Rare-earth ions, with relatively long luminescence lifetimes, have significant advantages for diagnostic application. In diagnostics the long luminescence lifetimes allow for extremely sensitive time-gated detection, where the difference in temporal behavior of scatter and background fluorescence and the long-lived rare earth luminescence is utilized. Unfortunately the absorption cross-section of rare earth ion transitions is extremely low. However, via sensitized excitation by means of a suitable organic molecule efficient excitation is obtained. It is shown that excitation in the visible part of the spectrum can be used to excite rare-earth ions which luminescent in the near-IR, such as ytterbium, neodymium and erbium, via a fluorescein-derivative as sensitizer. The advantages of this approach are manifold. Low cost light source are available for the visible part of the spectrum and interference's from the matrix are minimal. Detection in the near-IR is almost interference-free.
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