Anisotropic Nanostructures





Back Row (L-R): Yuan Liu, Pengcheng Chen, Qingyuan Lin, Liane Moreau, Kurinji Krishnamoorthy, Andy Wang

Middle Row (L-R): Jessie Ku, Taegon Oh, Lin Sun, Zack Urbach, Christine Laramy, Jarad Mason (leader) Jingshan Du, Mike Ashley (leader), Mike Ross, Ashlee Robison, Janet McMillan

Sitting (L-R): Ryan Thaner, Eileen Seo, Matt O’Brien, Lam-Kiu Fong





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 to self-assembly. In the Mirkin group, one of our primary goals is to develop synthetic strategies for creating a toolbox of structurally uniform anisotropic noble metal nanoparticles in high-yield with fine control over their shape and size. Our work focuses on two particular types of synthetic methodologies: seed-mediated and photo-mediated synthesis.

Seed-Mediated Synthesis. Seed-mediated syntheses utilize nanoparticle reactants, or seeds, as templates for the heterogeneous nucleation of anisotropic nanoparticle products. In this way, the processes of seed nucleation and subsequent nanoparticle growth can be separated to allow for better control of each step. Initial research in our group focused on the reaction conditions required to control shape, with a specific emphasis on halides1-3 and silver4 (via underpotential deposition) as shape-directing agents. This work led to a number of novel morphologies, including concave cubes, which are composed of twenty-four high-index facets,5 {110}-faceted bipyramids,6 and octahedra with hollow features,7 and a set of design rules for how to control shape.3

Subsequent research has focused on the mechanisms behind seed-mediated synthesis and how a deeper understanding can enable unprecedented control over the structural uniformity and yield of anisotropic nanoparticles.8 In particular, while the field has predominantly focused on how to transform an ill-defined initial state (<5 nm seeds) into a well-defined end state through manipulation of reaction conditions, we have instead developed chemistry to control the uniformity of the seeds. Recently, we reported how iterative reductive growth and subsequent oxidative dissolution can be used for the stepwise refinement of gold nanoparticle seeds used for anisotropic particle synthesis.8 This novel capability allowed us to systematically study how the size dispersity, shape variation, and crystalline structure of the seed influence anisotropic nanoparticle products and enables the synthesis of eight classes of single crystalline nanostructures from the same batch of seeds, each consisting of a different shape, where the shape and size uniformity exceeds that of all previously reported syntheses (Figure 1). Subsequently, we extended this work to two-dimensional particles, where a nonuniform mixture of triangular, truncated triangular, and hexagonal plates can be oxidized in a self-limiting, tip-selective reaction that converts each of these products into similarly sized circular disks, resulting in considerable particle homogenization and narrower plasmon resonances.9

O'Brien Fig 1.jpg

Figure 1: High quality seeds can be used interchangeably to generate eight different shapes. Each panel represents a different shape synthesized and is arranged counterclockwise from top left as three-dimensional graphic rendering of the shape; TEM image (scale bars are 100 nm); high-magnification SEM image of crystallized nanoparticles (scale bars are 500 nm) with FFT pattern inset. Moving clockwise from the top left, the shapes described are cubes, concave rhombic dodecahedra, octahedra, tetrahexahedra, truncated ditetragonal prisms, cuboctahedra, concave cubes, and rhombic dodecahedra.


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 2).10 We have shown that the triangular nanoprism morphology can be controlled photochemically by adjusting the wavelength11 of irradiation or chemically by varying the pH of the reaction solution.12-14 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.15 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 and penta-twinned rods. Through this work, we have begun to work out the mechanisms that underlie these unusual transformations.16

O'Brien Fig 2

Figure 2. 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).

