cancer cells	prostate cancer	[260]
ZnFe2O4 zinc ferrite nano-hollowspheres (NHSs)	solvothermal method	Sodium folate ligand modification for biocompatibility (folic acid small molecule vitamin)	Doxorubicin	cancer treatment	[318]
iron oxides (hematite, magnetite); carbonyl iron (due to its low size)	various (precipitation etc.)	polyethylene glycol (PEG); Magnetic liposomes; biodegradable polymers as stabilizers against oxidation (celluloseacetate; hydrogen phthalate)	Paclitaxel; Celecoxib; (Doxorubicin—poor distribution through BBB); Ferrocenyl diphenoltamoxifen; Gadd 153	Glioblastoma Multiforme management	[319]
SPION (magnetite, Fe3O4 nanospheres)	emulsion solvent evaporation method; polymer (ethylcellulose) coating	ascorbic acid (Vitamin C) capping	Carboplatin (CPt)	Breast cancer	[320]
multifunctional mesoporous silica nanoparticles (MSNs)	coating: Polydopamine (PDA) and graphene oxide (GO) double layer	fluorescent conjugates to yield FMSNs	ibuprofen and acetaminophen	Drug release	[321]
Fe3O4	dextran, PEG, Hyaluronic acid, Human serum albumin conjugate	polyvinyl alcohol (PVA) and PEG-derivatives/functional ligands: PEG(5)-nitrodopamine, PEG(5)-dopamine, PEG(5)-hydroxipyridine	Cetuximab and doxorubicin, Gallic acid, Erotinib, Actein, Quercetin	Lung cancer	[263]
Fe3O4-Dex-MA-g-P(NVI/NVCL) (pH-sensitive multi-functional magnetic nanocomposite)	Dex-MA (dextran modified by Glycidyl methacrylate)	Poly(N-vinylcaprolactam) (PNVCL), a temperature-sensitive biocompatible polymer	5-FLU (5-fluorouracil or 5-Fluoro-2,4-pyrimidinedione)	cancer	[322]
magnetic microspheres (MMS) based on Fe3O4	co-precipitation and water-in-oil-in-water (W1/O/W2) ternary emulsion solvent evaporation process	PLGA (poly-(D, L-lactide-co-glycolic acid)) microspheres; polymer coating tuned for required drug release rate	5-fluorouracil	cancer therapy, drug release	[323]
carbon-coated iron magnetic NPs; USPIOs; magnetite-gold nanocluster Fe3O4@AuNCs@ERL nanocomposite	various (precipitation followed by magnetic separation)	carbon coating; Au/polymeric coating	Erlotinib. ERL: N-(3-ethynylphenyl)-6,7-bis(2-methoxyethoxy)-4-quinazolinamine (epidermal growth factor receptor (EGFR) inhibitor)	metastatic non-small cell lung cancer; aggressive pancreatic cancer	[324]
magnetite nanoparticles (MNPs)	modified co-precipitation method (MNPs); glutaraldehyde (GA)/calcium chloride CaCl2 (crosslinker)	sodium alginate (SA)/polyvinylpyrrolidone-co-vinyl acetate (PVP-co-VAc) semi ipn microbeads	curcumin (CUR) (encapsulation by simple ionotropic gelation technique)	cancer treatment	[325]
SPIONs (Maghemite)	no organic dispersant	linking the keto-enol moiety of CUR with Fe atoms to form SPION@curcumin hybrids	curcumin	photodynamic therapy (PDT): photodynamic action against S. aureus using blue LED light.	[326]
iron oxide nanoparticles (bare Fe3O4 NPs)	Micellar-assisted aqueous stabilization: micelles (dhydrodynamic =120 nm): sodium dodecyl sulfate (SDS) and aniline hydrochloride (AHC)	-	curcumin	hyperthermia therapy (under AC magnetic field); drug (curcumin) delivery	[327]
multicore magnetic nanoparticles: magnetite (Fe3O4) and/or maghemite (γ-Fe2O3)	coprecipitation method; coating with SiO2 silica using TEOS, in a modified Stöber method	SiO2 coating of MNPs yields MNP@SiO2 (centrifugation)	Curcuminoids (CC) extracted from turmeric: curcumin (>50%), desmethoxycurcumin, and bisdemethoxycurcumin	theranostic nanoplatform; drug release; hyperthermia candidate	[328]
Fe3O4	chemical co-precipitation; folic acid labeling of MNPs	Polyethylenimine-graft-poly (maleic anhydride-alt-1-octadecene) coated, to yield Fe3O4@PIMF	curcumin (effect on MCF-7 and Helacells)	Drug delivery, MRI (negative signal enhancement in MRI)	[329]
NiFe2O4 in x(NiFe2O4)@(100−x)SiO2@HKUST-1 (10 ≤ x ≤ 60 wt.%)	Core-shell strategy; trichloroacetic acid (BTC) as the organic binding for MOF	Silica coating (by TEOS) and MOF functionalization; NiFe2O4@SiO2@HKUST-1 as Novel Magnetic Metal-Organic Framework Nanocomposites	Curcumin adsorption in mesoporous host	drug delivery	[330]
Magnetite Fe3O4 in PEGylated Fe3O4/hydroxyapatite (PMHA) nanocomposite	PEG coating	Hydroxyapatite (shell)	Curcumin (effect on A549, MCF-7, and MRC-5 cells) (1.9 mg/g loading, pH-dependent release)	MRI; drug release	[331]
Yttrium Y3+-Doped Iron Oxide Fe3O4 Nanoparticles	Co-precipitation (Fe3O4 and Y3+ xFe2+Fe3+2 − xO4)	-	(effect on 4T1 cells-mice mammary gland cancer cells; ATCC CRL2539)	Hyperthermia	[332]
Mesoporous Fe3O4 nanoparticles (SPIONs)	solvothermal method (PEG-diamine, hydrazine; for SPIONs); Folate encapsulation in PEG-diamine grafted NPs	Multifunctional polyethyleneglycol-diamine functionalized; 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride/Nhydroxysuccinimide	doxorubicin (DOX) (effect on breast cancer cells MCF-7); through electrostatic attachment to daunosamine (NH3+)	(breast) cancer treatment	[333]
iron oxides maghemite (γ-Fe2O3) and magnetite (Fe3O4) nanoparticles	ultrasonic irradiation assisted co-precipitation route (providing good dispersion)	-	-	Hyperthermia	[334]
Co/Li/Zn-mixed ferrites: Co0.76Zn0.24Fe2O4, Li0.375Zn0.25Fe2.375O4 and ZnFe2O4 mixed-structure ferrite	‘dry gel’ formed by a sol–gel auto-combustion method	-	-	Magnetic hyperthermia	[335]
FeNiCo ternary alloy nanoparticles	[FeNi]100−xCox (2.5 ≤ x ≤ 50) by polyol method	-	-	Hyperthermia	[336]
Co2+-doped magnetite, CoxFe3−xO4–carboxymethylcellulose conjugate ferrofluids	Cox-Fe3O4; (x = 3, 5, and 10% mol of cobalt)	carboxymethylcellulose (biocompatible macromolecular ligand) ferrofluids	(effect of AC magnetic field on human brain cancer cells U87)	magnetic hyperthermia, cancer therapy	[337]
magnetic hydrogel based on Fe3O4 NPs	co-precipitation method; Hydrogel formation	Gelatin formulation; Functionalization with methacrylic anhydride (GelMA), then copolymerization with (2-dimethylaminoethyl) methacrylate (DMAEMA) monomer	Doxorubicin (Dox)	breast cancer, hyperthermia	[338]
Copper Ferrite Nanoparticles CuFe2O4 MNPs	Co-precipitation, then magnetic separation; Silica coating (TEOS tetraethyl orthosilicate and CPTMS (3-chloropropyl)- trimethoxysilane)	Aromatic Polyamide Chains by polymerization of diamino-benzenes and -naphtalene with terephthaloyl chloride. Final nanocomposite: CuFe2O4@SiO2-poly(p-phenylene Terephthalamide) star-like polymers	-	Hyperthermia evaluation- suitable for mild hyperthermia (ΔT~4 °C)	[339]
Fe3 − xCoxO4 (X = 0–1) spherical nanoparticles (7 nm)	thermal decomposition or organometallic precursors: Fe(acac)3 and Co(acac)2 in 1,2 hexadecanediol, oleic acid, and olylamine (polyol)	hydrophilization of hydrophobic Fe3−xCoxO4 by TMAH (tetramethyl ammonium hydroxide, caping agent)	-	Hyperthermia (maximum SAR for x = 0.75)	[340]
