Raman scattering and red fluorescence in the photochemical transformation of dry tryptophan particles

Tryptophan is a fluorescent amino acid common in proteins. Its absorption is largest for wavelengths lambda less than or similar to 290 nm and its fluorescence emissions peak around 300-350 nm, depending upon the local environment. Here we report the observation of red fluorescence near 600 nm emerging from 488-nm continuous-wave (CW) laser photoexcitation of dry tryptophan (Trp) particles. With an excitation intensity below 0.5 kW/cm(2), dry Trp particles yield distinctive Raman scattering peaks in the presence of relatively weak and spectrally broad emissions with lambda similar to 500-700 nm, allowing estimation of particle temperature at low excitation intensities. When the photoexcitation intensity is increased to 1 kW/cm(2) or more for a few minutes, fluorescence intensity dramatically increases by more than two orders of magnitude. The fluorescence continues to increase in intensity and gradually shift to the red when photoexcitation intensity and the duration of exposure are increased. The resulting products absorb at visible wavelengths and generate red fluorescence with lambda similar to 650-800 nm with 633-nm CW laser excitation. We attribute the emergence of orange and red fluorescence in the Trp products to a photochemical transformation that is instigated by weak optical transitions to triplet states in Trp with 488-nm excitation and which may be expedited by a photothermal effect. (C) 2016 Optical Society of America