Magnetoliposomes: synthesis and application

8/9/2019 Magnetoliposomes: synthesis and application

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Ana Rita Oliveira Rodrigues

2013, April



INDEX

1.

Magnetoliposomes 1

1.1 Liposomes 2

1.2 Magnetic nanoparticles 5

1.3 Biomedical applications 8

2. Preparation techniques 11

2.1 Synthesis of magnetic nanoparticles by soft chemical methods 11

2.1.1 Nickel nanoparticles 12

2.1.2 Iron oxide magnetic nanoparticles 14

2.1

Liposomes preparation 17

2.2.1 Synthesis of aqueous magnetoliposomes by ethanol injection 18

2.2.2 Synthesis of dry magnetoliposomes 19

References 20



Magnetoliposomes: synthesis and applications MAP-fis

2

The flow of the MLs in blood results from the competition between magnetic

forces of the magnetic nanoparticles in blood circulation and magnetic forces applied by

the external magnetic field. The magnetic fluid is localized in the site of interest when

the magnetic forces are exerted on linear blood flow of the arteries or capillaries [8]. To

improve cell uptake, cationic amphiphiles (composed of biodegradable moieties and

specifically designed to exert minimal cytotoxic effects) can be inserted into

magnetoliposomes coat [9]. In drug delivery, after the localization of the

magnetoliposomes in the therapeutic site, drug liberation can be induced by different

physico-chemical environment variations, such as pH or temperature changes.

The magnetic force, important for magnetophoresis applications and the spin –

spin relaxation time depend on the nanoparticles concentration, and to how close they

are from each other [10]. This way, controlling the number and the time of encapsulated

magnetic nanoparticles in the magnetoliposomes is a key issue.

For in vivo applications, magnetoliposomes must be physically and biologically

stable in order to extend blood time circulation and improve their efficacy. The

encapsulated nanoparticles in MLs allow avoiding earlier dilution and undesirable

interactions with biological structures. There are some chemical adjustments that can be

made in liposomes in order to improve their physical and biological stability; this will

be discussed in the next chapter. The bioavailability of the magnetoliposomes is also

important for in vivo applications. That is, realize if the magnetic fluid leaves the

microcirculation and diffuses into the interstitial space, or if it remains unchanged in the

systemic circulation.

1.1 Liposomes

Liposomes were discovery in 1965 by Bangham, who found that when

phospholipids were dispersed in water, vesicular structures of hydrated bilayers with a

aqueous cavity are form spontaneously [11] (figure 2). This process is known as self-

assembling, a bottom-up mechanism of liposome synthesis that occurs due to different

interactions, such as, hydrophilic/hydrophobic electrostatic and van der Waals

interactions.

Liposomes act as encapsulation and transport system that can incorporate differentsubstances as nutrients, genes and drugs. Due to their amphipathic composition,




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incorporated substances can be either hydrophilic and/or hydrophobic, the first ones will

be incorporated in the aqueous cavity and the second inserted or adsorbed on the

membrane by solubilization in the lipid bilayer or by covalent linkage to active surface

groups in the liposome membrane. Anti-tumor drugs can also be transported by

magnetoliposomes using bifunctional reagents [12], taking advantage of magnetic

targeting.

Figure 2. Structure of liposomes and phospholipid molecule.

Liposomes are structurally flexible in size, composition and fluidity [13]. The

choice of phospholipids determines the rigidity and the charge of the bilayer. Saturated

phospholipids with long acyl chains form a rigid, rather impermeable bilayer structure,

while unsaturated species from natural sources (egg or soy bean phosphatidylcholine)

give much more permeable but less stable vesicles. Phase transition of the liposomes is

also an important characteristics. It is affected by the hydrocarbon length, unsaturation,

charge and headgroup species of the phospholipids. It defines the temperature required

to induce a change in the physical state of the liposomes, from the ordered gel phase,

where the hydrocarbon chains are fully extended and closely packed, to the disordered

liquid crystalline phase, where the hydrocarbon chains are randomly oriented and fluid.

According to size and number of lamellae liposomes are divided into different

subcategories (table 1 and figure 3).




4

Liposomes are very important for biomedical applications as they can overcome

many of the problems associated with other systems used for therapy, such as those

involving solubility, pharmacokinetics, in vivo stability and toxicity [15, 16].

