This paper reports the synthesis and self-assembly of monodisperse icosahedral Ag, Au, and Pd nanoparticles with tunable sizes. Ag nanoparticles were first prepared by a modified polyol method. The as-synthesized Ag nanoparticles were all icosahedral multiply twinned particles (MTPs) with notable size variations. The variability was reduced to less than 5% after a digestive ripening post treatment. The nearly monodisperse Ag nanoparticles were then used in replacement reactions with Au and Pd precursors to produce monodisperse icosahedral Au and Pd nanoparticles. The size of the monodisperse icosahedral Ag, Au, and Pd nanoparticles could also be increased thereafter by seed-mediated growth. The closely matched geometrical attributes of these icosahedral nanoparticles allowed them to easily self-assemble into 3-D superlattices either in the solution phase or on a solid substrate.

The replacement reaction between hydrophobized Ag nanoparticles and hydrophobized AuCl4- in toluene has been examined in detail. The conclusions obtained under our experimental conditions are different from those reported in the literature in three aspects: (1) a detectable contraction of the Ag nanoparticle sacrificial templates during the course of the reaction is shown; (2) the deposition of Au on the shrunken Ag templates inhibits further Ag oxidation, resulting in the formation of core-shell Ag-Au nanoparticles instead of Au nanoshells; and (3) the significant red-shift in the surface plasmon resonance (SPR) band is more of a consequence of shape and chemical composition changes rather than as an indication of Au nanoshell formation. Solvent and temperature are influential environmental factors that determine the structure and composition of nanoparticles formed by the replacement reaction.

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Root traits such as root angle and hair length influence resource acquisition particularly for immobile nutrients like phosphorus (P). Here, we attempted to modify root angle in rice by disrupting the OsAUX1 auxin influx transporter gene in an effort to improve rice P acquisition efficiency. We show by X-ray microCT imaging that root angle is altered in the osaux1 mutant, causing preferential foraging in the top soil where P normally accumulates, yet surprisingly, P acquisition efficiency does not improve. Through closer investigation, we reveal that OsAUX1 also promotes root hair elongation in response to P limitation. Reporter studies reveal that auxin response increases in the root hair zone in low P environments. We demonstrate that OsAUX1 functions to mobilize auxin from the root apex to the differentiation zone where this signal promotes hair elongation when roots encounter low external P. We conclude that auxin and OsAUX1 play key roles in promoting root foraging for P in rice.