Temporally and spectrally resolved imaging microscopy of lanthanide chelates

The combination of temporal and spectral resolution in fluorescence microscopy based on long-lived luminescent labels offers a dramatic increase in contrast and probe selectivity due to the suppression of scattered light and short-lived autofluorescence. We describe various configurations of a fluorescence microscope integrating spectral and microsecond temporal resolution with conventional digital imaging based on CCD cameras. The high-power, broad spectral distribution and microsecond time resolution provided by microsecond xenon flashlamps offers increased luminosity with recently developed fluorophores with lifetimes in the submicrosecond to microsecond range. On the detection side, a gated microchannel plate intensifier provides the required time resolution and amplification of the signal. Spectral resolution is achieved with a dual grating stigmatic spectrograph and has been applied to the analysis of luminescent markers of cytochemical specimens in situ and of very small volume elements in microchambers. The additional introduction of polarization optics enables the determination of emission polarization; this parameter reflects molecular orientation and rotational mobility and, consequently, the nature of the microenvironment. The dual spectral and temporal resolution modes of acquisition complemented by a posteriori image analysis gated on the spatial, spectral, and temporal dimensions lead to a very flexible and versatile tool. We have used a newly developed lanthanide chelate, Eu-DTPA-cs124, to demonstrate these capabilities. Such compounds are good labels for time-resolved imaging microscopy and for the estimation of molecular proximity in the microscope by fluorescence (luminescence) resonance energy transfer and of molecular rotation via fluorescence depolarization. We describe the spectral distribution, polarization states, and excited-state lifetimes of the lanthanide chelate crystals imaged in the microscope.