Liposomes stability is affected by different processes that can be chemical,

physical or biological. Chemical stability is related with liposome composition while

physical stability is associated with the aggregation and fusion of the vesicles. The

electrostatic repulsion reduces vesicle fusion and aggregation; this way, including

electrical charge in the liposomes will improve their physical stability. The addition of

cholesterol to the liposomes composition will prevent substance release as it will

decrease liposome permeability and improve their chemical stability. In fact, Egg-PC

(egg yolk phosphatidylcholine) liposomes with cholesterol, in (7:3) proportion, are

commonly used as models of cell membranes [17, 18]. On the other hand, biological

stability is concerned with interaction of the liposomes and so it is related with

administration pathway [19].

For in vivo applications liposomes must be very small (≤100 nm) in order to

reduce the recognition and phagocytosis, thereby increasing the probability of

penetration into the tissues of interest [20]. Recent studies have revealed that the

functionalization of the liposomes surface improves their efficacy. Functionalize the

lipid bilayer with FAB portions (a part of the antibody molecule that binds antigen),

make liposomes to be uptake only by cells that have specific antigens. In order to make

the system less recognizable by the immune system, a PEG (polyethylene glycol) crown

can be added. This will also provide stealth ability (long-circulating) and substance

accumulation in the site of interest [6]. It has also been reported that cationic liposomes preferably target vasculature [21], while anionic ones are captured by monocytes/blood

Table 1. Liposome classification based on structural parameters [14]. Figure 3. Schematic representation

of different liposomes.

http://www.all-acronyms.com/reverse/egg_yolk_phosphatidylcholine






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neutrophils [3]. This last behavior has been used to target the brain through the blood

brain barrier (BBB) [22].

1.2 Magnetic nanoparticles

Magnetic nanoparticles are a class of nanoparticles that have at least one

dimension less than 100 nm and one metallic component on its composition. This way,

its location can be manipulated with an external magnetic field. Due to their size,

nanoparticles have a large surface/volume ratio and thus the properties of mass and heat

transfer are better than in common materials [19].

Magnetic effects in the nanoparticles are caused by movements of particles as

electrons and protons that have both mass and electric charges; the spinning electric-

charged particle creates a magnetic dipole, so-called magneton. Magnetic behavior of

ferromagnetic materials like iron, nickel and cobalt arises from magnetic domain

structures. Domains result from the action of several interactions that exist in magnetic

materials, such as, exchange, anisotropy and dipolar interactions. Domains are regions

in a sample that has uniform magnetization where the individual magnetic moments are

aligned and point in the same direction [23]. When the size of a ferromagnetic material

decreases, it becomes single domain, where the magnetization is equal to the saturation

magnetization.

Since it is now accepted that magnetic fields are not especially contraindicated

for humans, the potential of magnetism has been recognized in many biological

applications [4]. Magnetic materials have attracted great interest in recent years because

of novel effects that arise due to size reduction, as is the case of superparamagnetism.

This type of magnetism is present in magnetic nanoparticles due to their size and it only

occurs below critical sizes in which nanoparticles are single domain. Critical sizes (dc)

of superparamagnetic nanoparticles depend on the material; iron nanoparticles have

superparamagnetic behavior below 20nm, while in nickel nanoparticles it occurs below

30nm [24, 25].

Superparamagnetic particles are characterized by an enormous magnetic

moment under an external magnetic field. However, when the external magnetic field is

removed, there is no remanescent magnetic moment and the net moment of the particles

is randomized to zero [26].




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Superparmagnetic behavior is easily observed by the hysteresis loop of the

material. Magnetic hysteresis is a function of the applied field at a given temperature

(M(H)). It is obtained on cycling the external field to values beyond the magnetic fields

where the magnetization reaches its saturation. The curves cross zero external field at

the remnant magnetization and the magnetization becomes zero at the coercive field.

Figure 4. Cobalt hysteresis for different size samples; the larger particles shows hysteresis while the smaller aresuperparamagnetic and do not have hysteresis [24].

