Geoscience Reference
In-Depth Information
and devices with new capabilities have been generated employing nanoparticles
based on metals, metal oxides, ceramics (both oxide and nonoxide), silicates,
organics, and polymers. One of the most important properties of materials in nano-
size regime is the changing physical properties. Nanoparticles possess unique opti-
cal and electronic properties not observed for corresponding bulk samples owing to
a substantial increase in the fraction of surface atoms and the increasing role of the
surface effects. The optical properties, and also other characteristics of materials
(structure of electronic energy levels and transitions, electron affinity, conductivity,
phase transition temperature, magnetic properties, melting points, etc.) become
dependent on the nanoparticle size and shape. Such size-dependent properties have
been exploited for biological tagging, for example, as fluorescent biological labels.
Hence, it is extremely important to control the size and also shape of the nanoparti-
cles/nanocrystals in order to obtain a desired physical property. Therefore, the field
of functionalization is almost endless and represents an enormous space to
explore. This in combination with molecular imaging can provide a new branch
of science, namely, nanobiotechnology imaging research. A major challenge in
the nanomaterials science is the accurate control of the size and shape, which in
turn is directly linked with the nanomaterials processing method. Nanoparticles
can be obtained from a great variety of processes involving the conversion of
solid to solid or liquid to solid or gas to solid. The stringent requirements for
the biological applications like therapeutic, bioimaging, hyperthermia, targeted
drug delivery system (DDS), biosensors, magnetic resonance imaging (MRI),
and microelectronics, insist on the control of the size and shape of nanomater-
ials. Hence, the solution techniques like hydrothermal are becoming the most
valuable nanomaterials fabrication tool in the recent years, and they have an
edge over all other processing methods because of the high quality of products.
By choosing the appropriate capping agents, the surface properties of the nano-
particles can be significantly altered from hydrophilic to hydrophobic and vice
versa. Also a perfect dispersion of the nanoparticles in the given solvent can be
achieved for making the self-assembly structures. The knowledge on the nucle-
ation, crystallization, self-assembly, and the growth mechanism of the nanocrys-
tals in hydrothermal solution media are rather complicated and are still not well
understood
[31]
.
Gold nanoparticles have been around since Roman times. As per the litera-
ture data, Michael Faraday
[32]
was the first to seriously experiment with gold
nanoparticles starting in 1850s. The development of new sophisticated tools like
Scanning tunneling microscope (STM), Transmission electron microscope
(TEM), Atomic force microscope (AFM), to observe, measure, and manipulate
processes at the nanoscale level gave a breakthrough to the nanotechnology.
Majority of the early hydrothermal experiments carried out during 1840s to
early 1900s mainly dealt with the nanocrystalline products, which were dis-
carded as failures due to the lack of sophisticated tools to examine the fine to
nanoproducts except in some chemical techniques
[21,33
35]
. Until the works
of Giorgio Spezia
[36]
in 1900, the hydrothermal technology did not gain much
importance in the growth of bulk crystals, as the products in the majority of the
