Current methods for synthesis of magnetic nanoparticles

Abstract

The synthesis of different kinds of magnetic nanoparticles (MNPs) has attracted much attention. During the last few years, a large portion of the articles published about MNPs have described efficient routes to attain shape-controlled and highly stable MNPs with narrow size distribution. In this review, we have reported several popular methods including co-precipitation, microemulsion, thermal decomposition, solvothermal, sonochemical, microwave-assisted, chemical vapor deposition, combustion, carbon arc, and laser pyrolysis, for the synthesis of magnetic nanoparticles.

Keywords::

• co-precipitation
• magnetic nanoparticles
• sonochemical
• thermal decomposition

Introduction

Nanoscience is one of the most important fields of research in modern science. Nanotechnology is beginning to allow researchers to work at the molecular and cellular levels, to achieve important developments in life sciences and healthcare (Akbarzadeh et al. 2012). Nanotechnology has been successfully applied for in vivo molecular imaging, disease diagnosis, and as an improved therapeutic platform (Yang et al. 2012).

NPs are submicron moieties with diameters ranging from 1 to 100 nm, made of inorganic or organic materials, which have many novel properties when compared to the bulk materials (LaConte et al. 2005, Wu et al. 2008).The use of nanoparticle (NP) materials offers major advantages, due to their unique size and physicochemical properties (Akbarzadeh et al. 2012). Nanomaterials have been attracting great attention owing to their excellent optical, electrical, magnetic, and catalytic properties. It is well known that the phases, sizes, and morphologies of nanomaterials have great impact on their properties and potential applications. In this context, the controlled synthesis of nanostructured materials with new morphologies has recently received much attention (Geng et al. 2006, Honig and Spalek 1998, Rao et al. 2007, Liu et al. 2013).

Currently, real uses of nanostructured materials in life sciences are unusual. However, the excellent properties of these materials provide a promising future for their use in this field (Akbarzadeh et al. 2012, Davaran and Entezami 1996, Spanhel et al. 1987, Steigerwald and Brus 1989, Steigerwald and Brus 1990). NPs have been used to deliver drugs to target tissues and to enhance stability against degradation by enzymes, such as superparamagnetic NP, which can be used by an external magnetic field to lead it to the target tissue (Mahdavi et al. 2013, Pourhassan-Moghaddam et al. 2013). With the recent progress of nanobiotechnology, magnetic nanoparticles (MNPs) have gained more attention for use in biomedical applications (Yang et al. 2012). MNPs are the group of engineered and specific materials of sizes less than 100 nm, that can be manipulated under the effect of an external magnetic field (Indira and Lakshmi 2010).

MNPs have attracted researchers from different fields such as biology, medicine, and physics, due to their multifunctional properties such as small size, superparamagnetism and low toxicity, etc. (Gu et al. 2006, Ahmadi et al. 2014, Roger et al. 1999, Wunderbaldinger et al. 2002). For biological and biomedical applications, magnetic iron oxide NPs are the best choice, for their biocompatibility, superparamagnetic actions, and chemical stability (Cabrera et al. 2008). Magnetic iron oxide NPs have been considered the best choice, and the application of small iron oxide NPs in in vitro diagnostics has been practiced, for almost half a century (Wu et al. 2008, Gupta and Gupta 2005).

As a kind of practical magnetic material, Fe₃O₄ nanomaterials have been used in many fields, because of their unique electric and magnetic properties (Liu et al. 2013, Zhu and Diao 2011, Hou et al. 2003). Magnetite (Fe₃O₄) NPs have attracted much interest, not only in the field of magnetic recording media, but also in the areas of medical care, such as medical applications, including magnetic resonance imaging (MRI), drug delivery systems, medical diagnostics, cancer therapy, microwave devices, magneto-optic devices, etc. (Silva et al. 2004, Sun et al. 2000, Sun 2006, Pankhurst et al. 2003, Davaran et al. 2013, Portet et al. 2001, Ito et al. 2005, Meng et al. 2009, Zi et al. 2009, Ghasemali et al. 2013, Kashevsky et al. 2008, El Ghandoor et al. 2012). The use of NPs with sizes smaller than 100 nm has some advantages, such as their higher effective surface areas, lower sedimentation rates, and better tissular diffusion (Akbarzadeh et al. 2012, Puntes et al. 2001, Park et al. 2005, Sadat Tabatabaei Mirakabad et al. 2014).

MNPs can bind to drugs, proteins, enzymes, antibodies, or nucleotides, and can be absorbed to an organ, tissue, or tumor using an external magnetic field, or can be heated in alternating magnetic fields for use in hyperthermia (Mahdavi et al. 2013).

Targeting of drugs by NPs is intended to decrease drug wastage, lessen the need for regular drug administration, and reduce side effects, providing prolonged and sustained drug delivery to the chosen target organ (Indira and Lakshmi 2010, Simsek and Akif Kilic 2005).

Magnetic iron oxide NPs have a large surface-to-volume ratio and consequently possesses high surface energy. Therefore, they tend aggregate, so as to minimize the surface energy. Moreover, the naked iron oxide NPs have high chemical activity, and are easily oxidized in air, generally resulting in the loss of magnetism and dispersibility. Therefore, it is important to provide proper surface coating and improve some effective protective approaches to retain the stability of magnetic iron oxide NPs. Particularly, for in vivo applications, the MNPs must be encapsulated with a biocompatible polymer during or after the preparation process, to avoid changes from the original structure, the formation of large aggregates, and biodegradation when exposed to the biological system (Akbarzadeh et al. 2012, Muldoon et al. 2005, Moghimi et al. 2001, Sosnovik et al. 2007).

