PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 6: High-Yield Synthesis of Gold Nanoparticles
6.3.1. Step-Addition Method
We have observed that the maximum yield of gold nanoparticles for a fixed concentration of
block copolymer occurs at a particular gold salt concentration, e.g. 0.008 wt% HAuCl 4 .3H 2 O
for 1 wt% P85 (see Section 4.3.1 in Chapter 4). This concentration corresponds to the block
copolymer-to-gold salt molar ratio of 11 and is related to the minimum number of block
copolymer molecules (n) required for the reduction of a gold ion to take place. At lower salt
concentrations when the molar ratio of block copolymer to salt ions (r) is much larger than n
= 11, the yield increases with salt concentrations up to 0.008 wt%. Beyond this concentration
the ratio r decreases and thus the probability of the reduction reaction in the sample to take
place decreases. The distribution of a large number of ions and block copolymers makes it
possible that a significant fraction of ions can still find n number of block copolymers and
become reduced even though the average ratio r is less than n. Based on this explanation the
yield is expected to be suppressed with increase in salt concentration, as has been
experimentally observed. UV-visible spectroscopy suggests the increase in the size of the
nanoparticles with the increase in the salt concentration beyond 0.008 wt% could be
understood in terms of nanoparticle aggregation occurring due to insufficient stabilization by
the block copolymers on the surface of the nanoparticles as the block copolymer to gold ion
ratio decreases with the increase in the gold salt concentration. As a result we have observed
that nanoparticles in these systems phase separate after some time (~ 1 day). At higher salt
concentrations (> 0.02 wt%) the ratio r is too low to provide any significant probability
within the system of being able to reduce the gold ions. Hence, UV-visible spectra only show
the strong peak of unreduced gold ions and no SPR peak of gold nanoparticles is seen. We
have seen that yield of gold nanoparticles that can be enhanced by increasing the
concentration of gold salt and block copolymer or both are limited. This arises as a result of
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