Fluorescence intensity of dye containing latex particles studied by near-field scanning optical microscopy

Latex particles of various sizes, containing a fluorescent dye and dispersed at low density in a poly(vinyl alcohol) matrix, were imaged by near-field scanning optical microscopy. The fluorescence intensity of the latex particles was studied quantitatively, by integrating the total intensity measured per particle. For all the samples, a high accuracy in determination of the fluorescence intensity could be achieved. The noticed difference in fluorescence intensity for particles of a similar size is mainly affected by the size distribution of the particles and by sample preparation. Particularly, the smaller particles (50-100 nm) show a significant difference in fluorescence intensity. This was confirmed by further study of the distribution of particle sizes by transmission electron microscopy and by examination of the correlation between the fluorescence intensity of the latex particles and their size. When the size difference is significant (e.g., when particles of 100 and 200 nm, which show a narrow size distribution, are present in the same sample), the two types of particles can be clearly distinguished. Furthermore, for smaller particles (50 and 100 nm) the differences in depth position in the film lead to differences in emission intensity.