Due to the extensive applications of MNPs in biotechnology, biomedicine, material science, engineering, and environmental areas, the synthesis of different kinds of MNPs has attracted much attention (Tartaj et al. 2003, Faraji et al. 2010, Niemeyer 2001). Recently, much research has been in progress on the synthesis of iron oxide NPs, and many reports have clarified efficient approaches of synthesis to produce shape-controlled, stable, biocompatible, and monodisperse iron oxide NPs (Wu et al. 2008). In the last decade, many efforts have been made to develop techniques and processes that would yield ‘mono-disperse colloids’ containing NPs that are uniform, both in size and shape (Bao et al. 2006, Kim and Park 2005, Kotitz et al. 1999). The synthesis of mono-dispersible nanocrystals with controllable sizes is very important, because the properties of these nanocrystals rely strongly on their dimensions, and it is also important to characterize the size-dependent physico-chemical properties of nanocrystals (Akbarzadeh et al. 2012, Portet et al. 2001, Kwon et al. 1997, Kwon et al. 1995, Denizot et al. 1999, Wormuth 2001). MNPs have been prepared with a number of different compositions and phases, involving pure metals, such as Fe, Co, and Ni (Puntes et al. 2001, Park et al. 2000, Sun et al. 2000); metal oxides, such as Fe₃O₄ and γ-Fe₂O₃ (Neveu et al. 2002, Davaran et al. 2014, Sun and Zeng 2002); ferrites, such as MFe₂O₄ (M = Cu, Ni, Mn, Mg, etc.) (Hu et al. 2007, Park et al. 2004); and metal alloys, such as FePt and CoPt (Sun et al. 2000, Faraji et al. 2010, Shevchenko et al. 2002). Highly magnetic materials, such as cobalt and nickel, are toxic and are liable to oxidation; hence, they are of little interest (Akbarzadeh et al. 2012, Choi et al. 2007, Murray et al. 1993, Peng et al. 2000).

Several methods have been developed to synthesize Fe₃O₄ particles with sizes in the nanometer range (Thapa et al. 2004, Berger et al. 1999). In nearly all uses, the method of synthesizing nanomaterials represents one of the most important challenges that will determine the shape, size distribution, particle size, and surface chemistry of the particles, and therefore their magnetic properties (Lopez Perez et al. 1997, Kouhi et al. 2014, Sjogren et al. 1997). In addition, the preparation method expresses to a great extent the degree of structural imperfections or impurities in the particle, as well as the distribution of such defects within the particle, hence defining its magnetic behavior (Akbarzadeh et al. 2012, Grossman et al. 2004, Chung et al. 2004).

In this review, we focus mainly on methods of preparation of MNPs for biomedical and biological applications. We want to modernize currently available methods for the synthesis of Fe₃O₄ nanomaterials with several morphologies; in addition, some important and novel findings reported earlier are also involved.

Synthesis of MNPs

There are several methods to synthesize MNPs, which have been reported in several papers (Sun et al. 2007). To date, various popular methods comprising co-precipitation, microemulsion, thermal decomposition, solvothermal, sonochemical, microwave-assisted, chemical vapor deposition, combustion, carbon arc, and laser pyrolysis, have been reported for the preparation of MNPs (Akbarzadeh et al. 2012). In addition, these NPs can also be synthesized by other methods such as electrochemical synthesis (Cabrera et al. 2008, Pascal et al. 1999), laser pyrolysis techniques (Bomati-Miguel et al. 2008), microorganism or bacterial synthesis (especially the magnetotactic bacteria and iron reducing bacteria) (Bharde et al. 2008, Roh et al. 2006), etc. (Wu et al. 2008). Several novel and effective methods have been developed to synthesize Fe₃O₄ nanomaterials with different shapes, such as nanorods, nanotubes, and hierarchical superstructures (Liu et al. 2013, Li et al. 2011, Barth et al. 2008, Abbasi et al. 2014, Liu et al. 2005, Gong et al. 2010).

Green synthesis of MNPs

Green nanotechnology has attracted a lot of attention and includes various processes which decrease or eliminate toxic substances to restore the environment. The biosynthesis of metal nanoparticles by plants is currently under development. The synthesis of metal NPs using inactivated plant tissue (Padil and Cernik 2013), plant extracts (Shameli et al. 2012), exudates (Lukman et al. 2011), and other parts of living plants (Pourhassan-Moghaddam et al. 2014), is a modern option for their production. Green synthesis of NPs makes use of environmentally friendly, non-toxic and safe components. The development of reliable, nontoxic, and eco-friendly methods for the synthesis of NPs is of extreme importance to develop their biomedical applications (Salam et al. 2012, Shankar et al. 2004, Mahdavi et al. 2013).

Biological methods of nanoparticle preparation using microorganisms (Klaus et al. 1999, Nair and Pradeep 2002, Abbasi et al. 2014), enzymes (Willner et al. 2006), fungi (Vigneshwaran et al. 2007), and plants or plant extracts (Chandran et al. 2006, Song and Kim 2009), have been recommended as possible eco-friendly substitutes to chemical and physical methods. Sometimes, the nanoparticle preparation using plants or parts of plants can prove advantageous over other biological processes, by eliminating the elaborate work involved in maintaining microbial cultures (Forough and Fahadi 2011).

