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Michael C. McAlpine & James R. Heath talk with 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.
McAlpine 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.

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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

Keywords: nanowire arrays, plastic substrates, flexible chemical sensors, single-crystal silicon nanowire sensor arrays, semiconductor nanowires, doped silicon, chemical sensors, nano-electronic noses, biological threat detectors, real-time pollution regulators, implantable health monitoring systems.


2008 : June 2008 - Fast Breaking Papers : Michael C. McAlpine & James R. Heath