RESEARCH NEWS

July/August 2005 15

Actuator materials change their

dimensions when subjected to stimuli

such as heat, electric voltage, or light.

The result is a mechanical response

much larger than the original input.

University of Cambridge researchers

have prepared mechanical actuators by

embedding multiwalled carbon

nanotubes (MWNTs) in a polymer matrix

[Ahir and Terentjev, Nat. Mater. (2005),

doi:10.1038/nmat1391]. Other

studies of nanotube actuators focused

on bending individual MWNTs under an

applied electric field, not a collection of

tubes in a continuous elastic matrix.

MWNT purity was >95% and surfaces

were not modified during processing.

The matrix was polydimethylsiloxane

crosslinked with a hydrosilane without

filler particles. The polymer is sheared

carefully to prevent air bubble

formation during MWNT addition.

MWNTs were dispersed at 0, 0.02,

0.5, 1, 4, and 7 wt.%, and films cast.

The input energy for actuation was

supplied by infrared radiation. For

small pre-strain, when stimulated by

photons, expansion of the composite is

orders of magnitude greater than that

of the parent polymer without MWNTs.

For larger pre-strain, the composite

contracts. The team suggests that the

two different behaviors result from the

relative amount of MWNT alignment.

Samples with different MWNT loadings

all cross over from expansion to

contraction at ~10% pre-strain. There

was no change in stress reading after

exposure to infrared radiation, so heat

transfer from the irradiated face into

the bulk is not significant. The

reproducibility indicates that no

degradation from nonradiative photon

decay occurs.

Team leader Eugene M. Terentjev thinks

that no other materials display such a

large, continuously reversible response.

John K. Borchardt

Photo-inducedactuationCOMPOSITES

In dye-sensitized solar cells (DSCs), transport ofphotogenerated electrons in a polycrystalline film ofrandomly connected oxide nanoparticles occurs bytrap-limited diffusion. But this is slow and limitscarrier collection efficiency, especially at redwavelengths. Now, Peidong Yang’s group at theUniversity of California, Berkeley and LawrenceBerkeley National Laboratory has reported the firstDSC with an ordered photoanode of continuous,crystalline conduits for electrons [Law et al., Nat.Mater. (2005), doi: 10.1038/nmat1387]. The DSC is based on a dense array of oriented,single-crystal ZnO nanowires. The nanowires’electron diffusivity is 0.05-0.5 cm2s-1, severalhundred times larger than for TiO2 or ZnOnanoparticle films. Currents are higher than for ZnOnanoparticle cells, and current densities the same,for equal surface area, as TiO2 nanoparticle cells. Efficiency is 1.2-1.5%. The aim is to grow arrayswith higher surface area (allowing higher dye

loadings) and exceed particle cell efficiencies of10%, says Yang. Nanowires are even more compelling for other typesof excitonic photocells, says Yang, so the group isalso working on inorganic-polymer hybrid cells. Mark Telford

Dye-sensitized solar cells get wired NANOTECHNOLOGY

Nanotube templates for lipid bilayers NANOTECHNOLOGY

Integrating carbon nanotubes with biologicalmachines and environments is necessary todevelop materials for applications such asnew types of biosensors andbionanoelectromechanical systems. However,carbon nanotube surfaces are quite resistantto functional attachment of biomolecules andshow a high degree of nonspecific adhesion,which is detrimental to device performance.Current attachment methods do not enablebiomolecules to be oriented or functionalized. Now, Aleksandr Noy and coworkers atLawrence Livermore National Laboratory andthe University of California, Davis havedeveloped lipid bilayers that spontaneouslyassemble in a continuous shell around ananotube template. The nanotubes arewrapped with hydrophilic polymer layers,which act as assembly templates forcommon phospholipids and provide a supportsurface to maintain the bilayer structure[Artyukhin et al., J. Am. Chem. Soc. (2005),127 (20), 7538]. These one-dimensional lipid bilayers are fluid.Fluorescence recovery measurements afterphotobleaching indicate the lipid moleculesare laterally mobile, despite the high bilayerstrain. The bilayers are stable enough tosurvive multiple bleaching-recovery cycles.

Strong coupling of the lipid mobility to theproperties of the underlying substrateprovides the ability to fine tune the diffusionproperties of the one-dimensional bilayers.These layers provide an efficient andbiocompatible barrier between the nanotubesurface and the surrounding solution. Membrane protein channels that penetratethe bilayers could serve to control access ofsmall molecules to the nanotube surface.Introduction of other types of proteins intothe lipid bilayers could enable the assemblyof more complex nanostructures. Thesestructures could lead to new classes ofbiosensors and bioelectronic devices. John K. Borchardt

Schematic of a carbon nanotube coated with hydrophilic polymer

layers then a lipid bilayer. (© 2005 American Chemical Society.)

Scanning electron micrograph cross section of a nanowire array

on F:SnO2 substrate. (Courtesy of Peidong Yang.)