The reason for selecting plants for biosynthesis is due to their reducing agents such as citric acid, ascorbic acids, flavonoids, reductases, dehydrogenases and extracellular electron shuttles, that may play an important role in the biosynthesis of MNPs (Pandey et al. 2012).

Awwad A. M. and Salem N.M. (Awwad and Salem 2012) recommended a rapid, non-toxic, facile and green synthesis method to prepare magnetite NPs in a single step reaction. Ferric chloride hexahydrate and ferrous chloride tetra hydrate, carob leaf extract, and sodium hydroxide, were the substances used in the synthesis experiments. Magnetite NPs can be obtained in a relatively low temperature range of 80–85°C. Magnetite NPs (Fe₃O₄) were prepared by a simple, rapid and green method, in a single vessel reaction. The average diameter of magnetite NPs is 4–8 nm, and they have good monodispersible properties. The magnetite NPs were coated by the carboxylic groups of amide I and amide II chains of the protein in carob leaf extract (Figure 1).

Figure 1. SEM micrograph of magnetite nanoparticles prepared with carob leaf extract as the initial substance (Awwad and Salem 2012).

Chin et al. (Eatemadi et al. 2014) attempted to prepare Fe₃O₄ NPs by the thermal decomposition method, without using toxic organic surfactants and solvents. Poly (ethylene glycol), PEO, was being used as both solvent and surfactant simultaneously, to prepare Fe₃O₄ NPs of controllable particle size and narrow size distribution. PEO has been widely used as a green solvent for several organic syntheses due to its low toxicity and high boiling point (Smith et al. 2005, Kidwai et al. 2010, Hosseininasab et al. 2014). The approach for MNP synthesis employs an environmentally friendly solvent, PEO, as an alternative to organic solvent. PEO has been applied both as a solvent and as a surfactant which inhibits the agglomeration of Fe₃O₄ NPs formed during synthesis (Eatemadi et al. 2014).

The green synthesis has many advantageous features for the synthesis of magnetite NPs; it is economical, environmentally friendly, non-toxic, and the treatment and size of the product, the magnetite NPs, can be controlled in a single-vessel reaction at mild conditions (Awwad and Salem 2012).

Precipitation from solution

One of the oldest techniques for the preparation of NPs is the precipitation of products from solutions. In precipitation reactions, the metal precursors are dissolved in an ordinary solvent, such as water, and a precipitating agent is added to generate an insoluble solid. The main advantage of precipitation reactions is that large quantities of particles can be obtained (Willard et al. 2004). Uniform particles are usually synthesized by a homogeneous precipitation reaction, a process that includes the separation of the nucleation and growth of the nuclei (Indira and Lakshmi 2010, Sugimoto 2000).

Co-precipitation

Co-precipitation is the most widely used and most proper method for the synthesis of MNPs of controlled sizes and magnetic properties (Sandeep Kumar 2013). It is extensively used for biomedical applications, because of the ease of application and less need for harmful materials and procedures (Indira and Lakshmi 2010). In this method, MNPs are prepared from aqueous salt solutions, by the addition of a base under an inert atmosphere at room temperatures or at high temperature (Faraji et al. 2010). The co-precipitation process is shown in the following Figure 2, and the reaction is simply as follows (Indira and Lakshmi 2010):

(1) $${\text{Fe}}^{2+}+2{\text{Fe}}^3+8{\text{OH}}^{-}\to \text{Fe}{(\text{OH})}_2+2\text{Fe}{(\text{OH})}_3\to {\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2\text{O}$$(1)

Figure 2: Procedure for the preparation of Fe₃O₄ nanoparticles (Sun et al. 2007).

There are two main approaches for the synthesis spherical MNPs in solution: partially oxidizing ferrous hydroxide suspensions with different oxidizing agents (Sugimoto and Matijevic 1980), and aging stoichiometric mixtures of ferrous and ferric hydroxides in aqueous media, which yield spherical magnetite particles that are homogeneous in size (Indira and Lakshmi 2010, Massart and Cabuil 1987). The size and shape of the iron oxide NPs depend on the type of salts used, such as chlorides, sulfates, nitrates, perchlorates, etc., the ratio of ferric and ferrous ions, the PH value, the reaction temperature, the ionic strength of the media, and the other reaction parameters (such as stirring rate, and dropping speed of basic solution) (Wu et al. 2008).

The pH ranging between 8 and 14 is the expected range for complete precipitation with a stoichiometric ratio of 2/1 (Fe³⁺/F²⁺) in a non-oxidizing oxygen environment (Faraji et al. 2010, Iida et al. 2007). In addition, it has been demonstrated that by adjusting the pH and the ionic strength of the precipitation medium, it is possible to control the mean size of the particles over one order of magnitude, in the range between 2 and 15 nm (Tartaj et al. 2003, Jolivet et al. 2000).

This method produces particles with extensive distribution of particle size, which sometimes requires secondary size selection. Kang et al. (Kang et al. 1996) reported a synthesis of uniform, monodisperse, Fe₃O₄ NPs with narrow size distribution, with the diameter of 8.5 ± 1.3 nm, by co-precipitation without surfactants; the reaction takes place in an aqueous solution with a molar ratio of Fe²⁺/Fe³⁺ equal to 0.5 and a pH of 11–12.The colloidal suspensions of the magnetite can be then directly oxidized by aeration to form colloidal suspensions of γ-Fe₂O₃ (Wu et al. 2008).

