Anisotropic Nanostructures

Standing (L-R): Bryan Mangelson, Daniel Park, Guoliang Liu, Tuncay Ozel, Eileen Seo, Matt O’Brien, Jessie Ku, Dan Eichelsdoerfer, Lam-Kiu Fong, Gilles Bourret, Mike Ross, Ryan Thaner. Sitting (L-R): Liane Moreau, Kaylie Young, Matt Rycenga (leader), Abrin Schmucker (leader), Keith Brown, Chad Shade.

Figure 1. Silver nanostructures prepared by plasmon-mediated syntheses. Left column: Solutions of triangular nanoprisms prepared at different excitation wavelengths and a representative transmission electron micrograph. Center column: Scanning electron micrograph of right triangular bipyramids and solutions of bipyramids prepared by different excitation wavelengths. Right column: Penta-twinned nanorods (top) and gold-core/silver-shell bimetallic icosahedra (bottom).

Background/Motivation.  As materials are reduced in size from the bulk to the nanoscale, they begin to exhibit new and unusual chemical and physical properties. Noble metal nanostructures are of particular interest because their chemical and physical properties can be controlled not only by varying material composition, but also by tuning size and shape. Anisotropic nanoparticles – non-spherical structures (e.g. prisms, rods, cubes) with unique shape-dependent properties and functionalities – can be utilized in a number of important applications ranging from catalysis to sensing. In the Mirkin group, one of our primary goals is to develop synthetic strategies for creating a toolbox of anisotropic, monodisperse noble metal nanoparticles in high-yield with fine control over their shape and size. Our work focuses on two particular types of synthetic methodologies: photo-mediated and seed-mediated synthesis.

Photo-Mediated Synthesis and the Concept of Plasmonic Seeds.  Anisotropic nanoparticle synthesis research in the Mirkin group began with our discovery and development of a photo-mediated (also referred to as a plasmon-mediated) synthesis for silver triangular nanoprisms, which was one of the first high-yield anisotropic nanoparticle syntheses ever developed (Figure 1).1 Because of their marked structural anisotropy, triangular nanoprisms have unusual interactions with light in terms of absorption and scattering phenomena. We have shown that the triangular nanoprism morphology can be controlled photochemically by adjusting the wavelength of irradiation2 or chemically by varying the pH of the reaction solution.3-5These methods enable the synthesis of uniform triangular nanoprisms whose optical features can be tuned throughout the visible and near-infrared regions of the spectrum by tailoring their edge length.

The photo-mediated synthesis also can be used to generate Au-core/Ag-shell structures by seeding the reaction with Au particles and irradiating the reaction solution at a wavelength equal to the plasmon resonance of the Au core.6-7 In this way, these particles serve as “plasmonic seeds” where light can be used to control particle growth and resulting shape. By changing the Ag precursor in the photo-mediated synthesis from Ag nanospheres to AgNO3, we have synthesized a variety of additional particle morphologies, such as right-triangular bipyramids with {100} facets8-9 and penta-twinned rods.10 The edge length of the bipyramids and length of the rods also can be controlled by varying the excitation wavelength.  Importantly, we have begun to work out the mechanisms that underlie these unusual transformations.

Figure 2. Selected Au particles synthesized using silver as a shape-directing additive. From left to right: octahedra, rhombic dodecahedra, truncated ditetragonal prisms, concave cubes, octahedra with hollow features. Scale bars: 200 nm (inset scale bar: 50 nm).

Seed-Mediated Synthesis. Efforts into developing seed-mediated nanoparticle synthesis methodologies in the Mirkin group originated with the synthesis of Au triangular nanoprisms.11 Current research directions focus on understanding the role of various components of the particle growth solution—such as surfactant, cationic and anionic additives, and pH—in controlling Au particle shape. Our most recent work has resulted in an improved understanding of the specific role of silver as a shape-controlling additive in the synthesis of gold nanoparticles (Figure 2).12 By systematically adjusting the amount of silver additive, we have synthesized a number of novel particle morphologies, including concave cubes, which are composed of twenty-four {730} high-index facets,13 {110}-faceted bipyramids,14 and octahedra with hollow features.15

Indeed, by developing an in-depth mechanistic understanding of shape-direction in nanoparticle synthesis, we have invented myriad protocols for creating monodisperse, anisotropic nanoparticles with properties and functions that can be synthetically defined by tuning architectural parameters.  We are currently investigating and characterizing the properties of these structures, in particular their novel photonic and plasmonic properties, and are applying them in assembly, sensing, and catalysis applications.


