Synthesis of Toroidal Gold Nanoparticles Assisted by Soft Templates Yong Yan, Pramod Padmanabha Pillai, Jaakko V. I. Timonen, Fateme S. Emami, Amir Vahid, and Bartosz A. Grzybowski* Department of Chemistry and Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States * S Supporting Information ABSTRACT: A three-component system comprising surfac- tant molecules and molecularly cross-linked metal centers assembles into nanoring structures. The thickness of the nanorings is determined by the dimensions of the surfactant bilayer while the dimensions of the ring opening depend on and can be regulated by the concentrations of the participating species. Once formed, these organic-inorganic hybrids can be transformed, by air plasma treatment, into all-metal nanorings exhibiting strong adsorption in the near IR. ■ INTRODUCTION Among a multitude of nanostructures synthesized to date, “toroidal/ring” particles have recently received considerable attention as potential mimics of biological transmembrane channels, 1-3 elements useful in information storage, 4 elec- tronics, 5 medicine, 6,7 and as constructs with which to investigate several fundamental properties of matter on the nanoscale. 8,9 In particular, metallic nanorings exhibiting unique plasmonic properties 10 have been considered in the context of confining and manipulating light, 11 telecommunication wave- guides, 12 optical data storage, 13 quantum information process- ing, 14 optical antennas, 15 and chemical and biological sensors 16 as in medical applications including photothermal ablation of cancer cells and tumors. 17 However, the synthesis of these nanorings in large quantities remains challenging; approaches such as imprinting lithography, 18 colloidal lithography, 19 and electron beam lithography 20 are relatively expensive and low- yielding, whereas solution-based methods are only in the proof- of-concept stages. 21 In this letter, we describe a straightforward and large-scale method for synthesizing gold nanorings from three-component toroidal templates comprising cationic surfactants, gold salt, and dithiol linkers. These components first assemble in solution into nanorings of discrete nanoparticles which then, upon electron beam irradiation or air plasma treatment, fuse into continuous structures. The thickness of the nanorings thus formed is constant at ∼20 nm; at the same time, the pore size depends on and can be regulated by the ratio of dithiol and gold salt concentrations, [dithiol]/[HAuCl 4 ]. ■ EXPERIMENTAL SECTION To prepare the nanorings, 1 mL of cationic surfactant, cetyltrimethy- lammonium bromide (CTAB, 10 mM), 1 mL of gold salt (HAuCl 4 , 10 mM), and 8 mL of deionized water were first mixed to give a turbid aqueous solution (at room temperature and with continuous stirring with a stir bar). This turbidity could be ascribed to the formation of large aggregates of CTAB-gold salt complexes (Figure S1) since the ratio of CTAB to gold salt was significantly lower than that commonly used in the synthesis of gold nanoparticle. (Normally, when the ratio is high enough, such as in the hundreds, the solution is clear with no signs of turbidity. 22 ) Upon addition of 1.0 μL of dithiol (99%, 1,3- propanedithiol) the mixture gradually became clear and the color changed from yellow to light white, indicating that the reduction of AuCl -4 was taking place (probably from Au 3+ to Au + ). 23 Once the solution became clear (∼300 s), 1 mL of sodium borohydride (NaBH 4 , 50 mM) was added to completely reduce all of the gold salt to gold nanoparticles. ■ RESULTS AND DISCUSSION As shown in the SEM images (Figure 1a), nanoring structures then formed in high yield and were relatively uniform in size. The TEM image in Figure 1b shows that these rings incorporated ∼4 nm nanoparticles which, under high-resolution TEM, showed lattice fringes of 0.23 and 0.20 nm, corresponding to gold {111} and {200} plane sets. Both poly- and single-crystalline particles were present (Figures 1c and S2). 24 The critical role of dithiol in the synthesis of gold nanorings was confirmed by a set of control experiments. In one of them, 1-propanethiol was used instead of a 1,3-propanedithiol despite its similar length, the thiol could not act as a bridging agent; consequently, no nanorings but only gold nanoparticles were formed (SEM in Figure S3). Second, only relatively large (∼40 nm) nanoparticles were formed when no thiols (neither dithiol nor monothiol) were added to the reaction mixture (Figure S4). In another control experiment, the amount of dithiol added was small ([dithiol]/[HAuCl 4 ] ≈ 0.5 and below); under these conditions, the rings were ill-defined, their size Received: May 30, 2014 Revised: August 4, 2014 Letter pubs.acs.org/Langmuir © XXXX American Chemical Society A dx.doi.org/10.1021/la5020913 | Langmuir XXXX, XXX, XXX-XXX