Materials Science: On Self-Assembly Building Blocks
ScienceWeek http://scienceweek.com


We are on the verge of a materials revolution in which entirely
new classes of "supermolecules" and particles will be designed
and fabricated with desired features, including programmable
instructions for assembly. These new building blocks will be the
"atoms" and "molecules" of tomorrow's materials, self-assembling
into novel structures made possible by their unique design.

MATERIALS SCIENCE: ON SELF-ASSEMBLY BUILDING BLOCKS

The following points are made by Sharon C. Glotzer (Science 2004
306:419):

1) Self-assembly -- the spontaneous organization of matter into
ordered arrangements -- is a governing principle by which
materials form (1). The patterns arising from self-assembly are
ubiquitous in nature, from the opalescent inner surface of the
abalone shell to the internal compartments of a living cell. Much
of materials science and soft condensed-matter physics in the
past century involved the study of self-assembly of fundamental
building blocks (typically atoms, molecules, macromolecules, and
colloidal particles) into bulk thermodynamic phases. Today, the
extent to which these building blocks can be engineered has
undergone a quantum leap. We are on the verge of a materials
revolution in which entirely new classes of "supermolecules" and
particles will be designed and fabricated with desired features,
including programmable instructions for assembly. These new
building blocks will be the "atoms" and "molecules" of tomorrow's
materials, self-assembling into novel structures made possible
solely by their unique design.

2) What happens when traditional atoms and molecules are replaced
with these new building blocks? What types of ordered structures
are possible, and what unique properties do they have? Colloidal
polyhedra (2), nanocrystals in the form of tetrapods (3) and
triangles (4), and tiny cubes of molecular silica (5) are just a
few examples of new building blocks being made today. In most
cases, these building blocks may not naturally assemble into any
desired structures. One emerging approach to confer upon
nanoparticles and colloids predetermined "instructions" for
assembly is to decorate the surface of the particles with "sticky
patches" made, for example, of synthetic organic or biological
molecules. This strategy takes its inspiration in part from
biology, where the precision of self-assembled structures such as
viruses and organelles originates in the selectivity of the
interactions between their constituents. According to computer
simulations, synthetic "patchy particles" should self-assemble
under the right conditions into structures atypical of
traditional materials.

3) On macroscopic scales, millimeter-sized plastic wedges
patterned with patches of solder and hydrophobic lubricant self-
assemble under surface tension when dispersed in water to form
tiny electronic devices whose structure resembles that of the
tobacco mosaic virus. Making patchy particles with precise
patterns of interactions on nanometer scales is much more
challenging, but exciting developments are being reported. For
example, Jackson et al (2004) synthesized gold and silver
particles 4 nm in diameter, using organic molecules to control
the size of the nanoparticles. Although the use of organic
stabilizing layers is commonplace in nanoparticle synthesis,
these researchers used a mixture of ligands that, on flat
surfaces, would tend to phase separate into bulk phases or random
domains. Instead, the ligands self-organized on the nanoparticle
surface into repeating patterns of stripes and dots with spacings
as small as 0.5 nm, imparting a controllable, precise, and
uniquely small pattern of attractive and repulsive patches to the
surfaces of the particles. Striped spheres and spheres with polar
patches were obtained, providing a striking demonstration of the
role of curvature in pattern formation. This method suggests an
exciting strategy for controlling the symmetry of nanoparticle
assemblies through anisotropic interactions achieved by
patterning. In another example, Mokari et al (2004) patterned
semiconductor tetrapods and nanorods with gold patches on the
tips, potentially providing a new way to assemble components for
nanocomputing devices.

4) Genetic engineering of biomolecules like DNA and proteins
opens up further possibilities for conferring recognition and
chemical specificity to particles, creating building blocks that
are potentially capable of assembling into hierarchically
arranged structures. In a recent twist, a new patchy particle was
synthesized by precisely positioning gold particles onto specific
sites on the surface of the cowpea mosaic virus, creating a new
type of building block with the potential for self- assembly.

References (abridged):

1. G. M. Whitesides, M Boncheva, Proc. Natl. Acad. Sci. U.S.A.
99, 4769 (2002)

2. V. N. Manoharan et al., Science 301, 483 (2003)

3. D. J. Milliron et al., Nature 430, 190 (2004)

4. N. Malikova et al., Langmuir 18, 3694 (2002)

5. R. M. Laine et al., J. Appl. Organomet. Chem. 12, 715 (1998)

Science http://www.sciencemag.org

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Related Material:

MATERIALS SCIENCE: ON MOLECULAR SELF-ASSEMBLY

The following points are made by W.J. Blau and A.J. Fleming
(Science 2004 304:1457):

1) On a molecular scale, the accurate and controlled application
of intermolecular forces can lead to new and previously
unachievable nanostructures. This is why molecular self-assembly
(MSA) is a highly topical and promising field of research in
nanotechnology today. MSA encompasses all structures formed by
molecules selectively binding to a molecular site without
external influence. With many complex examples all around us in
nature (ourselves included), MSA is a widely observed phenomenon
that has yet to be fully understood. Being more a physical
principle than a single quantifiable property, it appears in
physics, chemistry, and biochemistry, and is therefore truly
interdisciplinary (2).

