Discriminating bacteria from other atmospheric particles using femtosecond molecular dynamics

We investigated femtosecond pump–repump depletion schemes in biological fluorophors (tryptophan and riboflavin) in order to discriminate bioaerosols from organic interferents emitted by combustion (traffic related urban aerosols). Although fluorescence depletion is significative for riboflavin (Rbf, Vitamin B2), the most striking results have been obtained for the amino acid tryptophan (Trp). By using a 270 nm-pump 810 nm-repump femtosecond excitation, we showed that Trp exhibits fluorescence depletion up to 50%, contrary to naphthalene (<2%), despite almost identical absorption/emission spectra. We demonstrate that this process in Trp is so robust that it still occurs in living bacteria (Bacillus subtilis, Escherechia coli and Enterococcus fæcalis) but is absent for pure diesel fuel. This remarkable difference between biological and organic aerosols can be exploited to discriminate among them.