Porphyrin-sensitized photo-oxidation of hematoxylin in oxygenated solutions

This paper describes the oxidation of hematoxylin (HT) in oxygenated aqueous solutions at pH 4.0, upon irradiation with visible light, in the presence of protoporphyrin dimethylester (PPDME) as a photosensitizer. The redox potential for the lowest excited state of PPDME, estimated from its ground state oxidation potential [0.86 V with respect to normal hydrogen electrode (NHE)] and its low energy absorption edge (2.5 eV), was found to be -1.63 V (with respect to NHE). These values clearly indicate that the excited PPDME molecules could transfer their energy to the ground state oxygen in solution, producing the superoxide radical anion (Ο2˙ֿ) or singlet oxygen. Additional experiments carried out in the presence of sodium azide and/or benzoquinone, confirm that the oxidation of HT mainly proceeds through a superoxide radical based mechanism. Under the conditions described above, HT was found to be completely oxidised upon 24 hours of visible light irradiation.

Keywords: Hematoxylin, irradiation, photo-oxidation, photosensitizer, protoporphyrin dimethylester, superoxide anion, visible light.

doi :10.4038/jnsfsr.v35i4.1312

J.Natn.Sci.Foundation Sri Lanka 2007 35 (4): 239-244