[Spectral properties, protonation and fluorescence quantum yield of ciprofloxacin]

Fluorescence spectra, ultraviolet absorption spectra, and protonation of Ciprofloxacin (CIP) at different pH values have been studied. Fluorescence quantum yield of CIP under neutral condition has been measured. In HCl medium with [H+] > 1 mol x L(-1), CIP molecules (simplified as HL) may accept three protons to exist as H4L3+ with very weak fluorescence, and its maximum fluorescence emission wavelength (lambdamax) is 456 nm. In acidic solution of pH 0 to pH 2, CIP mainly exists as H3L2+ form with lambdamax at 450 nm, and fluorescence intensity is relatively weak and increases with increasing of pH. In the range of pH 2 to pH 4, CIP mainly exists as H2L+ form with a strong fluorescence, and lambdamax is still at 450 nm. When pH>4, lambdamax gradually blue-shifts to 414 nm, fluorescence intensity slightly decreases as pH increases, and at the same time an evident change in ultraviolet absorption spectrum is observed, indicating that H2L+ has lost proton to exist as dipole ion form HL. When pH>8, the fluorescence intensity decreases until disappearance as pH increases, indicating that HL has lost proton to exist as non-fluorescence anion ion form L-. In the molecular form changing process, the maximum excitation wavelength of CIP is essentially constant at 275 nm, but the maximum emission wavelength changes obviously. In a buffer solution with pH 7.0, and using quinine bisulphate as a reference, the fluorescence quantum yield of CIP at maximum excitation wavelength 275 nm was measured to be 0.12.