Superparamagnetic Iron Oxide Nanoparticles

Abstract

Thanks to their unique properties, inorganic nanostructures have become the center of modern material science. Among existing nanomaterials, magnetic iron oxide nanoparticles (IONP) have attracted a lot of interest. These nanoparticles are suitable for many technological applications such as contrast agent for magnetic resonance imaging. When comparing to molecular MRI probes (Gd-based organic complexes), IONP present many advantages such as a better sensitivity, a poor toxicity, and the possibility to easily modify their surface to develop some properties as multimodality, modulable half-life or specificity. However, one must stress that these properties are related to IONP’s composition, shape and stability (physical and chemical). In that paper, we will introduce some concepts related to IONP’s physico-chemical properties, the synthetic ways to obtain such structures and we will finish with some concepts governing their stability and surface modification.

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Superparamagnetic iron oxide nanoparticles have a hydrodynamic diameter ranging from 5 to 300 nm (Tables 5.1 and 5.2). These nanomaterials are coated with a natural or synthetic polymer or a small ionic molecule for ensuring a stable suspension [1, 2]. They have a very large magnetic moment and higher transverse relaxivity values than paramagnetic Gd complexes. This magnetic moment creates a local heterogeneity of the magnetic field in the region where water molecules are diffusing. This process leads to an acceleration of proton nuclear spin dephasing and a decrease of transverse relaxation time T₂. With a greater relaxivity r₂ than r₁, these contrast agents have a so-called negative T₂ predominant effect, leading to a decrease of the signal (dark MRI signal). Contrast induced by superparamagnetic agents is optimum using a T₂-weighted (increasing of TR and extending the TE). It is also possible, at low magnetic field to obtain a positive contrast using T₁-weighted sequences.

Table 5.1 Some examples of “ultrasmall superparamagnetic particle of iron oxide” (USPIO) (37 °C, 1.5 T) [2]

Table 5.2 Some examples of “superparamagnetic particles of iron oxide” (SPIO) (37 °C, 1.5 T) [2]

Two types of superparamagnetic nanoparticles were developed [1, 2] for MRI of liver or lymphatic system:

• Ultrasmall superparamagnetic iron oxide nanoparticle (USPIO, Fig. 5.1) and nanocompound monocrystalline iron oxide (MION) with a hydrodynamic diameter between 20 and 30 nm, those systems are monocrystalline (single crystal per particle). AMI-227, developed by Advanced Magnetics and Guerbet, is composed of particles with a hydrodynamic diameter of 17 nm and is obtained from a separation column of AMI-25 (Table 5.1).

  Fig. 5.1

  

  Schematic structures of USPIO and SPIO

• Superparamagnetic iron oxide nanoparticles (SPIO, Table 5.2, Fig. 5.1) have a hydrodynamic diameter higher than 60 nm and have several magnetic grains per particle. Among them are the AMI-25, known in Europe under the name Endorem^®.

These particles are composed of a polycrystalline iron oxide core entrapped in a crown of dextran. The interactions between the magnetic core and dextran are low. Stabilization can be improved by cross-linked dextran molecules to enhance their interactions with the crystal core (CLIO) [3]. Another class of versatile particles USPIO (VUSPIO) has also been developed [4]. In this case, the dextran molecules are covalently bound via silanol on the surface of nanoparticles.

The majority of iron oxide nanoparticles is often composed of magnetite (Fe₃O₄) and its oxidized form maghemite (γ-Fe₂O₄). These iron oxides exhibit an inverse spinel structure with a general formula of AB₂X₄ where A and B are cations and X are anions [5]. These crystals adopt a face-centered cubic crystalline structure. Anions delimit the network of the face-centered cubic crystal system and the cations are distributed in tetrahedral and octahedral sites. Specifically, the crystalline structure of magnetite is based on 32 anions of oxygen and contains both divalent and trivalent iron as cations. For one unit cell of magnetite containing 32 oxygens, 64 tetrahedral and 32 octahedral sites are generated. For each unit cell, 8 divalent atoms and 8 trivalent atoms of iron fill the octahedral sites of magnetite (Fig. 5.2) [6]. In the tetrahedral sites, 8 trivalent iron ions are present. Overall, each unit cell of magnetite contains 56 atoms in which there are 32 ions of oxygens, 16 ions of iron (III) and 8 ions of iron (II).

Fig. 5.2

Schematic illustration of inverse spinel of magnetite in which 8 octahedral sites are surrounded by 6 oxygens and 16 tetrahedral sites are surrounded by 4 oxygens. With permission of Ref. [7]. Available from: http://www.intechopen.com/books/modern-surface-engineering-treatments/surface-modification-of-nanoparticles-used-in-biomedical-applications

Iron oxide nanoparticles have been the subject of extensive scientific researches during the last decade, and several iron oxide based nanosystems have been approved as T ₂ agents. They include magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and other ferrites which have bivalent metal ions to replace Fe²⁺, such as MnFe₂O₄ [8], ZnFe₂O₄ [9] and CoFe₂O₄ [10].

5.1 Superparamgnetic Properties of Iron Oxide Nanoparticles

5.1.1 Relaxometric Properties

5.1.1.1 Relaxation Processes of Single Domain Iron Oxide Nanoparticles

Superparamagnetic iron oxide nanoparticles are composed of a single magnetic domain with a size of a few nanometers, the global size being much smaller than a Weiss domain of the corresponding bulk ferromagnetic materials [11, 12].

For these nanoparticles, the directions of the magnetization vector are generally aligned on preferred positions called “easy direction magnetization” or “anisotropy axes” in order to minimize the anisotropy energy of the system. The anisotropy energy can be defined as the energy required to move the magnetization between the positions of easy magnetization axis [13].

The anisotropy energy (E_a) of a single domain is proportional, in a first approximation, to its volume (V) [14]. In the simplest case of uniaxial anisotropy (Fig. 5.3), the energy barrier between two easy magnetization directions is proportional to K_aV and is defined as:

$$ E_{a} = K_{a} V\sin^{2} \theta $$

where K_a is the anisotropy constant and θ is the angle between both anisotropy axis positions.

Fig. 5.3

Uniaxial anisotropy axis for superparamagnetic materials

K_a constant can be influenced by four contributions for isolated nanoparticles: (i) the chemical composition and the crystallographic structure of the material, (ii) the nanoparticle shape, (iii) the coating nature and (iv) the inter-distance between the nanoparticles. In the case of agglomerated nanoparticles, only the distance between the nanoparticles contributes to K_a [15].

The anisotropy energy of magnetic crystal dramatically increases with the crystal volume and thus with the particle radius [16]. Consequently, for large crystal, the energy barrier is high and the transitions from an easy axis to another one are not favored. The magnetization vector is thus blocked on an anisotropy axis. On the contrary, for small particles, the transitions of the magnetization from one easy axe to another one are thus easier [17].

The changeover of magnetic moment vector from an easy axis to another is observed when the thermal energy is sufficient to break the anisotropy energy barrier. The average time to switch from one direction to another is called “Néel relaxation time (τ_N)” and is given by

$$ \tau_{N} = \tau_{0} E_{a} e^{{\frac{{E_{a} }}{{Tk_{b} }}}} $$

where τ_N is the Néel relaxation time, k_b is the Boltzmann constant, T is the temperature and τ₀ (E_a) corresponds to the preexponential factor of the Néel relaxation time expression.

The alignment of magnetization vector is influenced by two contributions: the anisotropy energy (K_aV) and the thermal agitation energy (k_bT). The ratio between both energies induces two situations as described below:

• If K_aV ≫ k_bT, the magnetic moment is locked in one of the easy directions. Consequently, the changeover of the magnetic vector to another easy axis involves the complete rotation of the particle.

• If K_aV ≪ k_bT, the thermal agitation is a dominant factor, the free transition of the magnetization vector on the easy axes occurs without the particle rotation.

For superparamagnetic crystals dispersed in a liquid medium, the return of the magnetic moment to equilibrium state is defined by two different processes: the Néel relaxation and the Brownian relaxation.

The Néel relaxation is related to the permanent changeover of the magnetization from an easy direction to another one and depends on temperature and crystal volume. The values of the Néel relaxation time τ_N are generally included in the range of 10⁻⁹–10⁻¹¹ s. When an external magnetic field is applied, the magnetic moments stay long enough on one easy magnetization axis to produce a resulting magnetization proportional to the strength of magnetic field. The behavior of these particles can be compared to the paramagnetic substances. However, the magnetic susceptibility of superparamagnetic materials is very much higher (100 or 1000 times) due to the collective interactions of electrons, and this explains why these compounds are called superparamagnetic.

For large crystals, the Néel relaxation time becomes longer and any changeover of the magnetization is observed because the magnetic moments are locked on an easy axis. The blocking volume (V_B) is defined as a crystal critical volume. This parameter leads to a lock of the magnetization during the measuring time at a given temperature. In this case, only the total rotation of particles submitted to the thermal agitation can be observed. The characteristic time of this rotation is given by the Brownian relaxation times (τ_B):

$$ \tau_{B} = \frac{3V\eta }{{k_{b} T}} $$

where V is the crystal volume, $\eta$ is the fluid viscosity, k_b is the Boltzmann constant and T is the temperature.

In summary, the global magnetic relaxation rate of colloidal suspensions (Fig. 5.4) is modulated by Néel and Brownian relaxation times following this equation:

Fig. 5.4

Illustration of water proton relaxation due to the presence of superparamagnetic particles

$$ \frac{1}{\tau } = \frac{1}{{\tau_{N} }} + \frac{1}{{\tau_{B} }} $$

The fastest process dominates the relaxation of system. The Néel relaxation is thus dominant for small crystals, whereas, the relaxation of large crystals is almost exclusively governed by Brownian contribution.

