nanotechnology: beyond the confines of templates

2
YOUNAN XIA & BYUNGKWON LIM C ontrolling the morphology of metal nanostructures is central to many appli- cations because it provides an effec- tive means of tailoring the electronic, optical and catalytic properties of those structures 1 . Reporting in Angewandte Chemie 2 , Kuroda and Kuroda introduce a new type of two- dimensional gold nanostructure that, intrigu- ingly, is made using a three-dimensional lattice of silica nanospheres as a template. The top and bottom surfaces of the plate-like nanostructure contain patterned arrays of small dimples that are useful for various applications, but diffi- cult to generate using other techniques. This work opens the door to the development of facile methods for producing new types of nanostructured materials. In a template-directed synthesis of nano- structures 3 , a template serves as a scaffold within (or around) which a material is formed and shaped into a nanostructure whose morphology is complementary to that of Figure 1 | Dimpled gold nanoplates. Kuroda and Kuroda 2 have made two-dimensional gold nanostructures using a three-dimensional template — a close-packed array of silica nanospheres. These scanning-electron microscopy images show: a, the surface (scale bar, 200 nm) and b, the cross-section (scale bar, 20 nm) of a gold nanoplate. c, This image shows a gold nanoplate sitting on the surface of a template before the silica spheres are removed. The pattern of dimples in the plate closely matches the pattern of spheres in the template (scale bar, 200 nm). (Images reproduced from ref. 2.) potentially epigenetic effect on the germ line of the F 1 generation — could indirectly promote a ‘generational phenotype’, such as an adult- onset disease in subsequent generations. Epidemiological evidence has long suggested that the environment has a significant effect on human health and disease. Examples include regional differences in disease frequency, dis- cordant diseases in identical twins, drastic increases in disease frequency in a popula- tion, and chemical exposures directly affecting adult-onset disease 2 . Numerous studies have also reported a maternal impact on disease in offspring — often due to fetal exposure. For instance, in the female Agouti mouse, diet can cause epigenetic alterations in a genomic region called the Agouti locus to promote, in the offspring, changes in coat colour and in the risk of adult-onset diseases, including obes- ity and diabetes 9 . However, the influence of environmental factors on the next generation through the father is not well documented. In light of Ng and colleagues’ observations 1 additional studies are required. For instance, it remains to be seen whether the generational phenotype these authors describe — the effect of environmental exposure of the F 0 -generation fathers on the F 1 -generation daughters — is transgenerational, being further transmitted to yet the next (F 2 ) generation (Fig. 1). Moreover, whether adult exposure can promote an epi- genetic transgenerational inheritance — beyond the effects of multigenerational exposure — should be more thoroughly investigated. Indeed, in this study 1 , the high frequency of disease in the F 1 generation, the reproduc- ibility of the diabetes-like condition and the extent to which the gene-expression profile is modified in the pancreatic islet cells all sug- gest the involvement of an epigenetic molecu- lar mechanism. Nonetheless, the direct role of epigenetics in the sperm-mediated process must be demonstrated experimentally. The dramatic increase in human metabolic disorders such as obesity and diabetes warrants considering the influence of environmental factors on the germ line. Ng and colleagues’ observations 1 certainly support a role for envi- ronmental factors and generational effects in contributing to metabolic disease. Epigenetics provides a molecular mechanism for environ- mental factors such as diet to affect health and to influence subsequent generations through the germ line 2 . It is likely, therefore, that epigenetic biomarkers would be useful to aid in the diagnosis and potential treatment of such cases. Michael K. Skinner is at the Center for Reproductive Biology, School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236, USA. e-mail: [email protected] 1. Ng, S.-F. et al. Nature 467, 963–966 (2010). 2. Skinner, M. K., Manikkam, M. & Guerrero-Bosagna, C. Trends Endocrinol. Metab. 21, 214–222 (2010). 3. Barton, T. S., Robaire, B. & Hales, B. F. Toxicol. Sci. 100, 495–503 (2007). 4. Aitken, R. J., Koopman, P. & Lewis, S. E. M. Nature 432, 48–52 (2004). 5. Sharpe, R. M. Phil. Trans. R. Soc. B 365, 1697–1712 (2010). 6. Du Plessis, S. S., Cabler, S., McAlister, D. A., Sabanegh, E. & Agarwal, A. Nature Rev. Urol. 7, 153–161 (2010). 7. Guerrero-Bosagna, C., Settles, M., Lucker, B. & Skinner, M. K. PLoS ONE 5, e13100 (2010). 8. Puri, D., Dhawan, J. & Mishra, R. K. Epigenetics doi:10.4161/epi.5.5.12005 (2010). 9. Jirtle, R. L. & Skinner, M. K. Nature Rev. Genet. 8, 253–262 (2007). NANOTECHNOLOGY Beyond the confines of templates The use of templates to control the morphology of nanostructures is a powerful but inflexible technique. A template that is remodelled during synthesis suggests fresh opportunities for fabricating new nanostructures. a b c or polymers, and voids in lattices crystallized from tiny spheres. In particular, crystalline lattices of colloidal spheres in the nanometre- to-micrometre size range have been used as templates to fabricate three-dimensionally ordered porous materials 4-6 , which are use- ful for applications ranging from photonics to separation and catalysis. These materials have been referred to as inverse or inverted opals because they exhibit porous structures complementary to those of opals. In general, it is necessary to selectively remove templates using a post-synthesis treatment (such as heat treatment or chemical etching) in order to harvest the resulting structures. Surprisingly, Kuroda and Kuroda 2 have found that two-dimensional gold nanostruc- tures with a plate-like morphology (Fig. 1a, b) the template. Various templates have been explored, including step edges on the surface of substrates, channels within porous materials, structures assembled from organic surfactants 21 OCTOBER 2010 | VOL 467 | NATURE | 923 NEWS & VIEWS RESEARCH © 20 Macmillan Publishers Limited. All rights reserved 10

