Gold nanorods are prepared in aqueous solution by an electrochemical method
and are shape controlled by using capping micelles as described by Yu et a
l. (J. Phys. Chem. B 1997, 101 (34), 6661). Transmission electron microscop
ic (TEM) images are determined for these gold nanorods, taken from solution
s heated to various temperatures in the range of 25-160 degrees C. Size and
shape analysis of their TEM images showed that the mean aspect ratio of th
e nanorods in solution decreases with increasing temperature, mostly as a r
esult of reduction in their length. Heating the dried nanorods themselves o
n a TEM slide to much higher temperatures does not produce any shape change
s. This suggests that the observed relative instability of the longer nanor
ods in the micellar solutions is attributed to the relative instability of
the micelles capping the longer gold nanorods. Following the change with ti
me of the longitudinal surface plasmon absorption band of the gold nanorods
at five different temperatures, the activation energy for the thermal deco
mposition of the micelles is found to be 21.0 +/- 1.0 K cal mol(-1). The mo
st stable rod-shaped micelle is found to have an aspect ratio of 2.0. The c
raft decomposition (dissolution) temperature of these micelles is found to
be similar to 155 degrees C. This study offers a new method in which the th
ermal stability of micelles of various shapes is used to change the size di
stribution of metal nanorods in solution. On the other hand, TEM or the abs
orption maximum of the longitudinal plasmon resonance of gold nanorods can
be used as a sensitive monitor in studying the physical properties of the h
ost micelles themselves.