Photophysical properties, singlet oxygen generation efficiency and cytotoxic effects of aloe emodin as a blue light photosensitizer for photodynamic therapy in dermatological treatment

Conventional photodynamic therapy (PDT) uses red light for deeper penetration. A natural compound, aloe emodin (AE) with anticancer and photosensitising capabilities, excited by blue light, is proposed to treat superficial diseases. The photophysical properties and singlet oxygen quantum yield (Phi(Delta)) of AE, as well as the cytotoxic effects of AE on human cells, were investigated. The absorption and emission spectra of AE were analyzed. The Phi(Delta) of AE was measured by a relative method. In order to study the relationship between Phi(Delta) and the oxygen concentration, the dependence of Phi(Delta) on the oxygen concentration was investigated. The cytotoxic effects of AE alone and AE-mediated PDT were compared. The relationship between cells' survival rate and PDT conditions was studied. According to spectral analysis, the energy levels of AE were identified. The maximum absorption peak of AE is in the blue region, which makes AE-mediated PDT suitable for superficial diseases. The Phi(Delta) of AE was determined to be 0.57(2), which was found to be dependent on the oxygen concentration. The studies under low oxygen concentration proved that there is no type I reaction between AE and the probe for singlet oxygen detection. The effect of AE-mediated PDT was significantly higher than that of AE alone and increased with the concentration of AE or fluence. AE-mediated PDT can provide a new strategy to treat superficial diseases using blue light, thus protecting deeper normal tissues.