Fluorescence from airborne microparticles: dependence on size, concentration of fluorophores, and illumination intensity

We measured fluorescence from spherical water droplets containing tryptopha n and from aggregates of bacterial cells and compared these measurements wi th calculations of fluorescence of dielectric spheres. The measured depende nce of fluorescence on size, from both droplets and dry-particle aggregates of bacteria, is proportional to the absorption cross section calculated fo r homogeneous spheres containing the appropriate percentage of tryptophan. However, as the tryptophan concentration of the water droplets is increased , the measured fluorescence from droplets increases less than predicted, pr obably because of concentration quenching. We model the dependence of the f luorescence on input intensity by assuming that the average time between fl uorescence emission events is the sum of the fluorescence lifetime and the excitation lifetime (the average time it takes for an illuminated molecule to be excited), which we calculated assuming that the intensity inside the particle is uniform. Even though the intensity inside the particles spatial ly varies, this assumption of uniform intensity still leads to results cons istent with the measured intensity dependence. (C) 2001 Optical Society of America.