2) The problem to date with research in the fundamental physics
behind MSAs has tended to be that prime examples of MSAs are
mainly found in the biological sciences. Biomolecular assemblies,
such as light-harvesting antenna complexes found in some
bacteria, are sophisticated and often hard to isolate, making
systematic and progressive analyses of their fundamental physics
very difficult. What in fact are needed are simpler MSAs, the
constituent molecules of which can be readily synthesized by
chemists to a high degree of purity -- high-quality sample
preparation, chemical purity, and known sample history that are
paramount in MSA research. These molecules should self-assemble
into simpler constructs that can be easily assessed with current
experimental techniques.

3) Weak intermolecular bonds, such as van der Waals bonds, that
selectively bind molecules to a site in an assembly are what make
MSAs so varied. It would be almost impossible to mimic MSA
complexity using synthetic, aggressive chemistry to join
molecules together via covalent bonding. Although a covalent bond
is much stronger, precursors, acidic/basic conditions, and high
temperatures are required for chemical synthesis. For MSAs,
synthetic chemistry is used only to construct the basic building
blocks (that is, the molecules), and weaker intermolecular bonds
are involved in arranging and binding the blocks together into a
structure. This weak bonding makes solution, and hence
reversible, processing of MSAs possible.

4) The current top-down approach to nanotechnology, whereby
nanostructures are created, manipulated, and modified by machine,
is incapable of offering the complexity and economy of scale that
MSA demonstrates in nature. Thus, solution processing and
manufacturing of MSAs offer the enviable goal of mass production
with the possibility of error correction at any stage of
assembly. It is well recognized that this method could prove to
be the most cost-effective way for the semiconductor electronics
industry to produce functional nanodevices such as nanowires,
nanotransistors, and nanosensors in large numbers (hundreds of
billions at a time). Although these nanodevices will be the first
to become available with MSA research, they are, in fact, the
simpler cousins to the complex integrated nanostructures to come,
in which many different molecules assemble (3-5).

References:

1. J. P. Hill et al., Science 304, 1481 (2004).

2. G. M. Whitesides, B. Grzybowski, Science 295, 2418 (2002).

3. F. Cacialli, P. Samori, C. Silva, Mater. Today 7, 24 (April
2004) [Abstract].

4. J. H. Wu, M. D. Watson, K. Mullen, Angew. Chem. Int. Ed. 42,
5329 (2003).

5. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, Science 297,
787 (2002)

Science http://www.sciencemag.org

--------------------------------

Related Material:

SELF-ASSEMBLY OF METAL NANOSTRUCTURES ON POLYMER SCAFFOLDS

The following points are made by W.A. Lopes and H.M. Jaeger
(Nature 2001 414:735):

1) Self-assembly is emerging as an elegant "bottom-up" method for
fabricating nanostructured materials. This approach becomes
particularly powerful when the ease and control offered by the
self-assembly of organic components is combined with the
electronic, magnetic, or photonic properties of inorganic
components.

2) The authors report a demonstration of a versatile hierarchical
approach for the assembly of organic-inorganic copolymer-metal
nanostructures in which one level of self-assembly guides the
next. In a first step, ultrathin diblock copolymer films form a
regular scaffold of highly anisotropic stripe-like domains.
During a second assembly step, differential wetting guides
diffusing metal ions to aggregate selectively along the scaffold,
producing highly organized metal nanostructures.

3) The authors report that in contrast to the usual requirement
of near-equilibrium conditions for ordering, the metal arranged
on the copolymer scaffold produces the most highly ordered
configurations when the system is far from equilibrium. The
authors delineate two distinct assembly modes of the metal
component -- each mode characterized by different ordering
kinetics and strikingly different current-voltage
characteristics. The authors suggest these results therefore
demonstrate the possibility of guided large-scale assembly of
laterally nanostructured systems.

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com


--
Best,
Frederick Martin McNeill
Poway, California, United States of America
mmcneill@fuzzysys.com
http://www.fuzzysys.com
http://members.cox.net/fmmcneill/
*************************
Phrase of the week :
For the real amazement, if you wish to be amazed, is this
process: You start out as a single cell derived from the coupling
of a sperm and an egg; this divides in two, then four, then
eight, and so on, and at a certain stage there emerges a single
cell which has as all its progeny the human brain. The mere
existence of such a cell should be one of the great astonishments
of the Earth. People ought to be walking around all day, all
through their waking hours calling to each other in endless
wonderment, talking of nothing except that cell. -- Lewis Thomas
(1913-1993)
:-))))Snort!)
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