Usage of the isotope effect for the synthesis of ultrahigh aspect ratio gold nanorods

Abstract

The synthesis of gold nanorods is commonly performed via seed-mediated growth processes, where typically tetrachloroaurate(III) is reduced with ascorbic acid in the presence of a surfactant (CTAB). However, in most cases nanorods with average aspect ratios below 4 are obtained, and reproducible processes which lead to average aspect ratios above 10 are widely missing. Using a modified seed-mediated method, we show that the average aspect ratio of gold nanorods increased from 8 to 19 when H₂O was replaced by D₂O as a solvent. The aspect ratio can be varied between these values by appropriate H₂O–D₂O mixtures. The increase in aspect ratio in the presence of D₂O is established rather by an increase in nanorod length than in diameter. Apparently, the increase in length is not caused by an influence of D₂O on the seed nanoparticles but is associated with the growth solution. It appears that in D₂O a slower reduction of gold(III) species by ascorbic acid, with involvement of an O–D bond (or an O–H bond in H₂O) in the rate determining step, is the origin of higher selectivity in the growth processes which lead to the different morphologies in nanoparticle formation.