Upload: byungkwon

Post on 28-Jul-2016

213 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Nanotechnology: Beyond the confines of templates

Y O u n a n X i a & B Y u n g K w O n l i M

Controlling the morphology of metal nanostructures is central to many appli-cations because it provides an effec-

tive means of tailoring the electronic, optical and catalytic properties of those structures1. Reporting in Angewandte Chemie2, Kuroda and Kuroda introduce a new type of two-dimensional gold nanostructure that, intrigu-ingly, is made using a three-dimensional lattice of silica nanospheres as a template. The top and bottom surfaces of the plate-like nanostructure contain patterned arrays of small dimples that are useful for various applications, but diffi-cult to generate using other techniques. This work opens the door to the development of facile methods for producing new types of nanostructured materials.

In a template-directed synthesis of nano-structures3, a template serves as a scaffold within (or around) which a material is formed and shaped into a nanostructure whose morphology is complementary to that of

Figure 1 | Dimpled gold nanoplates. Kuroda and Kuroda2 have made two-dimensional gold nanostructures using a three-dimensional template — a close-packed array of silica nanospheres. These scanning-electron microscopy images show: a, the surface (scale bar, 200 nm) and b, the cross-section (scale bar, 20 nm) of a gold nanoplate. c, This image shows a gold nanoplate sitting on the surface of a template before the silica spheres are removed. The pattern of dimples in the plate closely matches the pattern of spheres in the template (scale bar, 200 nm). (Images reproduced from ref. 2.)

potentially epigenetic effect on the germ line of the F1 generation — could indirectly promote a ‘generational phenotype’, such as an adult-onset disease in subsequent generations.

Epidemiological evidence has long suggested that the environment has a significant effect on human health and disease. Examples include regional differences in disease frequency, dis-cordant diseases in identical twins, drastic increases in disease frequency in a popula-tion, and chemical exposures directly affecting adult-onset disease2. Numerous studies have also reported a maternal impact on disease in offspring — often due to fetal exposure. For instance, in the female Agouti mouse, diet can cause epigenetic alterations in a genomic region called the Agouti locus to promote, in the offspring, changes in coat colour and in the risk of adult-onset diseases, including obes-ity and diabetes9. However, the influence of environmental factors on the next generation through the father is not well documented.

In light of Ng and colleagues’ observations1 additional studies are required. For instance, it remains to be seen whether the generational phenotype these authors describe — the effect

of environmental exposure of the F0-generation fathers on the F1-generation daughters — is transgenerational, being further transmitted to yet the next (F2) generation (Fig. 1). Moreover, whether adult exposure can promote an epi-genetic transgenerational inheritance — beyond the effects of multigenerational exposure — should be more thoroughly investigated.

Indeed, in this study1, the high frequency of disease in the F1 generation, the reproduc-ibility of the diabetes-like condition and the extent to which the gene-expression profile is modified in the pancreatic islet cells all sug-gest the involvement of an epigenetic molecu-lar mechanism. Nonetheless, the direct role of epigenetics in the sperm-mediated process must be demonstrated experimentally.

