Michael C. McAlpine &
James R. Heath talk with ScienceWatch.com and
answers a few questions about this month's Fast Breaking
Paper in the field of Materials Science. The authors
have also sent along images of their work.
Article Title: Highly ordered nanowire arrays on
plastic substrates for ultrasensitive flexible chemical
sensors
Authors:
McAlpine,
MC;Ahmad, H;Wang,
DW;Heath,
JR
Journal: NAT MATER
Volume: 6
Issue: 5
Page: 379-384
Year: MAY 2007
* CALTECH, Div Chem & Chem Engn, Pasadena, CA 91125
USA.
* CALTECH, Div Chem & Chem Engn, Pasadena, CA 91125
USA.
Why do you think your paper is highly
cited?
We believe that this paper is of general interest to the broad materials
community because it assimilates the areas of semiconductor nanowires,
chemical sensors, electronic noses, and high-performance electronics on
plastic substrates.
Our work was motivated by the challenge of overcoming the scientific
hurdles to these achievements, the commercial possibilities of these
advances, and concerns for health and safety of the general public. We
believe that the unique results presented in this paper could have
far-reaching impact on a host of sensing and medical applications.
Obtaining such high sensitivity sensors on plastic substrates is truly a
niche area for nanowires, and unprecedented in the literature. Immediate
applications include highly portable chemical and biological threat
detectors, and real-time pollution regulators. More intriguingly, since
plastic is biocompatible, the possibility exists for fully implantable or
wearable continuous health monitoring systems.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The paper describes a reliable and scalable method for constructing highly
ordered single-crystal silicon nanowire sensor arrays on plastic
substrates. The sensitivity of these sensors is competitive with the best
reported nanowire sensors on conventional inorganic substrates. We also
prepare a fully integrated nanowire sensing library, or "nano-electronic
nose," and demonstrate its utility. Many of these demonstrations are
"firsts."
Finally, this paper represents a key early step towards the long-term goal
of implementing wearable or even implantable biomolecular or chemical
sensing devices. In this sense, the work is a combination of discovery
(that nanowire sensors on plastic can have state-of-the-art sensitivities),
methodology (the process of transferring nanowires to biocompatible
plastic), and synthesis of knowledge (the integration of these sensors into
electronic noses).
Would you summarize the significance of your paper
in layman's terms?
We set out to show that highly ordered films of silicon nanowires can be
literally glued onto pieces of plastic to make flexible sensors with
state-of-the-art sensitivity to a range of toxic chemicals. Nanowires are
crystalline wires 1000x smaller than human hair, made out of doped
silicon—the mainstay of the computer industry.
James R. Heath
(Classic Science
Watch® Newsletter
interview.)
By etching nanowires into a wafer of silicon, and then peeling them off and
transferring them to plastic, we developed a general, parallel, and
scalable strategy for achieving high-performance electronics on low-cost
plastic substrates. Significantly, when we exposed these films to vapors of
the hazardous pollutant NO2 (car exhaust), the plastic sensors
detected concentrations as low as 20 parts-per-billion in air. This
performance is competitive with the very best sensors on rigid substrates,
and is less than half the EPA's health exposure metric (53 ppb). These
results should be appealing to both scientists and average consumers alike,
by providing a new platform for lightweight and portable sensors.
How did you become involved in this research, and
were there any problems along the way?
Our previous work in this area suggested that it should be possible to
obtain high-performance electronic systems on plastic substrates by a
simple nanowire assembly process. In general, achieving high-performance
electronics or sensors on plastic substrates is difficult, because plastics
melt at temperatures above ~120ºC. Unfortunately, high-quality
semiconductors (such as silicon) require high growth temperatures, so their
application to flexible plastics is prohibited.
Our approach bypasses this problem by first creating the nanowires from
high-quality silicon, and, in a separate step, assembling these nanowires
on plastic substrates under ambient conditions. This nanowire assembly and
the subsequent fabrication of an ultra-sensitive electronic nose required
unprecedented advances in assembly, integration, and device reliability of
nanoscale materials. Yet, the procedure involved only standard
microfabrication techniques, and thus is fully compatible with large-scale
industrial processes.
Where do you see your research leading in the
future?
In our view, the most significant challenge in this area is developing the
ability to detect chemicals with high specificity, and do it even in a
complex molecular mixture background. In other words, to design a universal
platform of sensors which can be programmably "tuned" to respond only to
analytes of interest and reject all others. Indeed, results since
submission of this work suggest that selectivity towards small molecules in
the gas phase is dramatically increased simply by chemically modifying the
nanowire surfaces with biorecognition peptides, with the result that our
selectivity towards small molecules in the gas phase is dramatically
increased.
In fact, the nanowire sensor arrays themselves may provide for the best
platform for identifying peptide binders to small molecules such as butane
and acetone. Such peptide screening against small molecules is a task that
could not be done using more traditional methods such as bead-based
fluorescence assays or surface-plasmon resonance.
Do you foresee any social or political
implications for your research?
We envision that such high-sensitivity sensors on biocompatible substrates
can be used for continuous, non-invasive monitoring of human health for the
prevention and early detection of diseases. I'll describe just one
application: continuous monitoring of human breath for specific disease
biomarkers for diseases ranging from cancers to heart disease. It turns out
that butane and acetone, among other molecules, are key biomarkers of
oxidative stress (such as can arise from cancer) and can be sensed in the
breath, but are present in sub-parts-per-million quantities.
Other than using gas chromatography/mass spec, there may not be another way
to detect such small, relatively unreactive molecules in the breath. In
fact, this application is our current focus. Finally, the work described in
our paper sets the initial foundation for the implementation of label-free
biological electronic-based sensors on biocompatible plastics, which could
be used for flexible or even implantable health monitoring systems.
Professor Michael C. McAlpine
Princeton University
Department of Mechanical and Aerospace Engineering
Princeton, NJ, USA
Professor James R. Heath
Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, CA, USA