Iida et al. (Davoudi et al. 2014) prepared Fe₃O₄ NPs by hydrolysis in an aqueous solution, including ferrous and ferric salts at various ratios, with 1,6-hexanediamine as the base. According to this study, when the ratio of Fe²⁺ to Fe³⁺ ions was increased, the formation of large hydroxide particles as precursors of Fe₃O₄ was promoted, which resulted in an increase in the size of Fe₃O₄ NPs from ∼9 to ∼37 nm. Furthermore, the saturation magnetization values of the samples prepared with both ferrous and ferric salts were 46.7 and 55.4 emu. g ^{− 1} for sulfate and chloride, respectively. These results are proved by the data in literature (Faraji et al. 2010, Babes et al. 1999, Tronc et al. 1992).

The size of MNPs decreased with increasing pH value and ionic strength in the medium (Jolivet et al. 2000). Both parameters affect the chemical structure of the surface, and consequently, the electrostatic surface charge of the particles (Tartaj et al. 2003).

In the preparation of Fe₃O₄, precipitation at temperatures below 60°C produces an amorphous hydrated oxyhydroxide that can be simply converted to Fe₂O₃, while higher reaction temperatures (> 80°C) favor the formation of Fe₃O₄ (Faraji et al. 2010, Ziolo et al. 1992, Govan and Gun'ko 2014).

Nitrogen gases bubbling through the solution help to protect magnetite NPs against critical oxidation. Moreover, N₂ reduces the particle size, in comparison with methods without oxygen removal (Faraji et al. 2010, Gupta and Wells 2004, Kim et al. 2001). Hong et al. (Hou et al. 2005) used N₂H₄ .H₂O as an oxidation-resistant reagent. According to their results, hydrazine can react with the dissolved oxygen to form [NH₃OH] ⁺, as in the following equation:

(2) $${\text{N}}_2{\text{H}}_4.2{\text{H}}_2\text{O}+4{\text{H}}^{+}+0.5{\text{O}}_2\leftrightarrow 2{[{\text{NH}}_3\text{OH}]}^{+}$$(2)

The cationic [NH₃OH] ⁺ can also react with Fe²⁺ to form Fe₃O₄, as follows:

(3) $$3{\text{Fe}}^{2+}+{[{\text{NH}}_3\text{OH}]}^{+}+6{\text{OH}}^{-}\to {\text{Fe}}_3{\text{O}}_4+{[{\text{NH}}_4]}^{+}+3{\text{H}}_2\text{O}$$(3)

Fe₃O₄ MNPs with narrow size distribution can be synthesized by controlling the titration rate of ammonium hydroxide (Indira and Lakshmi 2010, Zhao et al. 2008). The smallest particles can also be obtained after adding polyvinylalcohol (PVA) to the iron salts (Tartaj et al. 2003, Alimirzalu et al. 2014).

The trouble with the synthesis of Fe₃O₄ MNPs by chemical co-precipitation is the tendency of the particles to agglomerate because of extremely small particle size, leading to greater specific surface area and high surface energy; we must also consider the influence of alkali, emulsifier, and reaction temperature, which are the determining factors of the final product (Indira and Lakshmi 2010, Zhao et al. 2008). Moreover, the drawback in the synthesis of these aqueous solutions is that the high pH value of the reaction mixture has to be adjusted in both the synthesis and purification steps, yielding only very limited success the formation of uniform and monodisperse NPs (Wu et al. 2008). The common microstructure of MNPs prepared by this method is shown in Figure 4 (a).

Microemulsion

Microemulsion is the thermodynamically stable isotropic dispersal of two immiscible water and oil phases in the presence of a surfactant. The surfactant molecules can form a monolayer at the interface between the oil and water, with the hydrophilic head groups in the aqueous phase and the hydrophobic tails of the surfactant molecules dissolved in the oil phase (Wu et al. 2008, Solans et al. 2005).

This method has a series of advantages in comparison with other methods, namely, the use of simple equipment, the possibility of synthesizing a great variety of materials with a high degree of control over particle size and composition, the preparation of NPs with crystalline structure and high specific surface area, and the use of simple conditions of synthesis, and near ambient temperature and pressure (Woo et al. 2004). Particles produced by the microemulsion method are smaller in size and are higher in saturation magnetization (Wu et al. 2008, Chin and Yaacob 2007).

The properties of NPs prepared by the microemulsion method depend on the type and structure of the surfactant (Sanchez-Dominguez et al. 2012). The surfactant is an amphiphilic molecule which lowers the interfacial tension between water and oil, resulting in the formation of a transparent solution (Faraji et al. 2010). Quintela M. A. and Rivas J. (Lopez-Quintela and Rivas 1993) reported that magnetite NPs around 4 nm in diameter have been synthesized by the controlled hydrolysis of FeCl₂ with ammonium hydroxide and FeCl₃ aqueous solutions, within the inverse micelle nanocavities formed by using AOT (Sodium 2-ethylhexyl sulfosuccinate) as surfactant and heptane as the continuous oil phase (Tartaj et al. 2003). AOT-based systems are amongst the best characterized systems, and it has been found that the size of the inverse microemulsion droplets generated by this type of system increases linearly with the amount of water added to the system (Pileni 1998). It can increase from 4 nm to 18 nm with 0.1 M of sodium AOT surfactant (water/AOT/isooctane). The use of AOT-based systems is probably the best method for the synthesis of inorganic NPs in W/O microemulsions, for two reasons: good control of droplet size, and the large microemulsion regions found in the water/AOT/alkane systems, which give rise to a great deal of compositions available for NP preparation (Sanchez-Dominguez et al. 2012).