The dramatic increase in human metabolic disorders such as obesity and diabetes warrants considering the influence of environmental factors on the germ line. Ng and colleagues’ observations1 certainly support a role for envi-ronmental factors and generational effects in contributing to metabolic disease. Epi genetics provides a molecular mechanism for environ-mental factors such as diet to affect health and

to influence subsequent generations through the germ line2 . It is likely, therefore, that epigenetic biomarkers would be useful to aid in the diagnosis and potential treatment of such cases. ■

Michael K. Skinner is at the Center for Reproductive Biology, School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236, USA. e-mail: [email protected]

1. Ng, S.-F. et al. Nature 467, 963–966 (2010).2. Skinner, M. K., Manikkam, M. & Guerrero-Bosagna,

C. Trends Endocrinol. Metab. 21, 214–222 (2010). 3. Barton, T. S., Robaire, B. & Hales, B. F. Toxicol. Sci.

100, 495–503 (2007).4. Aitken, R. J., Koopman, P. & Lewis, S. E. M. Nature

432, 48–52 (2004).5. Sharpe, R. M. Phil. Trans. R. Soc. B 365, 1697–1712

(2010).6. Du Plessis, S. S., Cabler, S., McAlister, D. A.,

Sabanegh, E. & Agarwal, A. Nature Rev. Urol. 7, 153–161 (2010).

7. Guerrero-Bosagna, C., Settles, M., Lucker, B. & Skinner, M. K. PLoS ONE 5, e13100 (2010).

8. Puri, D., Dhawan, J. & Mishra, R. K. Epigenetics doi:10.4161/epi.5.5.12005 (2010).

9. Jirtle, R. L. & Skinner, M. K. Nature Rev. Genet. 8, 253–262 (2007).

n a n O T e c h n O l O g Y

Beyond the confines of templatesThe use of templates to control the morphology of nanostructures is a powerful but inflexible technique. A template that is remodelled during synthesis suggests fresh opportunities for fabricating new nanostructures.

a b c

or polymers, and voids in lattices crystallized from tiny spheres. In particular, crystalline lattices of colloidal spheres in the nanometre-to-micrometre size range have been used as templates to fabricate three-dimensionally ordered porous materials4-6, which are use-ful for applications ranging from photonics to separation and catalysis. These materials have been referred to as inverse or inverted opals because they exhibit porous structures complementary to those of opals. In general, it is necessary to selectively remove templates using a post-synthesis treatment (such as heat treatment or chemical etching) in order to harvest the resulting structures.

Surprisingly, Kuroda and Kuroda2 have found that two-dimensional gold nanostruc-tures with a plate-like morphology (Fig. 1a, b)

the template. Various templates have been explored, including step edges on the surface of substrates, channels within porous materials, structures assembled from organic surfactants

2 1 O c T O b E R 2 0 1 0 | V O L 4 6 7 | N A T U R E | 9 2 3

NEWS & VIEWS RESEaRch

© 20 Macmillan Publishers Limited. All rights reserved10

Page 2: Nanotechnology: Beyond the confines of templates

a S T r O n O M Y

Galaxy sets distance markA galaxy has smashed the record for the most distant object ever observed. The object sheds light on the nature of the sources that stripped electrons from hydrogen atoms during the reionization epoch. See Letter p.940

M i c h e l e T r e n T i

On page 940 of this issue, Lehnert et al.1 describe follow-up spectroscopic observations that identify one galaxy,

on a patch of sky called the Hubble Ultra Deep Field, as the most distant astronomical object known so far. The galaxy is located more than 4 billion parsecs from Earth, at a redshift z = 8.56. It breaks the previous redshift record, z = 8.2, held by a gamma-ray burst — a one-off powerful cosmic explosion that faded within a few hours of its peak luminosity2,3. Prob-ably composed of about one billion stars4, the galaxy formed within 600 million years of the Big Bang, and will continue shining at the level we see today for several tens of million years to come.

UDFy-38135539, as the galaxy is designated, was initially classified as a candidate galaxy at a redshift larger than 8 on the basis of imag-ing observations5,6 made at optical and near-infrared wavelengths with the Hubble Space Telescope’s Wide Field Camera 3 (Fig. 1). In fact, because of the absorption of photons by neutral hydrogen atoms in the intergalactic

medium between the source and the observer, a galaxy at redshift z has no observed flux at wavelengths shorter than the redshifted hydro-gen Lyman-α line limit, which is 0.1216(1 + z) micrometres.

Imaging in multiple spectral bands is there-fore routinely used to identify candidate galax-ies at high redshift through the detection of the Lyman-α limit. But because the filters that are generally used have a broad passband, the loca-tion of the Lyman-α limit is uncertain, and the redshift estimate only approximate (typically the error on the redshift estimate is about 0.5). In addition, there can be other sources — such as brown-dwarf stars and low-redshift galax-ies with unusual properties — that mimic the typical broadband properties of high-redshift objects. Spectroscopic observations are thus required to confirm the redshift estimates obtained from imaging.

