Detection of bacterial endospores by means of ultrafast coherent raman spectroscopy

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Title: Detection of bacterial endospores by means of ultrafast coherent raman spectroscopy
Author: Pestov, Dmitry Sergeyevich
Abstract: This work is devoted to formulation and development of a laser spectroscopic technique for rapid detection of biohazards , such as Bacillus anthracis spores . Coherent anti -Stokes Raman scattering (CARS ) is used as an underlying process for active retrieval of species -specific characteristics of an analyte . Vibrational modes of constituent molecules are Raman -excited by a pair of ultrashort , femtosecond laser pulses , and then probed through inelastic scattering of a third , time -delayed laser field . We first employ the already known time -resolved CARS technique . We apply it to the spectroscopy of easy -to -handle methanol -water mixtures , and then continue building our expertise on solutions of dipicolinic acid (DPA ) and its salts , which happen to be marker molecules for bacterial spores . Various acquisition schemes are evaluated , and the preference is given to multi -channel frequency -resolved detection , when the whole CARS spectrum is recorded as a function of the probe pulse delay . We demonstrate a simple detection algorithm that manages to differentiate DPA solution from common interferents . We investigate experimentally the advantages and disadvantages of near -resonant probing of the excited molecular coherence , and finally observe the indicative backscattered CARS signal from DPA and NaDPA powders . The possibility of selective Raman excitation via pulse shaping of the preparation pulses is also demonstrated . The analysis of time -resolved CARS experiments on powders and B . subtilis spores , a harmless surrogate for B . anthracis , facilitates the formulation of a new approach , where we take full advantage of the multi -channel frequency -resolved acquisition and spectrally discriminate the Raman -resonant CARS signal from the background due to other instantaneous four -wave mixing (FWM ) processes . Using narrowband probing , we decrease the magnitude of the nonresonant FWM , which is further suppressed by the timing of the laser pulses . The devised technique , referred to as hybrid CARS , leads to a single -shot detection of as few as 104 bacterial spores , bringing CARS spectroscopy to the forefront of potential candidates for real -time biohazard detection . It also gives promise to many other applications of CARS , hindered so far by the presence of the overwhelming nonresonant FWM background , mentioned above .
URI: http : / /hdl .handle .net /1969 .1 /85958
Date: 2008-10-10

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Detection of bacterial endospores by means of ultrafast coherent raman spectroscopy. Available electronically from http : / /hdl .handle .net /1969 .1 /85958 .

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