5.1.1.2 Superparamagnetic Relaxation: Theoretical Model

Due to their particular magnetic properties, superparamagnetic particles are used for MRI applications. The high magnetization of these compounds influences greatly the surrounding water relaxation rate.

The understanding of the superparamagnetic relaxation phenomena is possible with the accepted theory established by Roch-Muller-Gillis (commonly called RMG model or SPM model) [13, 18, 19]. This theory is based on the classical outer-sphere relaxation theory, but extended to include the presence of materials with a strong anisotropy. The relaxation induced by superparamagnetic nanoparticles is due to the coupling between the magnetic moments of water protons and the electron magnetic moments of particles. The modulation is caused by Néel relaxation (flip of the magnetization vector of particles from on easy axis to another) and the diffusion of water protons.

RMG model of large crystals will be firstly described and the model of small crystals will be then discussed.

• For the large crystals (particle radius > 7.5 nm)

The anisotropy barrier is high, inducing the lock of magnetic moment of superparamagnetic materials on one anisotropy axis when high magnetic fields are applied. Depending on the strength of external magnetic field, several limiting cases can be described as below.

At low magnetic fields, the magnetic moments can easily move from one anisotropy direction to an other causing drastic magnetic fluctuations on the water diffusion in the vicinity of magnetic particles. Consequently, the dipolar interactions between water protons and magnetic core are modulated by the translational correlation time of the water molecules (τ_D) and the Néel relaxation time (τ_N) as shown in Fig. 5.5. Both modulations can be gathered together in a global correlation time (τ_CI) as follows:

$$ \frac{1}{{\tau_{CI} }} = \frac{1}{{\tau_{N} }} + \frac{1}{{\tau_{D} }} $$

where τ_D is equal to the square of crystal radius (r) divided by the diffusion constant (D).

Fig. 5.5

Relaxation model for large magnetic nanoparticles with each contribution

The proton longitudinal and transversal relaxation rates can be expressed from Freed spectral density (J_F) [20]:

$$ R_{1} = \frac{1}{{T_{1} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left[ {10J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right)} \right] $$

$$ R_{2} = \frac{1}{{T_{2} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left[ {8J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 2J_{F} \left( {0,\tau_{D} ,\tau_{N} } \right)} \right] $$

$$ J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) = R_{e} \left[ {\frac{{1 + \frac{1}{4}\Omega ^{{\frac{1}{2}}} }}{{1 +\Omega ^{{\frac{1}{2}}} + \frac{4}{9}\Omega + \frac{1}{9}\Omega ^{{\frac{3}{2}}} }}} \right] $$

$$ \Omega = i\omega_{I} \tau_{D} + \frac{{\tau_{D} }}{{\tau_{N} }} $$

With γ is the proton gyromagnetic ratio, μ is the electron magnetic moment, N_A is the Avogadro number, C corresponds to the molar concentration of superparamagnetic compound, r is the crystal radius and ω_I is the proton angular frequency.

At high magnetic fields, the magnetization vector remains onto one of the easy axes. In this case, the Néel relaxation time is longer due to a high anisotropy. Therefore, the modulation of relaxation solely results from water proton diffusion. This modulation is described thanks to Ayant spectral density (J_A) [21] and the relaxation rates are described as below:

$$ R_{1} = \frac{1}{{T_{1} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left[ {9L^{2} \left( \alpha \right)J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right)} \right] $$

$$ R_{2} = \frac{1}{{T_{2} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left[ {\frac{9}{2}J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right) + 6J_{A} \left( 0 \right)} \right] $$

with

$$ J_{A} (\mu ) = \frac{{1 + \frac{5\mu }{8} + \frac{{\mu^{2} }}{8}}}{{1 + \mu + \frac{{\mu^{2} }}{2} + \frac{{\mu^{3} }}{6} + \frac{{4\mu^{4} }}{81} + \frac{{\mu^{5} }}{81} + \frac{{\mu^{6} }}{648}}} $$

At intermediate magnetic fields, the water relaxation induced by superparamagnetic particles is modulated by both previous contributions. A linear combination of the equations with Langevin function [L(α)] must be used to take into account that the magnetization is divided in two parts: the first one is locked with external magnetic field and the second one is influenced by Néel relaxation.

$$ \begin{aligned} R_{1} & = \frac{1}{{T_{1} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left\{ {\left( {\frac{L\left( \alpha \right)}{\alpha }} \right)21J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right)} \right. \\ & \quad \left. { + 9\left[ {1 - L^{2} (\alpha ) - 2\left( {\frac{L\left( \alpha \right)}{\alpha }} \right)} \right]J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 9L^{2} (\alpha )J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right)} \right\} \\ \end{aligned} $$

$$ \begin{aligned} R_{2} & = \frac{1}{{T_{2} }} = \frac{32\pi }{{405{,}000}}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left\{ {\left( {\frac{L(\alpha )}{\alpha }} \right)19.5J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right)} \right. \\ & \quad + \left[ {1 - L^{2} (\alpha ) - 2\left( {\frac{L(\alpha )}{\alpha }} \right)} \right]\frac{9}{2}\left[ { J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 6J_{F} (0,\tau_{D} ,\tau_{N} } \right] \\ & \quad \left. { + L^{2} (\alpha )\left[ {\frac{9}{2}J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right) + 6J_{A} (0)} \right]} \right\} \\ \end{aligned} $$

In summary, for large crystals, the relaxation is differently modulated depending on the strength of external magnetic field. This theory predicts the evolution of water proton relaxation rate with magnetic field. Each modulation is shown in Fig. 5.5.

The RMG model allows a good fitting of experimental points obtained for large crystals (Fig. 5.6a). However, this model does not fit the particles presenting a lower anisotropy energy (r < 7.5 nm). The experimental data show dispersion at low frequency that does not agree with the theoretical predictions. An example is shown in Fig. 5.6b. Consequently, an adaptation of RMG model has been elaborated.

Fig. 5.6

a Relaxation evolution of a suspension of large particles (Endorem^®: r = 15 nm) as a function of Larmor frequency and b relaxation evolution of a suspension of small particles (r = 5 nm) as a function of Larmor frequency. The line represents the theoretical fit developed for large magnetic crystals reproduced with permission of Ref. [2]

• For small crystals (particle radius < 7.5 nm):

These materials are characterized by a small anisotropy energy in which the magnetic moments can fluctuate from one easy magnetization axis to another one. Therefore, a new theory is required to explain the superparamagnetic relaxation of small magnetic cores. The objective is to insert the anisotropy energy as a quantitative parameter. However, the analysis requires a long calculation time which is not easy to practice. Therefore, Roch et al. [18, 19] developed an alternative heuristic method to include this parameter. This model reproduces the gradual vanishing of the low field dispersion through a linear combination of the rate for infinite and zero anisotropy energy.

$$ \begin{aligned} R_{1} & = \frac{1}{{T_{1} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left\{ {\left( {\frac{L(\alpha )}{\alpha }} \right)21PJ_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 21 (1 - P)J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right)} \right. \\ & \; \; \quad \left. { + 9\left[ {1 - L^{2} (\alpha ) - 2\left( {\frac{L(\alpha )}{\alpha }} \right)} \right]J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 9L^{2} (\alpha )J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right)} \right\} \\ \end{aligned} $$

$$ \begin{aligned} R_{2} & = \frac{1}{{T_{2} }} = \frac{32\pi }{405{,}000}\gamma^{2} \mu^{2} \left( {\frac{{N_{A} C}}{{r^{3} }}} \right)\left\{ {\left( {\frac{L(\alpha )}{\alpha }} \right)19.5J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right)} \right. \\ \; \; & \quad \left. { + \left[ {1 - L^{2} (\alpha ) - 2\left( {\frac{L(\alpha )}{\alpha }} \right)} \right]\frac{9}{2}\left[ { J_{F} \left( {\omega_{I} ,\tau_{D} ,\tau_{N} } \right) + 6J_{F} (0,\tau_{D} ,\tau_{N} } \right] + L^{2} (\alpha )\left[ {\frac{9}{2}J_{A} \left( {\sqrt {2\omega_{I} \tau_{D} } } \right) + 6J_{A} (0)} \right]} \right\} \\ \end{aligned} $$

where P is a weighing factor of the linear combination and ω_s is the electron angular frequency.

The modified model allows a good interpretation of the relaxation for small particles (Fig. 5.7). These predictions have been confirmed with the experimental results for materials with low anisotropy energy [2].

Fig. 5.7

NMRD profiles of superparamagnetic particles. With permission of Ref. [2]

5.1.1.3 Nuclear Magnetic Resonance Dispersion Profiles (NMRD)

NMRD profiles are essential tools to evaluate the relaxometric properties of MRI contrast agents as a function of magnetic field. The measurements are performed thanks to fast field cycling relaxometer. The method allows a rapid analysis of the properties for new contrast agents and can be exploited to control the reproducibility of the nanoparticle synthetic protocol. The fitting of NMRD profiles with suitable theoretical models provides information about the magnetic crystals (Fig. 5.7) such as their average radius (r), their saturation magnetization (M_s), their anisotropy energy (E_a) and their Néel relaxation times (τ_N).

