BioPAL is developing novel lanthanide reagents that can take advantage of time-resolved fluorescent technology.  Due to their very specific photophysical and spectral properties, complexes of lanthanide (rare earth) ions are of major interest for fluorescent applications in biology (1-3).  Time-resolved fluorescent reagents have been used in many biological systems as a means to reduce background fluorescence signal and increase sensitivity.  Under many experimental conditions autofluorescence of the sample can cause unwanted background fluorescence.  High background results in variable inter- and intra-experimental signals, as well as a decrease in sensitivity (4).  In histological samples observations in the regions of interest may be obscured by background fluorescence.  While several options are available to reduce background fluorescence, including different excitation and emission wavelengths, these measures often fail to adequately reduce the background signal.  Another approach to decrease background interference is to use time-resolved fluorescent reagents.

Lanthanide ions, particularly Europium, Samarium, Terbium, and Holmium, fluoresce at specific wavelengths.  Compared with traditional fluorescent compounds, the lanthanide compounds have a much longer emission lifetime.  These emission lifetimes are often measured in hundreds of microseconds whereas traditional organic reagents have lifetimes measured on the scale of several nanoseconds.  Time-resolved fluorescent technology takes advantage of this phenomenon as a means to reduce background fluorescence and increase sensitivity.  In addition, lanthanides exhibit a large Stokes shift, with excitation occurring by absorbance of UV light with emission wavelengths greater than 500 nm.  The emission peaks of several of the lanthanides of interest are provided in Table 1.  The fluorescent emission peak profiles are also quite sharp, with half-widths being 10 nm to 20 nm.