Iron Oxide Nanoparticles: Synthesis and Functionalization for Biomedical Applications Volume 62- Issue 2

Ihsan Ullah¹, Balqees Khan¹, Nabeela Aman², Nosheen¹, Syed Izaz Ali Shah¹, Ajmal Shah¹*, Nadia Aman¹, Marva Haroon³, Sunbal Haroon¹, Misbah Gul¹, Salwa Zeb¹ and Seeqal Aleena⁴

• ¹Department of Chemistry, Abdul Wali Khan University Mardan, Pakistan
• ²Department of chemistry, University of Swabi, Pakistan
• ³Department of Biochemistry, Abdul Wali Khan University Mardan, Pakistan
• ⁴Pmas arid agriculture university, Pakistan

Received: June 04, 2025; Published: June 11, 2025

*Corresponding author: Ajmal Shah, Department of Chemistry, Abdul Wali Khan University Mardan, Mardan 23200, KP, Pakistan

DOI: 10.26717/BJSTR.2025.62.009716

Abstract PDF

Iron oxide (Fe₂O₃) nanoparticles have garnered significant attention in biomedical research due to their unique magnetic properties, biocompatibility, and versatile surface chemistry. This review explores various synthesis methods for Fe₂O₃ nanoparticles, including co-precipitation, thermal decomposition, and hydrothermal techniques, with a focus on controlling particle size, shape, and crystallinity. Functionalization strategies-such as surface coating with polymers, ligands, or biomolecules—are discussed in the context of enhancing colloidal stability, targeting capabilities, and reducing toxicity. These engineered nanoparticles are increasingly employed in a range of biomedical applications, including magnetic resonance imaging (MRI), targeted drug delivery, hyperthermia treatment, and biosensing. Despite promising advances, challenges remain regarding large-scale production, long-term biocompatibility, and regulatory approval. This paper highlights current trends and future directions for the development of Fe₂O₃ nanoparticles as multifunctional platforms in nanomedicine (Figure 1).

Keywords: Fe₂O₃ Nanoparticles; Maghemite; Biomedical Applications; Hyperthermi; MRI Contrast; Targeted Drug Delivery

Figure 1

Nanotechnology has revolutionized biomedical science by enabling the design of materials at the nanoscale with properties that are not achievable at the bulk level. Among the various nanomaterials investigated, iron oxide nanoparticles (Fe₂O₃ or Fe₃O₄) have emerged as particularly promising candidates due to their unique magnetic properties, chemical stability, biocompatibility, and ease of surface modification. These characteristics make them attractive for a broad spectrum of biomedical applications, including magnetic resonance imaging (MRI), drug delivery, hyperthermia therapy, tissue engineering, and biosensing [1]. Iron oxide exists in several crystalline phases, primarily magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), and hematite (α-Fe₂O₃), each possessing distinct magnetic properties and degrees of biocompatibility. Superparamagnetic iron oxide nanoparticles (SPIONs), typically smaller than 20 nm, exhibit magnetization only in the presence of an external magnetic field and show no residual magnetism upon field removal. This superparamagnetic behavior is critical in biomedical contexts as it prevents particle aggregation and reduces potential cytotoxicity [2]. The successful application of Fe₂O₃ nanoparticles in medicine largely depends on their controlled synthesis and subsequent surface functionalization. Various synthesis routes have been developed to tailor particle size, morphology, crystallinity, and surface characteristics. Among them, co-precipitation is the most widely used method due to its simplicity and scalability, although it often yields particles with broad size distributions. Other advanced techniques such as thermal decomposition, solvothermal and hydrothermal synthesis, and microemulsion methods provide better control over particle uniformity and crystallinity but can be more complex or cost-intensive [3].