Fe3O4	polyol synthesis to give Fe3O4@Au@Cu2 − xS dumbbell heterostructures	hydrophobic-to-hydrophilic by two-step procedure (ligand exchange); thiol-polyethylene glycol coordinate Au and Cu2-xS surfaces and polycatechol–polyethylene glycol bind Fe3O4 surface; 64CuCl2 radiolabeling	-	Photo-Magnetic Hyperthermia and 64Cu Radio-Insertion (Tri-Modal Therapy); suggested efficient for skin cancer treatment	[341]
Mg1 − xCoxFe2O4 (0 < x < 1; Δx = 0.1)	chemical co-precipitation method	surface-functionalized: chitosan and chitosan-coated MNPs reported biocompatible behavior	(effect on HeLa cells showed no cytotoxicity)	hyperthermia and in vivo MR imaging	[342]
Coated Iron Oxide Nanoparticles (IONPs)	cross-linking with the adsorbed model drug (DOX)	Gelatin-coated (biocompatible natural polymer)	Doxorubicin (DOX); effect on MG-63 osteosarcoma cells	Cancer Treatment; potential hyperthermia effect	[343]
Magnetic nanoparticles (MNPs) Iron oxide (Fe3O4)	co-precipitation synthesis of magnetite Fe3O4; coated with four types of primary surfactants, polyethylene glycol 2000 (PEG 2000), oleic acid (OA), Tween 20, and Tween 80	-	Doxorubicin (high loading); effect on lung adenocarcinoma A549 cell line	cancer treatment	[344]
MNPs (Fe3+)	Mohr salt (NH4)2Fe(SO4)2(H2O)6 alkaline solution with sodium hypophosphite NaH2PO2 in the presence of NIPAM; Fe2+/PAA = 1/1	polyacrylic acid (30%), N-isopropylacrylamide (70%) (NIPAM) nanogel of ~150 nm	Doxorubicin (DOX)	anticancer activity	[345]
Magnetite Fe3O4	simple ionotropic gelationmethod (Gelatin-Coated)	Sodium Alginate (anionic polysaccharide)/Magnetite Nanoparticle Microbeads, doped with Mg2+and Al3+ ions	Doxorubicin (DOX)	drug delivery carriers and applications	[346]
MNPs—manganese ferrite MnFe2O4 nanoparticles	co-precipitation method (MnFe2O4)	citrate coating yielding Cit-MnFe2O4	Doxorubicin (effect on 60 male Wistar rats)	kidney injury (in rats); Chronic kidney disease (CKD)	[347]
MNPs (Magnetite Fe3O4 NPs)	co-precipitation (product recovery by magnetic decantation);	PEG coating to produce Fe3O4@PEG; Fe3O4@PEG immersion in Graphene quantum dots solution (by pyrolysis of citric acid, and 24 h still) to yield Fe3O4@PEG@GQD dispersed in PBS, then Fe3O4@PEG@GQD-DOX (magnetic separation)	Doxorubicin (effect on breast cancer MCF7 cells)	anticancer activity, drug release (in vitro)	[348]
-	unseeded stable cavitation (ultrasound)	-	Doxorubicin (or Adriamycin); effect on 4T1 murine mammary carcinoma	Murine Mammary Tumor Cells	[349]
Magnetite-based magnetic gelatin microspheres	co-precipitation method; using FeCl2 instead of FeSO4 produces higher Ms (61.6 emu/g); gelatin coating	fructose, glucose, genipin (most efficient) and 1-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC) as crosslinking agents of gelatin	Doxorubicin	drug delivery	[350]
MgFe2O4 ferrite MNPs	glycol-thermal method	Chitosan (CHI), polyethylene glycol (PEG) and polyvinyl alcohol (PVA); CHI-MNPs have highest DOX encapsulation (84.28%)	Doxorubicin; effect on human embryonic kidney (HEK293), colorectal adenocarcinoma (Caco-2), and breast adenocarcinoma (SKBR-3) cell lines	(pH controlled) drug release, cancer treatment	[351]
Fe3O4@SiO2@SBA-15	co-precipitation (and magnetic separation); PEG 400 coating	SiO2 silica coating (TEOS); PEI grafted	Doxorubicin	MCF-7 cell line drug delivery	[352]
MNPs	co-precipitation in alkaline media (NH4OH) of Fe2+/Fe3+/ethylene diamine (for introduction of -NH2 functionalization)	carboxymethyl chitosan (CMC) coating to yield MNPs-CMC-DOX	Doxorubicin	drug release	[353]
Superparamagnetic SPIONs (Fe3O4)	co-precipitation (using chloride sources); polymer-coated NPs, by polymerization of glycidyl methacrylate (GMA).	SiO2 and SiO2-NH2 functionalization with tetraethoxysilane (TEOS) and 3-(trimethoxysilyl) propyl methacrylate (TMSPM)	carboranes (by 1sopropyl-o-carborane immobilization)	boron neutron cancer therapy	[354]
MNPs magnetite Fe3O4	co-precipitation; Silica coating	surface-modification with N- (phosphonomethyl) iminodiacetic acid (PMIDA) to Fe3O4@SiO2@PMIDA	anti-CD4 monoclonal antibody (by bioconjugation)	positive selection of peripheral blood T CD4+ lymphocytes	[355]
γ-Fe2O3	bio-assisted method/aqueous co-precipitation; 3 morphologies of MNPs: nanospheres (NS), nanograsses (NG) and nanowires (NW)	green route: biosurfactant Furostanol Saponin (FS) from Fenugreek seeds extract	dopamine (DA) and uricacid (UA)	biosensors (molecular recognition platform for simultaneous detection of biomarkers)	[356]
Iron oxide MNPs	precipitation (under N2); coating and conjugation to yield Gem-PHB-MNPs hybrids	polyhydroxybutyrate coated	gemcitabine (effect on cell proliferation assay using SKBR-3 and MCF-7 breast cancer cell lines)	targeted drug delivery; treatment of breast cancer	[357]
SPION-type, reduced graphene oxide GO—Fe3O4	co-precipitation; In situ surface functionalization; coating with Pluronic F-127 (PF) to reduce cytotoxicity	delivery via an oriental fungus-type Ganoderma lucidum(provides stabilization); after drug Que loading: rGO-Fe3O4-GL-PF	Quercetin (Que), natural polyphenolic flavonoid with anti-cancer properties	cancer therapy; targeted drug delivery	[358]
Magnetite Fe3O4	precipitation in aqueous media with NH4OH of precursors, then oleic acid coating	PLGA–mPEG star-like block copolymers using biodegradable poly(lactic-co-glycolic acid) (PLGA) and methoxy poly(ethylene glycol) (mPEG)	Quercetin (conjugation to MNPs by dialysis method)	anticancer; nanocarrier for hydrophobic drugs	[359]
Magnetite Fe3O4	microemulsion-assisted co-precipitation method (for MNPs)	PEG-ylation (coating) to PMNPs	gallic acid	cancer treatment	[360]
Paramagnetic Fe3O4 nanoparticles	Monte Carlo simulated annealing scheme; molecular dynamics (MD)	PEG-ylation	5-fluorouracil	cancer treatment; drug delivery	[361]
MNPs (DEAE-FluidMAG; 5 mg, 200 nm, ChemicellTM)	enzyme encapsulation stable at 37 °C	-	CLytA-DAAO Chimeric Enzyme; effect against Hs766T, IMIM-PC-2 and RWP-1 pancreatic carcinoma cells, HT-29, SW-480and SW-620 colorectal carcinoma cell lines	cancer therapy (pancreatic and colorectal carcinoma and glioblastoma)	[362]
iron oxide nanoparticle	Quantum chemical analysis (B3LYP/6-31G(d,p) in aqueous solution; M06-2X dispersion correction)	-	5-aminolevulinic acid (anticancer drug); drug binding via advanced hydrogen bonding	cancer treatment	[363]
self-assembled magnetic nanospheres (MNS)	solvothermal method (MNS); Nintedanib (NTD) conjugated with MNS-APTES through the acid liable imine bond	Aminopropyltriethoxysilane (APTES) monolayer coating and functionalization	Nintedanib (NTD); targets human lung cancer cells L-132	anticancer	[364]
iron oxide (IO)	wet chemical co-precipitation (with enriched KNO3 content of FeSO4 solution prior to KOH precipitation)	APTES-Modified Nanohydroxyapatite (nHAp); Nanohydroxyapatite–Iron Oxide Composite (nHAp/IO) produces after APTES surface modification: nHAp/IO@APTES	effect on murine osteoblast precursor cell line (MC3T3-E1) and murine monocyte–macrophage cell line (RAW 264.7)	Early Osteogenesis, Reduces Inflammation and Inhibits Osteoclast Activity	[365]
