pyranine

Pyranine is the common name for 8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt (HPTS). It is a water-soluble fluorescent dye widely used as a pH indicator and tracer in biochemical and cell biology experiments. The molecule carries three sulfonate groups, giving a strong negative charge at physiological pH, which renders it largely membrane-impermeant and suitable for extracellular measurements or controlled delivery into compartments.

In fluorescence spectroscopy, pyranine is excited primarily around 460 nm and emits around 510 nm. Its fluorescence intensity and spectral ratio vary with pH, with a pKa near neutral (about 7.2–7.4), enabling ratiometric measurements across the physiological range. Two-excitation or dual-wavelength methods are common to obtain pH independent of dye concentration and path length.

Applications include its use as a pH indicator in cell culture media, for tracking intracellular pH when loaded appropriately, and in fluorescence microscopy. It is also employed as a soluble tracer in perfusion and microfluidics studies because it reports on solution mixing and flow without reacting with solutes.

Safety considerations follow standard laboratory dye practices: handle with appropriate precautions to avoid ingestion or inhalation, and dispose of according to applicable regulations.