Investigation of the role of NaBH₄ in the chemical synthesis of gold nanorods

An improvement in the previously reported seed-mediated chemical synthesis of gold nanorods (GNRs) is reported. Monodisperse GNRs have been synthesized in a one-step protocol. The addition of controlled quantity of sodium borohydride (NaBH₄) directly into the growth solution produced uniform GNRs, formed by in situ nucleation and growth. In order to arrive at the conclusion, we studied the formation of GNRs with various seeds, of metals of widely differing crystal structures, and there were no variations in the properties of the GNRs formed. The role of NaBH₄ in the growth of GNR, which has not been covered in previous reports, is discussed in detail. The dependence of longitudinal plasmon peak on the concentration of NaBH₄ is compared with the dependence of residual concentration of NaBH₄ in the seed solution, which is added to the growth solution in seed-mediated synthesis. The study shows that NaBH₄ plays an important role in the formation of GNRs. This proposed protocol offers a number of advantages: one-step preparation of GNRs, significant reduction in the preparation time to 10 min, high monodispersity of GNRs, and tailorability of the aspect ratio depending on NaBH₄ concentration. It is suggested that NaBH₄ added to the growth solution leads to in situ formation of the seed particles of the size of 3–5 nm which enables the growth of GNRs. The growth of GNRs suggested here is likely to have an impact on the preparation of other anisotropic structures. Our single-pot methodology makes the procedure directly adaptable for commercial-scale production of GNRs and for their synthesis even in undergraduate laboratories.