The nanodroplets of water containing reagents as nanoreactors, endure rapid coalescence allowing for mixing, a precipitation reaction, and an aggregation process, for the synthesis of MNPs (Figure 3). By mixing two identical water-in-oil microemulsions consisting of the chosen reactants, the microdroplets will continuously collide, coalesce and break again, and finally a precipitate forms in the micelles (Faraji et al. 2010). Pileni and co-workers (Jalil et al. 2014) synthesized MNPs with average sizes ranging from 4 to 12 nm, and standard deviation ranging from 0.2 to 0.3, using microemulsions. A ferrous dodecyl sulfate (Fe(DS)₂₎) micellar solution was used to generate nanosized magnetic particles whose size could be controlled by the concentration and temperature of the surfactant.

Figure 3. A schematic representation of the W/O microemulsion droplet. (Adapted with permission from (Moulik and Paul 1998)).

As in the binary systems (water/surfactant or oil/surfactant), self-assembled structures of various types can be generated, ranging, for example, from spherical and cylindrical micelles to lamellar phases and bicontinuous microemulsions, which can coexist with predominantly oil or aqueous phases (Solans et al. 2005). In this sense, the use of microemulsions and inverse micelles are routes that can be used to achieve the shape- and size-controlled iron oxide NPs (Wu et al. 2008).

The sequential preparation obtainable with reverse micelles is employed to first prepare an iron core, by the reduction of ferrous sulfate by sodium borohydride. After the reaction is completed, the micelles within the reaction mixture are expanded to accommodate the shell, using a larger micelle including additional sodium borohydride (Tartaj et al. 2003).

Vidal-Vidal et al. (Vidal-Vidal et al. 2006) have reported the preparation of monodisperse maghemite NPs by the one- vessel microemulsion method. The spherical-shaped particles, covered with a monolayer coating of oleylamine (or oleic acid), demonstrate a narrow size distribution of 3.5 ± 0.6 nm, are well crystallized, and have high saturation magnetization values (76.3 Am²/kg for uncoated NPs, 35.2 Am²/kg for oleic acid-coated NPs, and 33.2 Am²/kg for oleylamine-coated NPs) (Wu et al. 2008). The oil and water phases often comprise several dissolved components, and consequently, the selection of the surfactant depends upon the physicochemical characteristics of the system. The challenges in their scale-up procedures, and the adverse effects of the remaining surfactants on the properties of the particles, are the main disadvantages of the microemulsion method (Hasany et al. 2012). A common microstructure of MNPs prepared by the microemulsion method is shown in Figure 4(b).

Figure 4. Magnetic nanoparticles prepared in solution by: (a) co-precipitation (maghemite). (b) Polyol process (Fe-based alloy). Reprinted from (Sugimoto 2000). (c) Microemulsions (maghemite).Reprinted from (Jalil et al. 2014).

Polyol method

A very promising method for the synthesis of uniform NPs that could be used in biomedical applications such as magnetic resonance imaging, is the polyol technique. Fine metallic particles can be produced by reducing dissolved metallic salts and directly precipitating metals from a solution including a polyol (Tartaj et al. 2003, Sugimoto 2000, Nejati-Koshki et al. 2014, Matijevic 1993).

Fe₃O₄ MNPs have been synthesized by the co-precipitation method, combining a surface decoration process and the polyol process, and the factors affecting the adsorption of metal ions, such as pH, temperature, amount of adsorbent, and contact time, have been reported (Indira and Lakshmi 2010, Shen et al. 2009). The polyol method has also been a useful preparative technique for the synthesis of nanocrystalline alloys and bimetallic clusters (Willard et al. 2004). In the polyol method, the liquid polyol acts as a solvent for the metallic precursor, a reducing agent, and in some cases, as a complexing agent for the metallic cations. The solution is stirred and heated to a certain temperature, reaching the boiling point of the polyol, for less reducible metals. A better control of the mean size of the metal particles can be achieved by seeding the reactive medium with foreign particles (heterogeneous nucleation). In this way, the steps of nucleation and growth can be entirely separated, and uniform particles prepared (Tartaj et al. 2003).

By this method, precursor compounds such as oxides, acetates, and nitrates, are either dissolved or suspended in a diol, such as ethylene glycol or diethylene glycol. The reaction mixture is then heated to reflux between 180°C and 199°C. During the reaction, the metal precursors become solubilized in the diol and form an intermediate, and are then reduced to form metal nuclei, which form metal particles. Submicrometer-sized particles can be obtained by increasing the reaction temperature or inducing heterogeneous nucleation, by adding or forming foreign nuclei in situ. This method was also used to prepare nano-crystalline powders, such as Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Sn, Re, W, Pt, Au, (Fe,Cu), (Co,Cu), (Co,Ni), and (Ni,Cu), using different salt precursors. (Chow et al. 1995, Ebrahimi et al. 2014, Viau et al. 2001, Giri et al. 2000, Saravanan et al. 2001, Toneguzzo et al. 2000, Poul et al. 2000).

