In the stability of the nanosystem. The steric

the production of the magnetic colloidal nanosystems, stabilization is the
crucial step. The nanosystem should be stable against aggregation in both a
biological medium and a magnetic ?eld. This stability results from the
equilibrium between attractive and repulsive forces. Theoretically, four kinds
of forces can contribute to the interparticle potential in the system. Van der
Waals forces induce strong short-range isotropic attractions. The electrostatic
repulsive forces can be partially screened by adding salt to the suspension
(Laurent et al., 2008). Derjaguin-Landau-Verwey-Overbeek (DLVO) theory explains
about these forces and particle-particle interactions
(Derjaguin, 1941; Verwey, Overbeek & Overbeek, 1999). For magnetic suspensions, magnetic dipolar forces
between two particles must be added. These forces induce anisotropic
interactions, which are found to be globally attractive if the anisotropic
interparticle potential is integrated over all directions. Dependent
upon the pH of the solution, the surface of the magnetite will be positive or
negative. The isoelectric point is observed at pH 6.8 (Bacri, Perzynski, Salin,Cabuil & Massart, 1990) and
around this point point of zero charge (PZC), the surface charge density (?)
is too small and the particles are no longer stable in water and
?occulate.  Either
one or both of the two repulsive forces: electrostatic and steric repulsion are
required for stabilizing the magnetic particles. The force strength among these
particles dictate the stability of the nanosystem. The steric force is
dif?cult to predict and quantify (Napper,
1970; Fritz, Schädler, Willenbacher & Wagner, 2002). Stability
factor (W) is measured for the effectiveness of the potential barrier in
preventing the particles from aggregation. W is de?ned as the ratio of the
number of collisions between particles and the number of collisions resulting in
aggregation (Laurent et al., 2008). Stability factor is expressed as W = kfast/k,
where kfast is the rate constant describing rapid aggregation (every
collision leads to an aggregation) and k is the aggregation rate constant at
the salt concentration used (Baudry,
Bertrand, Rouzeau, Greffier, Koenig, Dreyfus, & Lequeux, 2004; Mylon, Chen
& Elimelech, 2004). Techniques that are used for the measurement of
stability factor are light scattering (static or dynamic) or turbidimetric
measurements. The stabilization of magnetic particles can be classified as
detailed in the Table No.2. and Figure 2 represents different surface coating
in magnetic nanoparticles.


stabilizers: Monomeric stabilizers can be tailored
for dispersibility into oil/hydrocarbon carrier ?uids or aqueous media. E.g. Carboxylates,
phosphates and sulphates. By co-ordinating via one or two of the carboxylate
functionalities, depending upon steric necessity and the curvature of the
surface, citric acid may be adsorbed on the surface of the magnetite
nanoparticles (Sahoo, Goodarzi,
Swihart, Ohulchanskyy, Kaur, Furlani & Prasad, 2005). By this method
one of the carboxylic acid group gets exposed to the solvent and makes the
surface negatively charged and hydrophilic.

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Materials: Inorganic coatings not only provide stability to
the nanoparticles in solution but also help in binding various biological
ligands to the nanoparticle surface. These nanoparticles have an inner iron
oxide core with an outer metallic shell of inorganic materials (Laurent et al.,
2008). An inert silica coating on the surface of magnetite nanoparticles
prevents their aggregation in liquid, improves their chemical stability, and
provides better protection against toxicity (Hosseini, et al., 2014). Silica shields the magnetic
dipole interaction with the silica shell and the silica nanoparticles are
negatively charged, hence the silica coating enhances the coulomb repulsion of
the magnetic nanoparticles. Tartaj and co-workers (2003) prepared submicronic
silica coated magnetic sphere aerosol by the pyrolysis method. The presence of
surface silanol groups reacts easily with various coupling agents to covalently
attach speci?c ligands to these magnetic particles, and stability is enhanced.
(Ulman, 1996; Liu et al.,

Coating: Two methods used in gold coating nanoparticles are
direct and indirect methods. In direct coating for nanoparticles (Silva, Tavallaie, Sandiford, Tilley
& Gooding, 2016), gold shell can be formed directly or indirectly
onto the magnetic core. The Au shell is formed directly on the core surface,
while in the indirect methods, a ”glue material” is used between core and Au
shell. Direct gold coating onto cores can be achieved using magnetite particles
that are in aqueous or organic phase. For particles that are in aqueous
solution the most common procedures reduce Au3+ by using reducing
agents such as sodium citrate and sodium borohydride. A shell is made by
attaching the gold atoms by using the sodium citrate reduction of gold chloride
(Ahmad,Bae, Rhee, Chang, Jin &
Hong, 2012; Chen, et
al.,2015; Ghorbani,
Hamishehkar, Arsalani & Entezami, 2015). Under vigorous stirring,
boiling gold chloride aqueous solution is mixed with synthesized magnetic
nanoparticles and sodium citrate is added wherein Au layer is formed. Sodium
citrate also confers stability against aggregations by coating the gold magnetic
nanoparticles by citrate moieties. Colour change from brownish to burgundy is
observed as Au shell is formed around magnetic nanoparticles (Zhou, Lee, Park, Lee, Park& Lee,
2012). Gold MNPs obtained by indirect gold coating are characterized by
having a ”glue layer”. The glue layer ensures the stability of the product and
has the ability to chelate metal ions that binds the magnetic core inorder to
promote gold shell growth. Hence the choice and preparation of the glue layer
is the keyfactor to obtain multifunctional properties of gold magnetic
nanoparticles. (Hu, Meng, Niu & Lu, 2013)


Stabilizers: Polymers functionalised iron oxide
nanoparticles are gaining lots of demand in biomedical applications. Polymer coating
increases repulsive forces to balance the magnetic and the van der Waals
attractive forces acting on the NPs. Moreover, polymer functionalized iron
oxide NPs have been extensively investigated due to interest in their unique physicochemical
properties. The saturation magnetization value of iron oxide NPs will decrease
after polymers functionalization. Monomer polymerization method or synthesis in
the microporous of polymeric microsphere gives uniform magnetic composite NPs
with high content of iron oxide NPs (Guin & Manorama, 2008). List of various polymers used in
stabilization of magnetic nanosystems are detailed in Table 3. (Wu et al.,