Selected References:

[1] R. Jin, Y. Cao, C.A. Mirkin, K. L. Kelly, G. C. Schatz, J. G. Zheng. “Photoinduced Conversion of Silver Nanospheres to Nanoprisms,” Science 2001, 294, 1901. DOI: 10.1126/science.1066541

[2] R. Jin, Y. C. Cao, E. Hao, G .S. Métraux, G. C. Schatz, C. A. Mirkin. “Controlling anisotropic nanoparticle growth through plasmon excitation,” Nature 2003, 425, 487. DOI: 10.1038/nature02020

[3] C. Xue, C. A. Mirkin. “pH-Switchable Silver Nanoprism Growth Pathways,” Angew. Chem. Int. Ed. 2007, 46, 2036. DOI: 10.1002/anie.200604637

[4] C. Xue, G. S. Métraux, J. E. Millstone, C. A. Mirkin. “Mechanistic Study of Photomediated Triangular Silver Nanoprism Growth,” J. Am. Chem. Soc. 2008, 130, 8337. DOI: 10.1021/ja8005258

[5] J. E. Millstone, S. J. Hurst, G. S. Métraux, J. I. Cutler, C. A. Mirkin. “Colloidal Gold and Silver Triangular Nanoprisms,” Small 2009, 5, 646. DOI: 10.1002/smll.200801480

[6] C. Xue, J. E. Millstone, S. Li, C. A. Mirkin. “Plasmon-Driven Synthesis of Triangular Core–Shell Nanoprisms from Gold Seeds,” Angew. Chem. Int. Ed. 2007, 46, 8436. DOI: 10.1002/anie.200703185

[7] M. R. Langille; J. Zhang; C. A. Mirkin. “Plasmon-Mediated Synthesis of Heterometallic Nanorods and Icosahedra” Angew. Chem. Int. Ed. 2011, 50, 3543. DOI: 10.1002/anie.201007755

[8] J. Zhang, S. Li, J. Wu, G. C. Schatz, C. A. Mirkin. “Plasmon-Mediated Synthesis of Silver Triangular Bipyramids,” Angew. Chem. Int. Ed. 2009, 48, 7787. DOI: 10.1002/anie.200903380

[9] J. Zhang, M. R. Langille, C. A. Mirkin. “Photomediated Synthesis of Silver Triangular Bipyramids and Prisms: The Effect of pH and BSPP,” J. Am. Chem. Soc. 2010, 132, 12502. DOI: 10.1021/ja106008b

[10] J. Zhang; M. R. Langille; C. A. Mirkin. “Synthesis of Silver Nanorods by Low Energy Excitation of Spherical Plasmonic Seeds.” Nano Lett. 2011, 11, 2495. DOI: 10.1021/nl2009789

[11] J.E. Millstone, S. Park, K. L. Shuford, L. Qin, G. C. Shatz, C. A. Mirkin. “Observation of a Quadrupole Plasmon Mode for a Colloidal Solution of Gold Nanoprisms,” J. Am. Chem. Soc. 2005, 127, 5312. DOI: 10.1021/ja043245a

[12] M. L. Personick; M. R. Langille; J. Zhang; C. A. Mirkin. “Shape Control of Gold Nanoparticles by Silver Underpotential Deposition.” Nano Lett. 2011, 11, 3394. DOI: 10.1021/nl201796s

[13] J. Zhang, M. R. Langille, M. L. Personick, K. Zhang, S. Li, C. A. Mirkin. “Concave Cubic Gold Nanocrystals with High-Index Facets,” J. Am. Chem. Soc. 2010, 132, 14012. DOI: 10.1021/ja106394k

[14] M. L. Personick; M. R. Langille; J. Zhang; N. Harris; G. C. Schatz; C. A. Mirkin. “Synthesis and Isolation of {110}-Faceted Gold Bipyramids and Rhombic Dodecahedra.” J. Am. Chem. Soc. 2011, 133, 6170. DOI: 10.1021/ja201826r

[15] M. R. Langille; M. L. Personick; J. Zhang; C. A. Mirkin. “Bottom-Up Synthesis of Gold Octahedra with Tailorable Hollow Features.” J. Am. Chem. Soc. 2011, 133, 10414. DOI: 10.1021/ja204375d