eli d. sone, eugene r. zubarev and samuel i. stupp- supramolecular templating of single and double...

8/3/2019 Eli D. Sone, Eugene R. Zubarev and Samuel I. Stupp- Supramolecular Templating of Single and Double Nanohelices

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Self-assembly

Supramolecular Templating of Single andDouble Nanohelices of Cadmium Sulfide**

Eli D. Sone, Eugene R. Zubarev, and Samuel I. Stupp*

Inorganic nanocrystals display size- and shape-dependent

properties that make them of both scientific and technologi-

cal interest.[1] While control

over particle size is by now

fairly advanced for a number

of systems, the ability to con-

trol the organization of inor-

ganic particles on the nano-

scale is not as well devel-

oped. One approach is to use

organic structures to influ-

ence the organization, and in

some cases growth, of the in-organic nanocrystals. The for-

mation of extended three-di-

mensional (3D) structures

using liquid crystals,[2] block

copolymers[3] and DNA link-

ers,[4] among others, has been

reported. In two dimensions,

nanoparticle organization is

often accomplished through

ordered crystal growth on

self-assembled monolayers.[5]

Supramolecular objects, with

widely varied, well-definedstructures and chemical func-

tionalities, provide excellent substrates for 1D nanoscale or-

ganization of inorganic materials. Examples of substrates

used include both biological[6] and synthetic[7] organic as-

semblies, but these have typically produced fairly simple 1D

inorganic structures, such as nanowires and nanotubes. We

recently reported on the templating of single helices of cad-

mium sulfide (CdS) from supramolecular ribbons.[7d] Here

we describe further insights into the system, including the

ability of the ribbons to template double helices of CdS. Al-

though helical inorganic structures from supramolecular

templates have been reported, these are either for amor-

phous materials such as silica,[8] or on a length scale that is

an order of magnitude larger[9] than the system reported

here.

The Dendron rodcoil (DRC) molecules developed by

our group have been shown to self-assemble into a network

of ribbons 102 nm in cross section and up to 10 mm in

length, leading to the gelation of various organic solvents at

low concentrations (%1 wt.%).[10] These ribbons, whose

cross section is made up of two hydrogen-bonded DRC

molecules packed head-to-head, can be either flat or twisteddepending on the solvent. A schematic representation of

the molecular packing in a twisted ribbon is shown in

Figure 1. We discovered that it was possible to exploit the

amphiphilic nature of the DRC in order to use the twisted

ribbons as templates for the growth of CdS in certain sol-

vents.[7d] In ethyl methacrylate (EMA), for example, we

added Cd(NO3)24 H2O in THF to suspensions of twisted

ribbons. Since the Cd2+ ions prefer the relatively hydrophil-ic phenolic groups of the DRC over the hydrophobic sol-

vent, this creates a supersaturation of Cd2+ ions on the sur-face of the ribbon. Upon exposure to hydrogen sulfide(H2S)

gas as a source of S2 ions, nucleation and growth of CdS lo-

Figure 1. Chemical structure of the DRC molecules and a molecular graphic model of a twisted ribbon inethyl methacrylate.

[*] Dr. E. D. Sone,+ Dr. E. R. Zubarev,++ Prof. S. I. Stupp

Department of Chemistry, Department of Materials Science and

Engineering, and Medical School

Northwestern University

Evanston, IL 60208-3108 (USA)

Fax: (+1)847-491-3010E-mail: [email protected]

[+] Current address:Department of Structural Biology

Weizmann Institute of Science

Rehovot, 76100 (Israel)[++] Current address:

Department of Materials Science and Engineering

Iowa State University

Ames, IA 50011 (USA)

[**] This work was supported by the U.S. Department of Energy (DoE,

grant no. DE-FG02-00ER45810), the Defense Advanced Research

Projects Agency (DARPA, grant no. MDA972-03-1-2003), and the

Air Force Office of Scientific Research - Multi-University Research

Initiative (AFOSR-MURI, grant no. F49620-00-1-0283). We

acknowledge the use of the Electron Probe Instrumentation

Center (EPIC) at Northwestern University. E.D.S. is grateful to the

Natural Sciences and Engineering Research Council of Canada

for a postgraduate scholarship.

694 2005 Wiley-VCH Verlag GmbH& Co. KGaA, D-69451 Weinheim DOI: 10.1002/smll.200500026 small 2005, 1, No.7, 694697

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calized to the ribbon occurs, leading to a templating effect.

