Ultra-violet fluorescence remains a cornerstone technique for the detection of biological agent aerosols. Historically, these UV based detectors have employed relatively costly and power demanding lasers that have influenced the exploitation of the technology to wider use. Recent advancements from the Defense Advanced Research Project Agency's (DARPA) Solid-state Ultra Violet Optical Sources (SUVOS) program have changed this. The UV light emitting diode (LED) devices based on Gallium Nitride offer a unique opportunity to produce small, low power, and inexpensive detectors. It may, in fact, be possible to extend the SUVOS technology into detectors that are potentially disposable. This report will present ongoing efforts to explore this possibility. It will present the Tactical Biological (TAC-BIO) detector as such a solution for low cost, low power, lightweight device for biological agent detection.
A number of strategies to meet the need for a small and inexpensive biosensor that mitigates military and civilian vulnerabilities to biological weapons are currently being pursued. Among them is UV induced biological fluorescence. UV induced biofluorescence is a potentially successful strategy because it involves no chemical consumables and it is an "on-line" detection method where particles can be interrogated without impaction onto a substrate or into a liquid. Indeed, there are already existing fluorescence based sensors already in place, yet these are limited by the cost and power consumption of the laser based UV excitation sources. Fortunately, inexpensive and low power solid state UV sources arising from the Defense Advanced Research Projects Agency's (DARPA) Semiconductor UV Optical Sources (SUVOS) project have become commercially available in wavelengths capable of exciting aromatic amino acids (e.g. tryptophan) and metabolic products (e.g. NADH). The TAC-Bio Sensor is capable of exploiting either source wavelength and will ultimately include both source wavelengths within a single sensor.
Initial work with the deep UV sources involves the correct optical filtering for the devices. The primary emission from both the 280 nm and 340 nm devices occurs at the design wavelength and is about 20 nm FWHM, however, there is a tail extending to the longer wavelengths that interferes with the fluorescence signal. A system of optical filters that sufficiently removes the long wavelength component from the UV source is designed and tested for the deep UV sources. Ongoing work with the sensor has confirmed that sensitivity to small biological particles is enhanced with the deeper wavelengths. When the 340 nm sources are placed in the TAC-Bio, it is capable of detecting 4 micron diameter Bacillus globigii (BG, Dugway, washed 4X) spore agglomerates. The deep UV sources show an improvement in signal to noise of 2, permitting the detection of 3 micron diameter BG agglomerates.
In light of the current state of detection technologies designed to meet the current threat from biological agents, the need for a low-cost and lightweight sensor is clear. Such a sensor based on optical detection, with real time responses and no consumables, is possible. Devices arising from the Defense Advanced Research Projects Agency's (DARPA) Semiconductor UV Optical Sources (SUVOS) are the enabling technology. These sources are capable of emitting UV wavelengths known to excite fluorescence from biological agent particles while costing a few dollars apiece and consuming low power. These devices are exploited in the TAC-Bio Sensor. A unique optical design is used to collect the usable portion of the LED emission and focus it into the probing region of the sensor. To compensate for the low UV power density relative to UV lasers, the TAC-Bio utilizes a unique opposed flow configuration to increase the interaction between particles and the UV beam. The current TAC-Bio sensor testbed is capable of detecting fluorescence Bacillus globigii (BG, an anthrax simulant) spore agglomerates down to 5 microns in diameter. Ongoing work is focusing on increasing signal to noise so that smaller particles, possibly single spores, can be detected, as well as on including additional data channels, such as light scattering, to increase selectivity of the sensor.