Detection and Identification of Individual Bioaerosol Microparticles
Abstract
Real-time detection and identification of biological aerosols, such as bacteria, viruses, or pollens is a key issue for both environmental and strategic purposes. UV-laser in- duced fluorescence (LIF) is a very efficient technique to detect biological tracers (e.g., amino acids) within airborne microparticles and thus identify bioagents in a mixture of aerosols. In order to obtain selectivity, the fluorescence spectrum of each particle has to be recorded individually. We present the LIF spectra of individual biological particles flowing in the air. The observed spectra reveal the signatures of tryptophan, riboflavin, and NADH. High sensitivity and counting rate are obtained using a novel detection design based on a shot-noise limited 32-anodes photomultiplier. While in- creasing the incident laser energy, parasitic non-linear processes can take place. In particular, we show that the fluorescence spectrum of riboflavin containing microparti- cles is modified by a 2-photon photodegradation- excitation process, which might lead to significant identification errors. However, using ultrashort laser pulses significantly reduces these artefacts since the deposited energy is low. Non-linear multiphoton ex- citation (photodegradation free) can even provide attractive features for bioaerosols identification. In particular, we demonstrated theoretically and experimentally that one-, two-, and three-photon excited fluorescence from dye molecules in spherical microdroplets has an asymmetrical angular distribution and is enhanced in the back- ward direction. Femtosecond excitation allowed us to illuminate the microparticles at high intensity without shape deformation and photodegradation. The enhancement ra- tios (of intensities at 180 and 90) reaches 9 for three-photon excitation. Calculations show a plateau above a given size (1-3 micrometers depending on the process order) under which the enhancement drastically decreases. This change in angular depen- dence might be of interest to distinguish between fine and coarse particles. Because of the reciprocity principle and concentration of the incident wave inside particles, the backward enhancement occurs even with nonspherical particles. This has been demonstrated in the case of clusters of polystyrene (PSL) spheres (monomers of 1 and 2 micrometer diameter) and solid tryptophan particles with a 1 photon excitation (at 266 nm). Backward enhancements are typ. 3 for the PSL clusters and 2 for the tryptophan microparticle.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27..861W