Long-lived visible luminescence of UV LEDs and impact on LED excited time-resolved fluorescence applications

We report the results of a detailed study of the spectral and temporal properties of visible emission from three different GaN-based ultraviolet (UV) light emitting diodes (UV LEDs). The primary UV emission in the 360-380 nm band decays rapidly (less than 1 νs) following switch-off; however, visible luminescence (470-750 nm) with a decay lifetime of tens of microseconds was observed at approximately 10^-4 of the UV intensity. For applications of UV LEDs in time-resolved fluorescence (TRF) employing lanthanide chelates, the visible luminescence from the LEDs competes with the target Eu³⁺ or Tb³⁺ fluorescence in both spectral and temporal domains. A UV band-pass filter (Schott UG11 glass) was therefore used to reduce the visible luminescence of the UV LEDs by three orders of magnitude relative to UV output to yield a practical excitation source for TRF.