Magnetic nanoparticles Fe3O4	co-precipitation method (using chloride iron sources); drug was loaded on MNP-CS through an amide bond between -NH2 groups (chitosan) and -COOH groups (TEL)	Chitosan coating; their solutions shaken gently for 2 h at 25 °C to obtain chitosan coated MNPs (MNP-CS)	Telmisartan (TEL), a water-soluble anticancer drug	cancer treatment	[366]
Fe3O4 in superparamagnetic graphene oxide (SPMGO) nanocomposite	chemical precipitation method, graphene oxide/magnetite nanocomposite	cyanuric chloride (CC), used as linker; final nanocarriers: SPMGO and SPMGO/CC, to yield SPMGO/MTX and SPMGO/CC/MTX	methotrexate (MTX); tested against Caov-4, HeLa and MCF-7 cell lines	cancer treatment	[367]
Mn0.5Zn0.5DyxFe2−xO4 (x ≤ 0.1) NPs	ultrasonic irradiation method	sonication in LB (Luria Bertaini) to achieve the suspended broth-drug solution	tested against Escherchia coli ATCC35218 as Gram-negative and Staphlyloccocus aureus ATCC29213, as Gram-positive bacteria; and Human colorectal or colon carcinoma cells (HCT-116)	anticancer; antifungal activity (vs. Candida albicans ATCC 14053, yeast)	[368]
magnetic silk nanoparticles	microfluidic device using silk fibroin and MNPs	Peptide-functionalization of magnetic silk NPs, with Antitumor peptide G3-a cationic amphiphilic anticancer peptide, G(IIKK)3I-NH2	Dimethylcurcumin (ASC-J9), androgen receptor inhibitor; tested against HCT 116 colorectal cancer cells	anticancer	[369]
metal ferrite NPs, MnFe2O4, CuFe2O4	one-pot solvothermal method (270 °C, polyol method, in situ CD formation, ethanolamine 1-amino-2-hydroxy-ethane as source)	oleyl amine surface coating and functionalization; carbon dots-metal ferrite hybrids, CDs-MNPs: CDs@MnFe2O4, CDs@CuFe2O4	(tested on HeLa cancer cells)	multipurpose marker agent of HeLa cancer cells	[370]
magnetic graphene oxide hybrid, based on MnFe2O4 magnetic core	nanocomposite (mGG3F) of graphene, MnFe2O4 NPs, poly(amidoamine) dendrons and folic acid	poly(amidoamine) dendron-functionalization	Pd(II) complex synthesized using Naphcon as a model drug, with entrapment efficiency (EE) 73.9% ± 0.08	cancer therapy	[371]
SPIONs (Fe3O4)	superparamagnetic iron oxide nanoparticles	polyamidoamine PAMAM-modified mesoporous silica-coating of SPIONS	folic acid (effect on MCF-7 cells); Indocyanine green (ICG) a near-infrared dye was loaded in M-MSN-PAMAM nanocarriers	cancer (photodynamic) therapy	[372]
Methionine Magnetic Nanoparticles Ni1−xCoxFe2O4@Methionine@PEG NPs	Ni1−xCoxFe2O4 NP coated with methionine using the reflux method (under N2); 1 mg of Ni1 − xCoxFe2O4@Methionine@PEG NPs could load 0.51 mg naproxen	PEG-coating by 30 min vigorously stirring PEG-6000 powder in a phosphate-buffered saline (PBS) with Ni1−xCoxFe2O4@Methionine	Naproxen (most potent COX-1 and COX-2 inhibitors)	cancer growth inhibition; controlled drug release	[373]
iron oxide nanocubes (IONCs)	one pot synthesis, from Fe(acac)3, decanoic acid and dibenzylether (DBE) in squalene (SQ) at 310 °C, then magnetic separation and centrifugation yield IONCs (15 nm ± 1 nm and 23 nm ± 5 nm edge length)	polycaprolactone fibers (electrospinning process)	doxorubicin; tested against Mouse embryonic fibroblast cell line (NIH 3 T3 cells), DOXOsensitive HeLa-WT cervical cancer cells and the DOXO-resistant MCF7 breast cancer cells	hyperthermia and cancer treatment	[374]
Iron Oxide nanocomposites with Fe2O3 core	commercial	Polyurethane diol/Polycaprolactone to yield PUD/PCL-Fe2O3 nanocomposites	-	catalytic effect (potential alternative use in fuel cells)	[375]
Iron Oxide (Fe3O4)	Two-Step LASER Ablation in Aqueous Media; TiO2 (core-shell), in both Fe3O4 and TiO2 pressed into pellets (commercial sources)	organic binder material	cytotoxicity against lung cancer cell lines (A549), Escherichia coli and Staphylococcus aureus	Antimicrobial and Anticancer	[376]
Iron MNPs	Co-precipitation method (MNPs); Polymer coatings were synthesized by two-stage melt polycondensation using BD:ADA:TBT as catalyst (molar ratio of 1:1:0.1), under N2.	biofunctionalized with poly(butylene adipate-co-terephthalate) (PBAT), and poly(butylene adipate) (PBA).	Absorption of DOPh and DBPh from aqueous medium	Phthalate absorption	[377]
SPIONs	dextran-coated PEG-COOH functionalized super-paramagnetic ironoxide nanoparticles, SPIONs (micromod Partikeltechnologie, GmbH); carbodiimide chemistry producing FGF2-SPIONs	dextran-coated PEG-COOH functionalized super-paramagnetic ironoxide nanoparticles, SPIONs (micromod Partikeltechnologie, GmbH, Rostock, Germany)	Fibroblast growth factor 2 (FGF2); effect studied on normal and cirrhotic human livers, Human hepatic stellate cells (LX2 cells)	treatment of acute liver injury (in vivo)	[378]
Manganese ferrite MnFe2O4 magnetic core	Microwave Driven Solvothermal Synthesis	Functionalization by oxidation polymerization process to yield polyrhodanine manganese ferrite PRHD@MnFe2O4 binary hybrids	effect against specific cell lines: macrophages (RAW 264.7), osteosarcoma cells line (UMR-106), and stromal progenitor cells of adipose tissue (ASCs); Antimicrobial activity against Escherichia coli and Staphylococcus aureus.	protection against Fenton’s reactions, and generation of highly toxic radicals; antimicrobial therapy	[379]
Zn2+Doped Magnetite Fe3O4 Nanoparticles	low-cost method oleic acid/alcohol/water system to synthesize Zn0.4Fe2.6O4 NPs; dimercaptosuccinic acid coated Zn2+ doped magnetite nanoparticles (DMSA-Zn0.4Fe2.6O4)	dimercaptosuccinic coating providing -SH functionalization	Spleen accumulation through translocation of oral medicine; in vivo study (rats)	(oral) drug delivery, MRI; evidence of drug translocation from oral to organ (liver, spleen) in non-toxic forms	[380]
magnetic microspheres with γ-Fe2O3 magnetic core	core-shell synthesis; doping with Tb3+ ions could sensitize the fluorescence of Enr	silica coating with -NH2 grafted functionality, and MOF and CMC- sodium carboxymethyl cellulose functionalization; γ-Fe2O3@SiO2-NH2-CMC/MOF5 and γ-Fe2O3@SiO2-NH2-CMC/IRMOF3 magnetic MOF nanoparticles	Enrofloxacin Enr (fluoroquinolone antibiotic, brand name: Baytril®); best results for γ-Fe2O3@SiO2-NH2-CMC/IRMOF3	treatment of bacterial infections	[381]
Ni(1−x)CoxFe2O4 NPs	reflux process (modified co-precipitation with NaOH under N2, reflux, then amino-acid addition)	Methionine (amino acid) coating during MNPs synthesis	Tetracycline (drug loading 0.33 mg in 1 mg of carrier); tested on Melanoma cancer cell line (A375) and HFF normal cell, Staphylococcus aureus, Escherichia coli.	drug delivery	[382]
