Time‐resolved microscopy for imaging lanthanide luminescence in living cells

Abstract

Time-resolved luminescence (TRL) microscopy can image signals from lanthanide coordination complexes or other probes with long emission lifetimes, thereby eliminating short-lifetime (\100 ns) autofluorescence background from biological specimens. However, lanthanide complexes emit far fewer photons per unit time than conventional fluorescent probes, making it difficult to rapidly acquire high quality images at probe concentrations that are relevant to live cell experiments. This article describes the development and characterization of a TRL microscope that employs a light-emitting diode (LED, k em 5 365 nm) for pulsed epi-illumination and an intensified charge-coupled device (ICCD) camera for gated, widefield detection. Europium chelate-impregnated microspheres were used to evaluate instrument performance in terms of short-lifetime fluorescence background rejection, photon collection efficiency, image contrast, and signal-to-noise ratio (SNR). About 200 nm microspheres were imaged within the time resolution limit of the ICCD (66.7 ms) with complete autofluorescence suppression. About 40 nm microspheres containing $400 chelate molecules were detected within $1-s acquisition times. A luminescent terbium complex, Lumi4-Tb 1 , was introduced into the cytoplasm of cultured cells at an estimated concentration of 300 nM by the method of osmotic lysis of pinocytic vesicles. Time-resolved images of the living, terbium complex-loaded cells were acquired within acquisition times as short as 333 ms, and the effects of increased exposure time and frame summing on image contrast and SNR were evaluated. The performance analyses show that TRL microscopy is sufficiently sensitive and precise to allow high-resolution, quantitative imaging of lanthanide luminescence in living cells under physiologically relevant experimental conditions. ' 2010 International Society for Advancement of Cytometry Key terms fluorescence microscopy; lanthanide; time-resolved; signal-to-noise ratio THE sensitivity and precision of fluorescence-based bioassays and microscopy is often diminished by autofluorescence background emitted from samples and containers or coverslips. Time-resolved luminescence (TRL) measurements can effectively eliminate autofluorescence when the detected analyte has an emission lifetime exceeding that of endogenous fluorophores (\100 ns) (1). TRL instrumentation uses a finite pulse of light to excite a sample (Fig. 1). Then, the detector is electronically switched on, or unshuttered after a short interval (the gate delay) during which sample autofluorescence has diminished. The detector remains on for a finite interval (the gate width), and the output from multiple excitation/emission cycles can be integrated to increase signal.The most common emissive probes for TRL-based bioassays are ligand-sensitized, coordination complexes of lanthanide cations. Lanthanide complexes (LCs) have long lifetimes (ls-ms) that facilitate TRL detection as well as large Stokes shifts ([150 nm) and multiple, narrow emission bands that make ...

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340 nm pulsed UV LED system for europium-based time-resolved fluorescence detection of immunoassays

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