Terahertz detection of Bacillus thuringiensis spores in DiPel®

Frequency domain terahertz (THz) spectroscopy was used to study DiPel^® PRO DF, a commercial insecticide containing 54% Bacillus thuringiensis subsp. kurstaki. The mixture contains Bt spores as well as Bt protein toxins. Microscopic imaging revealed that DiPel^® is composed of millimeter scale coarse particles and SEM photos indicate Bt spores are in these particles. The substantial attenuation from transmission measurements suggested strong signal scattering since the dimensions of particles (~1 mm-400 μm) are close to the THz wavelengths (~750-250 μm). Alternatively, periscope reflection measurements were carried out and a 917 GHz absorbance signature was observed. This is explained by the Mie theory that a portion of absorption is accompanied with scattering. Resonant absorption can be excited as long as internal vibration exists within particles. Therefore, absorption can be detected by the reflectivity measurements. To prove this hypothesis, Bt spores were separated from the large DiPel^® coarse particles and their presence was again confirmed with microscopy. A transmission scan of the extracted spore samples was then repeated. The 917 GHz absorbance signature was present and consistent with transmissions on culture-grown, freshly harvested Bt spore samples.