Nanowire electron scattering spectroscopy (NESS) has been proposed as the basis of a class of ultra-small, ultra-low-power sensors that could be used to detect and identify chemical compounds present in extremely small quantities. State-of-the-art nanowire chemical sensors have already been demonstrated to be capable of detecting a variety of compounds in femtomolar quantities. However, to date, chemically specific sensing of molecules using these sensors has required the use of chemically functionalized nanowires with receptors tailored to individual molecules of interest. While potentially effective, this functionalization requires labor-intensive treatment of many nanowires to sense a broad spectrum of molecules. In contrast, NESS would eliminate the need for chemical functionalization of nanowires and would enable the use of the same sensor to detect and identify multiple compounds.

Inelastic Scattering of Electrons by molecules on the surface of a nanotube would affect the currentversus- voltage characteristic of the nanotube.
NESS is analogous to Raman spectroscopy, the main difference being that in NESS, one would utilize inelastic scattering of electrons instead of photons to determine molecular vibrational energy levels. More specifically, in NESS, one would exploit inelastic scattering of electrons by low-lying vibrational quantum states of molecules attached to a nanowire or nanotube (see figure). The energy of the electrons is set by the voltage bias applied across the nanowire. When the electron energies correspond to particular molecular vibrational levels, enhanced electronic scattering will lead to a change in the differential conductance (dI/dV, where I is current and V is voltage) at that voltage. Thus changes in the conductance provide a direct readout of molecular vibrational energies, to enable spectroscopic identification of the attached molecules.

To realize a practical chemical sensor based on NESS, one would need a narrow-energy-band electron source, efficient coupling between the electrons and the molecules of interest, and the narrow vibrational bands in the molecules of interest. A carbon nanotube (CNT) provides a nearly ideal structure for satisfying the electron-source and coupling requirements for the following reasons: Even at room temperature, the energy bands in one dimensional carbon nanotubes are narrow, and low-energy electrons travel ballistically over distances of the order of a micron, so that injected electrons can have a nearly uniform kinetic energy. Because single-walled CNTs are essentially all “surface,” there is strong coupling between electrons and molecules on their surfaces.

Other than CNTs, nanowires of silicon and perhaps other materials may yield usable NESS signals, though the signals are expected to be smaller than those from CNT-based sensors. One might need non-CNT nanowire NESS sensors to detect molecules that do not readily bind to CNTs.

In order to simplify the interpretation of a complex spectrum from a mixture of compounds, a NESS-based sensor could be integrated with a microfluidic separation column. The column would enable separation and concentration of individual species, which would then be detected and identified at the column outlet by use of NESS.

This work was done by Brian Hunt, Michael Bronikowsky, Eric Wong, Paul Von Allmen, and Fabiano Oyafuso of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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This article first appeared in the July, 2009 issue of NASA Tech Briefs Magazine.

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