The dependence of each parameter is explained as follows:

• Average radius (r):

  At high magnetic field, the relaxation rate only depends on the diffusion correlation time (τ_D) as shown by the equation of Ayant spectral density. The inflection point corresponds to the condition of ω_I.t_D ≫ 1 in which ω_I is the proton Larmor frequency. The diffusion correlation time is defined as a ratio of the square of crystal radius divided by diffusion constant (τ_D = r²/D) and thus the average radius can be estimated. An increase of crystal size modifies the position of inflection point, which moves towards lower frequency.

• Saturation magnetization (M _s ):

  At high magnetic fields, the saturation magnetization is reached and can be estimated from the relaxivity maximum by M_s (R^max/cτ_D)^1/2, where c and R^max respectively correspond to a constant and the maximum relaxation rate.

• Crystal anisotropy energy (E _a ):

  The dispersion observed at low magnetic fields informs on the presence of crystals with low anisotropy energy. For materials with high anisotropy energy, no pitch dispersion is observed. The dispersion at low frequencies gives qualitative information on the magnitude of anisotropy energy of magnetic compounds in solution.

• Néel relaxation time (τ _N ):

  The Néel relaxation time obtained from the theoretical fitting is an approximate value. It can be used as qualitative information in addition to the crystal size and the specific magnetization by theoretical model.

The magnetic and relaxometric measurements give a complete description of the physico-chemical properties of iron oxide nanoparticles. It is important to notice that the RMG model is based on the assumption that nanoparticle size is uniform, and consequently, the theoretical parameters extracted from the theoretical fitting are average values. However, in the real situations, the size distribution of magnetic crystals is often inhomogenous. If the size distribution is large, the nanoparticle suspensions contain crystals with different sizes or agglomerated nanoparticles, and consequently, the obtained theoretical parameters don’t represent the effective characteristics of nanoparticle suspension.

5.2 Synthesis of Magnetic Nanoparticles

The preparation of magnetic materials in bulk form is easy. However, reducing the crystal size to the nanometer scale, brings some new complexities to synthesize these functional materials.

Due to their great potential in a wide range of domains, numerous synthetic methods were developed to produce iron oxide nanoparticles [22–28].

The synthetic way is very important and has a high impact on the characteristic of the SPIONs: size, size distribution, shape, surface chemistry, … Preparation routes can be classified into three main categories [29]: physical (aerosol, gas phase deposition, pulsed laser ablation, electron beam lithography, laser induced pyrolysis), biological (production from fungi, bacteria or proteins) and chemical (coprecipitation, thermal decomposition, microemulsion, sonochemical, electrochemical deposition) methods. Chemical methods represent most of the published protocols because they are easy, cheap and allow the best control on size, shape and composition.

The synthesis of magnetic nanoparticles in a solution by a chemical reaction consists in the formation of nuclei, followed by particles growth. When the formed monomers achieve saturation and then the supersaturation state, the nucleation is initiated such that monomers gather to form nuclei, followed by subsequent growth to result in the formation of nanoparticles. The relationship between supersaturation and nucleation and growth is defined by the following simplified equations,

$$ \begin{aligned} B & = k_{b}\Delta C^{b} \\ G & = k_{g}\Delta C^{g} \\ \end{aligned} $$

where B and G are the nucleation and growth rates, respectively; the subscripts b and g refer to nucleation order and growth order, respectively; k is a constant and ∆C is the supersaturation [30]. It is well acknowledged that a drastic burst of nucleation followed by slow controllable growth is critical to obtain monodisperse nanoparticles.

Synthesis of nanoparticles with uniform size and morphology is possible under conditions of rigorous control on kinetics of the nucleation and growth. The optimal control can be achieved when the two processes are separated in time, where nucleation needs to be completed before the beginning of growth of nuclei.

5.2.1 Coprecipitation Method

Coprecipitation technique is the most simple and efficient way to prepare iron oxide nanoparticles. In this method, nanoparticles are produced by heating 2/1 stoichiometric mixture of ferric and ferrous salts in alkaline medium [31, 32]. The size, shape, and composition of the particles depend on several parameters as the salts, the pH of the solution, the temperature and the ionic strength of the media [33–35]. Chemical reaction of magnetite (Fe₃O₄) formation can be written as:

$$ {\text{Fe}}^{{ 2 { + }}} + 2{\text{Fe}}^{{ 3 { + }}} + 8{\text{OH}}^{ - } \to {\text{Fe}}_{ 3} {\text{O}}_{ 4} + 4{\text{H}}_{ 2} {\text{O}} $$

A complete precipitation of Fe₃O₄ occurs at a pH between 9 and 14, and under a non-oxidizing oxygen-free environment. Fe₃O₄ is not very stable and is rapidly oxidized into γ-Fe₂O₃ in the presence of oxygen. Nitrogen is used to produce non oxidative environment but also to reduce the particle size [36, 37].

There are three phases in the coprecipitation process. In phase I, monomer concentration increases up to its saturation point. Then (phase II), small nuclei are formed when the concentration of the species reaches critical supersaturation. This second phase is followed by the growth of the crystal (phase III). The nucleation and growth steps need to be separated for producing nanoparticles [38, 39]. This phenomenum can be illustrated in the LaMer diagram (Fig. 5.8).

Fig. 5.8

LaMer diagram illustrating the nucleation and growth during the coprecipitation synthesis. The nucleation process rapidly occurs with a subsequent nuclei formation. With the permission of reference [40]. Available from https://www.researchgate.net/figure/215475567_fig13_LaMer-diagram

The presence of chelating organic molecules with negative charges (carboxylate ions, such as gluconic, citric, oleic acid, …) or polymers (natural or synthetic as dextran, polyvinyl alcohol, starch, polyethyleneglycol, …) during the formation of nanoparticles can help to control their size. Addition of organic ions can inhibit nucleation or crystal growth and lead to larger or smaller particles.

The most common synthesis is the Massart’s method [41]. FeCl₃ and FeCl₂ are mixed in alkali to produce the nanoparticles. In the original synthesis of Massart, the magnetite particles have a mean diameter of 8 nm [42]. The influence of the base (NaOH, NH₄OH or CH₃NH₂), the pH, the cations (Na⁺, Li⁺, K⁺, NH₄ ⁺, N(CH₃) ⁺₄ , and CH₃NH₃ ⁺), and Fe³⁺/Fe²⁺ ratio were studied. Depending on these different parameters, particles with size between 4.2 and 16.6 nm can be obtained. The influence of pH and ionic strength were also studied, and particle size between 2 and 15 nm were produced. The shape of the particles is correlated with the electronic surface density of the nanoparticles [43, 44].

The synthetic medium can be aqueous media or organic solvents [45]. Massart [42] reported also the synthesis of γ-Fe₂O₃ by a rapid method which allows various coatings, such as α-hydroxyacids, aminoacids, hydroxamates, or dimercaptosuccinic acid.

The influence of Fe²⁺/Fe³⁺ ratio on the size, shape, composition, and magnetic properties of the nanoparticles has been studied [46]. When the ratio is too small, FeO(OH) are formed in the solution. When the ratio is equal to 0.3, two phases are present. The first one has a Fe²⁺ and Fe³⁺ ratio of 0.07 and produces 4 nm particles; the other one has a ratio of 0.33 and gives larger particles. When this ratio is 0.35, only one phase exists. When this ratio is 0.5, the particles are homogeneous in size and in chemical composition.

The pH and ionic strength has a strong influence on the particle size. Due to their effect on the electrostatic surface charge, the particle size and size distribution width decreases when the pH and the ionic strength are increased [47].

The mixing rate can also influence the particle size: if the mixing rate increases, the size of the particles decreases. The influence of temperature has also been studied: the number of magnetite nanoparticles decreases when the temperature increases [33].

In aqueous synthesis, coprecipitation is widely employed to obtain different ferrites (MFe₂O₄, M = Fe, Mn, Co, Mg, Zn, Ni, etc.). Coprecipitation of thermodynamically favourable phases is propitious to obtain well-crystallized anhydrous oxides, even from diluted solutions.

The general reaction formula is:

$$ 2{\text{Fe}}^{3 + } + {\text{M}}^{2 + } + 8{\text{OH}}^{ - } \to {\text{MFe}}_{2} {\text{O}}_{4} + 4{\text{H}}_{2} {\text{O}} $$

Fe₃O₄, for example, has been synthesized by a simple coprecipitation of Fe³⁺ and Fe²⁺ in 2:1 molar ratio with NaOH above 70 °C [48]. Fe₃O₄ nanoparticles can also be obtained by coprecipitation using tetramethylammonium hydroxide with or without aqueous ammonia as the OH⁻ source [49]. ZnFe₂O₄ nanoparticles of 3–4 nm were similarly prepared from Zn²⁺ and Fe³⁺ solutions at temperatures ranging from 40 to 100 °C [50]. The relaxivities have been shown to be dependent on the size of magnetic particles, which can be controlled during the synthesis process by changing the conditions of coprecipitation. CuFe₂O₄ nanoparticles were obtained by coprecipitation in alkaline medium of a ferrite stoichiometric mixture of Cu²⁺ and Fe³⁺ [51]. By controlling the pH and ionic strength, variations of size and shape can be observed, without complexing agents [52]. The nanoparticles obtained from coprecipitation did not have to be annealed at a high temperature, which can avoid their agglomeration in some degree. It is especially helpful for the medical applications, where the agglomeration of particles should be avoided.

5.2.2 Hydothermal Method

In the hydrothermal way, reactions are performed in the aqueous media in rectors or autoclaves at high pressure (higher than 2000 psi) and high temperature (above 200 °C) [53, 54].

