Wavelength Comparison Study for Bioaerosol Detection

This paper reports on an investigation into optimal excitation and emission wavelengths for bioaerosol detection. Excitation/Emission Matrix (EEM) fluorescence data were gathered for a variety of materials, including biowarfare (BW) simulants, cell constituents, growth media and known interferents. These data were used to investigate multi- wavelength discrimination algorithms using pattern classification techniques. The results suggest that using two excitation wavelengths and narrower emission bands can improve discrimination between BW agents and interferents. Ultraviolet laser-induced-fluorescence (LIF) is a technique for discriminating between biological and non-biological aerosols based on detecting certain amino acids and enzymes present in living organisms. However, LIF-based biosensors suffer from high false alarm rates in some environments. The problem of false alarms in these sensors is due to the presence of interferents that mimic the fluorescence signatures of biological materials. Known interferents include diesel exhaust, cleaning solutions, clothing fibers, skin cells, and a variety of other substances. Previous studies carried out at Lincoln Laboratory have suggested that the use of multiple excitation wavelengths could improve discrimination between bioagents and interferents. We have been investigating optimal excitation and emission wavelengths for bio-aerosol detection in support of the DARPA-sponsored Semiconductor Ultraviolet Optical Sources (SUVOS) program. One area of investigation was to gather Excitation/Emission Matrix (EEM) data for a variety of materials including BW agent simulants, cell constituents growth media and known interferents. The other area was to develop multi-wavelength discrimination algorithms using these data. This paper reports on recent research results in these areas. The LIF principle as applied to bioaerosols is illustrated in Figure 1 for an anthrax spore. Spore constituents such as amino acids (Tryptophan, Tyrosine, and Phenylalanine) and coenzymes (NADH and flavins) fluoresce when excited at characteristic wavelengths. These excitation and emission wavelengths are illustrated in Figure 2, with the size of the typeface approximately indicating fluorescence intensity. This type of diagram is known as an Excitation/Emission Matrix (EEM). At first sight, it would seem to be simple to detect bioaerosols by simply exciting and sensing at the appropriate wavelengths. However, two problems occur. The first is that real bioaerosols do not fluoresce at exactly the wavelengths expected based on measurements of pure constituents. This is illustrated in Figure 3 which shows that the actual EEM signatures of various bioagent simulants differ somewhat from the signatures of pure constituents, which are indicated by the superimposed red boxes. The second problem is that interferents can occur in the environments that mimic the constituent signatures. Figure 4 illustrates this phenomenon for some common interferents such as diesel exhaust, detergent (which contains dye that causes fluorescence in clothing fibers), household dust and pollen. In particular, diesel exhaust has an EEM signature that mimics Tryptophan, and it has been observed that diesel exhaust indeed is associated with false alarms in LIF-based sensors.