Parallel fluorescence and phosphorescence monitoring of singlet oxygen photosensitization in rats

The time-and spectral-resolved set-up for measurements of weak infrared luminescence of photosensitizers(PSs)and singlet oxygen using optical lightguides was used on skin of laboratory animals in vivo.Wistar rats with abdominal incisions treated by methylis aminolevulinitis(MAL)were used as a model.A control group of animals with abdominal incisions was also tested.Spectrally resolved fluorescence of the PS was acquired during the treatment from the same spot.The intensity and spectral profile of the fiuorescence signal from the skin can be used to guide the detection setup to the investigated spots in the lesion.The rate of bleaching of Protoporphyrin IX band and appearance of a band of its photoproducts during the treatment can be characterized by the exposition ED,under which the latter becomes dominant feature infiuorescence spectrum.ED value differs statistically significantly between the normal skin and the lesion treated by MAL.No direct proportionality was found between the fluorescence signal and singlet oxygen production.Nevertheless,the strong fluorescence signal is necessary but not a su±cient condition for higher singlet oxygen production in vivo.ED value correlates rather well with production of singlet oxygen,but differently in lesion and normal skin.Lifetimes of singlet oxygen differ between spots outside and in the lesion.PS triplet state lifetimes exhibit weak di®erence between spots treated and untreated by MAL.

Thermally stimulated exciton emission in Si nanocrystals

Increasing temperature is known to quench the excitonic emission of bulk silicon,which is due to thermally induced dissociation of excitons.Here,we demonstrate that the effect of temperature on the excitonic emission is reversed for quantumconfined silicon nanocrystals.Using laser-induced heating of silicon nanocrystals embedded in SiO2,we achieved a more than threefold(4300%)increase in the radiative(photon)emission rate.We theoretically modeled the observed enhancement in terms of the thermally stimulated effect,taking into account the massive phonon production under intense illumination.These results elucidate one more important advantage of silicon nanostructures,illustrating that their optical properties can be influenced by temperature.They also provide an important insight into the mechanisms of energy conversion and dissipation in ensembles of silicon nanocrystals in solid matrices.In practice,the radiative rate enhancement under strong continuous wave optical pumping is relevant for the possible application of silicon nanocrystals for spectral conversion layers in concentrator photovoltaics.