The combination of photodynamic therapy (PDT) and fluorescence imaging provides a promising approach to theranostics. However, traditional photosensitizers (PSs) have low water solubility and lack active targeting ability. Our ingenious design used L-cys/ZnS:O (LZS) nanoparticles (NPs) modified with folic acid (FA), allowing them to easily enter tumor cells and accurately gather around the nucleus of cancer cells. L-Cysteine were used as intermediates, ZnS:O quantum dots and FA could be connected by a solid-state method and a coupling reaction. In doing so, the cytotoxicity of LZS NPs was further reduced, while the hydrophilicity and dispersibility were improved. Moreover, the as-synthesized FA@LZS NPs had a higher generation of reactive oxygen species (ROS) than commercial Ce6, and they killed HepG2 cells specifically in vitro. These findings give a clear way for the development of advanced PSs with homologous labeling functions. A template for NPs or other fluorophores modified by targeting groups is also provided.