Once synthesized, Fe₂O₃ nanoparticles require functionalization to improve their stability in biological fluids, reduce toxicity, and impart specific targeting or imaging capabilities. Functionalization strategies typically involve coating the nanoparticle surface with organic molecules, polymers (e.g., PEG, dextran), inorganic shells (e.g., silica or gold), or bioactive ligands (e.g., antibodies, peptides, or drugs). These surface modifications play a crucial role in enhancing circulation time, preventing immune clearance, and enabling site-specific delivery or imaging [4]. Despite the significant progress made in developing iron oxide-based nanomaterials, challenges persist. Issues related to reproducibility in large-scale synthesis, long-term in vivo safety, immune response, and regulatory approval continue to hinder clinical translation. A comprehensive understanding of the physicochemical properties, biological interactions, and functional behavior of Fe₂O₃ nanoparticles is essential for advancing their biomedical use [5]. This paper aims to provide a detailed overview of the synthesis techniques and surface functionalization strategies for Fe₂O₃ nanoparticles, with a focus on their application in biomedical fields. It also addresses current limitations and highlights potential future directions for research and development.

The synthesis strategy used for iron oxide (Fe₂O₃) nanoparticles fundamentally determines their size, crystallinity, magnetic behavior, surface characteristics, and biocompatibility-all of which are critical for biomedical applications such as targeted drug delivery, magnetic resonance imaging (MRI), hyperthermia therapy, and biosensing. Various chemical and alternative methods have been developed to achieve precise control over these features, and each method presents unique benefits and limitations.

Co-Precipitation Method

One of the most commonly employed methods is co-precipitation, which involves the simultaneous precipitation of Fe²⁺ and Fe³⁺ salts (typically chlorides or sulfates) in an alkaline aqueous medium. A base such as NaOH or NH₄OH is added under controlled temperature and pH conditions, leading to the formation of iron hydroxides that oxidize to yield Fe₃O₄ or γ-Fe₂O₃ nanoparticles [6]. This method is favored for its simplicity, scalability, and ability to operate in aqueous systems. Particle size, distribution, and surface charge can be modulated by adjusting synthesis parameters such as pH, ionic strength, temperature, and the use of surfactants or stabilizers (e.g., citric acid, dextran, polyethylene glycol). However, co-precipitation typically results in polydisperse nanoparticles with relatively low crystallinity, and further treatments are often required to improve uniformity and magnetic performance [7].

Thermal Decomposition

It is a more advanced approach that produces highly crystalline and monodisperse Fe₂O₃ nanoparticles. This method involves the high-temperature breakdown of iron precursors such as iron oleate, iron acetylacetonate, or iron pentacarbonyl in organic solvents like octadecene or benzyl ether, often in the presence of surfactants such as oleic acid or oleylamine. Controlled heating promotes nucleation and growth of uniform nanoparticles, with sizes tunable through reaction time, temperature, and precursor concentration. Although thermal decomposition yields nanoparticles with superior magnetic and structural properties, the particles are initially hydrophobic due to their organic surface coatings. Therefore, post-synthesis surface modification or phase transfer is required to make them dispersible and functional in aqueous biological environments [8].

Hydrothermal Synthesis

It provides another versatile method for producing Fe₂O₃ nanoparticles, utilizing a high-temperature and high-pressure reaction environment within a sealed autoclave [9]. In this method, iron salts react with water (or other solvents in solvothermal conditions) at temperatures typically ranging from 150–250 °C. These conditions promote crystal growth and allow for the formation of particles with controlled morphologies, including spheres, rods, and cubes. Parameters such as precursor type, reaction temperature, duration, and solvent composition play vital roles in determining particle characteristics. Hydrothermal synthesis yields highly crystalline nanoparticles with minimal defects, and it allows for the incorporation of dopants or functional additives. However, the need for specialized equipment, longer synthesis times, and relatively moderate scalability may limit its broader application [10].

