Capabilities for detection of biological and chemical threats are undergoing development.

Acontinuing effort that supplements and complements the one summarized in the immediately preceding article is dedicated to the development of nanowire-based sensor arrays, with emphasis on maximizing the utility of such arrays for real-time sensing of molecular species associated with chemical and biological threats. Like the sensors described in the immediately preceding article, most of these sensors are based on chemically functionalized semiconductor nanowires that are parts of fieldeffect transistors. In addition, some of these sensors are based on piezoelectric nanowire resonators.

This effort is oriented toward aggressively exploiting the unique electronic properties of nanowire-based electronic devices and the potential for integration of these devices with each other and with electronic readout circuitry to obtain the desired sensor-array functionalities. The effort has thus far yielded, and continues to yield, arrays of sensor devices that enable highly robust, ultrasensitive detection and identification of chemical species of interest while also effecting large reductions in the incidence of false positive sensor readings. The following is a summary of results achieved thus far in this effort:

• Detection of Biomolecules in Aqueous Solutions at Low Concentrations. Reversible detection of proteins and of single viruses in aqueous solution at concentrations somewhat less than 10^-12 M has been demonstrated. Concentration-dependent detection has been demonstrated over a range characterized by a ratio of somewhat more than 10⁴ between the extremes of high and low concentration. It was also demonstrated to be possible to detect a compound of interest with high selectivity and sensitivity, even when it is part of a complex mixture in which the other constituents are present at much greater concentrations.

• Detection of Chemical Threats at Low Concentrations. It has been demonstrated that chemical threats (more specifically, several explosive compounds) can be detected in a solution with high selectivity at concentrations somewhat below 100 parts per billion. Concentration-dependent detection of these compounds has been demonstrated over a range characterized by a ratio of somewhat more than 10³ between the extremes of high and low concentration.

• Multiplexing. Multiplexed detection from 10 or more nanowire sensor devices for simultaneous detection of multiple proteins and toxins has been demonstrated. Multiplexed detection from two nanowire sensor devices for simultaneous detection of two viruses has also been demonstrated. Methods of processing utilizing complementary components of electrical responses of multiplexed pand n-type-nanowire sensors to discriminate against false positive readings caused by electrical noise and nonspecific binding have been demonstrated.

• Analysis of Sensor Readings. Methods of computational analysis of raw sensor output signals to extract the maximum useful information have been developed and demonstrated to be, as intended, robust to all expected sources of electrical noise (e.g., faulty nanowires) and to chemical contamination.

This work was done by Charles M. Lieber of Harvard University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. AFRL-0025

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