PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 1: Synthesis, Characterization and Applications of Gold Nanoparticles
nature observe growth by this approach only. Bottom-up methods are chemically controllable
and non-destructive. The synthesis of nanoparticles from molecular solutions is a good
example of a bottom-up approach. The size of the nanostructures, which can be obtained with
a bottom-op approach, spans the full nano scale. An advantage of the bottom-up approach is
the better possibilities to obtain nanostructures with less defects and more homogeneous
chemical compositions. This is due the mechanisms utilized in the synthesis of nanostructures
reducing the Gibbs free energy, so that the produced nanostructures are in a state closer to a
thermodynamic equilibrium [1]. Some of the important methods involved are:
(i) Sol-gel method
(ii) Vapour phase deposition method
(iii) Chemical reduction method.
The bottom-up approach usually employs solution-phase colloid chemistry for the
synthesis. In a typical colloidal synthesis, atoms of the desired component are produced in the
solution at very high supersaturation to induce the assimilation of these atoms into particles to
reduce the system Gibbs free energy. Due to the flexibility in selecting different reducing
agents, particle capping agents, solvent systems as well as synthesis conditions, colloidal
synthesis offers a great variety of options for composition, shape, size and surface chemistry
control. The bottom-up approach is also suitable for controlling monodispersity of the
nanoparticles. With all these advantages, the bottom-up approach has become the main route
to nanomaterial production.
Among all bottom-up methods, the chemical reduction of the metal salt in an aqueous,
an organic phase or two phases, is one of the most popular routes as nanoparticles of a wide
range of sizes and shapes can be prepared by controlling the reaction conditions. The
reduction of gold salts in existing of a stabilizing agent is a facile and easy technique to
produce desired sizes of nanoparticles [37]. A stabilizing agent, also called as capping
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