Dependence of the Gold Nanorod Aspect Ratio on the Nature of the Directing Surfactant in Aqueous Solution

TLDR

The study proposes that surfactants bind as bilayers to growing gold nanoparticles, promoting elongation through a zipping mechanism. Gold nanoparticles were synthesized via a three‑step seed‑mediated growth using alkyltrimethylammonium bromides (CnTAB, n = 10–18) and cetylpyridinium chloride as stabilizers. Aspect ratios increased from 1 (C10TAB) to 23 ± 4 (C16TAB) as surfactant chain length grew, with plasmon peaks shifting from 520 nm for spheres to over 2000 nm for high‑aspect‑ratio rods.

Abstract

A three-step seed-mediated growth method was used to make gold nanoparticles. Different surfactants, alkyltrimethylammonium bromides (CnTAB, n = 10, 12, 14, 16, and 18) and cetylpyridinium chloride (C16PC), were chosen as stabilizers. In general, it was found that as the length of the surfactant chain increased, the resulting gold nanoparticles' aspect ratio increased: the aspect ratio was 1 (for C10TAB), 5 ± 2 (C12TAB), 17 ± 3 (C14TAB), and 23 ± 4 (C16TAB). The plasmon absorption maxima for the gold nanoparticles varied as a function of the shape, from 520 nm (spheres) to beyond 2000 nm (high aspect ratio nanorods). We propose that the surfactant binds as a bilayer to the growing nanoparticle and assists in nanoparticle elongation via a "zipping" mechanism.