Photodynamic therapy (PDT) presents an alternative non-invasive therapeutic modality for the treatment of cancer and other diseases. PDT relies on cytotoxic singlet oxygen that is locally generated through energy transfer between a photosensitizer and molecularly dissolved triplet oxygen. To minimize side-effects, i.e. damage of healthy tissue, targeted delivery to places of disease, high local photosensitizer concentrations, high singlet oxygen quantum yield, and rapid post-treatment clearance of photosensitizers are desired. Ultrasmall (sub-10 nm) organic-inorganic hybrid silica nanoparticles loaded with photosensitizer molecules, referred to as silica nanophotosensitizers (SNPSs), present a way to meet these requirements. Here, we investigate two different particle designs of ultrasmall poly(ethylene glycol) coated (PEGylated) SNPSs covalently binding the methylene blue derivate MB2. In the first approach (design one), MB2 is encapsulated into the silica matrix, while in the second approach (design two), MB2 is grafted on the particle surface in between chains of the stabilizing PEG corona. We compare both cases with regard to their singlet oxygen quantum yields, ΦΔ, with the effective ΦΔeff per particle reaching 111% and 161% for design one and two, respectively. Finally, we show that both particle designs allow functionalization with a targeting peptide, c(RGDyC), rendering SNPSs a promising platform for medical applications.