Superparamagnetic nanoparticles do not present remnant magnetization and are

characterized by the absence of hysteresis and coercivity (red line in figure 4) [27]. As a

balance between magnetic energy and thermal energy, superparamagnetism occurs in a

limited range. The underlying physics of this magnetic behavior is founded on an

activation law for the relaxation time, τ, of the new magnetization of the particle. It

depends on KV/k bT, where K is the particle anisotropy constant, V is particle volume, k b

is Boltzmann’s constant and T is the absolute temperature. When the size (volume) of

the particles is sufficiently small, KV becomes comparable to the thermal energy, k bT,

and so magnetization of the particle fluctuates rapidly from one direction to another due

to thermal agitation, leaving no net magnetic moment. In this condition, particles

present superparamagnetic behavior [26, 28]. Below a certain temperature, the blocking

temperature, the thermal energy is not enough to allow the spins to easily rearrange,

coercivity appears and particles have a ferromagnetic behavior. Blocking temperature

has a time component and is often defined as the point at which a particle dipole is able




7

to reorient under the influence of a specified magnetic field in 100 s (a typical timescale

for a measurement). This temperature is important because it represents the maximum

in susceptibility and the lower limit of superparamagnetic behavior [24]. Blocking

temperature is an important characteristic for many magnetic applications that depend

on particle size. A wide particle size distribution will result in a wide range of blocking

temperatures, and so, in a non-ideal magnetic behavior for the magnetic fluid.

Iron and nickel are considered metals of biological interest as they present

magnetic properties at room temperature [29]. However, particles of these metals have

some issues as its toxicity, high reactivity and also the fact that they are easily degraded

due to high surface/volume ratio. In order to overcome these problems and make them

compatible for biological applications, core-shell structures are used. Core-shell

structure consists of a metal or metallic oxide core, encapsulated in an inorganic or a

polymeric coating; silica is commonly used (figure 5). Silica coating will separate the

particles, thereby preventing a cooperative switching. It will also adjust the magnetic

properties of nanoparticles, as the extent of dipolar coupling is related to the distance

between particles and this, in turn, depends on the thickness of the inert silica shell.

Figure 5. Representation of core-shell nanoparticles structure.

With this structure, the undesirable effects are avoided and the properties of the

magnetic nanoparticles, such as thermal and chemical stability and solubility, are

improved. Core-shell structures also allow the conjugation of other molecules [30]. For

in vivo applications, nanoparticles must have high magnetization so that their movement

in blood circulation can be easily controlled. Nanoparticles with superparamagnetic

behavior are preferred because the risk of forming agglomerates is negligible at room

temperature [31, 32]. They can be guided to a therapeutic site and are not subject to




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strong magnetic interactions in the dispersion. Also, they are readily stabilized in

physiological conditions [28].

Colloidal stability of the magnetic fluid is important and it can be controlled by

size, charge and surface chemistry of the nanoparticles. Magnetic nanoparticles must be

small in order to prevent precipitation from gravitation forces and must be stable in

water at pH 7 and in physiological environment. The size of nanoparticles depends on

the preparation methods and can be controlled using a surfactant [23]. Citric acid avoids

nanoparticles aggregation and can preserve monodipersity [33]. Surface charges, which

give rise to both steric and coulombic repulsions [34], can also be controlled.

1.3

Biomedical applications

The high biocompatibility and versatile nature of liposomes have made these

systems keystone components in many biomedical research areas. Liposomes can be

combined with a large variety of nanomaterials, such as magnetic nanoparticles which

have greatest applications in biomedicine (figure 6).

Because the unique features of both the magnetic colloid and the versatile lipid

bilayer can be joined, the resulting so-called magnetoliposomes can be exploited in agreat array of biomedical applications. In therapy, the most promising applications of

magnetoliposomes are magnetic controlled drug delivery and hyperthermia [6, 7].

Otherwise, in diagnosis, magnetic nanoparticles have been used as contrast agents in

MRI (magnetic resonance imaging) [5].

Figure 6. Biomedical applications of magnetic nanoparticles [36].




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Cancer is a disease that causes more than six million deaths per year worldwide

[19]. In pharmaceutical industry new cytotoxic agents for cancer treatment have been

produced. However, the administration of these drugs continues to be a problem

because of toxicity to healthy cells. To overcome this problem, magnetoliposomes have

been proposed as carrier systems that protect and transport the drug to the target site by

means of an external magnetic field.