Two main routes are used for the formation of ferrites: hydrolysis and oxidation or neutralization of mixed metal hydroxides. These two reactions are similar, except that ferrous salts are used in the first method [55]. The solvent, the temperature and the reaction time have a large effect on the particle characteristics [56]. The size of the particles increases when the reaction time or when the amount of water increase. At higher temperatures, the nucleation process is faster than the crystal growth and the size of the particles decreases. With longer reaction time, the crystal growth dominates and larger particles can be formed. Magnetite nanoparticles with a size up to 27 nm have been obtaiend by hydrothermal method [57].

5.2.3 Thermal Decomposition Method

Thermal decomposition of iron precursors in the presence of hot organic surfactants is probably the best method to control the size, the size distribution, and to obtain a good crystallinity of the synthesized nanoparticles [58].

Numerous types of thermal decomposition of iron complexes (Fe(Cup)₃, Fe(CO)₅, or Fe (oleate)₃) have been developed. For exemple, Sun et al. described a high temperature mediated reaction of iron acetylacetonate (Fe(acac)₃) with 1,2-hexadecanediol in the presence of oleic acid and oleylamine. Resulting nanoparticles were monodisperse and size could be tuned from 4 nm to 20 nm [59].

The decomposition of iron pentacarbonyl in the presence of oleic acid at 300 °C allows the production of magnetite nanoparticles with a very good crystallinity, the size of the particles obtained range from 4 to 16 nm [58]. The size and shape of the particles are determined by the reaction time, temperature, precursors, concentration, ratios of the reactants, and solvent. The oleic acid on the surfaces of the particles is used to stabilize the colloid solution.

The thermal decomposition of iron carbonyl in the presence of octyl ether and oleic acid and using consecutive aeration can produce hydrophobic magnetite nanoparticles with narrow size distribution [60].

The thermal decomposition of iron oleate or iron pentacarbonyl in organic solvent at different temperatures can produce particles with size between 4 and 11 nm [61].

This method produced nanoparticles dispersible in organic solvents but not in aqueous medium. The decomposition of Fe(acac)₃ or FeCl₃ in refluxing 2-pyrrolidone can produce water-dispersible particles in acidic or basic media [62, 63]. The decomposition of Fe(acac)₃ in high boiling organic solvent allowed to obtain particles with size of 4, 6, 9 or 12 nm and very narrow size distribution. They are coated by 2,3-dimercaptosuccinic acid and are dispersible in water [64].

The decomposition of Fe₃(CO)₁₂ in the presence of oleic acid and diethylene glycol diethyl ether has also been described [65]. The particles are then annealed at 300, 700, and 900 °C. The annealing temperature helps to control the size, size distribution, composition, and magnetic properties of the particles.

To control the oxides crystal growth and to get uniform size and shape of nanoparticles, the decomposition is performed in low polar solvents and high boiling point. The solvents in thermolysis sometimes can be used as the stabilizing agent, for their high dielectric constant or high donor number [66, 67]. Fe(acac)₃ is mostly used as the Fe-source for forming Fe₃O₄ by thermolysis. To achieve the control over the nucleation and growth processes, assistant agents interacting with metal atoms and solvents are required, which mostly employ polymers with nucleophilic groups, capping ligands and organoaluminium agents. Superparamagnetic Fe₃O₄ nanoparticles with different sizes were synthesized from high-temperature reaction of iron acetylacetonate and 1,2-hexadecanediol in high-boiling phenyl ether solvent in the presence of oleic acid and oleylamine [59]. CoFe₂O₄ and MnFe₂O₄ nanoparticles were synthesized by the same method but with addition of Co(acac)₂ and Mn(acac)₂, respectively. It was found that particle diameter can be tuned from 3 to 20 nm by varying reaction conditions such as refluxing temperature or by seed-mediated growth. The key to obtain monodisperse nanoparticles was the pretreatment of mixture solution at 200 °C for some time before refluxing. Gao and his coworkers reported the synthesis of Fe₃O₄ nanoparticles via the thermal decomposition of Fe(acac)₃ dissolved in 2-pyrrolidone, which was used as the polar solvent and also the ligand agents for controlling the crystal growth and preventing the agglomeration [68]. For the application as MRI CAs, Fe₃O₄ nanoparticles obtained by thermolysis were transferred from hydrophobic to hydrophilic, using hydrophilic polymers like monocarboxyl-terminated poly(ethylene glycol) (MPEG-COOH) [69, 70] and by amphiphilic molecular like oleyl alcohol [71].

Size of the nanoparticles synthesized using by thermal decomposition method depends strongly of the reaction temperature, the iron to surfactant ratio and the reaction time. Nanoparticles obtained are soluble in organic solvents, they are stabilized and protected from aggregation by a surfactant surface coating as lauric or oleic acid: the polar head group of the surfactant is oriented on the nanoparticle surface and the hydrophobic tail is extended away from the nanoparticles as shown in Fig. 5.9 [72, 73].

Fig. 5.9

Representation of SPIONs synthesized by thermal decomposition method. SPION surface is coated with lauric acid during the synthesis

5.2.4 Sol-Gel Methods

The sol-gel process is based on the hydrolysis and condensation of precursors in a colloidal solution (sol). A metal oxide network (gel) can be obtained by removing solvent or chemical reaction: a colloidal gel is produced by basic catalysis and a polymeric form of the gel is produced by acidic catalysis [74].

The rates of hydrolysis and condensation influence the properties of particles formed. A lower and more controlled hydrolysis rate allows to produce smaller particles. The size of the particles can also be correlated to the solvent, concentration, pH, and temperature [75].

After a treatment at 400 °C, Fe₂O₃ particles with a size between 6 and 15 nm are formed [76]. With this method, Fe₂O₃ nanoparticles can be coated in an inert and inorganic silica matrix [77, 78].

5.2.5 Microemulsions

A microemulsion is a thermodynamically stable dispersion of two immiscible liquids. In water-in-oil microemulsion systems, the microdroplets of the aqueous phase surrounded by surfactant molecules are dispersed in a continuous oil phase [38]. The role of surfactant molecules is the limitation of the nucleation, the growth, and thus the agglomeration of the particles [79, 80]. The growth of the particles is due to an interdroplet exchange and nuclei aggregation [81, 82].

Water-in-oil (w/o) microemulsions are formed by well-defined nanodroplets of the aqueous phase, dispersed by the assembly of surfactant molecules in a continuous oil phase. Vidal-Vidal et al. [83] have reported the synthesis of monodisperse maghemite nanoparticles by the one-pot microemulsion method. Due to the nanometer size of the aqueous core, the particles formed usually have a size less than 15 nm and a very narrow size distribution. The biggest advantage of this method is the control of the particle size.

5.2.6 Polyol Methods

The polyols, like polyethyleneglycol, have some good properties: (i) a high dielectric constant, (ii) an ability to dissolve inorganic compounds, and (iii) a high boiling temperature. They offer a wide range of operating temperature for the production of inorganic compounds [84]. Nanoparticles can be produced by the reduction of dissolved iron salts and precipitation in the presence of polyol [85, 86].

The polyols are used as reduction agents and also as stabilizers, they can control the growth of particles and prevent aggregation. In this method, the precursor is suspended in the liquid polyol. The suspension is stirred and heated to reach the boiling point of polyol. The precursor becomes soluble in the diol during the process. The size and shape of the nanoparticles can be regulated by controlling the kinetic of the process.

The yield of iron oxide nanoparticles depends on the kind of polyols and ferrous salts, but also on the concentration and temperature. The size and yield of the particles are related to the reduction potential of the polyols [87].

Non-aggregated magnetite nanoparticles have been produced by a modified polyol method [88]. Different polyols, including ethylene glycol, diethyleneglycol, triethylene glycol, and tetraethylene glycol, have been used to react with Fe(acac)₃ at high temperature. The non-aggregated magnetite nanoparticles with narrow size distribution and uniformed shape are only produced when using triethylene glycol.

5.2.7 Electrochemical Methods

Nanoparticles with a size between 3 and 8 nm have been synthesized by electrochemical method [89]. In this method, the particles are prepared from an iron electrode in an aqueous solution of dimethylformamide and cationic surfactant; the nanoparticle size is controlled by the current density.

5.2.8 Aerosol/Vapor Method

Aerosol methods, including spray and laser pyrolysis, are continuous chemical processes allowing high rate production.

Spray pyrolysis consists in a process where a solution of ferric salts and a reducing agent in organic solvents is sprayed into a series of reactors, then the solvent is evaporated and the solute condenses. The size of the original droplets controls the particle diameter. Using different iron precursors in alcoholic solution, maghemite particles with size ranging from 5 to 60 nm and with different shapes can be produced.

Laser pyrolysis is a method to reduce the reaction volume. In this method, laser is used to heat a flowing gaseous mixture of iron precursor, and non-aggregated particles with small size and narrow size distribution can be obtained. By controlling the experimental conditions, maghemite particles can have a size between 2 and 7 nm and a very narrow size distribution [90, 91]. For example, maghemite nanoparticles with a size of 5 nm have been synthesized by continuous laser pyrolysis of Fe(CO)₅ vapors [92].

5.2.9 Sonolysis/Thermolysis

The decomposition of organometallic precursors by sonolysis can also produce iron oxide nanoparticles. Polymers, organic capping agents, or structural hosts allow to limit the particle growth [93, 94]. The rapid collapse of cavities generated by sonochemistry gives very high temperature hot spots. These spots allow the conversion of ferrous salts into magnetic nanoparticles. For example, magnetite nanoparticles have been produced by the sonolysis of Fe(CO)₅ aqueous solution in the presence of sodium dodecyl sulfate [95].

