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of individual particles may not be possible, also there is a poor dispersibility of the
modified particles and there is no control over the size and shape of the particles.
Hence, researchers proposed an in situ surface modification of the nanoparticles to
overcome all the above mentioned problems. Perhaps, the first steps in this direc-
tion leading to a rational nanoparticle assembly strategy were taken by Mirkin and
coworkers
[233,234]
and Alivisatos and coworkers
[235]
, who demonstrated that
DNA-modified colloidal gold nanoparticles could be assembled into superstructures
by hybridization of the complementary base sequences in the surface-bound DNA
molecules. The motive behind such studies was to obtain a perfect dispersion of
inorganic nanoparticles in solvents and plastics and to achieve a change in surface
properties to the nanoparticles to hydrophobic, then, it is necessary to control the
surface of the nanoparticles by organic modifiers and make a new generation of
advanced materials, namely hybrid organic
inorganic nanoparticles. Thus, a com-
bination of inorganic materials at the nanosize with the organic molecules could
solve several problems encountered in the application of nanoparticles, and it could
led to the emergence of the in situ surface modification of nanoparticles with a
great variety of organic surface modifiers, which would bring in a perfect disper-
sion of the nanoparticles in solvents or in polymers. So far, tremendous efforts
have been made to fabricate nanoparticle dispersed polymer, but it has been consid-
ered a difficult task to disperse the nanoparticles in organic solvents or in polymers,
especially for the particles synthesized under hydrothermal conditions. This is
because the metal oxide particle surface is hydrophilic, and for the case of nanopar-
ticles, it shows extremely high surface energy, which leads to the formation of
aggregates.
10.7.2 Supercritical Hydrothermal Organic
Inorganic
Hybrid Nanoparticles
In order to resolve the problem of particle aggregation, to achieve the perfect con-
trol over the size and morphology of the particles, and to obtain a desired surface
property of the nanoparticles, a new processing strategy has been proposed by
Adschiri and coworkers
[216,236
240]
utilizing the supercritical hydrothermal
technology.
Figure 10.52
shows a schematic representation of highly effective
strategy for the synthesis of metal oxide nanocrystals in the organic ligand-assisted
supercritical hydrothermal
technology
[236]
. The method yields perfect hybrid
organic
inorganic nanocrystals with very high dispersibility and a precise control
over the size and the shape of the nanoparticles. The organic components are intro-
duced into the system during the hydrothermal synthesis, and in situ surface modifica-
tion is obtained with an ultrathin layer of organics surrounding the inorganics
unlike the case of silane coupling on the metal oxides. The organic ligands and
supercritical water form a homogeneous phase, and it is known that under these
conditions water molecules themselves work as acid or base catalyst for various
organic reactions. Depending upon the applications of nanoparticles, one can select
suitable functional groups to introduce hydrophobicity or hydrophilicity property to
