Inorganic phosphors as new luminescent labels for immunocytochemistry and time-resolved microscopy

A new strongly luminescent marker consisting of inorganic crystals is described for time-resolved microscopy. These crystals, known as phosphors, show delayed luminescence, unlike prompt fluorescent labels such as FITC, TRITC and phycobiliproteins, and are therefore potentially suitable for time-resolved microscopy. The luminescence of these phosphors is strong and non-fading in comparison to FITC/TRITC, and not significantly influenced by pH or temperature. The phosphor yttriumoxisulfide activated with europium emits maximally at 620 nm with a typical half life-time of approximately 700 musec, upon excitation with near ultraviolet light (360 nm). Phosphors for immunocytochemical staining were made by ball milling and were stabilized in suspension with polycarboxylic acids. Proteins such as avidin, protein A or immunoglobulins were allowed to adsorb to the surface of the phosphors. The immunocytochemical properties of the conjugates were evaluated in a model system of latex beads with defined surface antigens and in a cellular system containing fixed human lymphocytes or erythrocytes. Specific cytochemical staining was observed in suspension as well as on glass slides. A specially constructed time-resolved microscope was used to suppress the fast decaying fluorescence, thereby permitting visualization of the specific, slowly decaying luminescence of the phosphor label without the necessity of integration. Finally, the use of multiple phosphors with different kinetic and spectral characteristics for multiparameter studies is indicated.