In another example, well-defined supermagnetic iron oxide nanoparticles have been produced by sonolysis [96]. Stable particles coated with oleic acid can be dispersed in chitosan solution and have a size of 65 nm.

For biomedical applications, the control of particle size, size distribution, particle shape, and magnetic properties is very important. Different physico-chemical characteristics can be obtained by different fabrication methods (Table 5.3).

Table 5.3 Summary of the synthetic method of iron oxide nanoparticles

According to the comparison of the different synthetic way to form iron oxide nanoparticles, thermal decomposition of metal iron precursors appears as the most profitable and the most promising synthetic method to produce monodispersed crystalline magnetic nanomaterials. This process forms well-controlled iron oxide particles without the use of a size selection step at the end of the synthesis.

5.3 Stabilization of Nanoparticle Suspensions

Colloidal suspensions are stable dispersions of small solid particles ranging in size from 1 nm to 1 µm. Stable nanoparticle dispersions are colloidal suspensions. The behavior of such solutions is dictated by the rules of interface and colloid science [97].

Due to their high surface-to-volume ratio, nanoparticles tend to aggregate in order to lower their interfacial energy. The destabilization is caused by interparticle attractions. The dispersion state of nanoparticle solutions is an important factor while synthesizing nanoparticles for biomedical applications because it tends to modify their physico-chemical properties. The DLVO theory (named after Dejaguin, Landau, Verwey and Overbeek) [98–104] describes the stability of colloid suspensions and will be presented later.

Production of stable nanoparticle suspensions is challenging and primordial for biomedical applications. Stability of nanoparticles in a colloidal system is related to their tendency to aggregate under the action of gravity. Destabilization results from attractive van der Waals interactions.

One way to stabilize nanoparticles is to introduce interparticle repulsions that avoid van der Waals attraction. This can be achieved (A) by charging particles so that the surface charge leads to interparticle repulsion forces or (B) by covering the nanoparticles with big molecules to create interparticle steric repulsions.

The first strategy is known as electrostatic stabilization and the second is the steric stabilization (Fig. 5.10).

Fig. 5.10

Representation of the two possible ways for particle stabilization. a Electrostatic stabilization, b steric stabilization

5.3.1 Stability of Charged Nanoparticles

This section highlights the factors that control stability. Nanoparticles can acquire a charge by four different mechanisms [97]:

• ionization of surface groups where the degree and nature of which are controlled by the pH of the solution,

• ion adsorption as for instance the adsorption of a cationic surfactant,

• isomorphous substitution,

• dissolution in ionic solids.

To understand the stabilization mechanism, it is primordial to study what happens when a charged particle is in solution. Let’s take the case of a positively charged nanoparticle. When placed in a solution containing inert ions, anions will be attracted by the nanoparticle surface and cations repelled from the interface. Electroneutrality is reached when the magnitude of the negative charge of the layer near the interface is equal to the magnitude of the nanoparticle’s surface charge.

This charged surrounding is known as the electrochemical double layer. It is characterized by a potential drop across the solid/liquid interface which is described by the Stern-Gouy-Chapman (SGC) model. According to this latter, the surface charge is counterbalanced by two distinctive regions, the Helmholtz layer (or inner layer) and the Gouy-Chapman layer (or diffuse layer) (Fig. 5.11). They both contribute to the total interfacial charge density.

Fig. 5.11

Representation of the electrochemical double layer in accordance with the SGC model. Reproduced from Ref. [105]

The innerlayer includes the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP). Ions directly adsorbed to the surface are located in the IHP and OHP is defined as the plane of closest approach of fully solvated ions. The charge in OHP is counterbalanced by a dynamic ionic atmosphere forming the diffuse layer.

The surface potential at the diffuse layer is called zeta potential (ε) and is an experimentally measurable value.

The SGC model considers that the solvent is a dielectric continuum and that ionic species are non-interacting point charges. It assumed that the total interfacial charge density depends on ionic species concentration in solution and on the permittivity of the solvent.

Stability of a whole colloidal system can be predicted by studying what happens when two nanoparticles come close together because of the Brownian motion. The colloidal pair potential governs the stability of colloidal dispersions, it is defined as the potential energy of interaction between two colloidal particles. The most relevant components for interaction between charged particles are the van der Waals and the repulsive forces (Fig. 5.12).

Fig. 5.12

Representation of two colloidal particles coming close together

The potential energy resulting from attractive forces (U_A) is proportional to the particle radius (a), a material constant named the Hamaker constant (A) and the inverse of the distance of separation (h) [106]. It is given by equation.

$$ U_{A} = - \frac{A}{12}\left[ {\frac{1}{x(x + 2)} + \frac{1}{{(x + 1)^{2} }} + 2\ln \frac{x(x + 2)}{{(x + 1)^{2} }}} \right] $$

where χ = h/2a

If the particles are very close (ℎ ≪ 2a), then the last equation can be simplified and the potential energy becomes,

$$ U_{A} = - \frac{Aa}{12h} $$

The Hamaker constant depends on the electronic polarisability and the density of the material. Table 5.4 provides the value of the Hamaker constant for some media and materials.

Table 5.4 Value of the Hamaker constant of some components [99]

$$ A = \left( {\sqrt {A_{particle} } - \sqrt {A_{medium} } } \right)^{2} $$

The electrostatic repulsion is an important stabilization mechanism of charged nanoparticles dispersed in aqueous solutions. When two of them come close together, two cases exist: their surface charge doesn’t change and the surface potential compensates or the opposite.

Hogg, Healy and Fuersteneau have developed equations describing the electrostatic interaction between two non-identical particles under constant charge $\left( {U_{R}^{\sigma } } \right)$ and constant potential $\left( {U_{R}^{\psi } } \right)$ [107]

$$ \begin{aligned} U_{R}^{\psi } & = \frac{{\varepsilon a_{1} a_{2} \left( {\psi_{01}^{2} + \psi_{02}^{2} } \right)}}{{4\left( {a_{1} + a_{2} } \right)}}\left[ {\frac{{2\psi_{01} \psi_{02} }}{{\psi_{01}^{2} + \psi_{02}^{2} }}\ln \left( {\frac{{1 + e^{ - \kappa h} }}{{1 - e^{ - \kappa h} }}} \right) + \ln \left( {1 + e^{ - 2\kappa h} } \right)} \right] \\ U_{R}^{\sigma } & = \frac{{\varepsilon a_{1} a_{2} \left( {\psi_{01}^{2} + \psi_{02}^{2} } \right)}}{{4\left( {a_{1} + a_{2} } \right)}}\left[ {\frac{{2\psi_{01} \psi_{02} }}{{\psi_{01}^{2} + \psi_{02}^{2} }}\ln \left( {\frac{{1 + e^{ - \kappa h} }}{{1 - e^{ - \kappa h} }}} \right) + \ln \left( {1 - e^{ - 2\kappa h} } \right)} \right] \\ \end{aligned} $$

where $\kappa^{ - 1}$ is the Debye length defined as the thickness of the electrical double layer, ε is the dielectric constant of the suspending medium, ψ₀₁, ψ₀₂ are the total double layer potentials of the respective particles, h is the distance between the particles and a₁ and a₂ are the particle radii.

For two identical particles, a₁ = a₂ = a, ψ₀₁₌ ψ_{02 =} ψ₀ and these two last equations become,

$$ \begin{aligned} U_{R}^{\psi } & = \frac{{\varepsilon a\psi_{0}^{2} }}{2}\ln \left( {1 + e^{ - \kappa h} } \right) \\ U_{R}^{\sigma } & = \frac{{\varepsilon a\psi_{0}^{2} }}{2}{ \ln }\left( {1 - e^{ - \kappa h} } \right) \\ \end{aligned} $$

and if $\kappa {\text{a}} < 3$, then the general expression is given by equation

$$ U_{R} = 2\pi \varepsilon \alpha \psi_{0}^{2} e^{ - \kappa h} $$

According to the DLVO theory, the colloidal stability of charged particles relies on the total potential resulting from the interaction of colloidal particles (Fig. 5.13). It is equal to the linear sum of the attractive and the repulsive potentials. The attractive potential results from attractive van der Waals forces and the repulsive one from repulsive electrical double layer forces. The result is a typical curve for charge-stabilized colloid particles.

Fig. 5.13

Representation of the DLVO theory showing the interparticle interaction as the result of the van der Waals attraction and the electrostatic repulsion

$$ U_{T} = U_{A} + U_{R} $$

From low to high interparticle distance, the shape of the curve presents, first, a primary minimum leading to irreversible coagulation (aggregation), then, a potential energy barrier and finally, a secondary minimum resulting in reversible flocculation (Fig. 5.14).

Fig. 5.14

Shape of the total potential energy (U_T) as a function of the distance (x)

Nevertheless, while the DLVO theory works reasonably well to predict the stability of colloidal solutions at law salt concentrations (< 5 × 10⁻² M), it has been shown to fail at higher ionic salt concentrations. The magnitude of forces varies greatly by changing counter-ion and is not in good agreement with the experiment anymore. This is explained by the fact that the theory ignores ion effects and dispersion forces acting on the ions.

At low salt concentration, electrostatic forces dominate so that the DLVO theory that describes interparticle interaction as the result of attractive VDW forces and repulsive electrical double layer forces is sufficient. But at higher salt concentration, buffer specificity is influent, dispersion forces are expected to dominate electrostatic forces and must be treated at the same level [108]. This is a major problem particularly when samples are devoted to biomedical applications because biological salt concentrations are more or less equal to 0.1 M. It can be solved by properly including dispersion forces acting on ions.

