Biomolecules organize nanomaterials: Nanobiotechnology

RESEARCH NEWS

May 200312

The optoelectronic properties of

conjugated polymers are promising for

a number of applications, but

nanoscale patterning of these polymer

films is essential for their use in

multicolored displays, integrated

electronics, or photonic structures.

Researchers at the University of

Cambridge and University College

London, UK, and at the Instituto

Superior Técnico, Portugal, have

developed a lithographic technique to

do just that.

Using a scanning near-field optical

microscope (SNOM), which the team

designed and constructed, well-defined

patterns can be written directly into

conjugated polymer films [R. Riehn,

et al., Appl. Phys. Lett. (2003) 82 (4),

526].

The method works by using thin films

of a soluble poly(p-phenylene vinylene)

or PPV precursor. Exposure to

ultraviolet (UV) light removes a labile

group, making the polymer insoluble.

After the SNOM UV writing step, the

nonexposed material is washed off.

The patterned polymer is then

converted into the fully conjugated PPV

by heating it at 220°C under vacuum.

Features with a minimum size of 160

nm were obtained in this way.

Franco Cacialli and coworkers have

demonstrated the potential of this

SNOM lithography by fabricating a

two-dimensional photonic crystal,

including intentional defects, with a

periodicity suitable for visible light

applications. The researchers claim

that the SNOM lithography technique

can be used to fabricate arbitrary

patterns in any photosensitive

conjugated polymer.

By modeling the near-field illumination

intensity within the polymer film, the

researchers were also able to explain

the effect of UV intensity on feature

size.

Writing withpolymersNANOFABRICATION

Mitsuhiko Shionoya and coworkers at the Universityof Tokyo and the Institute for Molecular Science inJapan have used DNA with modified bases toconstruct discrete one-dimensional metal arrays[Science (2003) 299, 1212]. Instead of thestandard bases, which pair to form the classic DNAdouble helix, the group synthesized DNA strandswith modified metal-binding bases. The introductionof the Cu2+ ions mediates the inter-strand base-pairing. In this way, the researchers aligned up tofive Cu2+ ions along the helix axis. The electronspins on adjacent Cu2+ centers coupleferromagnetically, forming a magnetic chain. Theresearchers hope, in future, to coordinate metalions in more complicated DNA structures.Researchers at The DuPont Company, Delawareand the Massachusetts Institute of Technology (MIT)have recently reported similar work showing thepotential of biological molecules to control thestructural organization of nanoelectroniccomponents. They used phage display, a standard

molecular biology technique, to select short peptidesequences that bind carbon nanotubes (CNTs) withhigh affinity [Nature Materials (2003) 2 (3), 196].The group describe the peptides as ‘specificchemical handles’ that could be used to manipulate,disperse, and functionalize CNTs.

Biomolecules organize nanomaterialsNANOBIOTECHNOLOGY

Inspiration from natureBIOMIMETICS

Learning from mineralization in biologicalsystems, Joanna Aizenberg and coworkers atBell Laboratories and Mat-Sim Research, NewJersey, have succeeded in growing large singlecalcite crystals patterned at the micrometerscale [Science (2003) 229999, 1205]. Theybelieve that their scheme could provide ageneral materials fabrication technique.Crystal formation in biology is normallycontrolled by organic frameworks that directthe nucleation, crystallographic orientation,and extent of mineral deposition. In the sameway, the researchers used an organicallymodified framework to control crystallizationfrom an amorphous CaCO3 phase. Theframework includes specific nucleation sitesand a square array of microscale posts. Theposts define the patterning of the singlecrystal, but also act to release stress andimpurities from the growing crystal phase.In an accompanying article [Science (2003)229999, 1192], Trevor Douglas of Montana StateUniversity says, “This novel model... points toa bright bio-inspired future for materialssynthesis.”

SEM images of micropatterned single calcite crystals in anechinoderm (top), and formed using a template (bottom). (Top image © 2003 American Association for the Advancement ofScience. Bottom image: courtesy of Joanna Aizenberg.)

Cu2+ mediated DNA duplex formation. (© 2003 AmericanAssociation for the Advancement of Science.)