Iron particles with a size of around 100 nm can be obtained by disproportionation of ferrous hydroxide in organic media (Ghalhar et al. 2014). Fe(II) chloride and sodium hydroxide react with ethylene glycol (EG) or polyethylene glycol (PEG), and the precipitation occurs at a temperature ranging between 80 ˚C–100 ˚C. Additionally, iron alloys can be synthesized by co-precipitation of Fe, Ni, and/or Co, in EG and PEG. Monodisperse, quasi-spherical, and non-agglomerated metallic particles, with an average size of around 100 nm, have been prepared without seeding (homogeneous nucleation), while particles of a size between 50 and 100 nm have been prepared using Pt as the nucleating agent (heterogeneous nucleation) (Tartaj et al. 2003, Viau et al. 1996).

Oxides can be prepared by modifying the polyol method with the addition of water, to act more like a sol-gel reaction (forced hydrolysis) (Jungk and Feldmann 2000, Feldmann and Jungk 2001, Feldmann 2001, Tabatabaei Mirakabad et al. 2014). For example, 6 nm of CoFe₂O₄ was synthesized by the reaction of ferric chloride and cobalt acetate in 1,2-propanediol, with the addition of water and sodium acetate (Willard et al. 2004, Rajamathi et al. 2002). A common structure of MNPs prepared by this method is shown in Figure 4(c).

Compared to aqueous methods, the polyol method was found to result in the synthesis of metallic NPs protected by surface-adsorbed glycol, thus minimizing the oxidation. The use of a non-aqueous solvent such as polyol also reduced the problem of hydrolysis of fine metal particles, a phenomenon that often occurs in the aqueous situation (Willard et al. 2004).

Thermal decomposition of organic precursors

The decomposition of iron precursors in the presence of hot organic surfactants has yielded improved samples with good size control, narrow size distribution, good crystallinity of individual and dispersible magnetic iron oxide NPs (Indira and Lakshmi 2010).

Nanoparticles with a high level of monodispersity and size control can be achieved by high-temperature decomposition of organometallic precursors, such as [Mⁿ⁺(acac)_n], (M = Fe, Mn, Co, Ni, Cr; n = 2 or 3, acac = acetylacetonate), M^x (cup)_x (cup = N-nitrosophenyl hydroxylamine) or carbonyls (such as Fe(CO)₅), using organic solvents and surfactants such as fatty acids, oleic acid, and hexadecylamine (Faraji et al. 2010). Alivisatos and co-workers (Rockenberger et al. 1999) have demonstrated that injecting solutions of FeCup₃ in octylamine into long-chain amines at 250˚C –300˚C resulted in the synthesis of nanocrystals of maghemite. These nanocrystals are crystalline, and are dispersible in organic solvents, and their sizes range from 4 to 10 nm in diameter (Figure 5) (Tartaj et al. 2003).

Figure 5. Maghemite nanoparticles prepared in solution by decomposition of organic precursors at high temperature: (a) FeCup₃. Reprinted from (Rockenberger et al. 1999). (b) Fe(CO)₅. Reprinted from (Hyeon et al. 2001).

Biomedical applications like MRI are extremely dependent on particle size, and thus MNP synthesis by this method could be potentially used for these applications (Tartaj et al. 2003). The temperature and time of the reaction, as well as the aging period, may also be vital for the precise control of size and morphology (Lu et al. 2007). The prepared NPs annealed at 300°C, 700°C and 900°C, and the annealing temperature allowed the control of size and size distribution of the particles, as well as their structure and magnetic properties (Indira and Lakshmi 2010, Amara et al. 2009).

It has been reported that monodisperse magnetite NPs with sizes from 3 to 20 nm, could be prepared by the high-temperature (265˚C) reaction of iron (III) acetylacetonate in phenyl ether in the presence of alcohol, oleic acid, and oleylamine. Success in accurate control of particle size of Fe₃O₄ NPs has only been achieved through thermal decomposition, using large quantities of toxic and expensive precursors and surfactants in organic solvent. Thermal decomposition of organometallic precursors, in which the metal is zero-valent in the composition (such as Fe(CO)₅), initially leads to a generation of metal NPs, but if followed by oxidation, can lead to high quality monodisperse metal oxides. In contrast, the decomposition of precursors with cationic metal centers (such as Fe(acac)₃) leads directly to metal oxides NPs (Lu et al. 2007).

Laborious purification steps are essential before the end product can be used in biomedical applications (Eatemadi et al. 2014). The other disadvantage of this method is the production of organic-soluble NPs, which limit the range of applications for their use in biological fields. In addition, surface treatment is needed after synthesis (Faraji et al. 2010); moreover, the resulting NPs are generally only dissolved in nonpolar solvents (Wu et al. 2008).

Hydrothermal method

The hydrothermal method, also called the solvothermal method, is a preparation method for the synthesis of MNPs and ultrafine powders, as described in literature (Butter et al. 2005, Mao et al. 2006, Zhu et al. 2007, Fekri Aval et al. 2014, Gozuak et al. 2009, Wang et al. 2009). This technique is one of the most successful ways to grow crystals of many different materials (Faraji et al. 2010). As an alternative, the hydrothermal method contains various wet-chemical technologies of crystallizing material in a sealed container, from aqueous solution at the high temperature range of 130°C to 250°C, and at high vapor pressure, generally in the range of 0.3 to 4 MPa (Wu et al. 2008).

