Microsensors Focus Area
Optical, piezoresistive, and piezoelectric based sensors are being developed by the SST Partnership for a variety of physical, chemical, and biological applications. Among the piezoactive devices being developed are thin film strain sensors and pressure sensors for use at elevated temperatures. Optical sensors based on fluorescence and chemiluminescence and piezoelectric sensors have been developed for testing for pathogens in food. In one type of sensor, pathogen contamination in seafood can be detected at much lower levels than can be achieved with other types of sensors. Cost effective temperature and humidity sensors are also being developed within the SST Partnership, as well as optical sensors to measure the cumulative strain in structural components for damage assessment.
An example of one type of microsensor is a biosensor. An effort by one of the SST research teams in this area is described below.
Biosensors use antibodies or enzymes to interact selectively with targeted toxins or pathogens and then one of several possible transduction mechanisms to detect that interaction. In this inherently multidisciplinary field, the SST has been developing rapid and sensitive sensors for food pathogens such as Salmonella typhimurium and Escherichia coli. Food-borne illnesses afflict millions of people and incur costs of hundreds of millions of dollars each year, but standard laboratory testing methods are expensive and require two or three days to process. Food-processing plants and wholesale and retail sales outlets have a great need for rapid, on-site sensors that are sensitive enough to detect trace amounts of food-borne pathogens. Several kinds of biosensors are being studied by SST, including those based on fluorescence, surface-enhanced infrared absorption (SEIRA) spectroscopy, and quartz crystal microbalance.
One approach is to use an acoustically enhanced fluorescence sensor. It starts with a standard "sandwich" immunoassay in which a primary antibody captures a pathogen (e.g., Salmonella typhimurium), which, in turn, captures one or more fluorescently labeled antibodies. For efficient pathogen capture, the primary antibodies must be distributed throughout the sample volume; however, for efficient detection of the sandwich complexes, the concentrations must create the correct optical excitation-to-detection volume ratio. To circumvent this problem, the primary antibodies are immobilized on polystyrene microspheres 5-10 mm in diameter. These microspheres can be distributed throughout the cell initially and then manipulated with an ultrasonic standing wave into the immediate vicinity of a step-tapered optical fiber aligned along the axis of the cell. The detected fluorescence signal can be increased by a factor of 20 this way. This research is led by professors Chris Brown (chemistry), Stephen Letcher (physics), and Garth Rand (food science); students that have been involved with the research have included Sibel Babacan, He Cao, Gi-Ho Kim, James Lyons, John Seelenbinder, and Chongua Zhou.
The U.S. Department of Agriculture provides partial funding for this research.