Drug delivery based on magnetoliposomes is one of the most promising

applications with largest impact on the scientific community. The efficacy of

magnetically-controlled drug targeting (MDT) depends on physiological parameters

[35].

In order to specifically target cancer cells, it is possible to attach antibodies [37],

creating a subclass which is known as immuno-MLs, use PEG-folate conjugate to target

folate receptors (overexpressed in cancer cells) [38], and incorporate small peptides that

target specific membrane proteins [22]. Recently, it has been shown that the use of MLs

tagged with folate results in superior doxorubicin cell uptake [39]. The inclusion of the

anionic lipid cholesteryl hemisuccinate (CHEMS) in the magnetoliposomes allows them

to be pH-sensitive, which is important as the tumor cells have a lower pH than normal

cells. These liposomes fuse after the pH is lowered below a critical value between 4.0

and 6.7 [40]. It has also been reported that the incorporation of the tripeptide arginine-

glycine-aspartic acid (RGD) into liposomes resulted in specific and efficient binding of

the liposomes to integrin-expressing endothelial and melanoma cells [41].

The concomitant use of magnetic targeting greatly increased drug loading

efficiency [38], by use of a simultaneous biological and physical targeting. MDT has

been widely studied, due to the large potential and advantages, such as [42]:

the ability to target specific locations in the body;

the reduction of the quantity of drug needed to attain a particular

concentration in the vicinity of the target;

the decrease of the concentration of the drug at non-targeted sites, that

minimize severe side effects.

Magnetoliposomes have also been used in thermotherapy, as a chemotherapy

alternative [7]. Tumor cells are more sensitive to high temperatures than healthy ones




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[42, 43] and hyperthermia makes use of magnetic nanoparticles to deliver toxic amount

of thermal energy to targeted tumors. Therefore, magnetic nanoparticles are used as

hyperthermia agents. For a better temperature control, nanoparticles should be uniform

in size and shape. Also, they should be subdomain in order to absorb much more power

at tolerable AC (alternating current) magnetic fields [44, 45].

In hyperthermia, magnetic nanoparticles are placed in alternating current and so

magnetic fields randomly flip the magnetization direction between the parallel and

antiparallel spin orientation. This random orientation allows the transfer of magnetic

energy, in the form of heat, to the particles. Fixing the magnetic nanoparticles in a

tumor, this property can be used to increase the temperature of the tumor tissues and so

destroy the pathological cells by hyperthermia [23]. The greatest advantage associated

with thermotherapy is that it allows the heating to be restricted to the tumor area and so

healthy cells are preserved.

In diagnosis, magnetoliposomes are a valuable system for magnetic resonance

imaging (MRI). MRI is one of the most powerful non-invasive imaging modalities used

in clinical medicine today. Magnetoliposomes, with the benefits of the magnetic

nanoparticles, could improve image quality, as they are used as contrast agents to

enhance nuclear magnetic resonance. Magnetoliposomes based on magnetite have been

reported as the less toxic formulation, with a maximum load of 67pg Fe/cell, that

corresponds to a minimum of 50 cells/microliter detection limit by MRI [46], settling

the high efficient induced contrast already reported [47].

Reticuloendothelia system or mononuclear phagocyte system is a network of

cells, part of the immune system, responsible for remove foreign substances from

bloodstream located in reticular connective tissue. Tumor cells do not have the effective

reticuloendothelial system as healthy cells, and so their relaxation times are not altered

by contrast agents [5]. This theory has been used for diagnosis of malignant lymph

nodes [48], liver tumors [49], and brain tumors [50].

Body tissues contains lots of water, so, when a magnetic field is applied, the

average magnetic moment of the many water protons become aligned with the direction

of the field. When a radio frequency current is briefly turned on, an electromagnetic

field with resonance frequency is absorbed, flipping the spin of the protons in themagnetic field. When the field is turned off, the spins of the protons return to the




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thermodynamic equilibrium and the bulk magnetization is reorganized with the static

magnetic field. Protons in different tissues return to their equilibrium state at different

relaxation rates. The image is obtained from different variables, as spin density and T1

and T2 relaxation times (T1 is the spin-lattice or longitudinal relaxation time, and T2 is

the spin-spin or transverse relaxation time) that are used to create a MRI contrast which

relies on the differential uptake of different tissues.

