Nanoparticle platform to modulate reaction mechanism of phenothiazine photosensitizers

Herein, we report on the synthesis of photosensitizing nanoparticles in which the generation of different oxidizing species, i.e., singlet oxygen (1O2) or radicals, was modulated. Sol gel and surface chemistry were used to obtain nanoparticles with specific ratios of dimer to monomer species of phenothiazine photosensitizers (PSs). Due to competition between the reactions involving electron transfer within dimer species and energy transfer from monomer triplets to oxygen, the efficiency of 1O2 generation could be controlled. Nanoparticles with an excess of dimer have an 1O2 generation efficiency (S(delta)) of 0.01 while those without dimer have a S(delta) value of 0.4. Furthermore, we demonstrate that the PS properties of the nanoparticles are not subjected to interference from the external medium as is commonly the case for free PSs, i.e., PS ground and triplet states are not reduced by NADH and ascorbate, respectively, and singlet excited states are less suppressed by bromide. The modulated 1O2 generation and the PS protection from external interferences make this nanoparticle platform a promising tool to aid in performing mechanistic studies in biological systems. Also, it offers potential application in technological areas in which photo-induced processes take place.