Laser Ablation

physical approach such as laser ablation offers a unique route to produce iron oxide nanoparticles without the use of chemical precursors or surfactants. In this technique, a high-energy laser beam is directed at a bulk iron target submerged in a liquid medium (e.g., water or ethanol), causing material to vaporize and condense into nanoparticles. Laser ablation produces ultrapure, surfactant-free nanoparticles with minimal contamination—an advantage for sensitive biomedical applications where high purity and surface reactivity are essential. However, this method suffers from low productivity, high equipment costs, and limited control over particle size and crystallinity, making it more suitable for specialized applications or fundamental research [11].

Green Synthesis

A rapidly growing and environmentally friendly alternative is green synthesis, which leverages natural biological agents such as plant extracts, microorganisms, or biopolymers to facilitate nanoparticle formation [12]. In this approach, phytochemicals like polyphenols, flavonoids, and alkaloids act as both reducing and capping agents to convert iron salts into Fe₂O₃ nanoparticles. The method typically involves mixing an iron precursor solution with a bioextract under mild conditions, resulting in the formation of stable, biocompatible nanoparticles. Green synthesis is notable for its sustainability, low energy consumption, and elimination of toxic reagents, making it highly attractive for biomedical use. However, achieving consistent particle size, crystallinity, and yield across different biological sources remains a challenge, and the method is still under active development for large-scale, reproducible production [13-17] Table 1.

Table 1: Fe₂O₃ Nanoparticle Synthesis Methods.

Although iron oxide (Fe₂O₃) nanoparticles offer immense promise for biomedical applications, their unmodified or “bare” forms face several challenges in biological environments. Chief among these is their tendency to aggregate rapidly in physiological fluids due to strong magnetic dipole-dipole attractions and high surface energy. Such aggregation can lead to rapid clearance from the bloodstream, reduced targeting efficiency, and increased cytotoxicity. To overcome these limitations, surface functionalization is essential. Functional coatings can improve colloidal stability, reduce opsonization and immune clearance, extend circulation time, and provide active targeting or imaging capabilities [18].

Polymer Coatings

Polymer coatings are among the most common and effective strategies for functionalizing Fe₂O₃ nanoparticles. Polyethylene glycol (PEG), a biocompatible and hydrophilic polymer, is widely used to create a “stealth” layer on the nanoparticle surface, reducing recognition by the mononuclear phagocyte system (MPS) and thereby extending systemic circulation time. This process, known as PEGylation, also imparts steric stabilization, minimizing aggregation and protein adsorption [19]. Similarly, natural polysaccharides such as dextran and chitosan are often employed due to their non-toxic, biodegradable, and water-soluble nature. These polymers enhance the colloidal stability of the nanoparticles, improve biocompatibility, and provide functional groups for further conjugation with targeting or therapeutic agents [20].

Inorganic Shells

Inorganic shells represent another powerful approach to surface functionalization, offering structural robustness and multifunctionality. Silica (SiO₂) coatings, for example, form a chemically inert and porous layer around Fe₂O₃ cores, preventing oxidation of the iron oxide core while allowing for the encapsulation or adsorption of drugs and biomolecules. The surface of silica can be readily modified with various chemical groups (e.g., amines, thiols) to tailor drug release profiles or attach bioactive ligands [21].

Targeting Ligands

Beyond passive stabilization and protection, functionalization with targeting ligands allows for active, site-specific delivery of nanoparticles. Biomolecules such as antibodies, peptides, and small-molecule ligands can be attached to the nanoparticle surface to facilitate selective binding to diseased cells or tissues [22]. For instance, the use of monoclonal antibodies or tumor-targeting peptides significantly enhances nanoparticle accumulation in tumors through receptor-mediated endocytosis. Folic acid is another commonly used targeting agent, exploiting the overexpression of folate receptors on the surface of many cancer cells. Such ligand-mediated targeting improves therapeutic efficacy while minimizing off-target effects [23- 26] Table 2.

Table 2: Functionalization Strategies.

