A Rhodamine Based Fluorometric and Colorimetric Probe for Detection of pH in Aqueous Medium

Abstract

Present study involves a rhodamine based fluorescent probe, which is reported to exhibit high fluorescence and colorimetric response in the pH range 2.0-5.5, where 55-fold fluorescence intensity change was identified with the lowering of pH of the medium. The working principle of the probe is based on its reversible structural inter-conversion between spirocyclic ring-close (non-fluorescent) and spirocyclic ringopen (fluorescent) forms of rhodamine base along with a reversible color change between colourless and pink with change in the pH medium. The closed spirocyclic form (AX) of the probe converts gradually into the spirocyclic open-ring (HAX) with increasing acidity of the medium and which in turn results in the strong orange fluorescence emission along with visible colorimetric responses. The calculated pKa ~ 4.0 for such conversion indicates that the probe can be useful to detect the pH of the aqueous medium in the range 3.0-5.0. The probe displays high sensitivity, good photostability, and reversible pH dependence. Detailed analysis of the pH sensing capability of the probe were performed by means of UV-vis spectroscopy, steady state and time resolved fluorescence techniques and DFT/TD-DFT based theoretical calculations.