MNPs Fe3O4	co-precipitation	SH functionalization via (3-Mercaptopropyl) and trimethoxysilane	Coenzyme Q0 (CoQ0, 2,3-dimethoxy-5-methyl-1,4-benzo-quinone); effect on Saos, MCF7 and Hela cell lines	antitumoral effect; anti-inflammatory, anticancer, and antioxidant	[383]
MNPs Fe3O4	coprecipitation of iron sulfate salts in basic media; 2-step strategy for nanohybrid	APTES linker between MNPs and the stearyl moiety (amide bond)	(R)-9-Acetoxystearic Acid (9-HSA);	biomedical (antiproliferative agent active against different cancer cells)	[384]
iron oxide NPs	co-precipitation of Fe(III) and Fe(II) in alkaline medium (MNPs); ceftriaxone (CFT)-loaded Nʹ-methacryloylisonicotinohydrazide (MIH)-functionalized magnetic nanoparticles(CFT-MIH-MNPs)	high functionalization degree	ceftriaxone (oral administration, brand name Rocephin, a third-generation cephalosporin antibiotic) in vitro stability using simulated gastrointestinal tract (GIT) fluids	treatment of bacterial infections; high drug entrapment, gradual drug release; enhanced oral delivery of CFT.	[385]
α -Fe2O3/Gadofullerene (GdF) Hybrid	simple chemical precipitation method	chitosan	chitosan-α-Fe2O3/GdF hybrid composites-antibacterial resistance against Escherichia coli, Pseudomonas aeruginosa, Bacilus subtilis, and Staphylococcus aereus, and P. aeruginosa (inducing pneumonia)	Treatment of Antibiotic-Resistant Bacterial Pneumonia	[386]
α-Fe2O3	chemical precipitation method	chitosan	Chitosan/α-Fe2O3 nanocomposite; antibacterial activity against Staphylococcus aureus and Escherichia coli.	antibacterial treatment	[387]
SPION Fe3O4	co-precipitation with sonication in thermostatic bath (under Ar)	chitosan coating, collagen functionalization	-	biomedical and technological applications, scaffolds for tissue regeneration	[388]
Fe2O3	chitosan coating	Fe2O3/chitosan/montmorillonite (MMT, polymer layered silicate); after encapsulation, QC release is pH-dependent and follows Weibullkinetic model.	Quercetin (QC) delivery (potential adjuvant in COVID-19 medication); Fe2O3/CS/MMT NPs tested against MCF-7 cells	drug delivery; cancer treatment	[389]
Fe3O4@PAA@MIL-100(Cr) metal-organic framework	50 wt% drug CIP encapsulation in Fe3O4@PAA@MIL-100(Cr)@CIP	PAA@MIL-100(Cr)	ciprofloxacin (CIP)(fluoroquinolone antibiotic); tested by disk diffusion method against Escherichia coli and Staphylococcus aureus	antibacterial	[390]
Ag-coated MNPs; Fe3O4/Ag and Fe3O4@SiO2/Ag (33.2–35.1 nm)	chemical reduction method (co-precipitation)	Ag coating; -NH2 functionalization via APTES	trimethoprim (antibiotic); sulfamethoxazole; effect on Escherichia coli and Staphylococcus aureus	antibiotic treatment; drug release	[391]
CoFe2O4–BaTiO3, CoFe2O4–Bi4Ti3O12 and Fe3O4–BaTiO3 core-shell magnetoelectric nanoparticles	core-shell type magnetoelectric nanoparticles	PNIPAm. -functionalized	methotrexate MTX (model drug; its adsorption best described by Freundlich model)	drug delivery	[392]
magnetic nanoparticles MNPs	Co-precipitation of Fe2+/Fe3+ = 1/2, with NH4OH	Tryptophan (amino acid involved in metabolic functions); 99mTc labeling afforded evaluation of the biodistribution and the blood kinetics	indoleamine 2,3 dioxygenase (IDO) and L-type amino acid transporter (effect on cell lines A-549, MCF-7)	tumor treatment; cancer treatment (ovarian, lung, colorectal)	[393]
Maghemite (γ-Fe2O3)	core-shell magnetic nanoparticles;	γ-Fe2O3@SiO2, γ-Fe2O3@SiO2-NH2 and γ-Fe2O3@SiO2-NH2-COOH MNPs via TEOS, APTMS and glutaric anhydride (GA). Further functionalization: Core-Shell package of Tb-BDC-NH2 and Tb-BDC, with ligands 2-aminoterephthalic acid (H2BDC-NH2) and terephthalic acid (H2BDC).	norfloxacin (Nor); via coordination between γ-Fe2O3@SiO2-NH2-COOH/Tb-BDC and Nor	antibiotic	[394]
MNPs (Fe)	For C-coating, Fe is more biocompatible and less toxic than Fe3O4	Graphene-encapsulated to produce Fe@C (biocompatible graphene shell)	ferulic acid (pharmaceutical ingredient found in the traditional Chinese herb Angelica sinensis)	diabet (mice); (controlled) drug release	[395]
Ionic magnetic Fe2O3 core-shell nanoparticles	coprecipitation of Fe3+ and Fe2+ ions at a molar ratio of 2:1. with NH4OH. Oxidation occurs due to air exposure (24 h)	silica shell and functionalized with alkylimidazolium organic halide: Guerbet imidazoles (to yield NpFeSi MNPs)	DNA extraction and stabilization against fragmentation	promising platform for therapeutic delivery; DNA extraction	[396]
superparamagnetic iron oxide nanoparticles Fe3O4 (APTMS@SPIONs)	co-precipitation method from iron salts Fe3+/Fe2+: 2/1 (mol ratio) using NH4OH in presence of APTMS at 85 °C (Ar flow)	3-aminopropylsilane coating (APTMS) for cationic APTMS@SPIONs; after ICG encapsulation, hydrodynamic size increased from 18 to 35 nm for ICG-APTMS@SPIONs	indocyanine green (ICG) (25 μg mL−1); effect evaluated on planktonic cells and biofilms of Gram-negative (E. coli, K. pneumoniae, P. aeruginosa) and Gram-positive (S. epidermis) bacteria	Antimicrobial photodynamic therapy (aPDT) and antimicrobial photothermal therapy (aPTT)	[397]
MNPs (Fe3O4) as Fe3O4@SIO2/SH/NH2	Chemical co-precipitation for core-shell MNPs (under N2)	Silica-thiol coating; -NH2 functionalization via hydrolysis/condensation of APTES solution, CPTES and MPTES.	methotrexate (MTX) and cysteine (Cys); up to 65% drug absorption	drug release (tested at 37° and 25 °C)	[398]
Magnetite Fe3O4	(commercial powder, from Sigma-Aldrich Ltd., ≥97% trace metal basis, particle size 50–100 nm.)	chemical-free, pulsed laser ablation (PLAL) to give ibuprofen:magnetite composites 4:1, 3:1 and 2:1 (wt)	ibuprofen	inflammation and pain management; targeted drugdelivery	[399]
MNPs Fe6(OH)18(H2O)6	quantum chemical study (using GAUSSIAN 09 and LANL2DZ basis set)	drug approaches TPZ via NH2 (MNP/TPZ1), NO (MNP/TPZ2-3) and intraring N-atom (MNP/TP4) functional groups/-NH2 mechanism leads to the thermodynamically- stable product via reaction to surface -OH groups (MNPs)	Tirapazamine (TPZ), experimental anticancer drug activated to a toxic radical only at hypoxia (low [O2])	cancer treatment	[400]
core-shell magnetic nanoparticles (NPs): Fe3O4@SiO2/NH2 and Fe3O4@CS	co-precipitation under Ar atmosphere; chitosan coating	silica coating and -NH2 functionalization using APTES (3-(triethoxysilyl)-propylamine)	goat anti-HBsAg antibody (with NaIO4 activation procedure)	antibody immobilization; sensing nanoplatforms (detection of HBsAg)	[401]
iron oxide nanoparticles (Magnetite Fe3O4 with hydroxyl endings)	standard co-precipitation technique (MNPs); silane coating with APTES by silanization reaction; Galactosylated coating	lactobionic acid (LBA)-functionalized: MNP-LBA	ceftriaxone (CFT)	controlled drug release for the oral delivery of CFT	[402]
bare iron oxide NPs (IONs): magnetite (cubic)	-	-	lasioglossin III (short cationic peptide) from bee venom	drug delivery (Escherichia coli tests show higher antimicrobial activity of bound lasioglossin)	[403]