However, despite several studies to find proper ligands to synthesize monodisperse nanocrystals in a hydrophilic environment (Xu and Wang 2012), hydrothermal approaches still fail to obtain quality nanocrystals smaller than 10 nm with hydrophilic surface properties (Stojanovic et al. 2013). For example, Zheng et al. (Zheng et al. 2006) have prepared Fe₃O₄ NPs with a diameter of 27 nm using the hydrothermal method in the presence of a surfactant, sodium bis (2-ethylhexyl) sulfosuccinate. Wang et al. (Zohre et al. 2014) reported that the nanoscale Fe₃O₄ powder with a diameter of 40 nm, can be prepared by using the hydrothermal method at 140°C for 6 hours, having a saturation magnetization of 85.8 emu. g^{− 1}, which is lower than that of the corresponding bulk Fe₃O₄ (92 emu. g^{− 1}) (Wunderbaldinger et al. 2002).

The particle size and size distribution increased with precursor concentration. However, the residence time had a more significant impact on the average particle size than the feed concentration. Monodisperse particles were produced at short residence times (Xu et al. 2008, Osuna et al. 1996). The study of changing the precursor (ferric nitrate) concentration from 0.03 to 0.06 M, when all other variables were kept constant, showed that at the precursor concentration of 0.03 M, spherical particles with an average particle radius of 15.6 ± 4.0 nm were obtained (Hasany et al. 2012). One of the main problems of the conventional hydrothermal method is the slow reaction kinetics at any given temperature. Using microwave heating can increase the kinetics of crystallization during the hydrothermal synthesis (Faraji et al. 2010, Komarneni and Katsuki 2002).

Chemical vapor deposition (CVD)

In vapor-phase preparation of NPs, conditions are created where the vapor phase mixture is thermodynamically unstable relative to generation of the solid material to be prepared in nanoparticulate form. This method has wonderful flexibility in producing a wide range of materials, and can take advantage of the enormous database of precursor chemistries that have been developed for CVD processes. The precursors can be solid, liquid, or gas at ambient conditions, but are delivered to the reactor as a vapor (Swihart 2003). Wegner et al. (Wegner et al. 2002) presented a detailed, systematic modeling and experimental study of this method, as applied to the synthesis of bismuth NPs. They reported that they could control the particle size distribution by controlling the flow field and the mixing of the cold gas with the hot gas carrying the evaporated metal.

Gas phase methods for preparing nanomaterials depend on thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, or other reactions, to cause precipitation of solid products from the gas phase (Faraji et al. 2010, Pierson 1999). Spray and laser pyrolysis have been shown to be excellent techniques for the direct and continuous production of well-defined MNPs (Indira and Lakshmi 2010).

Spray pyrolysis

Spray pyrolysis is a method in which a solid is prepared by spraying a solution into a series of reactors where the aerosol droplets endure evaporation of the solvent, with condensation of the solute within the droplet, followed by drying and thermolysis of the precipitated particle at a higher temperature (Messing et al. 1993). Recently, this method has been used to synthesize colloidal aggregates of superparamagnetic maghemite NPs in the form of hollow or dense spheres, with the possibility of having a surface enriched in silica (Tartaj et al. 2003, Tartaj et al. 2001, Tartaj and Serna 2002, Tartaj et al. 2004). Their high rate of production can indicate a promising future for the synthesis of MNPs useful in in vivo and in vitro applications (Tartaj et al. 2003). Most of the pyrolysis-based processes used to generate maghemite NPs start with an Fe³⁺ salt and some organic compound that acts as the reducing agent. In this procedure, Fe³⁺ is partially reduced to a mixture of Fe²⁺ and Fe³⁺ by organic compounds, with the formation of magnetite, which is finally oxidized to maghemite (Pecharroman et al. 1995). In alcoholic solutions, uniform γ-Fe₂O₃ particles can be obtained with an extensive variety of particle morphologies and sizes in the range from 5 to 60 nm, depending on the nature of the iron precursor salt (Gonzalez-Carreno et al. 1993). Dense aggregates with a spherical shape composed of γ-Fe₂O₃ subunits, with an average diameter of 6 and 60 nm, have been generated using Fe(III) nitrate and Fe(III) chloride solutions, respectively (Tartaj et al. 2003).

Laser pyrolysis

An alternate means of heating the precursors to persuade reaction and homogeneous nucleation is the absorption of laser energy. This method allows highly localized heating and rapid cooling in comparison with heating the gases in a furnace.

The laser pyrolysis method includes heating a flowing mixture of gases with a continuous wave CO₂ laser, which initiates and sustains a chemical reaction (Veintemillas-Verdaguer et al. 2002). Biocompatible magnetic dispersions have been synthesized from γ-Fe₂O₃ NPs (5 nm) by continuous laser pyrolysis of Fe(CO)₅ vapors (Veintemillas-Verdaguer et al. 2004). Nanoparticles of many materials have been obtained by this method, such as the synthesis of Si NPs synthesized by Ledoux et al. (Ledoux et al. 2002, Ledoux et al. 2002). They used a pulsed CO₂ laser, thereby shortening the reaction time and allowing the synthesis of even smaller particles (Swihart 2003).

Sonochemical reactions

The sonochemical method has been widely used as a competitive alternative to synthesize novel materials with unusual properties (Wu et al. 2008, Faraji et al. 2010). The chemical effects of ultrasound arise from acoustic cavitation, that is, the generation, growth, and implosive collapse of bubbles in liquid. The conditions designed in the hotspots which are formed by the implosive collapse of the bubble, show temporary temperatures of 5000 K, pressures of 1800 atm, and cooling rates in excess of 10¹⁰ K/s(Wu et al. 2008, Suslick 1990). Figure 6 illustrates the common steps of iron oxide synthesis by the sonolysis method.

