Thermal reshaping of gold nanorods in micelles

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.