Recommendations about the diameter of latex particles depend on the detection method:
For a naked-eye or a spectrophotometric detection, the sensitivity, i.e., the minimal number of particles that can be detected per trial, increases as function of particle diameter. For the same number of particles, the optical density measured at a given wavelength (380 nm) increases as function of particle diameter (Fig. 2).
Nonfluorescent particles with a diameter above 300 nm are readily detected under microscope. Using larger particles allows to work with an objective of lower magnification and to improve sample speed analysis and/or accuracy since a larger surface can be analyzed. Much smaller fluorescent particles (<100 nm) are detectable with a fluorescent microscope since it is not necessary to resolve them optically for individually counting them. Sensitivity is linked to the density, intensity, and stability of fluorophores entrapped inside the particle or chemically bound to the surface.
Requirements concerning colloidal stability and surface functionality are the same than for magnetic particles, i.e., an excellent colloidal stability after ligand binding and a high density of reactive sites. Experimentally, the density of reactive groups can be estimated with 35S-radiolabeled molecules.
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