YE ET AL . VOL. 6 ’ NO. 3 ’ 2804–2817 ’ 2012 www.acsnano.org 2804 February 29, 2012 C 2012 American Chemical Society Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives Xingchen Ye, †,^ Linghua Jin, ‡,^ Humeyra Caglayan, § Jun Chen, ‡ Guozhong Xing, § Chen Zheng, ‡ Vicky Doan-Nguyen, ‡ Yijin Kang, † Nader Engheta, § Cherie R. Kagan, †,‡,§ and Christopher B. Murray †,‡, * † Department of Chemistry, ‡ Department of Materials Science and Engineering, and § Department of Electrical and Systems Engineering, University of Pennsylvania, Pennsylvania 19104, United States. ^ These authors contributed equally to this work. T he field of plasmonics has been one of the most exciting and active research areas in nanoscience and nano- technology. 1À6 The resonant excitation of surface plasmons on subwavelength metal- lic nanostructures provides ways to manip- ulate electromagnetic waves beyond the diffraction limit and to tailor lightÀmatter interactions at the nanoscale, which may enable a rich variety of new technologies including plasmonic solar cells, 4 surface- enhanced spectroscopies, 7À10 sensing, 11À13 bioimaging and therapeutics, 14À16 drug delivery, 17,18 superlenses, 19 nanolasers, 20 op- tical recording, 21 and plasmonic rulers. 22 Chemical synthesis of metal nanostruc- tures with well-controlled size and morpho- logy represents a versatile bottom-up ap- proach for accessing nanoscale plasmonic building blocks. 23À25 Gold nanorods (NRs), with a broadly tunable aspect-ratio- dependent longitudinal surface plasmon resonance (LSPR), are one of the most studied colloidal plasmonic nanostruc- tures. 26À28 Seeded growth of gold NRs in aqueous medium, pioneered by Murphy et al . and later improved by El-Sayed et al ., 29À31 has been well-documented in terms of control over NR size and shape uniformity and the spectral tunability of the LSPR. The original idea was to use micelles formed by the cationic surfactant hexadecyltrimethylammonium bro- mide (CTAB) as a “soft template” to direct NR formation. Since then, factors including temperature, 28 pH, 32À34 crystallinity of seed particles, 35,36 concentration of react- ants, 33,37À39 single-component surfactants other than CTAB, 40,41 binary surfactant mixtures, 31,37 and the presence of iodide ions in the growth solution 42À46 have been care- fully evaluated by several research groups, and many modifications of the initial recipe have been carried out to help gain a better understanding of the reaction mechanism. These efforts have led to the speculation that rather than micelle-templated growth, prefer- ential binding of CTAB molecules onto certain facets of developing NRs seems to be opera- tive in the growth of gold NRs. 26,27 Impor- tantly, Murphy et al. and Jin et al. demon- strated that the high concentration of CTAB (0.1 M) needed for gold NR formation is indeed a requirement for the high concentra- tion of bromide counterions, although in both reports, control over NR monodispersity un- der reduced CTAB concentration was not * Address correspondence to cbmurray@sas.upenn.edu. Received for review January 22, 2012 and accepted February 29, 2012. Published online 10.1021/nn300315j ABSTRACT We report an improved synthesis of colloidal gold nanorods (NRs) by using aromatic additives that reduce the concentration of hexadecyltrimethylammonium bromide surfactant to ∼0.05 M as opposed to 0.1 M in well-established protocols. The method optimizes the synthesis for each of the 11 additives studied, allowing a rich array of monodisperse gold NRs with longitudinal surface plasmon resonance tunable from 627 to 1246 nm to be generated. The gold NRs form large-area ordered assemblies upon slow evaporation of NR solution, exhibiting liquid crystalline ordering and several distinct local packing motifs that are dependent upon the NR's aspect ratio. Tailored synthesis of gold NRs with simultaneous improvements in monodispersity and dimensional tunability through rational introduction of additives will not only help to better understand the mechanism of seed-mediated growth of gold NRs but also advance the research on plasmonic metamaterials incorporating anisotropic metal nanostructures. KEYWORDS: monodisperse . gold nanorods . CTAB . salicylic acid . self-assembly . superlattice . plasmonics ARTICLE