.

As one may observe from Table 1, many MNPs have a core derived from either Fe₃O₄ (magnetite) or from Fe₂O₃ (maghemite). After successful drug loading, the release parameters are essential for the evaluation of efficient therapeutic drug levels in biological systems. Various release mechanisms have been proposed in the literature, and research data suggest some are more appropriate than others. For instance, when ibuprofen or acetaminophen was conjugated to multifunctional mesoporous silica nanoparticles (MSNs) containing APTMS and PDA/GO double layer functionalization, the drug loading was facilitated by π–π stacking interactions (Figure 5).

The drug release profiles seemed pH-dependent, and this consideration is typically valid for a plethora of drugs; the release ability under slightly acidic pH (4–6) seems to be considerably higher than under near-neutral or slightly basic pH values (pH > 7.4). However, the latter is more important since the biological pH usually is maintained by living organisms at around 7.4, with coma or convulsions whenever the pH deviates by more than 0.3 pH units from this value. Therefore, ongoing efforts concentrate on enhancing kinetic release profiles under this biologically relevant value. Analysis of ibuprofen or acetaminophen from NPs (FMSNs) bearing a single-layer coating of PDA (FMSNs@PDA) or a double-layered coating of PDA and GO (FMSNs@PDA@GO) revealed that the most relevant release mechanisms are Fickian, Kp and Higuchi models (Figure 6), as illustrated the goodness of fit (GoF) parameter [321]. The results can serve as reference data for nanoparticulate systems that bear similar surface coating and functionalization to MNPs; their behavior in drug-release systems reveals many similarities [321,322].

