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

Latex particles of various sizes, containing a fluorescent dye and disperse d at low density in a poly(vinyl alcohol) matrix, were imaged by near-field scanning optical microscopy. The fluorescence intensity of the latex parti cles was studied quantitatively, by integrating the total intensity measure d per particle. For all the samples, a high accuracy in determination of th e fluorescence intensity could be achieved. The noticed difference in fluor escence 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 fluore scence intensity. This was confirmed by further study of the distribution o f particle sizes by transmission electron microscopy and by examination of the correlation between the fluorescence intensity of the latex particles a nd their size. When the size difference is significant (e.g., when particle s 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.