Many characteristics of nanoscale materials are size-and shape-dependent, including their optical, electronic, and catalytic properties. [1±4] Spherical gold nanoparticles are easily produced by the chemical reduction of gold salts. [5,6] Anisotropic metal nanoparticles have been prepared using electrochemical [7] and photochemical [8] reduction methods in aqueous surfactant media, porous alumina templates, [9±10] polycarbonate membrane templates, [11] and carbon nanotube templates. [12,13] We recently reported the wet chemical syntheses of relatively monodisperse gold and silver nanorods, as well as silver nanowires. [14±16] Our method involves surfactantdirected growth of nanorods from spherical seeds. [17,18] Seeds are prepared by reduction of a metal salt with a strong reducing agent; the growth steps involve the addition of more metal salt to the seed solution, with a weak reducing agent, in the presence of a surfactant. The aspect ratio of the nanorods can be controlled by varying the ratio of seed to metal salt [14] as well as the time delay between steps of the syntheses. Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 have been prepared reproducibly by our method. [14] One drawback to this method is that it requires extensive separation of nanorods from spherical and plate-like gold nanoparticle side-products (Fig. 1a), achieved by up to a half-dozen rounds of centrifugation and washings. The separation of gold nanorods from other shapes is time consuming, and the yields of gold nanorods are low,~4 %. [14] We here report an improved synthetic methodology that produces monodisperse gold nanorods of high aspect ratio in~90 % yield in comparison to the initial method that produced gold nanorods iñ 4 % yield. Transmission electron microscopy (TEM) images of gold nanoparticles produced by the previous method and by the new method, before purification, are shown in Figure-s 1a and 1b, respectively.In our previous method, [14] gold nanorods were synthesized at pH 2.8. The method involved a three-step seed-mediated growth process. In the first step, 4 nm gold nanoparticles are produced by the reduction of HAuCl 4 with sodium borohydride in the presence of sodium citrate. The 4 nm gold nanoparticles were used as seed for the next growth steps. The seed is added to a growth solution containing the surfactant cetyltrimethylammonium bromide (CTAB), HAuCl 4 , and ascorbic acid. The ascorbic acid is the weak reducing agent, incapable 414

We report the preparation of polystyrene and silica-coated Au nanorods (aspect ratio 13 with 16 nm short axis). The coating resulted in a modification of the optical properties of the Au nanorods. Dissolution of the Au core from the polystyrene-and silica-coated Au nanorods using KCN enabled the formation of hollow polystyrene and silica nanotubes.Metal nanoparticles of different shapes and sizes, particularly gold, silver, and copper, have attracted considerable attention from the inorganic, materials, and physical community. 1 For nanoparticles made in solution, uncontrollable aggregation effects can be detrimental to many applications (e.g., electronics). Common stabilizers such as polymers, surfactants, and dendrimers have been used to increase nanoparticle stability. 2 Coating nanoparticles by in situ polymerization reactions provides an alternative method to increasing nanoparticle stability.3 Another potential use of coated nanoparticles is to make precursors to hollow nanoparticles. Hollow nanoparticles are an interesting and increasingly important class of materials with diverse applications, ranging from drug delivery, 4 cell and enzyme transplantation, 4b,5 contaminated waste removal, 6 and gene therapy. 7 Controlling the shape of the hollow nanoparticles is a great challenge. 8 Most of the work done in this field makes use of spherical inorganic cores. We have recently synthesized high aspect ratio cylindrical Au nanorods. 9 We here report a method of coating these nanorods with polystyrene and silica, followed by subsequent dissolution of the Au nanorods to form hollow nanotubes of the coating material.A solution of Au nanorods in the presence of the surfactant cetyltrimethylammonium bromide (CTAB) was prepared as described earlier. 9 The particle concentration in terms of Au atoms was 2.5 × 10 -4 M. Polystyrene coating of the particles was accomplished by the emulsion polymerization of styrene in the presence of CTAB-coated Au nanorods. Typically, 1 µL of styrene (Aldrich) was added to 10 mL of the Au nanorod solution. The solution was stirred for 5 min and 4 µL of a 0.1 M solution of the initiator ammonium persulfate (APS) was added. The polymerization was allowed to take place, and after 15 min the stirring was stopped. The solution was allowed to sit at room temperature for about 1 h before analysis. The polystyrene-coated Au nanoparticles were separated from spheres and excess CTAB by centrifugation and resuspended in deionized water for characterization by TEM and UV-visible absorption spectroscopy.For silica coating, Au nanorods were first separated from spheres and excess CTAB as described earlier.9 Next, a Au nanorod solution that was 0.01-0.1 wt % nanorod concentration was prepared. For typical silica coating, 10 µL of a

ABSTRACTWe show here that silver nanoparticles (AgNP) were intrinsically antibacterial, whereas gold nanoparticles (AuNP) were antimicrobial only when ampicillin was bound to their surfaces. Both AuNP and AgNP functionalized with ampicillin were effective broad-spectrum bactericides against Gram-negative and Gram-positive bacteria. Most importantly, when AuNP and AgNP were functionalized with ampicillin they became potent bactericidal agents with unique properties that subverted antibiotic resistance mechanisms of multiple-drug-resistant bacteria.

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Gold nanorods have been prepared in aqueous solution using a seed-mediated growth approach in the presence of surfactant. We observe the formation of liquid crystalline phases in concentrated solutions of high aspect ratio (13-18) gold nanorods by polarizing microscopy, transmission electron microscopy, and small angle X-ray scattering. These phases, which are stable up to 200 uC, exhibit concentration-dependent orientational order.