The single helices thus produced had a morphology (coiled

helix) and pitch (%50 nm) that were both consistent with

our proposed mechanism involving growth of CdS on only

one face of the twisted ribbon. We also expected that under

certain conditions it would be possible to produce double

helices of CdS resulting from growth along both faces of the

ribbon structure. By studying the early stages of CdS nucle-

ation and growth on DRC ribbons and exploring variables

in ribbon preparation and mineralization, we have been

able to produce double-helical CdS morphologies that con-

firm our templating hypothesis and further our understand-

ing of the mineralization process.

Once samples are exposed to H2S, the diffusion of the

gas through the sample can be monitored by a color change

from colorless to yellow, due to the formation of CdS. Since

it takes %10 min for the gas to diffuse through the sample,

ribbons at different levels in the suspension are exposed to

S2 ions for different amounts of time, thus leading to heli-

ces of different thicknesses. Ostwald ripening effects may

also play a role in the distribution of sizes of CdS helices.As such, at any given time there exist ribbons at various

stages of mineralization. Using transmission electron micro-

scopy (TEM), we can directly image the different stages of

the process. Figure 2 shows ribbons from various samples

that have undergone different amounts of CdS nucleation

and growth. In Figure 2a, the ribbon itself is still clearly visi-

ble, as are the much darker CdS nanocrystals that decorate

its surface. The 510 nm nanocrystals are generally isolated

from each other, providing clear evidence that nucleation

occurs at many points along the ribbon, rather than at a

single node. Evidently, heterogenous nucleation on the

DRC ribbon occurs more quickly than CdS growth at this

stage. Although the crystals are isolated, the nucleation pat-tern is not random; rather, a hint of a helical pattern is al-

ready evident, which suggests that the crystals are nucleat-

ing on only one face of the ribbon. In the micrograph in Fig-

ure 2 b, further nucleation and growth have occurred, and

many of the CdS crystals have coalesced. Although the or-

ganic ribbon is still visible at this stage, the helical form of

the mineralized product is now clear. We note that high-res-

olution (HR) TEM of a similarly mineralized ribbon (Fig-

ure 3 a) does not show lattice fringes, suggesting that the

CdS is still amorphous, or at least not strongly crystalline, at

this stage. As the CdS continues to grow, it eventually be-comes wider than the DRC ribbon template, completely ob-

scuring the less electron-dense structure (Figure 2c). By this

point, the mineral is certainly crystalline, as evidenced by

lattice fringes visible in HRTEM (Figure 3 b) and a selected-

area electron diffraction pattern consistent with the CdS

zinc blende structure (not shown). We did not observe any

correlation between orientations of neighboring crystals.

Further growth of the CdS helix leads to a thickening of the

helix, eventually resulting in mature structures such as the

one shown in Figure 2 d. This single-coiled helix morphology

clearly appears to be the result of nucleation and growth on

one face of the twisted ribbon template.

Editorial Advisory Board Member

Professor Samuel Stupp earned his PhD

in materials science and engineering

from Northwestern University in 1977.

After spending 18 years at the University

of Illinois at Urbana-Champaign, he

returned to Northwestern (1999) as a

Board of Trustees Professor of Materials

Science, Chemistry, and Medicine, and

was later appointed Director of North-

westerns Institute for BioNanotechnolo-

gy in Medicine. He is, amongst others, a

member of the American Academy of

Arts and Sciences and a fellow of the American Physical Society. His

awards include the Department of Energy Prize for Outstanding Ach-

ievement in Materials Chemistry, the Materials Research Societys

Medal Award, and the American Chemical Society Award in Polymer

Chemistry. His areas of research include self-assembly, electronic/

photonic properties of organic nanostructures, biomolecular mineral-

ization, templating chemistry of inorganic nanostructures, and bio-

materials for regenerative medicine.

Figure 2. Various stages of the mineralization of DRC ribbons: a) Iso-

lated CdS nanoparticles on the organic ribbon; b) CdS particles that

have grown a little and have begun to coalesce in a helical pattern;c) a CdS helix that has grown wide enough that it completely

obscures the ribbon template; d) a mature CdS single helix. Scale

bar is identical in all micrographs (50 nm).

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Although the single-helical structures can be rational-

ized in terms of the twisted ribbon template, we expected

that it should be possible for CdS to nucleate and grow on

both faces of the ribbon, leading to double helices (shown

schematically in Figure 4). In fact, when we mineralized rib-

bons from gels that had been aged for several months, we

observed large numbers of such structures, as shown in

Figure 5. The braided pattern clearly visible in the inset is

what one would expect from a 2D projection of two inter-twining coils of CdS. Furthermore, the dimensions of these

structures are consistent with those of the single helices,

having a repeat distance along the edge (%30 nm) that is

approximately half the pitch of the single helices and similar

to the pitch of the twisted ribbons (%20 nm). The templat-

ing fidelity exhibited by these inorganic objects with regular

nanoscale features that repeat for several micrometers is

quite remarkable.

