Iodide impurities in hexadecyltrimethylammonium bromide (CTAB) products: lot-lot variations and influence on gold nanorod synthesis.

Recent reports [Smith and Korgel Langmuir 2008, 24, 644-649 and Smith et al. Langmuir 2009, 25, 9518-9524] have implicated certain hexadecyltrimethylammonium bromide (CTAB) products with iodide impurities, in the failure of a seed-mediated, silver and surfactant-assisted growth protocol, to produce gold nanorods. We used two of the three "suspect" CTAB products and a "good" CTAB product in the protocol, varying silver nitrate solutions in the growth solutions. We obtained excellent gold nanorod samples as witnessed in signature longitudinal plasmon peaks in optical extinction spectra, which we substantiated using electron microscopy. Analysis of these samples using inductively coupled plasma mass spectroscopy (ICP-MS) failed to detect iodide. We subsequently learnt from discussions with Smith et al. that different lot numbers within the same product had been analyzed by our respective laboratories. We can conclude that iodide impurities can vary significantly from lot to lot within a product, to such an extent that there is no guarantee that gold nanorods can be synthesized with one or other CTAB product. Conversely, labeling a CTAB product, identified by a product number or supplier name, as one whose use precludes the formation of nanorods, is also hasty.