Biosynthesis of Metallic Nanoparticles and Their Applications - Intracellular Delivery: Fundamentals and Applications - page 375

first and, subsequently, formed into NPs or nanostructures - the final product. Some
of the major assets of the bottom-up approach are resulting NPs without structural
defects and with homogenous chemical compositions. Using “top-down” techniques,
starting bulk material is reduced in size by mean of mechanical or chemical methods.
The major drawback of this approach is often imperfection of the obtained structure.
The surface defects have significant impact on physical properties and surface
chemistry behavior.

2.1 Physical Methods

Commonly utilized are physical (or mechanical) methods such as attrition, evapora-
tion/condensation, laser ablation and pyrolysis. In attrition, particles are ground by
a size-reducing mechanism (e.g. ball mill). Resulted NPs are critically affected by
the used starting material, time of drilling and medium which is used as atmosphere.
This method is typical representative of “top-down” approach. Inert gas conden-
sation can be used in atom creation (e.g. for the preparation small clusters of gold
atoms). As a result of interatomic collisions with the gas atoms in the chamber, the
evaporated metal atoms lose their kinetic energy and condense in the form of small
crystals (Lee et al. 2007 ). Laser ablation method is used to produce NPs by using the
pulsed laser irradiation on the metal target in liquid or gas environment (Yang 2007 ).
In pyrolysis, the precursor solution is atomized into series of droplet “reactors”.
These very small droplets are introduced by a carrier air gas into a hot-wall region
under atmospheric pressure conditions. The solvent in the droplets evaporates inside
the furnace, and the remaining solutes cause precipitation, thermal decomposition
and intraparticle reactions to form product particles (Widiyastuti et al. 2010 ).
The advantage of these methods is narrow particle size distribution of the
produced NPs, while its limitation is the need for expensive equipment (lasers etc.).
Also the production rate is lower compare to chemical methods. Higher energy
consumption for maintaining the pressure and temperature conditions used in the
aforementioned procedures are an additional handicap.

2.2 Chemical Methods

The second route is a chemical approach (usually “bottom-up” methods), particu-
larly the wet chemical procedure - the metal ions of dissolved precursor (e.g.
AgNO 3 ) is reduced in defined conditions which allows the subsequent formation
of small clusters or aggregates of metal atoms (Khomutov and Gubin 2002 ;
Oliveira et al. 2005 ). Similar procedures can be used also for creating of metallic
molecules such as sulfides or oxides (Seoudi et al. 2010 ). Specifically, methods
such as the sonochemical method (Salkar et al. 1999 ; Shchukin et al. 2010 ), the
polyol process, the solvent-reduction method (Bonet et al. 2000 ), the template method