Dependence of the gold nanorod aspect ratio on the nature of the directing surfactant in aqueous solution

A three-step seed-mediated growth method was used to make gold nanoparticles. Different surfactants, alkyltrimethylammonium bromides (C(n)TAB, 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 C(10)TAB), 5 +/- 2 (C(12)TAB), 17 +/- 3 (C(14)TAB), and 23 +/- 4 (C(16)TAB). The plasmon absorption maxima for the gold nanoparticles varied as a function of the shape, from 520 mn (spheres) to beyond 2000 mn (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.