Figure 6. Flow chart of the sonochemical synthesis of iron oxide (Ashokkumar et al. 2007).

Commonly, volatile precursors in solvents of low vapor pressure are used to improve the particle yield. Acoustic irradiation is carried out with an ultrasound probe, such as a titanium horn, operating at 20 kHz (Willard et al. 2004). This method has been applied for the synthesis of several nanocomposites, and its versatility has been successfully confirmed in iron oxide NP synthesis (Bang and Suslick 2007). Vijayakumar et al. (Vijayakumar et al. 2000) reported a sonochemical synthetic route for producing pure nanometer-sized Fe₃O₄ powder with a particle size of 10 nm. The Fe₃O₄ NPs obtained are superparamagnetic, and their magnetization is very low (< 1.25 emu g^{− 1}) at room temperature (Roger et al. 1999). The powders prepared by this method are usually porous, amorphous, and agglomerated. For example, an amorphous iron powder was synthesized by the sonication of iron carbonyl in decalin (Suslick et al. 1991, Suslick et al. 1996, Suslick et al. 1996), producing a powder with a surface area of 120 m². g^{− 1}. Annealing this powder at 350°C under nitrogen resulted in α-Fe with a diameter of 50 nm (Willard et al. 2004). The addition of stabilizers or polymers added during or after sonication, resulted in metal colloids (Suslick et al. 1996, Suslick et al. 1995, Kataby et al. 1998).

Kim et al. (Hee Kim et al. 2005) prepared Fe₃O₄ NPs using the sonochemical and co-precipitation methods. The crystallinity and magnetic properties of the products generated using these two methods were compared, and the results achieved indicated that the Fe₃O₄ NPs prepared by the sonochemical method had a higher crystallinity and saturation magnetization than those achieved in the NPs prepared by the co-precipitation method (Figure 6).

Sol-gel reaction

The sol-gel method is a proper wet route for the preparation of nanostructured metal oxides. This method is based on the hydroxylation and condensation of molecular precursors in solution, initiating a “sol” of nanometric particles. Additional condensation and inorganic polymerization lead to a three-dimensional metal oxide network, denominated as wet gel. Extra heat treatments are needed to obtain the final crystalline state (Laurent et al. 2008, Safarik et al. 2011), because these reactions are performed at room temperature.

The sol-gel process includes hydrolysis and condensation of metal alkoxides. Metal alkoxides are good precursors, due to their endurance in the face of hydrolysis, i.e. the hydrolysis step replaces an alkoxide with a hydroxide group from water and a free alcohol is generated. Factors that need to be considered in a sol-gel method are the solvent type, temperature, precursors, catalysts, pH, additives and mechanical agitation. These factors can affect the kinetics, growth, and hydrolysis and condensation reactions (Willard et al. 2004, Scherrer and Brinker 1990). For metal oxides, the sol-gel process offers some advantages compared to other methods, including good homogeneity, low cost, and high purity. Moreover, the sol-gel method has been developed for the synthesis of magnetite NPs using metallo-organic precursors (Shaker et al. 2013). Shakeel Akbar et al. (Akbar et al. 2004) have successfully synthesized NPs, primarily of α-Fe₂O₃, by the sol-gel process, and studied their magnetic characterization. They reported that by using a modified sol-gel method, they achieved best results for obtaining alpha phase particles in two conditions. Particles were synthesized at a citric acid concentration of 0.2 M and 0.1 M of iron nitrate, with aging of the dry precursors (gel) at 90°C for about 16 hours in an open atmosphere. For samples annealed at high temperatures, pure alpha phase particles were achieved for an annealing temperature of 180°C, with a concentration of 0.1 M of both iron nitrate and citric acid. Sara Shaker et al. (Shaker et al. 2013) reported that magnetite NPs (Fe₃O₄) were successfully prepared the via sol-gel technique combined with annealing using inexpensive, nontoxic ferric nitrate and ethylene glycol, at temperatures of 200°C, 300°C, and 400°C. The characterization results indicated that the size of Fe₃O₄ NPs could be changed by changing the annealing temperature.

The sol-gel method has some advantages. In this method, it is possible to prepare pure amorphous phases, with monodispersity and good control of the particle size, and also to obtain materials with a predetermined structure based on experimental conditions. Moreover, the microstructure and the homogeneity of the reaction products are controllable. The sol-gel method includes pollution from by-products of reactions, as well as the need for post-treatment of the products. The disadvantage of this method is that it produces 3D oxide networks, and hence, it is limited in its efficiency (Hasany et al. 2012).

Conclusion

The investigation for novel routes of synthesis or the improvement of established ones which are able to fabricate MNPs with the proper characteristics of improved colloidal stability and biocompatibility, is continuously developing. Therefore, we focus mainly on routes of synthesis of magnetic nanoparticles for application in nanobiomedicine. We would like to modernize currently available methods for the preparation of Fe₃O₄ NPs with several morphologies; however, some major and new findings reported earlier are also involved.

Authors’ contributions

AA and MSG conceived of the study and participated in its design and coordination. SM, FZS, and SMF participated in the sequence alignment and drafted the manuscript. All authors read and approved the final manuscript.

Acknowledgments

The authors thank the Department of Medical Nanotechnology, Faculty of Advanced Medical Science of Tabriz University for all support provided. This work is funded by the 2014 Drug Applied Research Center Tabriz University of Medical Sciences Grant.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.