Anisotropic Nanoparticle Assembly. With this incredible toolbox of anisotropic nanoparticle building blocks, we have investigated a number of different properties, ranging from plasmonics to assembly. Assembly represents a particularly interesting avenue of research, as shape can be used to introduce directional interactions between particles. We first investigated the effect of shape in the context of DNA-mediated nanoparticle crystallization.17-20 In this work, DNA is used as a surface ligand to mediate interparticle interactions through sequence specific hybridization events.  The DNA forms a densely packed, conformal shell around the nanoparticle, such that the shape controls the strength, number and directionality of the “DNA bonds” for a given particle. Our initial work showed that a given shape (e.g. triangular prisms, rods, octahedra, rhombic dodecahedra) will crystallize with a lattice symmetry that maximizes the number of face-to-face interactions between particles (and thus maximizes the number of DNA hybridization events).17 We have subsequently investigated how the DNA bond strength between particles changes as a function of shape21 and how differences in bonding strength can be exploited to separate otherwise difficult to purify mixtures.22 Recently, we have developed a Langmuir-based assay to elucidate the binding thermodynamics of nanoparticles as a function of shape and size.23 Outside of DNA, our group has also looked into the use of directional depletion forces based on particle shape to assemble and purify nanoparticles.24


[1] Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L. D.; Schatz, G. C.; Mirkin, C. A. “Observation of a quadrupole plasmon mode for a colloidal solution of gold nanoprisms” J. Am. Chem. Soc. 2005, 127, 5312-5313. Doi 10.1021/Ja043245a.

[2] Millstone, J. E.; Wei, W.; Jones, M. R.; Yoo, H. J.; Mirkin, C. A. “Iodide ions control seed-mediated growth of anisotropic gold nanoparticles” Nano Lett. 2008, 8, 2526-2529. Doi 10.1021/Nl8016253.

[3] Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A. “Defining Rules for the Shape Evolution of Gold Nanoparticles” J. Am. Chem. Soc. 2012. Doi 10.1021/ja305245g.

[4] Personick, M. L.; Langille, M. R.; Zhang, J.; Mirkin, C. A. “Shape Control of Gold Nanoparticles by Silver Underpotential Deposition” Nano Lett. 2011, 11, 3394-3398. Doi 10.1021/Nl201796s.

[5] Zhang, J. A.; Langille, M. R.; Personick, M. L.; Zhang, K.; Li, S. Y.; Mirkin, C. A. “Concave Cubic Gold Nanocrystals with High-Index Facets” J. Am. Chem. Soc. 2010, 132, 14012-14014. Doi 10.1021/Ja106394k.

[6] Personick, M. L.; Langille, M. R.; Zhang, J.; Harris, N.; Schatz, G. C.; Mirkin, C. A. “Synthesis and Isolation of {110}-Faceted Gold Bipyramids and Rhombic Dodecahedra” J. Am. Chem. Soc. 2011, 133, 6170-6173. Doi 10.1021/Ja201826r.

[7] Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A. “Bottom-Up Synthesis of Gold Octahedra with Tailorable Hollow Features” J. Am. Chem. Soc. 2011, 133, 10414-10417. Doi 10.1021/Ja204375d.

[8] O’Brien, M. N.; Jones, M. R.; Brown, K. A.; Mirkin, C. A. “Universal Noble Metal Nanoparticle Seeds Realized Through Iterative Reductive Growth and Oxidative Dissolution Reactions” J. Am. Chem. Soc. 2014, 136, 7603-7606. Doi 10.1021/ja503509k.

[9] O’Brien, M. N.; Jones, M. R.; Kohlstedt, K. L.; Schatz, G. C.; Mirkin, C. A. “Uniform Circular Disks With Synthetically Tailorable Diameters: Two-Dimensional Nanoparticles for Plasmonics” Nano Lett. 2015. Doi 10.1021/nl5038566.

[10] Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. “Photoinduced conversion of silver nanospheres to nanoprisms” Science 2001, 294, 1901-1903. Doi 10.1126/science.1066541.

[11] Jin, R. C.; Cao, Y. C.; Hao, E. C.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. “Controlling anisotropic nanoparticle growth through plasmon excitation” Nature 2003, 425, 487-490. Doi 10.1038/Nature02020.