Lehnert et al.1 used the SINFONI Integral Field Unit Spectrograph on the Very Large Telescope in Chile to observe UDFy-38135539 for a total of 16 hours. From the resulting spectrum, they detected, with robust statisti-cal confidence (6 sigma), an emission line at

can be produced from a three-dimensional lattice of silica nanospheres 40 nanometres in diameter. To explain this, the authors sug-gest that partial cleavage of the lattice might occur because of the great strain exerted on the lattice as gold structures grow inside it. This partial cleavage creates a two-dimensional crack that acts as a template for the growth of nano-metre-thick plates of gold. Hexagonal arrays of dimples form on both sides of the gold nano-plate, caused by imprints of the spheres that make up the surfaces of the crack. The authors went on to construct hierarchical assemblies in which gold nanoparticles were positioned in the dimples of the plates, so generating two-dimensional periodic arrays of nanoparticles.

Kuroda and Kuroda obtained a microscopy image of a gold nanoplate still attached to a silica template (Fig. 1c), which shows dimples on the exposed gold surface whose arrange-ment corresponds well with that of the under-lying silica spheres. The authors interpret this as evidence that partial cleavage of the crystalline lattice of silica nanospheres occurred simultaneously with the growth of the nanoplate, ruling out the possibility that the plate could have formed elsewhere and then moved to its present position. But another possibility is that gold is deposited along pre-formed, planar cracks in the crystalline lattice that can easily develop when the silica nano-sphere assembly is dried before its use as a template. The mechanism responsible for the formation of cracks in the template lattice, and thus for the development of surface-patterned nanoplates, deserves further investigation.

A similar example of template fragmentation during synthesis has previously been observed7 in the formation of gold nanostructures within a three-dimensional lattice constructed from iron nanospheres assembled on a magnetic stirrer bar. As the volume of gold in the inter-stitial space of the lattice grew, the magnetic attraction between the iron nanospheres gradually weakened until the template lattice spontaneously disassembled, leaving behind branched gold nanostructures.

The spontaneous cleavage and/or dis-assembly of a colloidal lattice template during the replication process of a template-mediated synthesis offers an intriguing way of prepar-ing unconventional metal nanostructures. Mani pulation of the stress and strain in the lattice may enable one to control the surface topography of a replicated nanostructure, or to generate nanostructures that have new mor-phologies. ‘Smart’ templates, whose structures are remodelled or disassembled spontaneously and advantageously during synthesis, could be used for preparing metal nanostructures for applications including catalysis, sensing, imaging and device fabrication. The synthetic strategy could also be extended to other mater-ials, such as alloys, semiconductors, polymers, carbon and composites.

Leaving aside any smart features, colloidal

lattice templates will also be useful for a range of fundamental studies examining the nuclea-tion and growth of nanostructures. Metal nanostructures prepared from solutions are generally considered to grow by the addition of atoms to ‘seed’ structures that form naturally in solution, and usually exhibit poly hedral shapes such as a truncated octahedron. Anisotropic growth of metal nanostructures in solution is often explained by the preferential growth of seeds along specific directions, which can be guided by the presence of capping agents or polymeric stabilizers1. The chemical reduc-tion of a metal salt inside a colloidal lattice template — as used by Kuroda and Kuroda2 — presents an interesting system for study-ing the nucleation and growth of metal nano-structures in highly confined spaces. In such systems, metal nanostructures may follow a different growth pathway from those observed in solution. A deeper understanding of the mechanisms underlying the nucleation and growth of metal nanostructures within and beyond the confinement of a colloidal lattice

template might make it possible to design and synthesize nanostructures more complex than anything currently available. ■

Younan Xia is in the Department of Biomedical Engineering, Washington University, St Louis, Missouri 63130, USA. Byungkwon Lim is in the School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, South Korea. e-mails: [email protected]; [email protected]

1. Xia, Y., Xiong, Y., Lim, B. & Skrabalak, S. E. Angew. Chem. Int. Edn 48, 60−103 (2009).

2. Kuroda, Y. & Kuroda, K. Angew. Chem. Int. Edn 49, 6993−6997 (2010).

3. Martin, C. R. Acc. Chem. Res. 28, 61−68 (1995).4. Velev, O. D., Tessier, P. M., Lenhoff, A. M. & Kaler,

E. W. Nature 401, 548 (1999).5. Johnson, S. A., Ollivier, P. J. & Mallouk, T. E. Science

283, 963−965 (1999).6. Stein, A. & Schroden, R. C. Curr. Opin. Solid State

Mater. Sci. 5, 553−564 (2001).7. Li, Z., Li, W., Camargo, P. H. C. & Xia, Y. Angew. Chem.

Int. Edn 47, 9653−9656 (2008).

9 2 4 | N A T U R E | V O L 4 6 7 | 2 1 O c T O b E R 2 0 1 0

NEWS & VIEWSRESEaRch

© 20 Macmillan Publishers Limited. All rights reserved10