2. Preparation techniques

Because of the widespread applications of magnetoliposomes in biomedicine and

engineering, much attention has been paid to the synthesis of different kinds of

magnetic nanoparticles [51-53] and liposomes [54].

Each potential application requires different properties. Synthesis methods of

magnetoliposomes and their constituents will determine their final shape, size

distribution, surface chemistry and magnetic properties [55-57].

2.1 Synthesis of magnetic nanoparticles by soft chemical methods

Magnetic nanoparticles with spherical shape can be synthesized by plasma

atomization, wet chemistry, from gas phases and aerosols. Depending on the

mechanism of formation, spherical nanoparticles obtained in solution can be crystalline

or amorphous, if they result from a disordered or ordered aggregation, respectively.

Synthesis method also determines a great extent of the structural defects or impurities in

the particle, as well as the distribution of such defects within the particles, therefore

determining their magnetic behavior [58, 59].

Techniques for magnetic nanoparticle synthesis have been developed to yield

monodispersed colloids, consisting of uniform nanoparticles both in size and shape. In

these systems, the entire uniform physicochemical properties directly reflect the

properties of single particles [60, 61].

As mentioned in the previous chapter, for biomedical applications, nanoparticles

with core-shell structure should be used in order to reduce adverse effects. After

purification, synthesized metal nanoparticles can be covered with a silica shell that can




12

be obtained by hydrolysis of TEOS (tetraethyl orthosilicate, a compound that is easily

converted into silicon dioxide). Different shell sizes are obtained by the addition of

different TEOS concentrations, either in a dispersed nanoparticle solution in ethanol, or

in AOT/cyclohexane. MDA (mercaptododecanoic acid) is usually used to promote the

binding between TEOS and the metal nanoparticles.

2.1.1 Nickel nanoparticles

Simple soft chemical methods have been widely used as they do not demand

extreme pressure or temperature control, are easy to handle, and do not require special

or expensive equipment. Nickel nanoparticles can be obtained with different soft

chemical methods, namely by the reduction of nickel chloride with hydrazine (N2H4):

Surfactants can act as size control agents in the synthesis of nanoparticles.

Superparamagnetic nickel nanoparticles with mean diameter between 10 and 36 nm

were obtained by the reduction nickel chloride with hydrazine in an aqueous solution of

the cationic surfactant CTAB (cetyltrimethylammonium bromide) [5] (figure 7).

However, size monodispersity is not tightly controlled. Nanoparticles size is influenced

by nickel chloride and hydrazine concentration.

Figure 7. Nanoparticles synthesis in aqueous solution of CTAB.

Size monodispersity can be improved using water-in-oil microemulsions in

metallic nanoparticles synthesis [62]. Microemulsion method is one among the various

low-temperature routes to tailor nanoparticles. It is a soft technique that can be used

with almost all chemical reactions which have been studied to obtain nanoparticles in

homogeneous solutions.




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A microemulsion is a stable dispersion of reverse micelles of water in an organic

solvent, that is, two immiscible liquids in the presence of a surfactant that form

nanometer-size water droplets which are dispersed in a continuous oil medium and

stabilized by surfactant molecules accumulated in the oil/water interface. These are

categorized as water-in-oil (w/o) microemulsions.

In this process, reagents are separated in two different microemulsions that are

then mixed under agitation. After mixing, the reverse micelles collide among

themselves to exchange the reactants solubilized in the nanoreactors (individual reverse

micelles) and then again break apart (figure 8). This way, the reagents undergo

homogeneous mixing and the polydispersity of the particles decrease. Decoalescence

ensures the presence of the protective coating for the controlled nucleation and growth

and it also prevents aggregation [63].

Figure 8. Mechanism showing the intermicellar exchange for the formation of nanoparticles.

The main problem associated with the synthesis of small magnetic nanoparticles

are the magnetic attractions of the metallic nanoparticles. This contributes to

agglomeration/coalescence into larger particles and their subsequent settling out of the

reaction environment. A novel route for the synthesis of smaller magnetic nanoparticles




14

has been proposed. This method is based on the reduction of metal chloride ionic

clusters in the confined space of the anionic surfactant AOT (bis(2-

ethylhexyl)sulfosuccinate) reverse micelles [64].

