Progress has been made in the development of compact sensor arrays containing molecular electronic devices for detecting molecules of interest (especially biomolecules) with high sensitivity and selectivity. As described in somewhat more detail below, the sensory devices in these arrays are based on chemically functionalized semiconductor nanowires. Because of the small sizes of nanowire-based devices, these arrays could be extremely dense, enabling simultaneous detection of multiple molecular species of interest. In addition, in some cases, it should be possible to extend the limits of detectability to quantities as small as a single molecule.

A Basic Sensor Device of the type described in the text — one of many in an array — includes a semiconductor nanowire that serves as the channel of a field-effect transistor. When molecules of interest become bound to receptors on the nanowire, the electrical conductance of the nanowire changes.
Each device in such an array (see figure) is configured as a field-effect transistor wherein a semiconductor nanowire serves both as part of the gate and as the channel between the source and drain electrodes. The nanowire is chemically functionalized with one or more receptor( s) specific to a molecular species or a group of related molecular species that one seeks to detect. Receptors can range from highly specific nucleic-acid reporters (ribozymes and deoxyribozymes) to simple chemical reporters that respond to multiple related compounds. The binding to, or modification of, the receptor(s) by molecules of interest causes a change in electric charge on the surface of the nanowire, thereby changing the electrical conductance of the nanowire. Thus, a molecular process occurring near the surface of the nanowire is transduced into a measurable electric current along the nanowire.

If a suitable potential (VSD) is applied between the source and drain electrodes and if a suitable bias potential (VGS) is applied between the gate layer and the source electrode, then the resulting change in the current (I) between the source and drain electrodes could serve as an indication of the number of molecules of interest detected. Depending on the specific application, it may be possible to use more than one receptor per nanowire to optimize the response of the device with respect to competing requirements to maximize sensitivity, selectivity, and speed while minimizing device size. By setting the gate bias potential VGS at a suitable value, one could set the electron-donor or -acceptor energy levels in the nanowire so that electric charge would be either drawn into or ejected from the nanowire when the binding occurs. This effect could be exploited to increase sensitivity and selectivity for detecting specific molecules. Alternatively, the gate bias potential could be chosen to modify or restrict certain molecular-recognition events.

An important recent product of this development is a computerized strategy for rapidly generating numerous different deoxyribonucleic acid (DNA)-sensing and ribonucleic acid (RNA)-sensing ribozymes. This strategy involves exploitation of switching and logic functions that have been found to be performed by DNA and RNA molecules and are naturally used in many organisms to control the expression of genes.

Another important recent product of this development effort is an approach for selective functionalization of electrically conducting and semiconducting lithographically patterned sensor components (e.g., semiconductor nanowires) that eliminates the need for alignment and, therefore, is scalable to any size. This approach is exemplified by electropolymerization of derivatized phenols to functionalize the desired patterned surfaces with amine, aldehyde, and carboxylic acid groups. It has been demonstrated that these groups can covalently bind molecular targets, including proteins and DNA. This approach also enables sequential deposition of a myriad of chemical or biochemical receptors at high density on desired surfaces with minimal cross-contamination.

This work was done by Mark Reed, Ronald Breaker, Chongwu Zhou, Robert Penchovsky, Benjamin Boese, Tyler Ames, Lixia Guo, Guosheng Cheng, Elena Cimpoiasu, Ryan Munden, Stan Guthrie, Chao Li, Fumiaki Ishikawa, Steven Jay, James Bertram, Daniel Turner-Evans, Carl Dietz, David A. LaVan, and Tarek Fahmy of Yale University; Ilona Kretzschmar of The City College of New York; and Tadeusz Malinski of Ohio University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at under the Electronics/Computers category. AFRL-0024

This Brief includes a Technical Support Package (TSP).
Dense Functionalized-Nanowire Biosensor Arrays

(reference AFRL-0024) is currently available for download from the TSP library.

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This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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