The drug transport can show concomitant effects from both swellings of the polymer chain and diffusion of the drug from the matrix. Since typically polymeric coatings can be protonated under acidic pH if their functional groups allow it (such as imidazole, for instance), the enhanced drug release is dominated under acidic conditions (pH < 7) by swelling of the polymer [322]. This aspect is important, and evaluation of drug release under acidic pH is useful since tumors usually feature an acidic microenvironment that can trigger further invasion of organs and metastasis through many possible mechanisms (intracellular pH_i = 7.0–7.2, while extracellular pH_e = 6.4–7.0 or even lower). As such, when magnetite MNPs (Fe₃O₄) coated with glycidyl methacrylate, dextran and then N-vinylcaprolactam and N-vinylimidazole monomers (used for inducing temperature and pH sensitivity, respectively) were conjugated with 5 FU drug in Fe₃O₄-Dex-MA-g-P(NVI/NVCL), the release profile confirms a much better release under acidic pH values (pH = 5) rather than near-neutral conditions (pH = 7.4) when just ~20% of the drug was being eliminated by 62 h mark (Figure 7) [322].

Notably, due to the immobilization of a water-soluble drug, 5-fluorouracil (5 FU), the drug release may occur also due to a chemical potential gradient [322]. A typical release profile such as those in Figure 5 or Figure 6 exhibits an initial burst release due to drug diffusion at the solid–liquid interface. The Korsmeyer–Peppas fit of drug release ($\frac{M_t}{M_{\infty }}=k{t}^n$) bears important mechanistic significance, especially when referring to the value of “n”. Under acidic conditions, which are indicative of tumoral site behavior, n = 0.585 (R² = 0.997), pointing to both swelling and diffusion mechanisms when reaching the acidic tumoral environment, but the value of “n” is much lower for release under bloodstream conditions (pH = 7.4), n = 0.302, so only minute amounts of drug will be released during blood circulation [322]. This divergent behavior of drug-loaded microcarriers will be beneficial for reaching maximum target efficiency.

Other release models under consideration are the 0th (constant amount of drug released per unit time, irrespective of drug concentration) and 1st order ($c=c_0{e}^{-kt}$) kinetics, linear equation (y = ax + b), Weibull ($\frac{M_t}{M_{\infty }}=1-e^{-a{t}^b}$), Michaelis–Menten ($\nu =\frac{V_{max}\left[S\right]}{K_m+\left[S\right]}$) or Hill equation ($y=\frac{y_{max}{x}^{\alpha }}{c^{\alpha }+x^{\alpha }}$) [329].

An interesting functionalization process is depicted in Figure 8. The magnetite NPs obtained from the co-precipitation method are covered by hydroxyl groups (from either NH₄OH or MOH, where M = Na, K), and this provides chemical anchors for Si(OEt)₄ (tetraethoxysilane), forming a silica-coated MNPs as a result (product 1, Figure 8). Then, there is a further reaction with 3-(trimethoxysilyl) propyl methacrylate (TMSPM) to produce Fe₃O₄/TEOS/TMSPM hybrids containing C=C bonds for grafting subsequent functional polymers. Subsequently, polymerization of Fe₃O₄/TEOS/TMSPM with glycidyl methacrylate (GMA) initiated by benzoyl peroxide occurs, and a reactive suspension of Fe₃O₄/TEOS/TMSPM/GMA is obtained, that reacts with isopropyl-o-carborane to produce Fe₃O₄/TEOS/TMSPM/GMA/Carborane (product 4, Figure 8). The cytotoxicity of these nanohybrids was assessed towards tumoral cell lines: HeLa (cervical cancer), BxPC-3 (pancreatic cancer) and MCF-7 (breast cancer) with promising results motivating a shift to boron cancer therapy. Interestingly, Mossbauer spectra of starting Fe₃O₄ differ significantly from that of Fe₃O₄/TEOS pointing to a change in phase composition upon coating and further functionalization. Nevertheless, the saturation magnetization registered a decrease upon siloxanic coating, from 65.2 emu/g (Fe₃O₄) to 48.6 emu/g in final nanocomposite Fe₃O₄/TEOS/TMSPM/GMA/Carborane [354].

Another typical example depicting MNPs formation is coating with biocompatible chitosan (MNP-CS) and further binding Telmisartan (an anticancer drug that is water-soluble and contains carboxyl -COOH moieties) (Figure 9).