From this method, smaller nanoparticles, with diameters lower than 100 nm, were

obtained (figure 9). The resulting particles are covered with an AOT layer that prevents

particles agglomeration, due to electrostatic repulsions of the reverse micelles.

Figure 9. SEM images of Ni nanoparticles synthesized in the confined space of AOT reversed micelles.

2.1.2 Iron oxide magnetic nanoparticles

Iron oxide-based magnetic nanoparticles (Fe3O4, γFe2O3, …) are prepared through

bottom-up strategies, including co-precipitation, microemulsion approaches,

hydrothermal processing and thermal decomposition techniques.

Table 2. Summary comparison of different nanoparticles synthesis methods [66].

Iron-based nanoparticles can be obtained by the precipitation of suitable mixtures

of Fe(II) and Fe(III) chlorides (figure 10). Co-precipitation is a simple and suitable




15

method to synthesize iron oxides from aqueous Fe2+ and Fe3

+ salt solutions by the

addition of a base under inert atmosphere at room temperature or at elevated

temperature:

Fe2+ Fe3

+ 8HO− → Fe3O 4H2O

Magnetite nanoparticles characteristics, like size and shape, depend on the

Fe2+/Fe3

+ ratio, the reaction temperature, pH value and ionic strength of the media.

Figure 10. Fe3O4 nanoparticles synthesized by co-precipitation method, the scaler bar is 30 nm [65].

Maghemite nanoparticles can be easily obtained from the deliberate oxidation of

the resulting magnetite nanoparticles by the dispersion of magnetite nanoparticles inacidic medium, followed by addition of iron (III) nitrate. The maghemite particles

obtained are then chemically stable in alkaline and acidic medium [66].

The experimental challenge in the preparation of iron oxide nanoparticles by co-

precipitation lies in control of particles size and polydispersity. This method suffers

from a narrow particle size distribution and poor crystallinity. Small molecules and

amphiphilic polymeric molecules can be introduced to enhance the ionic strength of the

medium, protect the synthesized nanoparticles from further growth, and stabilize the

magnetic fluid [67]. Recent studies have revealed that a short burst of nucleation and

subsequent slow controlled growth is crucial to the synthesis of monodisperse

nanoparticles. Organic additives that act as stabilization and/or reducing agents can be

used to prepare monodisperse magnetite nanoparticles of different sizes [66].




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A modification of the co-precipitation method is the microemulsion method, in

which the Fe(II) and Fe(III) salts are precipitated with bases in reverse micelle droplets

stabilized by surfactant. The final size and shape of the nanoparticles prepared by this

technique can be tuned by the adjustment of surfactant or reactants concentration [68].

Small magnetic iron oxide nanoparticles with sizes lower than 10nm were

synthesized by this method [69]. Also, MnFe2O4 nanoparticles with sizes between 4nm

and 15 nm were synthesized in microemulsions of water-in-toluene reverse micelles

with surfactant sodium dodecylbenzenesulfonate (NaDBS). The synthesis of MnFe2O4

nanoparticles in microemulsions of water-in-toluene starts with a first clear aqueous

solution consisting of Mn(NO3)2 and Fe(NO3)3. A NaDBS aqueous solution is mixed

with the metal salt solution, and subsequent addition of a large volume of toluene forms

reverse micelles [70].

The disadvantages of this method are the low yield and poor crystallinity of the

nanoparticles, which limit its practical use. Also, the large amount of organic solvents

used would lead to not only higher costs, but also a non-friendly impact in the

environment.

In order to improve monodispersity and crystallinity, magnetic nanoparticles can

be prepared by thermal decomposition method (figure 11). In this method, the

organometallic complexes are decomposed in refluxing organic solvent in the presence

of surfactant [71]. Metallic iron nanoparticles with size between 2 and 10nm and good

polydispersity were obtained by thermal decomposition of Fe(CO)5 and in the presence

of polyisobutene in decalin [72].

Figure 11. Fe3O4 nanoparticles prepared by thermal decomposition of iron oleate Fe(OA)3 [65].




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In the thermal decomposition technique, nanoparticles size and morphology can

be controlled by temperature, reaction time, surfactant concentration and type of

solvent. The result nanoparticles are usually hydrophobic. Recently, it has been

demonstrated that using strong polar solvents (e.g. dimethylformamide or 2-

pyrrolidone) resulted in hydrophilic iron oxide nanoparticles, which are preferred for

biomedical applications [73].

