Figure 2
From: Real-time detection of viable microorganisms by intracellular phototautomerism
Acid-base equilibrium and fluorescence properties of salicylic acid. (A) Chemical structures of salicylic acid in its fully protonated form (at pH < pKa) and in its singly charged anion form (at pH > pKa). (B) Upon excitation, intramolecular electron rearrangements enable the proton which is associated with the phenolic group to be transferred to the carboxylic group, changing the fluorescence to that of the excited singly charged anion, ionized at the phenolic group; this phenomenon known as phototautomerism or excited state intramolecular proton transfer (ESIPT). (C) Fluorescence excitation and emission spectra of salicylic acid at pH 7.5. (D) Titration curve showing the pH-dependence of salicylic acid fluorescence emission.