Controlled post synthesis growth and etching of gold nanorods

Abstract

It is important to control the aspect ratio of individual gold nanorod as tiny change in aspect ratio and shape of gold nanorod can lead to different optical properties of gold nanorods. In this work, we report the synthesis of gold nanorods having the various aspect ratios. Increase in the pH causes the blue shift of the LSPR of gold nanorods indicating the decrease in the aspect ratio of gold nanorods whereas the trasverse SPR of gold nanorods remains unaffected to the pH of the solution. Seed mediated approach for the synthesis of gold nanorods inhibits to look upon the properties of individual gold nanorod as distribution of the nanorod sizes takes place in this method. Till now post synthesis growth and/or etching of the gold nanorods has been done in bulk solution. Herein, we report the post synthesis growth and/or etching of individual gold nanorod by isolating them over the glass surface and adding the growth solution containing the HAuCl4, AgNO3, Ascorbic acid in CTAB and sodium oleate in order to do the post synthesis growth and adding the ferric chloride solution in order to etch them. It will not only be novel approach to control the aspect ratio of gold inanorods but can also be used to control the nanosize’s gap between the dimer of gold nanorods which can be used to detect the molecules with weak emission yield without labeling them with a bright fluorophore just by placing it in the nanosize’s gap of the dimer of gold nanorods. We also report the effect of CTAB and ferric chloride concentration on the etching rate of gold nanorods respectively in the bulk solution keeping all other things constant/same. Ferric chloride oxidises the gold nanorods from the tips selectively. We found that on decreasing the CTAB and ferric chloride concentration in the solution, etching rate decreases and etching rate also decreases as the time passes by. Etching leads to the significant decrease in the length but little change in the diameter of the gold nanorods.