Spectral Characterization of Purpurin Dye and Its Application in pH Sensing, Cell Imaging and Apoptosis Detection

Purpurin (1,2,4-trihydroxy-9,10-anthraquinone) is a natural red dye obtained from the red madder plant that is widely used in food and dyeing industries. The present study investigated the characteristics of purpurin and its application as a pH-sensitive probe to detect the pH of solutions and intracellular pH of mammalian and bacterial cells. Purpurin exhibited high pH-sensitive behavior, low analytes interference, high stability with pKa of 4.6 and visible colorimetric change. ¹H NMR and FTIR studies indicated protonation of phenolic hydroxyl group under acidic condition with hypsochromic shift in the absorption and fluorescence spectra relative to that of basic condition. Cell culture studies using HeLa cells revealed that purpurin is well tolerated by the cells and the fluorescent imaging result indicated excellent cell permeability with possible use of the dye to detect the pH fluctuations in living cells under various physiological conditions such as apoptosis. Microbiological studies indicated that the dye could be used for visualization of bacteria under acidic condition.