PHYS01200704032 Debes Ray - Homi Bhabha National Institute

utilized in the formation of nanoparticles. The substantial decrease in the stability for higher
block copolymer concentration arises because of large size of the nanoparticles formed in
these systems.
Self-assembly is a characteristic structural phenomenon of amphiphilic block
copolymers and its role in the synthesis of gold nanoparticles has been examined in Chapter
5 [15]. The self-assembly has been tuned by varying the chemical structure of the block
copolymer and solution temperature. The synthesis is carried out using three different block
copolymers P85, F88 (EO 103 PO 39 EO 103 ) and P105 (EO 37 PO 56 EO 37 ), which not only have
varying molecular weight but also differ in hydrophobicity to hydrophilicity ratio. It has been
found that the yield of gold nanoparticles increases in the order F88 < P85 < P105. The fact
that P105 has higher tendency to self-assemble, it results in higher yield as compared to P85
and F88. The formation rate is also found faster for the system having higher yield. The P105
block copolymer, having the highest tendency of self-assembly, forms the nanoparticles with
largest sizes whereas the size of the nanoparticles decreases for P85 and F88 and are mostly
consists of block copolymer cross-linked small gold nanoparticles. Block copolymers selfassembly
leads to a sphere-to-rod micellar shape transition at higher temperatures (e.g. 75 o C
for 1 wt% P85) and its effect on the gold nanoparticle synthesis has been studied. Our results
show that irrespective of shape of block copolymer micelles the synthesis carried out at
different temperature gives rise to the synthesis of similar spherical nanoparticles. This is
believed to be because of the differences in the driving force of block copolymer to selfassemble
to micelles and in synthesizing nanoparticles. However, it has been found that
increasing temperature increases the reaction kinetics drastically, resulting in much faster
synthesis of nanoparticles (within few minutes).
All the synthesis carried out so far show relatively low yield in block copolymermediated
synthesis of gold nanoparticles and is understood to be limited by the capability of
the reduction of gold ions by block copolymers. Chapter 6 discusses the development of two
methods of high-yield synthesis of gold nanoparticles, where the yield can be enhanced by
manyfold [16-18]. The first method based on step addition, the gold salt is added in small
steps to maintain the continuous formation of nanoparticles. The fact that there exists a
minimum ratio of block copolymer-to-gold salt (r min ) for the reduction to take place and most
of the block copolymers remain free from the gold nanoparticles, the step size is decided by
the gold salt concentration little less than that of the r min . The time between two steps is
decided by the time required to complete the synthesis in each step (about 2–3 hrs). In this
method thus the yield can be increased by number of steps used. However, the application of
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