2.2 Liposomes preparation

Liposomes can be obtained by means of several different methods. The properties

of the liposomes depend on the composition and concentration of the lipids and also on

the preparation method (figure 12). Liposomes incorporating different types of

substances could be manufactured by passive or active loading. The methods involving

the loading of the entrapped agents before or during the manufacturing procedure are

called passive loading. Otherwise, in active loading, certain type of compounds with

ionizable groups, and those which display both lipid and water solubility, are introduced

into the liposomes after the formation of intact vesicles.

Figure 12. Scheme of the methods for liposome preparation.

Lipid film hydration by

hand shaking, non-hand

shaking or freeze drying.

Micro-emulsification

Sonication

French pressure cell

Membrane extrusion

Dried reconstituted

vesicles

Freeze-thawed liposomes

Ethanol injection

Ether injection

Double emulsion

Reverse phase

evaporation vesicles

Stable pluri lamellas

vesicles

Detergent (Chocolate,

alhylglycoside, triton x-

100) removal from

mixed micelles by:

- Dialysis

- Column

chromatography

- Dilution

- Reconstituted sendai

virus enveloped

Methods of Liposomes preparation

Passive loading Active loading

Mechanical dispersion

methods

Detergent removal

methods

Solvente dispersion

methods




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The adequate choice of liposome preparation depends on the following parameters

[74, 75]:

1. the physicochemical characteristics of the material to be entrapped and

those of the lipids;

2. the nature of the medium in which the lipid vesicles are dispersed;

3.

the effective concentration of the entrapped substance and its potential

toxicity;

4. additional processes involved during application/delivery of the vesicles;

5.

optimum size, polydispersity and shelf-life of the vesicles for the intended

application;

6.

reproducibility and possibility of large-scale production of safe and

efficient liposomal products.

2.2.1 Synthesis of aqueous magnetoliposomes by ethanolic injection

One of the simplest methods used to obtain magnetoliposomes for in vivo

application is the so-called ethanolic injection. This is a simple technique, which

provides a reasonably homogeneous population vesicle, although rather dilute.

Figure 13. Representation of ethanolic injection.

In the ethanolic injection method (figure 13), a lipid solution in ethanol is rapidlyinjected in a buffer solution under vortex. The buffer solution must be at a temperature




19

above the transition temperature of the lipid [76-78]. Small liposomes in a range of 30-

110 nm are obtained. The advantages of ethanolic injection include its potential for

scale up, the simplicity of the procedure, low cost and low expenditure of time [79]. On

the other hand, the drawbacks of this method are that the population is poorly

homogeneous, liposomes are dilute and it is also difficult to remove all the ethanol [54].

Magnetoliposomes result from the encapsulation of magnetic nanoparticles into

liposomes, by the injection of the ethanolic lipid solution in an aqueous solution of

nanoparticles. The magnetoliposomes obtained have an aqueous pool inside and are

known as aqueous magnetoliposomes [47].

2.3. Synthesis of dry magnetoliposomes

From the alkaline co-precipitation of iron salts in the presence of the lipid DOPG

(1,2-Dioleoyl- sn-glycero-3-[phospho-rac-(1-glycerol)]) phospholipid molecules,

magnetite nanoparticles covered by a monolayer of DOPG molecules were synthesized

[8]. Dry magnetoliposomes are based on the growth of a second lipid layer around these

nanoparticles. Adding slowly another volume of lipid solution, equal to that used in the

synthesis of those nanoparticles, a second phospholipid layer is formed around the iron

oxide core [8]. This way, a lipid bilayer is created around the nanoparticles and dry

magnetoliposomes are obtained. Dry magnetoliposomes can also be obtained by mixing

SUV liposomes (obtained by strong sonication with tip) with MNPs, followed by

dialysis [6].

In order to decrease toxic effects, magnetoliposomes are washed by magnetic

decantation, taking advantage of the magnetic properties. Also, centrifugation or gel

permeation chromatography can be performed by magnetophoresis. This way, the non-

encapsulated nanoparticles are removed and the toxicity of the magnetic fluid is

reduced.




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