b2815 Tissue Engineering and Nanotheranostics “9.61x6.69”
164 Tissue Engineering and Nanotheranostics
(Ag+) in aqueous or nonaqueous solutions were reduced by various
reducing agents, such as sodium borohydride (NaBH 4 ), sodium cit
rate and polyol to formate metallic silver (Ag 0 ). Following that, Ag 0
agglomerate into oligomeric clusters and eventually formate metallic
colloidal silver particles.70–72 It is important to use protective agents to
stabilize dispersive nanoparticles during the course of metal nanopar
ticle preparation and protect the nanoparticles that can be absorbed
on or bind onto nanoparticle surfaces, avoiding their agglomeration.
The presence of surfactants comprising functionalities (e.g. thiols,
amines, acids, and alcohols) for interactions with particle surfaces can
stabilize particle growth and protect particles from sedimentation,
agglomeration,^73 or losing their surface properties. One of popular
approaches to quickly generate silver colloids was reported by Lee and
Meisel in 1982.^74 In their method, boiling AgNO 3 was reduced by
citrate in an aqueous solution to generate silver nanoparticles.75,76
During synthesis, the citrate ions serve as a reducing agent and a
stabilizer, and meanwhile, they may also bind with Ag+ ions or Ag 2
dimers in the early stages of the reaction.29,77–80 Additionally, changes
of pH will slightly alter the shape of Ag nanoparticles. Besides, the
polyol process represents a robust and versatile method for generating
Ag nanostructures with wide different and welldefined shapes.77–81
Achievement on tunable control over nucleation, growth and final
shape were realized by varying the reaction conditions, such as tem
perature, reagent concentration, and presence of trace ions.72,82–86 In
a typical synthesis as displayed in Fig. 2, ethylene glycol serves as the
source of reductant to generate metal atoms from AgO 3 at an elevated
temperature, meanwhile, a polymeric capping agent, such as poly
(vinyl pyrrolidone) (PVP), was added to facilitate control on nanopar
ticles’ shape.79,87–90 In this method, seeds with different sizes grow
into nanoparticles with different shapes.^79 Small clusters of fluctuating
structure aggregates into larger clusters for seeds. There are three
predominant seed structures: single crystalline, single twinned, and
multiply twinned. Typically, the single crystalline seeds grow into
cuboctahedrons or nanocubes, depending on relative growth rates
along the {100} and {111} directions.^85 Additionally, through oxida
tive etching, these cuboctahedrons or nanocubes can be applied as
