Scientists synthesized a 1,3,5,7-tetramethyl-8-phenyl-BODIPY modified with polyethylene glycol (PEG) chain to be used for temperature sensing and live cell imaging. The dye travels through the body and shows increase in nonradiative decay as temperature increases, which directly impacts its fluorescence lifetime. When PEG-BODIPY makes contact with cells, it spontaneously enters and stains cells (but not the nucleus).

Researchers utilized fluorescence-lifetime imaging microscopy (FILM) to view fluorescence that occurs at a specific temperature. BODIPY fluoresces only while inside a cell and relies on temperature changes and viscosity. However, since the inside of cells are typically high-viscosity environments, in this situation BODIPY fluorescence lifetime relies on temperature alone.

Fluorescence-lifetime Imaging: microscopy that produces an image based on the differences in exponential decay rate of a fluorescent sample. The images produced by FILM yield pixels with unique intensities based on the fluorescent decay rate of materials. This technique can be used in a number of systems including confocal microscopy and two-photon excitation microscopy.

A unique characteristic of the rotor system is its ability to wobble. Instead of being allowed to rotate fully, researchers constrained the molecule to go back and forth. “What we measure is how long the molecule stays in the excited state, which depends on how fast it wobbles,” Martí said. “If you increase the temperature, it wobbles faster, and that shortens the time it stays excited.”

The excitation effect is independent of concentration of BODIPY molecules and of photobleaching (the point at which molecules can no longer fluoresce). Moreover, the effect of viscosity is eliminated by constraint of the rotation of the motor.

A potential application of this technique could be applied to tumor ablation therapies where heat is used to destroy cancer cells. Alternatively, since the metabolism of cancer cells are generally greater than others resulting in increased heat generation, this technique could be used to measure the presence of cancers in the body. The researchers want to explore if they can identify cancers by the heat they produce and successfully differentiate them from normal cells.

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