Photobleaching of Phthalocyanine Molecules within a Complex with Colloidal Quantum Dots

Fluorescent nanoparticles are currently considered as promising enhancers of photodynamic activity of traditional photosensitizers in procedures for photodynamic therapy of cancer and inactivation of pathogens. On the one hand, such nanoparticles act as a nanoplatform for target delivery of dye molecules to cells; on the other hand, they act as light-harvesting antennas that increase the effective absorption cross-section of the dye. This paper investigates the effect of semiconductor CdSe/ZnS quantum dots coated with a polymer shell on the photostability of zinc or aluminum polycationic phthalocyanines. It was shown that the rate of photobleaching of phthalocyanines increases significantly in the presence of quantum dots both under direct illumination of phthalocyanine by red light and under the quantum dot-mediated illumination by blue light. This effect can be explained by the fact that phthalocyanine inside the polymer shell of the quantum dot is the primary target for the attack by reactive oxygen species. The rate of bleaching of the chemical trap of reactive oxygen species, 4-nitroso-N,N-dimethylaniline, in a phthalocyanine solution increases in the presence of the quantum dot under red light illumination. We believe that the chemical trap is concentrated in the polymer shell of the quantum dot, which increases the probability of its damage by active oxygen species generated by phthalocyanine. Since the diffusion of reactive oxygen species from the polymer shell of the nanoparticle into the surrounding solution is slow, the use of quantum dots as an enhancer of the photodynamic action of phthalocyanines can be effective only in the absence of significant steric obstacles to the diffusion of reactive oxygen species to the target molecules of photodynamic inactivation.