Environmental Engineering Reference
In-Depth Information
formation of the nanoparticles, owing to its coordinating ability and low melting point
that provides a molten lux-like condition, making this synthesis a solventless method.
The decomposition of carbonaceous precursors, HDA, TPA, and Fe(CO)
5
, leads to the for-
mation of the carbonaceous shell coating the FeAs nanoparticles.
14.2.3.3 Polymer Stabilizers
Both
in situ
coatings and postsynthesis coatings have been developed for coating iron oxide
nanoparticles [104-107]. For
in situ
coatings, nanoparticles are coated during the synthesis
process. The postsynthesis coating method involves grafting the polymeric surfactants.
Now, many polymers have been introduced for coating iron oxide nanoparticles, such as
dextran, carboxymethylated dextran, carboxydextran, starch, arabinogalactan, glycosami-
noglycan, sulfonated styrene-divinylbenzene, polyethylene glycol (PEG), polyvinyl alco-
hol (PVA), poloxamers, and polyoxamines.
The formation of dextran-covered magnetite was irst reported by Molday and Mackenzie
by the Molday coprecipitation method with
in situ
coating by dextran-40 (MW = 40,000)
[106]. In this case, the dextran was functionalized to create more hydroxyl groups to allow
for the binding of the amino groups. Recently, laser pyrolysis and the coprecipitation
method have been proposed for fabricating dextran surface modiication of pure SPIONs
[107]. It is believed that the favored mechanism of adsorption of dextran on the surface of
maghemite nanoparticles is the collective hydrogen bonding between dextran hydroxyl
groups and the iron oxide particle surface.
PEG, a hydrophilic, water-soluble, biocompatible polymer, is another important func-
tional medium for coating Fe
3
O
4
nanoparticles [108,109]. For example, PEG-coated iron
oxide nanoparticles can be fabricated via hydrolysis of FeCl
3
·6H
2
O in water and the sub-
sequent treatment with PEG-poly(aspartic acid) block copolymer [110]. PVA coating onto
the particle surface can prevent their agglomeration, giving rise to monodisperse particles
[111,112]. The surface of Fe
3
O
4
nanoparticles can be modiied with PVA by precipitation of
iron salts in PVA aqueous solution, and the bonding of PVA onto the surface is inevitable
[113]. As is known, PVA is a unique synthetic polymer that can transform into a polymer
gel [114]. Albornoz and Jacobo have reported the fabrication of an aqueous ferroluid and
the preparation of a magnetic gel with PVA and glutaraldehyde [115].
Since alginate is an electrolytic polysaccharide with many carboxyl groups, it is expected
that the COO- of alginate and iron ion can interact and that the excess COO- can lead to
good dispersion of the MNMs and a high adsorption effect onto heavy metals. Recently,
several investigations have referred to the preparation of iron oxide nanoparticles with
alginate [116]. The standard chemical synthesis consists of the following three steps:
(i) gelation of alginate and ferrous ions, (ii)
in situ
precipitation of ferrous hydroxide by the
alkaline treatment of alginate, and (iii) oxidation of ferrous hydroxide by using an oxidiz-
ing agent. The method is relatively complex. A new modiied two-step coprecipitation
method has been developed and the typical iron oxide nanoparticles are Fe
3
O
4
with a core
diameter of 5-10 nm.
Nowadays, the preparations of MNPs encapsulated in chitosan are also of great inter-
est for its alkaline, nontoxic, hydrophilic, biocompatible, and biodegradable properties
[117]. Such kind of MNMs can be fabricated by various approaches such as sonochemical
method [118] or embedding Fe
3
O
4
i n chitosan [119].
Another important approach to synthesize polymeric core-shell MNPs is to use pre-
formed synthetic polymers as a matrix for controlling the formation of magnetic cores
[120,121]. For example, Underhill and Liu have offered a synthetic method for preparing
