Abstract: Fluorescent nanoparticles such as quantum dots are incorporated into paper and other web products such as plastics to achieve cross-direction and machine direction on-line detection of selected components during manufacture. Fluorescent nanoparticles markers are added in known proportions into product formulations along with the selected components of interest. By detecting the fluorescence from the nanoparticles, the selected components can be traced at various stages of production. In addition, by using different fluorescent nanoparticles that emit radiation at different wavelengths, data from individual materials or layers in a composite structure can be ascertained simultaneously with a single sensor. The technique is particularly suited for monitoring expensive and difficult-to-measure components that may be present only in trace quantities.