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 _nTAB, 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 ₁₀TAB), 5 ± 2 (C₁₂TAB), 17 ± 3 (C ₁₄TAB), 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.