As world governments have grown more attentive to the potential threat of biological weapons, a premium has been placed on developing technologies that can detect and identify airborne pathogens. Such technologies also would be useful for public health, spotting allergens and bacteria in the atmosphere. Now a team of researchers from Université Claude Bernard Lyon 1 in Villeurbanne, France, has demonstrated that two-photon-excitation lidar is suitable in principle for the remote detection and identification of biological aerosols.
   Currently, the favored approach for such detection is polymerase chain reaction, said Jean-Pierre Wolf, a professor at the university. Samples are collected in the field and returned to the lab for analysis. This method, however, is time-consuming, and the results do not easily yield information with which scientists can predict the spread of a bio-aerosol.

The Teramobile nonlinear lidar system yielded a fluorescence signature for riboflavin that was clearly different from that of pure water, indicating that the approach should be suitable for the remote detection of bio-aerosols.

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   In theory, lidar is an attractive tool for the detection and identification of these particles in nearly real time. Microscopic biological materials can produce a backscattered fluorescence signal that indicates their presence and their location in three-dimensional space – relative to the lidar system – and that carries characteristic spectral information enabling their identification.
   Unfortunately, many interesting molecules in these bio-aerosols, such as amino acids, are directly excited to fluoresce by ultraviolet radiation, to which the atmosphere is relatively opaque because of Rayleigh scattering and ozone absorption. The researchers, for example, calculate that the detection limit of a lidar system using the best commercially available 266-nm Nd:YAG laser would be on the order of only a few hundred meters in a high-ozone urban environment.
   Nonlinear lidar, in contrast, does not suffer from this limitation. In the process, the biological molecule is excited to fluoresce not by one high-energy photon, but by two lower-energy ones. Crucially, the atmosphere is much more transparent at 530 nm, greatly increasing the detection limit of a two-photon-excitation lidar system, in much the same way that 800-nm excitation sources in two-photon microscopy exploit the lower scattering by tissues in the near-IR to achieve better penetration than 400-nm sources. The team estimates that a system employing frequency-doubled Nd:YAG lasers (which would have to display pulsed performance better than is possible today) would be capable of detecting tryptophan in bio-aerosols at concentrations of 10 bacteria per cubic centimeter at a distance of 4 km in the typical urban environment.
   In a proof-of-principle demonstration of the technique, the scientists induced fluorescence in plumes of 1-µm-diameter droplets of an aqueous solution containing 0.02 g/l of riboflavin. As an excitation source, they employed the Teramobile 5-TW mobile lidar system, which is based on a Thales Optics chirped pulse amplified Ti:sapphire laser that produces 80-fs pulses of 800-nm radiation at a repetition rate of 10 Hz. Spectral data from the plume were collected from a distance of 45 m and displayed a clear fluorescence signature for riboflavin.
   Wolf said that the experiment illustrated the importance of controlling the parameters of the excitation pulses. If the pulse were too short, the fluorescence response would be lost in a white-light signal generated in the air. If the pulse were too long, however, the on-target intensity would be insufficient to induce fluorescence. In the work, 1-ps pulses, corresponding to on-target intensities of 10¹¹ W/cm², produced the best response.
   A practical system will require an excitation source that can produce similar intensities on the scale of kilometers but at a wavelength of approximately 530 nm. Until such lasers emerge, the researchers plan to continue their investigations, including studying pump-probe schemes that would enable them to measure the size as well as composition of airborne particles so that they may distinguish bio-aerosols and soot.

by Daniel S. Burgess