5.3.2 Steric Stabilization

The colloidal stability resulting from steric stabilization, for example the adsorption of polymer chains on the particle surface, is based on steric hindrance. The total potential function is the linear sum of the attractive potential (U_A) due to van der Waals interaction and a steric potential (U_s) and is given by the following equation.

$$ U_{S} = 4\pi akT(0.5 - \chi )\Gamma ^{2} e^{{\left( {1 - \frac{h}{2\sigma }} \right)}} $$

where h is the interparticle distance, Γ is the amount of adsorbed or grafted polymer, σ is the layer thickness and $\chi$ is the Flory polymer/solvent interaction. In the ideal case of stability, $\chi < 0.5$ and U_S is repulsive; if $\chi > 0.5$, U_S is attractive and the solution is not stable.

$$ {\text{U}}_{\text{T}} = {\text{U}}_{\text{A}} + {\text{U}}_{\text{S}} $$

5.3.3 The Surface of Iron Oxide Nanoparticles

In order to describe the different stabilization methods, it is essential to define the surface nature of iron oxide nanoparticles. During the nanoparticle formation, surface is covered by epoxy functions which naturally react with water molecules to form hydroxyl groups on surface (Fig. 5.15).

Fig. 5.15

Schematic representation of hydroxyl formation on iron oxide surface

The hydroxyl groups on nanoparticle surface exhibit amphoteric properties and thus their charge can be modified with pH variation. These charge modification can modulate the surface with positive or negative charges as illustrated by following relations:

As a function of pH, acido-basic equilibriums are:

$$ \begin{array}{*{20}c} {{\mathbf{Fe}}{ - }{\mathbf{OH}}_{2}^{ + } \leftrightarrow {\mathbf{Fe}}{ - }{\mathbf{OH}} + {\mathbf{H}}^{ + } } & {{\text{with}}\;k_{a}^{ + } = \frac{{[{\text{Fe}} {-} {\text{OH}}]\left[ {{\text{H}}^{ + } } \right]}}{{\left[ {{\text{Fe}} {-} {\text{OH}}_{2}^{ + } } \right]}}} \\ {{\mathbf{Fe}}{ - }{\mathbf{OH}} \leftrightarrow {\mathbf{Fe}}{ - }{\mathbf{O}}^{ - } + {\mathbf{H}}^{ + } } & {{\text{with}}\;k_{a}^{ - } = \frac{{[{\text{Fe}} {-} {\text{O}}^- ]\left[ {{\text{H}}^{ + } } \right]}}{{[{\text{Fe}} - {\text{OH}}]}}} \\ \end{array} $$

The superficial electrical charge (q₀) is given by:

$$ q_{0} = \left( {\frac{F}{S}} \right)\left\{ {\left[ {{\text{Fe}} {-} {\text{OH}}_{2}^{ + } } \right] - \left[ {{\text{Fe}} {-} {\text{O}}^{ - } } \right]} \right\} $$

where S is nanoparticle area by volume unit (m² L⁻¹), F is Faraday constant (96,500 C mol⁻¹) and the second factor correspond to the difference between the concentrations of both charged species (mol L⁻¹).

The charge of nanoparticle surface depends on the pH and on the ionic force of the solution: the superficial electrical charge can be positive, negative and neutral as a function of the medium condition. Zero Point Charge (ZPC) corresponds to the pH value which the electrical charges cancel out [109]. ZPC depends on material nature and is represented as an average between the dissociation constants of species present on surface:

$$ ZPC = \frac{{pk_{a}^{ + } + pk_{a}^{ - } }}{2} $$

In the case of magnetite (Fig. 5.16), ZPC is around to 6.5 [110–112]. Below the ZPC, hydroxyl sites absorb the protons and become positive by the formation of FeOH₂ ⁺ species. Above the ZPC, the protons are desorbed and the deprotonation leads to negative species Fe–O⁻.

Fig. 5.16

Schematic representation of the charges present on nanoparticle surface as a function of pH

When iron oxide nanoparticles are put in contact with a solution, a biphasic system is created with solid/liquid interface. At this interface, some phenomena can appear and influence the colloidal stability, especially, the creation of a double electrical layer.

In the presence of water molecules, the surface can be hydrated by the surrounding water molecules. A water layer will thus be absorbed on nanoparticle surface and the structure is stabilized by hydrogen bonds as shown in Fig. 5.17 [113]. Depending on the oxide nature, the thickness of solvated layer can vary from 1 to10 Å.

Fig. 5.17

Schematic representation of solvated surface of iron oxide nanoparticles

The surface depends on two parameters: the charge and the hydration states. However, it is also important to distinguish the difference between the ZPC and the isoelectric point (IEP). ZPC is the pH value at which the surface is neutral by the absence of positive or negative charges. On the contrary, the isoelectric point is the pH value when the surface charge is exactly offset by the counter-ions.

The structure of the double electrical layer is described by different models [114–119]. In our work, Stern model was used to explain the composition of the interface between the nanoparticles and the solution (Fig. 5.18). It is based on the models of Helmholtz and Gouy-Chapman. The double electrical layer is composed of two distinct layers: a compact layer (Stern layer) and a diffuse layer (Fig. 5.18b).

Fig. 5.18

a Schematic representation of the electrical double layer and the evolution of electrostatic potential as a function of the distance with negative charges on particle surface. b Schematic representation of the water absorption on the surface of iron oxide nanoparticles where A⁻ and C⁺ are the anions and the cations present in solution respectively. IHP is the interne Helmholtz plan and OHP is the Outer Helmholtz plan. Wih pemission of Ref. [120]

The compact film of the double electrical layer corresponds to the Stern layer in which the ions (H⁺ or OH⁻) can protonate or deprotonate the hydroxyl groups bonded on iron oxide surface. The Stern layer can be separated into two sets:

• (A) The first part of Stern layer is composed of ions presenting a high affinity with surface. Their main role is to neutralize charges of the surface. These ions are away from the surface until a distance called Interne Helmholtz Plan (IHP).

• (B) The second part of Stern layer can be considered as a frontier between the IHP layer and the diffuse layer. This layer is included in the Stern layer until the Outer Helmholtz Plan (OHP).

Beyond the Outer Helmholtz Plan, the second layer of the double electrical layer starts. This one is called the diffuse layer and is mainly composed of the counter ions. The global electric charge of the diffuse layer leads to the charge compensation, inducing the electric neutrality of the nanoparticles. These different layers explain why the hydrodynamic diameter of nanoparticles is always larger than the diameter measured by the transmission electron microscopy.

The electrostatic potential decreases as a function of the distance with the nanoparticle surface. However, this decrease is more affected by the Outer Helmholtz plan also called slipping plan as illustrated in Fig. 5.18a. At this plan, the potential observed is named the zeta potential (ξ). This value is the unique measurement of an observable and measurable potential. It is used as a reference to evaluate the stability of colloidal suspension.

The magnitude of zeta potential gives an indication of the stability of a nanoparticle suspension. In the case of a high value of zeta potential (negative or positive), the colloidal suspensions are stable and the presence of agglomerates is not observed. On the contrary, when low values of zeta potential are observed, flocculation generally occurs in solution. This parameter is strongly influenced by pH value. Zeta potential value varies and allows thus to estimate the pH region where the suspension is stable or not. At a given pH value, the zeta potential is close to zero when all charges are compensated by the counter-charged molecules versus to the surface; this point is the isoelectric point (previously described). Although the pH is the most important factor influencing the zeta potential properties, the ionic strength of the media also affects the value of zeta potential, and especially, the nature of the ions interacting with the surface.

The thickness of the double layer depends on the ion concentrations in solution and can be correlated to the ionic strength of the medium. According to the ionic force, the double layer can be collapsed (at high ionic force) or extended (at low ionic force).

Two kinds of ions can interact with a charged surface:

• The nonspecific ions exhibit a low electrostatic interaction with the surface. When their concentrations increase in solution, the thickness of the electrical double layer is reduced without any effect on the value of the isoelectric point;

• The specific ions are attracted towards the surface with the electrostatic force inducing their adsorption on the surface and the isoelectric point is thus modified. In some cases, the adsorption of specific ions can affect the nature on charge surface.

The knowledge and the study of the surface state give indications on the stability of colloidal suspensions. Moreover, the measurements of zeta potential are generally used to confirm the surface modification process.

5.3.4 Stabilization Strategies

The surface of nanoparticle is the first area interacting with the body fluids when iron oxide suspensions are injected. This is the major reason why the nanoparticle surface is extensively studied and characterized. The problematic of uncoated nanoparticles is their trend to agglomerate. This explains why the development of stabilization methods are required to promote the repulsive forces between the particles that are opposed to attractive Van der Waals forces. The strategy generally used is to insert capping agents on nanoparticle surface.

Two complementary approaches can be envisaged: electrostatic stabilization and steric stabilization (Fig. 5.19) [121–124].

Fig. 5.19

Schematic representation of stabilization strategies: electrostatic and steric protections

The goal of the surface protection is to decrease the interfacial energy in order to obtain a strong barrier that limits the distance of closest approach between nanoparticles. In the literature, the main coating agents are polymers (synthetic or naturals), small ionisable organic molecules, silica coating and biological molecules. Some examples are illustrated in Fig. 5.20.

Fig. 5.20

Summary of the main coating agents used for the stabilization of iron oxide nanoparticles

The following part briefly describes some examples for each stabilizing agent.