Diagnostic Imaging

γ-Fe₂O₃ (maghemite) nanoparticles are widely used as contrast agents in magnetic resonance imaging (MRI) due to their superparamagnetic properties. They significantly improve image clarity by altering the local magnetic environment, enhancing contrast especially in soft tissues. Moreover, these nanoparticles are increasingly integrated into multimodal imaging systems, combining MRI with techniques like positron emission tomography (PET), computed tomography (CT), or optical imaging. This allows for more comprehensive diagnostics by leveraging the strengths of different imaging modalities in a single platform [27].

Therapeutic Delivery

Fe₂O₃ nanoparticles offer promising capabilities for drug delivery, especially when functionalized to carry therapeutic molecules. Through magnetic targeting, an external magnetic field can be used to direct these particles to a specific location in the body, such as a tumor site, improving the precision of treatment and minimizing off-target effects. Additionally, these nanoparticles can be engineered for stimuli-responsive drug release, where specific internal or external triggers-such as changes in pH, temperature, or enzymatic activity- prompt the release of drugs. This facilitates site-specific and controlled delivery, enhancing therapeutic efficacy and reducing systemic toxicity [28].

Hyperthermia & Theranostics

In magnetic hyperthermia therapy, Fe₂O₃ nanoparticles generate localized heat when exposed to alternating magnetic fields. This heat can selectively destroy cancerous cells without harming surrounding healthy tissue. This approach is minimally invasive and can be combined with traditional therapies like chemotherapy or radiotherapy for synergistic effects. Furthermore, Fe₂O₃ nanoparticles are integral to theranostic platforms—systems that combine therapeutic and diagnostic functions. These enable real-time monitoring of treatment progress while simultaneously delivering therapy, offering a highly integrated and efficient approach to disease management [29].

Emerging Applications

Beyond conventional applications, Fe₂O₃ nanoparticles show strong antimicrobial activity by disrupting bacterial cell membranes and generating reactive oxygen species (ROS), which can lead to bacterial death. This property makes them potential candidates for use in coatings, wound dressings, and implantable devices to prevent infections. Additionally, they are being explored for stem cell tracking, where their magnetic properties allow for the non-invasive, longterm monitoring of stem cell migration and localization in vivo. This is particularly valuable in regenerative medicine and cell-based therapies, where tracking the fate of transplanted cells is crucial for understanding and optimizing therapeutic outcomes [30].

1) Despite the promising potential of Fe₂O₃ nanoparticles in biomedicine, several critical challenges must be addressed to enable their successful clinical translation. One of the major concerns is toxicity, which is often dose-dependent and influenced by factors such as particle size, surface charge, and coating materials. Uncoated nanoparticles tend to aggregate and are rapidly recognized by the immune system, leading to oxidative stress and cellular damage. Surface modifications with biocompatible polymers like PEG or dextran are therefore essential to mitigate these effects and improve safety profiles.

2) Another significant hurdle lies in biodistribution. Fe₂O₃ nanoparticles commonly accumulate in the liver and spleen due to uptake by the mononuclear phagocyte system, reducing their availability at the target site. Adjusting the nanoparticle’s size and surface properties can help improve circulation time and reduce off-target accumulation. In terms of production, many synthesis techniques lack scalability and exhibit batch-to-batch variability, which hampers reproducibility and regulatory approval. Standardizing synthesis protocols and developing scalable methods are vital steps toward clinical application.

3) Looking ahead, future research is likely to focus on the development of hybrid nanostructures that combine Fe₂O₃ cores with therapeutic agents or other functional materials for enhanced multifunctionality. Moreover, artificial intelligence and machine learning are expected to play an increasing role in the rational design and optimization of nanoparticles, accelerating innovation and improving clinical success rates (Figure 2).

Figure 2

Fe₂O₃ nanoparticles offer exceptional versatility in nanomedicine. Surface engineering is vital for in vivo stability, while theranostic platforms promise integrated diagnosis and therapy. Overcoming toxicity and manufacturing barriers remains crucial for clinical adoption. Future innovations will expand their role in personalized medicine.

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