The chitosan is loaded by gently shaking the as-prepared Fe₃O₄ MNPs (by co-precipitation from chloride iron sources) with an acylated chitosan solution (chitosan reaction with CH₃COOH). The obtained hybrid, MNP-CS, was magnetically separated from the reaction mixture. The actual loading of Telmisartan by a classical condensation reaction between an amine and an acid, produces an amidic bond -NH-C(=O)- when kept in an incubator shaker for 24 h at 100 rpm, at room temperature [366].

In this case, the magnetization curves of MNPs and MNP-CS show a close resemblance, and, more importantly, the M_S is only slightly reduced by coating from 59 emu/g (Fe₃O₄) to 50 emu/g (Fe₃O₄@chitosan) (Figure 10a). The magnetization curves show no coercivity, indicative of the superparamagnetic behavior of Fe₃O₄. The reduction of magnetic response and M_S is potentially problematic in polymeric coatings of MNPs since this coating procedure leads to a decrease in M_S. Ideally, the coating agent should have a thin coverage such that the magnetic behavior would remain largely unchanged [366].

An example of cytotoxicity assessment involved testing on PC-3 human prostate cancer cell line. The cell viability showed dependence on pH (drug release occurring at acidic pH) and dose administered (Figure 10b). The coating/functionalization techniques described in Figure 8 and Figure 9 represent a strategy with wide applicability for the conjugation of many other drugs (Table 1).

5.2. Cell Drug Uptake

The porous structure containing mesopores (D_p = 2–50 nm) enhances remarkably the efficiency of MNPs to uptake different biologically active molecules, such as paclitaxel (PTX), better known under its commercial name of Taxol, a chemotherapy medication for the treatment of many types of cancer [404], among others. There is a fine line delimiting cell uptake and accumulation, and MNPs should only accumulate at the target site where cancer or tumor cells lie [405].

Other in situ/operando characterization methods have been employed to better visualize the uptake process of NPs by cancer cells and Raman spectroscopy has served recently this purpose when cancer cells were subjected to Co-NPs [406,407]. Improvements in cell uptake were recorded when using biomimetic MNPs, attesting once again that the key to overcoming biologic barriers is to use other bio-inspired mechanisms [408].

5.3. Hyperthermia

Magnetic nanoparticles investigated for biomedical and hyperthermia applications have been reviewed recently [409,410,411,412]. Of particular interest are hyperthermia applications, since they can offer new horizons in cancer treatment and disease detection, management and treatment [70,335,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428]. Many critical aspects have been investigated regarding the use of ferrofluid media and critical heat transfer issues [429,430]. At the core of MFH interpretation lies the proper establishment of the magnetic state of the examined NPs, and the main parameter is the blocking temperature (T_B), the vast majority of SPIONs being found in a superparamagnetic state at room temperature: $T_B=\frac{K_{eff}V}{k_{B}ln\left(\frac{\tau }{{\tau }_0}\right)}$, with V as the mean NP volume, and t as the measurement time, typically ~10⁻⁶ s, K_eff the anisotropy constant and k_B the Boltzmann’s constant 1.38 × 10⁻²³ J K⁻¹ [76]. In order for MNPs based on biomagnetic core-shell Zn_δMn_{1 − δ}Fe₂O₄@γ-Fe₂O₃ composition (δ = 0.2 and 0.5) to exhibit a blocking temperature near room temperature (T_B = 300 K), their size was estimated to be 32–25 nm; hence, the actual MNPs synthesized with sizes 7.2–9.2 nm were all in superparamagnetic state at 300 K [76].

Aiming for a more effective hyperthermia procedure to combat tumors would imply a synergic effect of drug delivery (tuned for the specific disease), corroborated with the hyperthermia effect itself providing an efficient tool to locally increase the temperature to a range where cancer/tumor cells are most sensitive to and eventually die (41–46 °C) [431,432].

DFT modeling using the Monte Carlo model is also available for clinical hyperthermia applications [433] or magnetic gel behavior under magnetic guidance simulation [434]. Mechanistic studies on the effect and interplay of the essential heat release mechanisms have been presented in detail [23,435,436], as well as the effect of Ti atoms on Néel relaxation of MNPs [437], the drug-release modeling such as the 0th order, 1st order, Higuchi model or Korsmeyer–Peppas kinetic models [438], the magnetization reduction essential in sustainable hyperthermia [439], or others [440].

Theoretical simulations have revealed interesting features of MNPs related to their performance in hyperthermia experiments, and some of these investigations pointed to an optimal aspect ratio for maximum heating effect [441]. Closing the loop in the heating process, the temperature reduction mechanism was shown to greatly depend on the heating rate of a core-shell magnetite NP system [442].

5.4. Hypoxia

Hypoxia is a medical condition where the oxygen levels in body tissues are low and represents a turning point in cancer treatment that is resistant to traditional or targeted therapies. Oxygen binds to hemoglobin to form oxyhemoglobin and deoxyhemoglobin, and these two forms experience concentration modifications that can be visualized by functional MRI (fMRI) technique: BOLD (blood oxygenation-dependent imaging) MRI [144]. Biodegradable Fe-based nanohybrids have been used for hypoxia-modulated tumor treatment when H-MnFe(OH)_x hydroxide nanocapsules were designed with high loading capacity, for instance using a chemotherapeutic drug, doxorubicin (DOX) with in vitro and in vivo proof-of-concept anticancer synergy [443,444].

5.5. MDT: Magnetic Drug Targeting

Targeting cancer and tumors with an effective treatment therapy is at the forefront of biomedical research and diagnosis [24,141,208,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464]. This is the result of traditional chemotherapy treatments being non-specific, and by this, the healthy tissues could also be harmed by the aggressive anticancer drugs. MNPs respond to an externally applied magnetic field, offering the means to guide the transport and delivery of cytotoxic drug-loaded magnetic nanocarriers at the target organ/tissue. Many variables complicate this process, including blood flow velocity, drug immobilization strategy on MNP carriers, poor diffusion control after intravenous injection, geometry and depth of affected organ among others. Moreover, in order to beat the odds, early diagnosis is an essential moment—vital for the life of affected patients, especially when the drug formulation has to target specific organs/tissue [261,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495].

The flow behavior in MDT was studied by several groups [496,497,498,499,500], as well as their ability to penetrate different types of tissues, including eye tissue [501]. The modeling of flow behavior is even more important when the targeted disease is localized at the arterial segment (atherosclerosis), where control of NP aggregation is vital and is addressed by numerical solving of vorticity stream-function formulation [497]. Other research groups studied the potential removal of MNP-tagged cytokine during cardiopulmonary bypass, by employing simulation methods based on Navier–Stokes equations [498], or the feasibility study of introducing multicore MNPs of ~50 nm through the eye tissue using a magnetic field gradient of 20 T m⁻¹ [501]. Many of the research groups concluded that the best results in MDT can be obtained only when affected tissues are close to the body surface, hence the depth factor of the tumor seems to be prevalent. Moreover, the behavior of MNPs in the coronary circulatory system is of vital importance to understand and optimize the drug-delivery nanovehicles [502].