We note that samples in which we found double helices

always contained varying amounts of single helices as well.

However, we very rarely found ribbons that were partly

mineralized as single helices and partly as double helices,

which is somewhat surprising given the length of the ribbons

and the fact that nucleation occurs at so many spots along

the ribbon. We also determined that the duration of miner-

alization had no effect on the relative pro-

portions of single and double helices by

extracting samples at various times after

H2S exposure. This suggests that there is a

structural difference between different

ribbons that leads to the different CdSstructures, rather than being due to a dif-

ference in the kinetics of nucleation and

growth. In ribbons with a perfect twisted

structure, for instance, both faces of the

ribbon would be equivalent, and equally

able to nucleate and grow CdS. Twisted

ribbons with a slightly coiled axis, howev-

er, would have one face more exposed to

the solvent, and would thus be more sus-

ceptible to CdS nucleation and subse-

quent growth. An alternative explanation

is that the two faces of the ribbon are initially equivalent,

but a nucleation event distorts the structure so as to renderthem nonequivalent. However, this seems unlikely consider-

ing that many nucleation events occur along the length of

the ribbon, so such a distortion would have to propagate for

the full length of the ribbon to result in a single helix of

CdS growing along its entire length. Furthermore, this

would not explain why some ribbons are able to grow

double helices.

A structural difference in ribbons may be partly related

to aging, but also seems to be sensitive to conditions of gel

formation; there exist variations between ribbons from dif-

ferent gels in mineralization behavior, both in terms of the

ratio of single-to-double helices and in the fidelity of tem-

Figure 3. HRTEM micrographs of mineralized ribbons: a) A ribbon at

an early stage of mineralization. The inset shows the absence of

lattice fringes, indicating that the CdS is amorphous or only weakly

crystalline at this stage; b) a more fully mineralized ribbon. The inset

shows lattice fringes from the CdS.

Figure 4. Schematic representation of templating pathways. Nucleation and growth on one side of

the twisted ribbons (blue) leads to single helices of CdS (yellow), while nucleation and growth on

both sides of the ribbon leads to double helices.

Figure 5. TEM micrograph of CdS double helices. The inset shows an

enlargement in which the expected braided appearance is clearly

visible.

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plating, under identical mineralization conditions. Gel for-

mation tends to be a kinetic process, and may be extremely

sensitive to minor changes in variables such as temperature

and time. We know that DRC ribbons can adopt different

structures in different solvents, and it may be that there are

minor structural variations among ribbons in EMA that

lead to significantly different mineralization results. Al-

though it is possible to image the ribbons using TEM, it is

not possible to detect extremely subtle differences in struc-

ture for noncrystalline organic materials with this technique.

Our results suggest that supramolecular structures, analo-

gously to covalent structures, can have conformations that

in some cases may be controlled by subtle forces. In the

system studied here, two different conformations seem to

result in the mineralization of the supramolecular structure

into either a single or a double helix. More sensitive techni-

ques, such as circular dichroism spectroscopy, may provide a

more complete understanding of DRC structure that will be

needed to fully explain the details of its mineralization; a

chiral variant of the DRC would be needed for such an ex-

periment.Supramolecular templating is a powerful approach to

the nanoscale organization of inorganic crystals that enables

nucleation, growth, and organization in a single reaction by

using structures with rationally designed functionality. Our

system is unique in producing complex helical organizations

of inorganic nanocrystals with high fidelity on such a small

length scale. Ultimately, such structures may prove to have

interesting and potentially useful properties for nanoscale

devices.

Experimental Section

Gels of the DRC in ethyl methacrylate (EMA) were prepared

at 3 wt.% by breaking up the DRC (30 mg) into a fine powder in

a glass vial, to which was added EMA (1.0 g). The sealed vial

was immediately sonicated, followed by heating in an oil bath at

%758C for 5 min. Gels formed after allowing the solution to sit

undisturbed at room temperature overnight.

DRC ribbons from EMA gels were typically mineralized from

suspensions of a 3 wt.% gel (20100 mg) in EMA (2.0 g).

Cd(NO3)24 H2O in THF (20 mg of a 0.2m solution) was added to

these suspensions and the samples were exposed to H2S gas for

515 min. TEM samples were prepared 120 min after exposure

to H2S by diluting the suspension in EMA (%1:20), depositing a

drop onto a holey carbon grid (SPI Supplies), and wicking awaymost of the drop to leave a thin layer of solvent that evaporated

quickly. Samples were imaged at 100 or 200 kV on a Hitachi H-

8100 TEM. Images were taken on Kodak SO-163 negatives, and

digitized with a Umax PowerLook scanner at a resolution of

1200 dpi.