[12] Millstone, J. E.; Hurst, S. J.; Metraux, G. S.; Cutler, J. I.; Mirkin, C. A. “Colloidal Gold and Silver Triangular Nanoprisms” Small 2009, 5, 646-664. Doi 10.1002/Smll.200801480.

[13] Xue, C.; Metraux, G. S.; Millstone, J. E.; Mirkin, C. A. “Mechanistic study of photomediated triangular silver nanoprism growth” J. Am. Chem. Soc. 2008, 130, 8337-8344. Doi 10.1021/Ja8005258.

[14] Xue, C.; Mirkin, C. A. “pH-switchable silver nanoprism growth pathways” Angew Chem Int Edit 2007, 46, 2036-2038. Doi 10.1002/Anie.200604637.

[15] Xue, C.; Millstone, J. E.; Li, S. Y.; Mirkin, C. A. “Plasmon-driven synthesis of triangular core-shell nanoprisms from gold seeds” Angew Chem Int Edit 2007, 46, 8436-8439. Doi 10.1002/Anie.200703185.

[16] Langille, M. R.; Personick, M. L.; Mirkin, C. A. “Plasmon-Mediated Syntheses of Metallic Nanostructures” Angew. Chem. Int. Ed. 2013, 52, 13910-13940. Doi 10.1002/anie.201301875.

[17] Jones, M. R.; Macfarlane, R. J.; Lee, B.; Zhang, J. A.; Young, K. L.; Senesi, A. J.; Mirkin, C. A. “DNA-nanoparticle superlattices formed from anisotropic building blocks” Nat. Mater. 2010, 9, 913-917. Doi 10.1038/Nmat2870.

[18] Macfarlane, R. J.; O’Brien, M. N.; Petrosko, S. H.; Mirkin, C. A. “Nucleic Acid-Modified Nanostructures as Programmable Atom Equivalents: Forging a New “Table of Elements”” Angew. Chem. Int. Ed. 2013, 52, 5688-5698. Doi 10.1002/anie.201209336.

[19] Jones, M. R.; Osberg, K. D.; Macfarlane, R. J.; Langille, M. R.; Mirkin, C. A. “Templated Techniques for the Synthesis and Assembly of Plasmonic Nanostructures” Chem. Rev. 2011, 111, 3736-3827. Doi 10.1021/Cr1004452.

[20] Jones, M. R.; Seeman, N. C.; Mirkin, C. A. “Programmable Materials and the Nature of the DNA Bond” Science 2015, 347, 1260901, Doi 10.1126/science.1260901

[21] Jones, M. R.; Macfarlane, R. J.; Prigodich, A. E.; Patel, P. C.; Mirkin, C. A. “Nanoparticle Shape Anisotropy Dictates the Collective Behavior of Surface-Bound Ligands” J. Am. Chem. Soc. 2011, 133, 18865-18869. Doi 10.1021/Ja206777k.

[22] Jones, M. R.; Mirkin, C. A. “Bypassing the Limitations of Classical Chemical Purification with DNA-Programmable Nanoparticle Recrystallization” Angew. Chem. Int. Ed. 2013, 52, 2886-2891. Doi 10.1002/anie.201209504.

[23] O’Brien, M. N.; Radha, B.; Brown, K. A.; Jones, M. R.; Mirkin, C. A. “Langmuir Analysis of Nanoparticle Polyvalency in DNA-Mediated Adsorption” Angew. Chem. Int. Ed. 2014, 53, 9532-9538. Doi 10.1002/anie.201405317.

[24] Young, K. L.; Jones, M. R.; Zhang, J.; Macfarlane, R. J.; Esquivel-Sirvent, R.; Nap, R. J.; Wu, J. S.; Schatz, G. C.; Lee, B.; Mirkin, C. A. “Assembly of reconfigurable one-dimensional colloidal superlattices due to a synergy of fundamental nanoscale forces” Proc. Nat. Acad. Sci. USA 2012, 109, 2240-2245. Doi 10.1073/Pnas.1119301109.