The Investigation of Fluorescence Spectra and Fluorescence Quantum Yield of Enrofloxacin

In this paper, the fluorescence spectra of Enrofloxacin (ENR) in different pH conditions was studied in order to determine its structural changes due to protonation with pH changes. The ENR two-step dissociation constant is calculated and the fluorescence quantum yield under acidic conditions is measured. In the strong acidic conditions, ENR exists of H3L2+ form of which maximum emission wavelength is at 450 nm. At the condition of pH 2.45 to 4.23, ENR exists of H2L+ form with strong and steady fluorescence. The maximum emission wavelength is still 450 nm. At the condition of pH more than 4.23, the maximum emission wavelengths are gradually blue shifted to 445 nm and the fluorescence intensity decrease with the increase of pH which shows that H2L+ loses one proton with the increase of pH and exists in the form of bipolar ion HL. When the pH is more than 12.28, the fluorescence intensity are weakened to nearly disappear with the increase of pH value, indicating that HL gradually loses the proton with the conversion to the anion of L- which is weaker fluorescence. In the buffer solution of pH 3.00, with quinine sulfate as reference, the fluorescence quantum yield of ENR at excitation wavelength of 274 nm is 0.125