Gold Nanorods to Nanochains: Mechanistic Investigations on Their Longitudinal Assembly Using α,ω-Alkanedithiols and Interplasmon Coupling

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Abstract
Mechanistic investigations on the end-to-end assembly of Au nanorods to nanochains, in the presence of α,ω-alkanedithiols, were reported. A decrease in the longitudinal plasmon absorption was observed along with a concomitant formation of a new red-shifted band above a critical concentration of dithiol, which is attributed to the interplasmon coupling in assembled nanorods. However, no noticeable spectral changes were observed below the critical concentration, and the TEM studies indicate that the nanorods remain isolated and randomly distributed. This step is ascribed as an incubation step wherein one of the thiol groups of α,ω-alkanedithiol preferentially binds onto the edges of the nanorods, leaving the other thiol group free. Above the critical concentration, a chain up process proceeds through the interlocking of nanorods, initially to dimers and subsequently to oligomers, which results in longitudinal interplasmon coupling. The dimerization step follows second-order kinetics which deviates with time due to oligomerization. The rate constants for dimerization of nanorods possessing various dithiols and their energy of activation were determined. The large activation energy for the dimerization further confirms that the process is not diffusion but activation controlled.