5.6. On-Demand Drug Release

Nano-engineered hybrid formulations can release the loaded molecule on-demand [378,503,504,505,506,507,508,509,510,511,512,513], but also leach divalent cations from ferrite composites [514]. The leaching test mentioned is an important piece of evidence regarding magnetic core integrity on its passage through the biological system, and ideally, it should retain its integrity, as some data in the literature has shown.

5.7. Apoptosis

The process of programmed cell death (apoptosis) is another important achievement of utilizing SPIONs; HT-29 cells have shown apoptosis by stimulative oxidative stress by iron oxide SPIONs [515,516,517].

6. Biocompatibility and Toxicity

Biocompatibility and toxicity features of anti-cancer formulations are nowadays treated with equal care and thoroughly investigated [518,519]. Oftentimes, endocytosis is not an easy task because various immune responses from the cells can decompose the nanocarrier before entering the cell membrane. Biomimetics was exploited as a tool to ease the incorporation of MNPs into living tissue [450,520,521].

Liposomes resemble the structure of cell membranes and are spherical vesicles composed of multiple phospholipid bilayers, and the incorporation of MNPs or SPIONs into such vesicles can ease the design and efficiency of drug-delivery systems since both hydrophilic and lipophilic drugs can be encapsulated [522,523]. Liposome use was expanded to vaccine formulation and stabilization [524]. This bio-inspired strategy typically affords swift entry of the drug by endocytosis with no to minimal damage.

6.1. Cytotoxicity

Oftentimes, the therapeutic effect was the main concern of the clinician; however, the toxicity that MNPs can have towards neighboring tissue or during its magnetically-guided path to the tumor site is less investigated, yet is just as important. The non-magnetic shell typically covering MNPs was shown to be essential to the reduced overall toxicity of polymer-coated metal oxide-based MNPs [525,526,527], drug-loaded nanoparticles [528] or that of bare MNPs [529,530] and derived ferrofluids with biological administration relevance [531]. The antioxidant effect was assessed by recent reviews in relation to the potential toxicity issues of MNPs [532].

The effect of CoFe₂O₄ cobalt ferrite on Channel catfish ovary cells was reported in 2022 by Srikanth et al. [533]. However, magnetite Fe₃O₄ can reduce CdCl₂-induced toxicity by oxidative stress as shown in a test on small intestine cells of mice, while orally-administered nano-Fe₃O₄ showed no toxicity at all [534]. The biodistribution and cytotoxicity of oral iron supplements, which are particularly relevant for patients suffering from iron-deficiency-induced anemia, have been investigated [535], and the effect on the human adipose tissue-derived stromal cell system of very small SPIONs [536,537] or targeting Parkinson’s disease [538]. Other coatings of magnetite showed no gene toxicity, making them suitable candidates for biomedical applications when guar gum-coated Fe₃O₄ MNPs obtained by co-precipitation (Fe₃O₄-GG nanocomposites) were used on Drosophila melanogaster (fruit fly), a leading invertebrate model system used in aging research [539]. Regarding anemia treatment options, MNPs are now available in a commercially, FDA-approved drug formulation, ferumoxytol (Feraheme as brand name), with intravenous administration.

6.2. NPs Accumulation in Tissue: The Fate of MNPs in Biological Systems

Accumulation of MNPs is an issue for MDT practices, as well as understanding the complete picture of MNP fate in the biological systems [540]. It was also found that the injection rate can affect the efficiency of MNPs accumulation [541,542,543]. However, the natural evolution in the treatment of cancerous cells would be to increase localized MNPs concentration, a theme tackled by various groups [544], as well as investigating its suitability to stimulate the activation of a targeted immune response [545].

Other research directions use model fish systems such as the common carp (Cyprinus carpio) to evaluate the accumulation of SPIONs in tissue and subsequent antioxidant and immune responses to iron oxides [546,547], or the effect of KFeO₂ nanoparticles on MCF-7 cell lines [548]. General toxicology studies have been also performed [526]. Other metal ferrites such as NiFe₂O₄ NPs have been investigated regarding histopathological mediated toxicity and the oxidative stress induced in rabbits [549]. The final fate of MNPs introduced in biological systems is complex and still not completely understood but it constitutes a theme of great practical significance [550,551,552,553].

7. Conclusions and Outlook

Various types of MNP platforms exist today and they are under rapid development. The MNPs can be tailored for detection (MR imaging contrast agents) and treatment therapies of various diseases, including early-stage and advanced forms of cancer. Surface manipulation (silica, gold or biocompatible polymers such as PEG or dextran) can yield stable MNP systems with minimal aggregation or opsonization, providing minimal systemic response and a high likelihood of passage through biological barriers (reticuloendothelial, vascular endothelium or blood–brain barrier). Achieving enhanced biocompatibility, precise targeting and increased accumulation of target cells for proper biological response remain the main goals. Multifunctional MNPs can offer diverse therapeutic strategies for healthcare providers.

Improvements to MNP formulations are still possible, and they tackle enhanced magnetic features of the magnetic core, new bio-inspired coatings and/or multifunctional drug loading. When a suitable system is identified, it will undergo scrutiny from quality, reproducibility, efficacy and stability criteria—all necessary prerequisites for scaling up processes and further pre-clinical implementation and testing. However, a number of unknowns still linger on: the uncertain fate of the MNPs after reaching the biological system, or the interaction mechanism of MNPs in vivo, an insight to further enhance the great opportunities that MNPs could provide, namely detection and treatment of various diseases, including cancer. Recent years have witnessed advances in drug-delivery systems being optimized and expanded by incorporating new functionalization agents for multifunctional MNPs synthesis and applications, as well as expanding the scope of therapeutic choices by employing highly-effective drugs that would be otherwise hard to deliver at the tumoral site due to many possible shortcomings, solubility issues being one example.

Moreover, a recent research direction aims to use magnetic (nano)particle imaging (MPI) in diagnostic imaging and guided treatment therapy, effectively linking image contrast and quality to relaxation mechanisms while emphasizing a safety administration profile. With no depth attenuation, MPI based on magnetic nano tracers—usually SPIONs—could provide excellent imaging contrast, spatial and temporal resolution and excellent signal-to-noise ratio. At its core, the emerging field of MPI relies heavily on the successful implementation of magnetic tracers, and this endeavor can take advantage of current development in MNPs used for drug-delivery applications.