FAST CARS: Engineering a laser spectroscopic technique for rapid identification of bacterial spores

Abstract

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time. There is an urgent need for the rapid assay of chemical and biological unknowns, such as bioaerosols. Substantial progress toward this goal has been made over the past decade. Techniques such as fluorescence spectroscopy (1, 2) and UV resonant Raman spectroscopy (3-7) have been successfully applied to the identification of biopolymers, bacteria, and bioaerosols.At present, field devices are being engineered (1) that will involve an optical preselection stage based on, e.g., fluorescence radiation as in Fig. 1. If the fluorescence measurement does not give the proper signature then that particle is ignored. Most of the time the particle will be an uninteresting dust particle; however, when a signature match is recorded, then the particle is selected for special biological assay (see Fig. 1b). The relatively simple fluorescence stage can very quickly sort out some of the uninteresting particles whereas the more time-consuming bio-tests will be used for only the ''suspects.''The good news about the resonance fluorescence technique is that it is fast and simple. The bad news is that although it can tell the difference between dust and bacterial spores it cannot differentiate between spores and many other organic bioaerosols (see Fig. 1c).However, despite the encouraging success of the abovementioned studies, there is still interest in other approaches to, and tools for, the rapid identification of chemical and biological substances. To quote from a recent study (8): ''Current [fluorescence-based] prototypes are a large improvement over earlier stand-off systems, but they cannot yet consistently identify specific organisms because of the similarity of their emission spectra. Advanced signal processing techniques may improve identification.'' Resonant Raman spectra hold promise for being spore specific as indicated in Fig. 2b. This is the good news, the bad news is that the Raman signal i...

Theory of selective excitation in stimulated Raman scattering

“…In [9] the use of adaptive techniques in FAST CARS is explored for an identification of related biological objects such as bacterial spores.…”

Comparison of coherent and spontaneous Raman microspectroscopies for noninvasive detection of single bacterial endospores

“…The choice of bacteria for our CARS experiments is mostly driven by the necessity of fast remote detection and recognition of anthrax (1)(2)(3). In particular, we study Bacillus subtilis endospores, which resemble anthrax endospores, and are, unlike anthrax, harmless.…”

“…1 shows microscopic images of B. subtilis endospores, obtained by regular and phase-contrast microscopy. As is well known, the calcium salt of 2,6-pyridinedicarboxylic acid [or dipicolinic acid (DPA)] is a major chemical component of bacterial endospores, accounting for Ͼ15% of its molecular weight (1). Recently, it was shown that the sensitivity of CARS spectroscopy is sufficient to allow discrimination of DPA against other similar molecules (4).…”

Femtosecond coherent anti-Stokes Raman scattering measurement of gas temperatures from frequency-spread dephasing of the Raman coherence

“…The fact that this ratio is approximately equal for all three temperatures indicates that all the Raman transitions are excited with the same phase, as would be expected for near-transformlimited pulses. 8 The excitation of the Raman coherence is discussed in detail by Scully et al 9 For longer delay times, the CARS signal decays because the Raman coherences for the various transitions oscillate at slightly different frequencies, resulting in a decay of the initial macroscopic Raman polarization. At higher temperatures the N 2 population is spread over a greater range of rovibrational levels.…”