Polymer-functionalized platinum-on-gold bimetallic nanorods.

Physical and chemical properties of nanosized crystals are known to be a function of their size, morphology, and chemical composition. Hence, over the past decade many research groups have focused on controlling the shape of inorganic nanostructures, and many low-symmetry nanocrystals have been produced. In this respect, one-dimensional structures are particularly interesting objects whose optical and catalytic properties strongly depend on their shape anisometry. This unique feature of metallic nanorods has already found several applications in nanotechnology and biomedical research. Moreover, four examples of bimetallic nanorods have been described in the last few years. It is believed that bimetallic platinum-on-gold and palladium-on-gold nanostructures may open a new direction in the area of catalysis. However, all bimetallic nanorods reported to date are only soluble in water and therefore cannot catalyze any reactions in organic solvents. In fact, there are no examples of any platinum or palladium nanostructures covalently functionalized with polymer chains, which could render them soluble in organic media. Herein we describe the first example of polymer-functionalized platinum-on-gold nanostructures. We use gold nanorods as templates and demonstrate how a polycrystalline continuous shell of platinum can be deposited on their surface and subsequently functionalized with organic molecules. In addition, we provide conclusive evidence of the functionalization of the platinum shell by direct visualization of the polymer surface layer by electron microscopy and its detection by H NMR spectroscopy. The seed-mediated method developed by Murphy and coworkers and later modified by El-Sayed and Nikoobakht produces single-crystalline gold nanorods (Au NRs). However, the conversion of Au ions to Au in this method is only 10–15%, and the majority of ions remains in solution after the growth of Au NRs is complete. Our systematic studies revealed that the amount of ascorbic acid used for the reduction of Au ions is critically important, and addition of 10 mol% ascorbic acid after completed nanorod growth makes their size distribution much narrower. Figure 1a shows a representative TEM image of Au NRs prepared by this modified method. The analysis of 250 nanorods shown in Figure 1a revealed that their average length and width measure 52.8 and 11.4 nm, respectively, and the standard deviation (SD) for both dimensions is less than 9%. These results represents a significant improvement in comparison with Au nanorods prepared under the standard conditions (SD 20%). Further introduction of ascorbic acid allows conversion of Au ions remaining in the growth solution into metallic gold and its deposition onto the surface of Au NRs. However, this rapid addition of ascorbic acid results in a nonuniform deposition of gold, and the shape of the nanorods changes to a so-called dogbone (Figure 1b). It has been suggested that the preferential deposition on the tips of the nanorods is due to a denser packing of CTAB molecules on the side facets, which have a much lower curvature. Upon the conversion of nanorods to dogbones, the length increases from 53 to 63 nm, whereas the average width changes from 11.4 to 18.5 nm. This transformation corresponds to a decrease in the aspect ratio of the nanorods from 4.6 to 3.4 and an almost twofold increase in their mass. After the gold ions were reduced (ca. 2 h), we added excess ascorbic acid and subsequently 25 mol% Pt ions. Under these conditions, a slow reduction of platinum takes place at 30 8C within 12 h. The color of the dogbones solution gradually changes from deep red to dark gray. A TEM analysis revealed that the deposition of platinum proceeds mainly on the tips of the dogbones and converts them into more symmetrical dumbbell-shaped structures (Figure 2a). These bimetallic objects measure on average 67 23 nm (SD 15%) and have a polycrystalline structure composed of multiple particles of Pt, which are oriented randomly and fused together near the tips of the gold core. This behavior is in contrast to single-crystalline bimetallic Au/Pt nanorods prepared by reduction of Pt ions. The deposition of platinum can be resumed and repeated several times if a new portion of Pt ions is introduced. This procedure allows us to increase the coverage of dumbbells and produce more uniform Au/Pt nanorods with a nearly rectangular shape. Figure 2b shows the structures obtained Figure 1. TEM images of a) CTAB-stabilized Au NRs and b) Au dogbones. CTAB=cetyltrimethylammonium bromide.