Rings of Nanorods

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Finding new ways to assemble nano-objects into finite superstructures is an important task because their properties depend not only on their size and shape but also on the spatial arrangement and the degree of order among the individual building blocks. Ringlike assemblies are fairly common and have been documented for various polymers, small organic molecules, and spherical inorganic nanoparticles. 13] Several mechanisms have been proposed to explain the formation of rings, including the Marangoni effect, the hole-nucleation mechanism, “2D gas bubbles”, and the “breath figures” method (BF). The latter mechanism takes advantage of the condensation of water microdroplets from moist air and uses them as templates. Thus, the BF method typically produces a honeycomb structure, which can be viewed as an array of air bubbles embedded into a continuous 2D matrix formed by polymer chains or spherical nanoparticles. One can see that if the concentration of solute is reduced, the perforated film should transform into a set of isolated rings, as was recently demonstrated in the case of linear polymers. However, rings composed of anisometric nanocrystals, especially inorganic rods, have never been observed. Nearly all reports on the self-assembly of nanorods describe their packing in a parallel fashion. Theoretical models also predict a side-by-side packing of rods regardless of their overall morphology. In stark contrast to that, we describe herein a spontaneous assembly of hybrid gold/ polymer core–shell nanorods into ringlike arrays. A systematic investigation shows that the rings of rods described here are templated by water droplets that condense on the surface of nonpolar solvents from humid air, akin to the classical BF method. The high yield and the well-defined structure of rings are a result of the presence of polystyrene (PS) chains covalently attached to the surface of the gold nanorods. The polymer shell insures high solubility of the hybrid 1D nanostructures and keeps the AuNR(PS)n rods in solution during the evaporation until they are concentrated around the circumferences of water droplets. This method is surprisingly simple and can be used for organizing nanorods into circular arrays in a nearly quantitative yield. Our recent studies revealed that carboxyl-terminated polymers can be covalently attached to phenol-functionalized gold nanoparticles under mild esterification conditions. To test the versatility of such an approach, we attempted a synthesis of rodlike gold/polymer core–shell nanostructures. Unlike spherical nanoparticles, the functionalized nanorods cannot be directly prepared by reduction of gold ions in the presence of thiols. For that reason, we first prepared gold nanorods stabilized by cetyltrimethylammonium bromide (CTAB) surfactant using a modified seed-mediated growth method. However, a seemingly trivial step of exchanging CTAB for a functional thiol was found to be a serious challenge. For example, the removal of either water or CTAB surfactant causes an irreversible agglomeration of nanorods and the subsequent multiple attempts to attach thiols in organic media are completely unsuccessful. In addition, many functional thiols are not soluble in water and their dispersion in an aqueous solution of CTAB-stabilized nanorods does not result in ligand exchange. After many trials, we found that dropwise addition of a concentrated solution of 4-mercaptophenol in THF directly into an aqueous growth solution of rods led to CTAB–thiol exchange and a slow precipitation of phenol-functionalized nanorods. After centrifugal purification, AuNR(SC6H4OH)n rods could be dispersed in dichloromethane and covalently coupled with carboxybiphenyl-terminated polystyrene (Mw= 5000 gmol ; Figure 1). The reaction proceeded within several hours after the addition of 4-(N,N-dimethylamino)pyridinium-4-toluenesulfonate (DPTS) and 1,3-diisopropyl carbodiimide (DIPC). Remarkably, the reaction could be monitored by TLC as the starting material AuNR(SC6H4OH)n with a retention factor of Rf= 0 gradually transforms into AuNR(PS)n product, which moves on a TLC plate as a single dark-red spot (Rf= 0.6 in 15% THF in CH2Cl2). The product was isolated by centrifugal ultrafiltration of the THF solution using a regenerated cellulose membrane (MWCO=30 kDa). Importantly, the complete removal of linear polystyrene was confirmed by size-exclusion chromatography of the THF-soluble AuNR(PS)n product, whereas the presence of covalently attached PS arms was confirmed by H NMR spectroscopy (see the Supporting Information). The typical weight gain of such coupling reactions is about 40%, which allows us to estimate the grafting density of PS arms (5 kDa) on gold nanorods (10 nmC45 nm as determined by transmission electron microscopy (TEM)). The grafting density is about 2.1 chainsnm , which means that approximately 3000 PS chains are covalently attached to a given nanorod (see the Supporting Information). The value of the grafting density is comparable to that reported for spherical Au nanoparticles (2.9 chainsnm ). The presence of a dense polymer shell ensures high solubility and stability of the nanorods in organic solvents. [*] B. P. Khanal, Prof. E. R. Zubarev Department of Chemistry Rice University Houston, TX 77005 (USA) Fax: (+1)713-348-5155 E-mail: zubarev@rice.edu