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Nanostructured Sensors for Homeland Security Applications

Written by: 
Jeff Morse, PhD.

Golightly Figure 1c
Particle-based approaches for SERS: SEM image of silver octahedra assembled into a close-packed film.
An important beneficiary of emerging nanomanufacturing techniques has been in the area of detection and sensing for homeland security and defense applications. Examples of sensor technologies directly benefiting from nanostructured materials and processes include ultra-microelectrode arrays, which effectively increase the surface area of an electrochemical detection electrode by incorporation of three-dimensional (3D) nanostructures, or Surface Enhanced Raman Scattering (SERS), which detects biological or chemical species by analyzing the shift in SERS spectrum due to species adsorbing on 3D nanostructures over the detection surface. Other sensors utilize nanowires or nanotubes of specific materials that exhibit a change in resistance as a result of chemical or biological molecules adsorbing to the surface.

While the methods for applying these and other sensors remain application specific, issues associated with specificity, selectivity, sensitivity, and operating environment remain. As the understanding of sensor operations has improved, advances in nanofabrication techniques have rendered the design and fabrication of the sensor devices more precise. Combined with improved theoretical models for optimization and prediction of sensor performance, the ability to produce nanostructured sensors by design is rapidly advancing these technologies from laboratory science a few years ago, to manufacturing and deployment today.

An important factor behind this trend is the transition out of the laboratory of various approaches for patterning and synthesis of nanostructured materials on a specific surface. For example, early approaches for SERS sensors used gold or silver nanoparticles dispersed over a surface. Subsequent developments synthesized novel shapes of SERS-active nanoparticles and structures, combining this with nanopatterned templates to precisely position the structure on the surface. Additional approaches combining top-down processes, such as electron beam lithography, with bottom-up processes, such as electrodeposition or self-assembly, have proven successful in producing ordered arrays of high aspect ratio nanostructures over large areas. Further incorporation of appropriate surface functionality to the nanostructured elements can enhance the specificity of the sensor, which may be critical for detection of unknown threats from complex environments.  As such, the sensor technologies and methods have reached a critical stage wherein the nanofabrication and synthesis route to achieve the desired structured has been established, and critical understanding of the limitations (selectivity, sensitivity, environmental, etc.) of the sensor is being investigated. These are critical steps necessary for implementing the sensor technology in the field.

As further examples of the visibility that nanostructured materials have received recently for sensor applications, a symposium titled “Nanorods and Microparticles for Homeland Security” was organized as part of the 238th ACS National Meeting in Washington D.C. The symposium of invited talks brought together experts from academia, government and industry to highlight the progress made in sensor R&D for the detection of biological and chemical substances relevant to homeland security and defense applications, as well as fundamental advances in the field. Additionally, Golightly et. al. published a recent review article summarizing the highlights of this event, along with an overview and comparison of other methods for fabrication and implementing specific sensor technologies. The key take away message is that nanofabrication and manufacturing methodologies have emerged to enable the design, synthesis, and subsequent scale-up manufacturing of sensors utilizing nanostructured elements. This now allows sensor R&D to focus on application specific issues and  continue the development of this important technology for wide spread application and deployment.


Image reproduced with permission from Golightly RS, et. al. 2009. Surface-Enhanced Raman Spectroscopy and Homeland Security: A Perfect Match? ACS Nano 3(10):2859-2869. DOI: 10.1021/nn9013593. Copyright 2009 American Chemical Society.