Anisotropic Materials Subgroup
Back Row: Gengfeng Zheng, Matt Banholzer, Xiaodong Chen, Ari Atkinson, Andrew Senesi, Seongpil Hwang, Wei Wei, Jongkuk Lim
Front Row: Jake Ciszek, Can Xue, Louise Giam, Jill Millstone, Hyojong Yoo, Jae-won Jang
Not Pictured (but very important!) : Lidong Qin, Jae-seung Lee, Xiaoyang Xu
Anisotropic materials are crazy! :
Anisotropic Nanostructures
Nanoscience and technology focuses on the unique properties that arise in common materials when they are shrunk to nanoscale dimensions. For example, a spherical silver nanoparticle may appear yellow in solution, whereas a solution of triangular silver nanoparticles appears blue. These types of color changes reflect significant alterations in the chemical and physical properties of the nanostructures as result of their morphology. The relationship between physical dimensions and material properties then emphasizes the significance of fully characterizing these nanostructures, and highlights the role of this characterization in both fundamental science questions and future technological applications based on nanomaterials. In our group, we are particularly interested the effect of shape on nanoparticle properties and applications, and we focus our energy on two main morphologies: nanoprisms and nanorods.
Our nanoprism worked began with our development of a photochemically-mediated synthesis for silver triangular nanoprisms. This synthesis allowed the observation of the unique optical signature of Ag triangular nanoprisms, and marked one of the first high-yielding anisotropic nanoparticle syntheses ever developed (1). Since our initial work, we have shown that nanoprism morphology can be controlled photochemically by mediating either the wavelength of irradiation or by pH (2,3), as well as controlled thermally by adjusting redox conditions (4). These methods allow us to produce nanoprisms with tailorable optical features across the visible and near-infrared spectrum. We have now demonstrated the use of Ag nanoprisms in chemically-responsive films (5), and as starting materials for advanced anisotropic structures (6). Additionally, we have now developed a method to produce a gold analogue to these nanoprisms, and this high-yield synthesis has allowed us to observe the unique optical features Au nanoprisms in solution for the first time (7,8). Promising applications of nanoprisms include uses as spectroscopic enhancers, materials for advanced biodiagnostics, and agents for thermal ablation therapeutics.
For our nanorod work, we use a template-directed, electrochemical synthesis that allows us to control nanorod composition, length, and diameter. With this method, we have produced a variety of structures ranging from nanodiodes to self-assembled microtubes (9,10,11), and we have elucidated many unique nanorod properties including the observation of multipolar plasmon resonances (12). Additionally, we have developed a new lithographic technique based on these materials, termed On-Wire Lithography (OWL) (13), that can generate linear disk arrays with remarkable plasmonic and optical features (14,15).
<Photoconversion of Silver Nanoparticles into Nanoprisms
>For instance we have prepared single-crystalline triangular silver nanoprisms by photoconversion of 8 nm diameter silver nanoparticles in solution. Because of their large structural anisotropy, the resulting nanoprisms have striking optical properties, such as light-absorption, -scattering, and surface-enhanced Raman spectroscopy.
Figure 1-1. The silver nanoprism growth mechanism (top) and A. EELS mapping analysis showing the flat-top morphology of the Ag nanoprisms. B. Stacks of Ag nanoprisms assembled in a top-to-base manner on a carbon film-coated Cu grid.
Figure 1-2. The unimodal growth of nanoprisms. a, Schematic diagram of dual-beam excitation. b, The optical spectra (normalized) for six different-sized nanoprisms (1-6 edge length: 38 7 nm, 50 7 nm, 62 9 nm, 72 8 nm, 95 11 nm and 120 14 nm) prepared by varying the primary excitation wavelength (central wavelength at 450, 490, 520, 550, 650 and 750 nm, respectively; width, 40 nm) coupled with a secondary wavelength (340 nm; width, 10 nm). c, The edge lengths as a function of the primary excitation wavelength. d-f, TEM images of Ag nanoprisms with average edge lengths of 38 7 nm (d), 72 8 nm (e) and 120 14 nm (f). Scale bar applies to panels d-f.
Another intriguing possibility for new nanoparticle compositions lies in combinations of materials, for example in a core-shell format. Such materials are likely to have physical properties which are very different from particles of the individual components, and the potential for tunable properties by varying composition ratios. We have devised a low-temperature method for generating one type of core-shell particle consisting of a core of Ag and a monolayer shell of Au. The desirable optical properties of the silver core particles are preserved. However, now the particles can be readily functionalized with thiol-modified oligonucleotides because of the thin gold shell. We are currently extending this strategy to prepare other particles such as copper and platinum to create a series of core-shell particles with tailorable physical properties by virtue of the choice of core but the surface chemistry and stability of the native, and oligonucleotide-modified, pure gold nanoparticles.
References:
Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. "Photo-Induced Conversion of Silver Nanospheres to Nanoprisms," Science, 2001, 294, 1901-1903.
Metraux, G. S.; Cao, Y. C.; Jin, R. C; Mirkin, C. A. Triangular nanoframes made of gold and silver, Nano Lett. 2003, 3, 519-522.
Jin, R.; Cao, Y.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Controlling anisotropic nanoparticle growth through plasmon excitation, Nature 2003, 425, 487-490.
Living Templates for Hierarchical Assembly of Nanoparticles
A novel approach of using fungi to template the assembly of pre-synthesized and oligonucleotide -functionalized nanoparticles into ordered structures is presented. Once assembled, secondary structure can be introduced into the living materials, either through hybridization events on the surfaces of the
Figure 2-1. Schematic illustration of the use of a living hypha of a filamentous fungus as the template for the assembly of oligonucleotide-funtionalized Au nanoparticles into ordered microscopic structures.
Figure 2-2. (a)The culture media with a ~7 nM 13 m Au colloid immediately after introducing the fungal spores. (b)UV-Vis spectra of the media during the mycelium growth process. (c) A reddish purple fungal mycelium formed due to the fungal growth and Au nanoparticle assembly. (d) Microscope image of the mycelium in (b). Regular hyphae (4~6 µm in diameter, tens of µm long) can be seen.
References:
Li, Z.; Chung, S. W.; Nam, J.-M.; Ginger, D.; C. A. Mirkin, C. A. Living templates for the hierarchical assembly of gold nanoparticles, Angew. Chemie. Int. Ed. 2003, 42, 2306-2309.