5.3.4.1 Silica

The most common strategy used for the surface modification is the formation of a silica shell. Protocols including trialkoxysilane molecules or tetraethyl orthosilicate (TEOS) are often reported [125]. The advantage of these molecules is that the silane groups can be covalently grafted onto nanoparticle surface through the reaction between the hydroxyl groups present on iron oxide surface and the alkoxysilane functions (–Si–O–R where R is methyl or ethyl groups) [126]. The cross-linking reactions induce the formation of a silica layer surrounding the particles [127]. A large choice of terminal functional groups (alcohol, amine, epoxy, thiol or carboxylate) [128–130] can be used to protect and stabilize iron oxide nanoparticles (Fig. 5.21).

Fig. 5.21

General silica coating method and TEM images of silica coated iron oxide nanoparticles [131]

Coating agents with precious metal (such as gold) can also provide an effective protection to avoid the surface oxidation and to reduce nanoparticle agglomeration in aqueous solution [132, 133].

5.3.4.2 Polymers

Another important and widely employed stabilizing agent is polymer (synthetic or natural). This type of coating allows a biocompatible and biodegradable surface but also improves the blood circulation times depending on the polymer nature [134, 135]. The polymers can be natural [136] or synthetic [134, 137].

One of the most used natural polymers is dextran [138]. This biocompatible and biodegradable polysaccharide can be strongly absorbed on nanoparticle surface, due to a strong interaction with hydrogen bonds formed between the hydroxyl functions on the polymer chains and the surface of iron oxide cores [139, 140]. Several MRI contrast agents have been elaborated with a dextran coating or its derivatives (carboxydextran and carboxymethyl dextran) [141–144]. Some dextran coatings are based on the cross-linked polymer after the nanoparticle attachment using epichlohydrin [145, 146]. The system has demonstrated a high circulation half-life in blood with no acute toxicity.

Duguet et al. [147–149] described the modification of the dextran structure with silane molecules. This strategy allows a covalent grafting on the surface. Other polymers can be used such as chitosan [150–152], gelatin [153], alginate [154] and pullulan [155] as stabilizing agents.

Another natural and biodegradable polymer, polylactic acid, has been used for the preparation of stable colloid suspension with a typical range of hydrodynamic diameter included between 10 and 180 nm [156, 157].

Among the synthetic polymers, the polyethylene glycol (PEG) is widely used due its properties such as the increase of the blood circulation time, the hydrophilicity and the biocompatibility. PEG can also be used for the coupling with other polymer to increase the hydrophilic properties. Two approaches are currently used to coat nanoparticles and consist in the addition of the surfactants during the synthetic process or post-synthesis. Other polymers and copolymers which have been used to coat magnetic nanoparticles are polyvinyl alcohol (PVA) [158, 159], polystyrene (PS) [160, 161], poly(vinylpyrrolidone) (PVP) [162, 163], poly(acrylic acid) (PAA) [164], poly(ethylenimine) (PEI) [165, 166], PAA-chitosan [167], …

5.3.4.3 Small Organic Molecules

Small organic molecules are frequently used to obtain stable colloidal suspension. The chemical functions used generally are carboxylates, phosphates and sulfates due to their high affinity for iron oxide surface. These strong interactions result from an ionic interaction between the acidic functions of the coating agents and the surface of nanoparticles. The most used carboxylic acids are citric and dimercaptosuccinic acids (Fig. 5.22) [168–172]. These polyacids form a stable colloidal suspension resulting from the high coordination on metal surface. However, the ionic bonds between the carboxylic functions and the iron oxide surface are labile and can be easily broken by the elevation of temperature or by carboxylic compounds presenting a much higher affinity with the surface.

Fig. 5.22

Some examples of the nanoparticle stabilization with small organic molecules as citric acid, dimercaptosuccinic acid and mono- or bis-phosphonate molecules

Phosphate and phosphonate derivatives also are promising stabilizing molecules. Their absorption on metal surface is very stable and they are able to form a strong interaction in aqueous solution [173–176]. Bisphosphonate compounds (Fig. 5.22) are preferred in order to anchor double functions on the metal surface, involving the strengthening of the nanoparticle stability [177–179].

5.3.4.4 Biological Molecules

The stabilization performed by biological molecules is not a common method. Some examples describe the surface covering with proteins such as avidin [180] or human serum albumin (HSA) [181, 182]. This process allows the formation of stable and biocompatible magnetic fluids.

5.3.5 Recent Advances on the Stabilization of Hydrophobic Iron Oxide Nanoparticles

The biocompatibility and the colloidal stability in water are primordial characteristics in order to use nanoparticles for clinical applications. Iron oxide nanoparticles synthesized in organic medium present the advantages to have controlled magnetic properties depending on the size and on the size distribution.

However, it is essential to modify the surface in hydrophilic one by the use of appropriate coating.

Two main ways are: ligand addition or ligand exchange (Fig. 5.23).

Fig. 5.23

Transfer of hydrophobic iron oxide nanoparticles in water media thanks to the ligand addition or ligand exchange

5.3.5.1 Stabilization by Ligand Addition

The ligand addition approach consists in the addition of amphiphilic molecules interacting with the initial stabilizing agent. The hydrophobic segments interact with the post-synthesis hydrophobic capping groups and the hydrophilic segments contribute to a better dispersion of nanoparticles in water. As a result, this interaction forms a double layer on the nanoparticle surface (Fig. 5.23). In this way, the stabilizing molecules are considered as phase transfer agents that allow the transition from an organic phase to an aqueous solution. The reported main surfactants are polymers, alkylammonium salts and lipids; this part briefly describes some main examples.

5.3.5.1.1 Polymers

Many strategies have been developed using polymers as a surfactant. An example widely used is pluronic copolymers composed of a hydrophobic poly(propylene oxide) (PPO) chain flanked on each side with hydrophilic poly(ethylene oxide) (PEO) chains [183–187]. This is a triblock polymer (PEO-PPO-PEO) where hydrophobic PPO segments are located between the oleic acid chains and the PEG segments (Fig. 5.24).

Fig. 5.24

Water stabilization of iron oxide nanoparticles by pluronic polymers (with permission of reference [184])

Several similar processes have been described in the literature for diblock or triblock copolymers [188–192].

5.3.5.1.2 Alkylammonium Salts

Another common class of phase transfer agents is alkylammonium compounds. For example, cetyltrimethylammonium bromide (CTAB) is usually used [193, 194] as shown in Fig. 5.25.

Fig. 5.25

Ligand addition of CTAB on oleic acid coated iron oxide nanoparticles

A better water dispersion is observed when CTAB interacts with initial oleic acid coating. The nanoparticle size is inevitably increased.

5.3.5.1.3 Lipids

A similar approach is exploited by the addition of lipids (Fig. 5.26) [195, 196] on nanoparticle surface. Phospholipids and fatty acids are the most widely used as amphiphilic ligands in order to transfer hydrophobic particles in water.

Fig. 5.26

Stabilization of hydrophobic nanoparticles by the use of fatty acids or phospholipids

Deng et al. [197] reported a strategy to functionalize the hydrophobic surface of iron oxide nanoparticles with phospholipids such as 1,2-dioleyl-snglycero-3-phosphoethanolamin-N(succinyl) (carboxylated phospholipid). This coating was selected for several reasons. This phospholipid is amphiphilic, biocompatible and able to disperse nanoparticles in water when it interacts with long carbon chains initially present on nanoparticle surface. Other phospholipids as phosphatidylcholine and phosphatidylethanolamine were also used [198].

Fatty acids (with 10–18 carbons) are also used to form a hydrophobic-hydrophilic bilayer for the transfer of hydrophobic nanoparticles in water [199, 200].

In summary, ligand addition transfers hydrophobic particles in water by the use of amphiphilic compounds (polymers, alkylammoniums, lipids, …). The strong hydrophobic interaction of amphiphilic molecules with nonpolar organic surface of nanoparticles leads to a complete encapsulation of the magnetic core and preserves the original coating. As a result, the agglomeration phenomena are generally minimized due to the conservation of the initial coating and a strong steric stabilization. However, some drawbacks are observed such as the increase of nanoparticle size due to the addition of new coating and the destabilization of double layer by the elevation of temperature [159].

5.3.5.2 Stabilization by Ligand Exchange

Ligand exchange is defined as a replacement of initial capping agent with other coating, which is put onto the nanoparticle surface through exchange reactions. This approach is considered as a most versatile process to provide a better colloidal stability. According to the ligand nature, the link between the coating molecules and the nanoparticle surface can be covalent or ionic. In the following part, some major examples are described such as the ligand exchange by small charged molecules, by polymers or by silica.

5.3.5.2.1 Small Charged Molecules

The main small charged molecules are carboxylic acid derivatives. As previously described, the interaction between the nanoparticle surface and the carboxylic acid of oleic acid can be easily broken by temperature increase or by exchange with another carboxylic acid compound. An easy way is to select carboxylate molecules containing many carboxylic functions so that it could be anchored by two or more sites on the particle. Citric acid [159, 201] and dimercaptosuccinic acid (DMSA) [202–205] are the most widely used polyacids in which a multiple coordination of carboxylate functions on magnetic core can be observed. These ligands give a better stability in water by a strong ionic interaction.

Tetramethylammonium hydroxide (TMAOH) is a second common transfer agent used in order to replace oleic acid. This small molecule electrostatically stabilizes nanoparticles in water. The phase transfer is composed of a mixture between nanoparticles dispersed in non-polar solvent and an aqueous TMAOH solution. A double phase is created and the solution can be mixed either by magnetic agitation or by the use of ultrasounds [206–208].

