Demonstration of true-color high-contrast microorganism imaging for terbium bioprobes.

Lanthanide bioprobes offer a number of novel advantages for advanced cytometry, including the microsecond luminescence lifetime, sharp spectral emission, and large stokes shift. However, to date, only the europium-based bioprobes have been broadly studied for time-gated luminescence cell imaging, though a wide range of efficient terbium bioprobes have been synthesized and some of them are commercially available. We analyze that the bottleneck problem was due to the lack of an efficient microscope with pulsed excitation at wavelengths of 300-330 nm. We investigate a recently available 315 nm ultraviolet (UV) light emitting diode to excite an epifluorescence microscope. Substituting a commercial UV objective (40×), the 315 nm light efficiently delivered the excitation light onto the uncovered specimen. A novel pinhole-assisted optical chopper unit was attached behind the eyepiece for direct lifetime-gating to permit visual inspection of background-free images. We demonstrate the use of a commercial terbium complex for high-contrast imaging of an environmental pathogenic microorganism, Cryptosporidium parvum. As a result of effective autofluorescence suppression by a factor of 61.85 in the time domain, we achieved an enhanced signal-to-background ratio of 14.43. This type of time-gating optics is easily adaptable to the use of routine epifluorescence microscopes, which provides an opportunity for high-contrast imaging using multiplexed lanthanide bioprobes.