Inthe production of the magnetic colloidal nanosystems, stabilization is thecrucial step.
The nanosystem should be stable against aggregation in both abiological medium and a magnetic ?eld. This stability results from theequilibrium between attractive and repulsive forces. Theoretically, four kindsof forces can contribute to the interparticle potential in the system. Van derWaals forces induce strong short-range isotropic attractions. The electrostaticrepulsive forces can be partially screened by adding salt to the suspension(Laurent et al., 2008).
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory explainsabout these forces and particle-particle interactions(Derjaguin, 1941; Verwey, Overbeek & Overbeek, 1999). For magnetic suspensions, magnetic dipolar forcesbetween two particles must be added. These forces induce anisotropicinteractions, which are found to be globally attractive if the anisotropicinterparticle potential is integrated over all directions. Dependentupon the pH of the solution, the surface of the magnetite will be positive ornegative. The isoelectric point is observed at pH 6.8 (Bacri, Perzynski, Salin,Cabuil & Massart, 1990) andaround 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. Eitherone or both of the two repulsive forces: electrostatic and steric repulsion arerequired for stabilizing the magnetic particles. The force strength among theseparticles dictate the stability of the nanosystem.
The steric force isdif?cult to predict and quantify (Napper,1970; Fritz, Schädler, Willenbacher & Wagner, 2002). Stabilityfactor (W) is measured for the effectiveness of the potential barrier inpreventing the particles from aggregation. W is de?ned as the ratio of thenumber of collisions between particles and the number of collisions resulting inaggregation (Laurent et al., 2008). Stability factor is expressed as W = kfast/k,where kfast is the rate constant describing rapid aggregation (everycollision leads to an aggregation) and k is the aggregation rate constant atthe salt concentration used (Baudry,Bertrand, Rouzeau, Greffier, Koenig, Dreyfus, & Lequeux, 2004; Mylon, Chen& Elimelech, 2004). Techniques that are used for the measurement ofstability factor are light scattering (static or dynamic) or turbidimetricmeasurements. The stabilization of magnetic particles can be classified asdetailed in the Table No.
2. and Figure 2 represents different surface coatingin magnetic nanoparticles. Monomericstabilizers: Monomeric stabilizers can be tailoredfor dispersibility into oil/hydrocarbon carrier ?uids or aqueous media.
E.g. Carboxylates,phosphates and sulphates. By co-ordinating via one or two of the carboxylatefunctionalities, depending upon steric necessity and the curvature of thesurface, citric acid may be adsorbed on the surface of the magnetitenanoparticles (Sahoo, Goodarzi,Swihart, Ohulchanskyy, Kaur, Furlani & Prasad, 2005). By this methodone of the carboxylic acid group gets exposed to the solvent and makes thesurface negatively charged and hydrophilic. InorganicMaterials: Inorganic coatings not only provide stability tothe nanoparticles in solution but also help in binding various biologicalligands to the nanoparticle surface. These nanoparticles have an inner ironoxide core with an outer metallic shell of inorganic materials (Laurent et al.,2008).
An inert silica coating on the surface of magnetite nanoparticlesprevents their aggregation in liquid, improves their chemical stability, andprovides better protection against toxicity (Hosseini, et al., 2014). Silica shields the magneticdipole interaction with the silica shell and the silica nanoparticles arenegatively charged, hence the silica coating enhances the coulomb repulsion ofthe magnetic nanoparticles.
Tartaj and co-workers (2003) prepared submicronicsilica coated magnetic sphere aerosol by the pyrolysis method. The presence ofsurface silanol groups reacts easily with various coupling agents to covalentlyattach speci?c ligands to these magnetic particles, and stability is enhanced.(Ulman, 1996; Liu et al.,1998).GoldCoating: Two methods used in gold coating nanoparticles aredirect and indirect methods. In direct coating for nanoparticles (Silva, Tavallaie, Sandiford, Tilley& Gooding, 2016), gold shell can be formed directly or indirectlyonto 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 Aushell. Direct gold coating onto cores can be achieved using magnetite particlesthat are in aqueous or organic phase. For particles that are in aqueoussolution the most common procedures reduce Au3+ by using reducingagents such as sodium citrate and sodium borohydride. A shell is made byattaching the gold atoms by using the sodium citrate reduction of gold chloride(Ahmad,Bae, Rhee, Chang, Jin &Hong, 2012; Chen, etal.
,2015; Ghorbani,Hamishehkar, Arsalani & Entezami, 2015). Under vigorous stirring,boiling gold chloride aqueous solution is mixed with synthesized magneticnanoparticles and sodium citrate is added wherein Au layer is formed. Sodiumcitrate also confers stability against aggregations by coating the gold magneticnanoparticles by citrate moieties. Colour change from brownish to burgundy isobserved as Au shell is formed around magnetic nanoparticles (Zhou, Lee, Park, Lee, Park& Lee,2012). Gold MNPs obtained by indirect gold coating are characterized byhaving a ”glue layer”. The glue layer ensures the stability of the product andhas the ability to chelate metal ions that binds the magnetic core inorder topromote gold shell growth. Hence the choice and preparation of the glue layeris the keyfactor to obtain multifunctional properties of gold magneticnanoparticles. (Hu, Meng, Niu & Lu, 2013) PolymerStabilizers: Polymers functionalised iron oxidenanoparticles are gaining lots of demand in biomedical applications.
Polymer coatingincreases repulsive forces to balance the magnetic and the van der Waalsattractive forces acting on the NPs. Moreover, polymer functionalized ironoxide NPs have been extensively investigated due to interest in their unique physicochemicalproperties. The saturation magnetization value of iron oxide NPs will decreaseafter polymers functionalization.
Monomer polymerization method or synthesis inthe microporous of polymeric microsphere gives uniform magnetic composite NPswith high content of iron oxide NPs (Guin & Manorama, 2008). List of various polymers used instabilization of magnetic nanosystems are detailed in Table 3. (Wu et al.,2008).