These strategies are generally easy to perform. However, their stabilization is often not sufficient in order to conserve nanoparticle suspensions during long time storage.

5.3.5.2.2 Polymers

Polymers have been also widely used for the surface modification by ligand exchange. Firstly, the polymer chains can be attached on nanoparticle surface thanks to available chemical functions to create a strong interaction between the nanoparticles and the polymer.

Several chemical functions [209] can be used to graft the polymer on the surface. Amongst others, we can quote dopamine [210–212], cysteine [213], phosphonic acid, carboxylic acid [214] and alkoxysilane [128, 129, 215] (Fig. 5.27).

Fig. 5.27

Main chemical functions for the attachment of polymers on nanoparticle surface. With permission of Ref. [134]

As an example, the PEGylation of oleic acid coated nanoparticles can be performed via the reaction with dopamine modified PEG diacid (DPA-PEG-COOH) (Fig. 5.28). The first step is the modification of PEG diacid derivative with dopamine. Then, the resulting DPA-PEG-COOH is used to replace oleate compound and the particle surface is converted from hydrophobic to hydrophilic [212, 216].

Fig. 5.28

Surface modification of oleic acid coated nanoparticles with dopamine modified PEG diacid and TEM images before and after their surface modification [212]

Another method to form water-soluble iron oxide nanoparticles from the preparation of high thermal decomposition is to combine two ligand exchange strategies (Fig. 5.29) (small charged molecules and polymers) [159].

Fig. 5.29

General scheme of stabilization strategies described by Lattuada et al. [217]. Steps 1A and 1B: ligand exchange reactions with polyelectrolytes. Step 2: acylation of hydroxyl groups to prepare the surface with polymerization initiators. Step 3A: surface-initiated the polymerization with l-lactide. Step 3B: polymerization start up via the addition of monomers. Step 4: deprotection or additional reaction after polymerization. Step 5: grafting of end functionalized PEG chains onto nanoparticle surface using amidation chemistry. With permission of Ref. [217]

In the first steps, authors proposed the use of polyelectrolyte compounds such as ricinoleic acid or citric acid to remove oleic acid on nanoparticle surface, ensuring the nanoparticle dispersion in water (Fig. 5.29). The second step promotes the formation of a polymer shell. Two general pathways are described: (i) the first one links the polymer chains by the reaction of carboxylate functions available on nanocrystal surface or (ii) the polymerization process can be directly initiated on nanoparticle surface by addition of initiator.

Another alternative is to combine the effect of small charged molecules and polymers in using a unique compound. Short hydrophilic polyelectrolyte molecules such as poly(acrylic acid), poly(allylamine) or poly(sodium styrene sulfonate) exhibit several functional groups presenting a strong coordination on nanocrystal surface and are thus able to remove initial hydrophobic coating.

For example, Ge et al. [216, 218] proposes a simple process using polyacrylic acid. This method is performed at high temperature in glycol solvent (Fig. 5.30). Due to a high boiling point (245 °C), a high dissolving capability for polyelectrolytes and a high miscibility with water or with organic solvents, diethylene glycol is widely used.

Fig. 5.30

Schematic representation of ligand exchange of oleic acid with polyacrylic acid in diethylene glycol [216, 218]. Reproduced with Ref. [216]

This approach presents several advantages such as:

• the high reaction temperature favors the desorption of oleic acid;

• the polyelectrolyte bonds are strong with multiple anchoring points on nanoparticle surface;

• due to the presence of the large quantities of the polar end functionalities, the final particles are water-soluble.

5.3.5.2.3 Silica

Silica coating is extensively used for nanoparticle stabilization in water. Indeed, silica shell is chemically inert and biocompatible [219]. Due to the ability to interact covalently with the surface, they are excellent candidates.

Major methods described in the literature for the silanization of hydrophobic nanoparticles can be separated into three general approaches:

• Direct silanization in non-polar solvents or a mixture of polar and non-polar solvents;

• Silanization by reverse microemulsion;

• Two-steps silanization: phase transfer followed by silanisation.

For the direct silanisation, several research groups proposed to replace oleic acid initially present on nanoparticle surface by organosilanes.

De Palma et al. [127] described a versatile method to functionalize the nanoparticles with different silane coupling agents. This group developed a method consisting in ligand exchange with oleic acid and silanes bearing different chemical end (Fig. 5.31). Briefly, the reaction occurs by the mixing of particles dispersed in n-hexane and selected trialkoxysilane in presence of acetic acid. The resulting solution is then mixed during three days at room temperature. Numerous silanes with different chemical functions were used such as amine, aldehyde, thiol, carboxylic acid and cyano groups. The authors reported the efficient phase transfer of nanoparticles in water.

Fig. 5.31

Ligand exchange method to make hydrophilic iron oxide nanoparticles with silane agents. Reproduced with permission of Ref. [127]

Another similar approach was proposed by Huang et al. [220]. The reaction is performed in tetrahydrofuran (THF) at 50 °C during a time varying from 100 to 210 min. Two types of silane were tested: N-(6-aminohexyl)-aminopropyltrimethoxysilane (AHAPS) and 3-(triethoxysilyl)propylsuccinic anhydride (TEPSA). Interestingly, this strategy keeps the initial narrow size distribution (Fig. 5.32).

Fig. 5.32

TEM images of iron oxide nanoparticles stabilized with AHAPS from a THF, b ethanol or c water at pH = 3. With permission of Ref. [220]

The ligand exchange with silane is time consuming. Therefore, Bloemen and collaborators [221] recently proposed an alternative method to reduce the reaction time. Thanks to the sonication during 5 h, the nanoparticles are dispersed in toluene and are mixed with trimethylamine and selected silane. After the reaction, a silane coating was formed.

Although the direct silanization is an efficient method to stabilize the nanoparticles, the control of the silica layer thickness is a poorly controlled parameter.

Consequently, the reverse microemulsion system was investigated in order to control this parameter. The reverse micelle system can be compared to nanoreactors allowing the control of the size, the shape and the reactant concentration.

Widely inspired by the works on hydrophobic semiconductor nanoparticles [222–225], Narita et al. [226] reported an easy method to control the silica layer thickness by tuning the reaction time and y limiting the reaction rate rather than by changing the solution components. Influenced by the reaction time and the pH, the thickness of silica layer can be tuned from 2 to 14 nm. However, the formation of silica coated particles with multiple magnetic cores has been observed.

Recently, Ding et al. [227] described a similar approach. The preparation of a single magnetic core encapsulated with different silica shell thicknesses for different size of iron oxide nanoparticles was reported. The reverse microemulsion system used is composed of five compounds: ammonia, igepal-CO520, TEOS, cyclohexane and the hydrophobic iron oxide nanoparticles. IgepalCO520 is an amphiphilic copolymer made of hydrophilic polyethyleneglycol chains and a short hydrophobic alkylphenyl derivative. It is used as a surfactant to form reverse micelle in cyclohexane.

TEOS can be used in order to initiate the formation of silica shell after its hydrolysis by ammonia in solution. The general reaction is illustrated in Fig. 5.33.

Fig. 5.33

Schematic representation of the silica coating mechanism on the surface of iron oxide nanoparticles via the reverse microemulsion method published by Ding et al., with permission of Ref. [227]

Thanks to the control of experimental parameters, authors demonstrated the ability to produce silane coated iron oxide nanoparticles with thickness control. Additionally, the silane coverage of magnetic particles presenting different sizes was also proven (Fig. 5.34).

Fig. 5.34

TEM images of silica coated iron oxide nanoparticles: a with different silica shell thicknesses of 2 nm (a), 6.3 nm (b), 14.1 nm (c) and 19.8 nm (d) and b with magnetic core sizes of 10.1 nm (e), 13.5 nm (f) and 19.1 nm (g). With permission of Ref. [131]

Reverse microemulsion system is an efficient and innovative method to produce nanoparticles covered with a controlled silica shell. Although the silane thickness can be modulated, the preparation of a large quantity needs the use of large surfactant and solvent amounts. Moreover, nanoparticles initially prepared with TEOS often require an additional step in order to functionalize the surface for future targeting [226].

The last method uses two steps. Typically, the first step is a ligand exchange or a ligand addition in order to transfer hydrophobic particles in water. The second step involves the formation of silica shell.

For example, Kim et al. [193, 228] developed a two-steps method to modify hydrophobic nanoparticles with cetyltrimethylammonium bromide (CTAB) in order to disperse nanomaterials in water (Fig. 5.35). The carbon chains of CTAB interact with oleic acid chains through hydrophobic interactions and the polar group of CTAB (ammonium function) is oriented towards aqueous solution. This first step is a phase transfer via a ligand addition. The subsequent reaction is a ligand exchange in order to form silica coating by addition of TEOS performed in water medium.

Fig. 5.35

Schematic representation of two-steps silanization and TEM image of the resulting materials. Reproduced with permission of Ref. [228]

Some authors used this method of phase transfer with polymers as a primary step before the silanization. As an example, Shen et al. [229] proposed the use of silane modified poly(ethylene glycol). Firstly, methoxy-poly(ethylene glycol) silane (MPEG-sil) replaces oleic acid. The nanoparticles are thus coated with polymer and can be dispersed in a more hydrophilic media. The silane function present on the polymer allows the formation of silica coating by the addition of TEOS.

In a similar way, it has been reported that the primary ligand exchange step can be also realized by poly(acrylic acid) (PAA) [230]. After the first ligand exchange, nanoparticles can be dispersed in an aqueous ethanol solution and then, the particles can be easily encapsulated with a uniform silica coating using TEOS as a silica source.