membrane-less fuel cell

1. RESEARCH NEWS
March 200314
Materials systems for information
storage require the ability to be
switched between states of
comparable free energy. A group of
researchers from the University of
Edinburgh, UK, and the Institute for
Nanostructured Materials Studies and
Universitá degli Studi di Bologna, Italy
have investigated thin films of
rotaxanes, believing that the structure
of these molecules will allow
switchable molecular motion. They
show that rotaxane films can be
induced to change structure and so
have potential for novel information
storage systems [Science (2003) 299,
531].
Rotaxanes consist of a molecular
thread that runs through the center of
a cyclical molecule or macrocycle.
Bulky groups on each end of the thread
lock the macrocycle onto the thread.
The authors compare this arrangement
to an abacus, with the macrocycle able
to move up and down the thread.
Atomic force microscopy (AFM) of the
thin films of rotaxane molecules
reveals that, above a threshold load
force, continuously scanning the AFM
tip along a line results in the
production of a series of regularly
spaced dots of uniform size. The
number of dots depends only on the
scan length, and so it is possible to
write information as strings of bits.
Molecular modeling shows the
rotaxane can switch between two
nearly degenerate structures with a
small activation energy. The
researchers believe the AFM provides
this energy. The ease of rotaxane
rearrangement then allows nuclei of
reorganized molecules to coarsen and
give the dots observed.
“With such an approach, information
storage on a thin film could reach
densities of 1-10 Gb/in2,” claim the
authors.
Molecular
abacus
INFORMATION STORAGE
There is currently much interest in the
use of nanoparticles for biomedical
applications, from drug and
biomolecule delivery to biosensors.
While many groups have investigated
the use of spherical particles,
researchers from the University of
Florida and VTT Biotechnology, Finland
are putting the case for nanotubes.
Charles R. Martin and coworkers have
developed a template synthesis
method that produces nanotubes with
a number of advantages. The template
allows control of the dimensions of the
nanotubes, and enables them to be
made out of nearly any material.
Importantly for biomedical
applications, the nanotubes have inner
voids that can be filled with small
molecules or large proteins, and the
inner and outer surfaces can be
modified separately to give distinct
functionalities.
Using silica nanotubes, the group has
performed a series of proof-of-concept
experiments that show the potential of
this approach [JACS (2002) 124,
11864]. First, by derivatizing the outer
surface of the nanotubes, the
researchers controlled partitioning of
the nanotubes into either an organic or
aqueous phase. Second, nanotubes
with hydrophilic outer surfaces and
hydrophobic inner surfaces efficiently
extracted lipophilic molecules from an
aqueous solution. Third, immobilizing
an antibody fragment to the nanotubes
allowed the recognition and molecule-
specific extraction of an enantiomer
from a racemic mixture. Finally, the
group was able to attach enzyme
molecules to give biologically active
nanotubes.
The researchers are keen to stress the
applicability of their method to most
materials, which means that
biodegradable nanotubes for in vivo
conditions could be made.
Doctoring
nanotubes
NANOBIOTECHNOLOGY
Switching surfaces
SURFACE SCIENCE
Using an electrical potential to trigger a conformational change
in a self-assembled monolayer (SAM), researchers from the
Massachusetts Institute of Technology (MIT), University of
California at Santa Barbara, and at Berkeley have shown the
reversible switching of surface wettability [Science (2003) 299,
371]. The group believes the dynamic control of other surface
properties, such as adhesion, friction, and biocompatibility, may
be possible. “This opens the door to a variety of applications,
including novel drug-delivery systems and smart templates for
the bioseparation of one molecule from another,” says Robert
Langer of MIT, who led the team.
The alkanethiolate (16-mercapto)hexadecanoic acid (MHA) has a
hydrophobic chain capped by a hydrophilic carboxylate group
and forms a SAM on a Au surface. The chains have a straight,
upright equilibrium conformation, presenting the carboxylate
groups to the surrounding medium. Under an applied electric
potential, the carboxylate groups are attracted to the Au
surface, causing the molecules to rearrange and expose the
hydrophobic chains.
This approach relies on obtaining a low density monolayer on
the Au surface to give enough space for the conformational
change. Synthesizing an MHA derivative with a bulky endgroup
gives a self-assembled monolayer with the required density. This
end-group is then removed. Sum-frequency generation
spectroscopy confirms the reversible molecular switching in
response to the electrical potential and contact angles for the
SAM showed switching of the macroscopic hydrophilicity.
“This is the first time to our knowledge that anyone has
created a truly reversible switch of a surface’s property
exploiting monomolecular layers,” says Joerg Lahann of MIT.
Membrane-less fuel cell
ELECTROCHEMISTRY
A millimeter-scale fuel cell without the membrane that normally
separates the anode and cathode compartments has been designed
and fabricated by chemists at Harvard University [JACS (2002) 124,
12930]. Instead, the laminar flow at low Reynolds number of two
liquids, one oxidizing and one reducing, is exploited to stop the mixing
of the two fuels.
Fuel cells with this feature have a channel with two inlets and one
outlet running past graphite electrodes. V(II) and V(V) aqueous
solutions flow into the channel to give the redox couples V(III)/V(II) at
the anode and V(V)/V(IV) at the cathode. Three of these cells in series
were able to power a light-emitting diode (LED).
The group, led by George M. Whitesides, also demonstrated some of
the advantages and disadvantages of these membrane-less fuel cells.
The system shows poor efficiency in terms of the ratio of fuel
consumed at the electrodes to that delivered to the cell. However, it
eliminates the ohmic losses and fouling of a membrane, is comparable
in performance to macroscopic fuel cells, and could prove a general
format for electrochemistry in microsystems.