Keywords:mineralization nanostructures self-assembly

semiconductors template synthesis

[1] a) A. P. Alivisatos, Science 1996, 271, 933 937; b) L. Brus,

Curr. Opin. Colloid Interface Sci. 1996, 1, 197 201; c) C. B.

Murray, C. R. Kagan, M. G. Bawendi, Annu. Rev. Mater. Sci.

2000, 30, 545610.

[2] a) P. V. Braun, P. Osenar, S. I. Stupp, Nature 1996, 380, 325

328; b) G. S. Attard, C. G. Goltner, J. M. Corker, S. Henke, R. H.

Templer, Angew. Chem. 1997, 109, 13721374; Angew. Chem.

Int. Ed. Engl. 1997, 36, 1315 1317; c) S. W. Lee, C. B. Mao,

C. E. Flynn, A. M. Belcher, Science 2002, 296, 892895;

d) B. M. Rabatic, M. U. Pralle, G. N. Tew, S. I. Stupp, Chem.

Mater. 2003, 15, 1249 1255.

[3] a) R. E. Cohen, Curr. Opin. Solid State Mater. Sci. 1999, 4, 587

590; b) S. Frster in Colloid Chemistry 1, Vol. 226 (Ed.: M. Anto-

nietti), Springer, Berlin, 2003, pp. 1 28.

[4] C. A. Mirkin, R. L. Letsinger, R. C. Mucic, J. J. Storhoff, Nature

1996, 382, 607609.

[5] a) H. Bekele, J. H. Fendler, J. W. Kelly, J. Am. Chem. Soc. 1999,

121, 7266 7267; b) A. Berman, D. Charych, J. Cryst. Growth

1999, 199, 796801.

[6] a) D. D. Archibald, S. Mann, Nature 1993, 364, 430 433;

b) A. P. Alivisatos, K. P. Johnsson, X. G. Peng, T. E. Wilson, C. J.

Loweth, M. P. Bruchez, P. G. Schultz, Nature 1996, 382, 609

611; c) J. L. Coffer, S. R. Bigham, X. Li, R. F. Pinizzotto, Y. G. Rho,

R. M. Pirtle, I. L. Pirtle, Appl. Phys. Lett. 1996, 69, 38513853;

d) W. Shenton, T. Douglas, M. Young, G. Stubbs, S. Mann, Adv.

Mater. 1999, 11, 253256; e) M. Knez, A. M. Bittner, F. Boes,

C. Wege, H. Jeske, E. Maiss, K. Kern, Nano Lett. 2003, 3, 1079

1082; f) T. Scheibel, R. Parthasarathy, G. Sawicki, X. M. Lin, H.

Jaeger, S. L. Lindquist, Proc. Natl. Acad. Sci. USA 2003, 100,

45274532; g) H. Yan, S. H. Park, G. Finkelstein, J. H. Reif, T. H.

LaBean, Science 2003, 301, 1882 1884.

[7] a) S. Kobayashi, K. Hanabusa, N. Hamasaki, M. Kimura, H.

Shirai, S. Shinkai, Chem. Mater. 2000, 12, 15231525; b) H.

Matsui, S. Pan, B. Gologan, S. H. Jonas, J. Phys. Chem. B 2000,

104, 9576 9579; c) J. D. Hartgerink, E. Beniash, S. I. Stupp,

Science 2001, 294, 16841688; d) E. D. Sone, E. R. Zubarev,

S. I. Stupp, Angew. Chem. 2002, 114, 1781 1785; Angew.

Chem. Int. Ed. 2002, 41, 1705 1709.

[8] a) J. H. Jung, Y. Ono, K. Hanabusa, S. Shinkai, J. Am. Chem. Soc.2000, 122, 50085009; b) K. Sugiyasu, S. Tamaru, M. Takeu-

chi, D. Berthier, I. Huc, R. Oda, S. Shinkai, Chem. Commun.

2002, 1212 1213.

[9] a) S. L. Burkett, S. Mann, Chem. Commun. 1996, 321322;

b) R. R. Price, W. J. Dressick, A. Singh, J. Am. Chem. Soc. 2003,

125, 11259 11263.

[10] a) E. R. Zubarev, M. U. Pralle, E. D. Sone, S. I. Stupp, J. Am.

Chem. Soc. 2001, 123, 4105 4106; b) E. R. Zubarev, M. U.

Pralle, E. D. Sone, S. I. Stupp, Adv. Mater. 2002, 14, 198203.

Received: January 21, 2005

Published online on May 11, 2005

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