Cancer is one of the most serious threats to human health, not only because of the frequency of disease but also because of the severe side effects experienced by the patience during the chemotherapy and radiotherapy treatments, such as immunosuppression and drug resistance. Surgery, as a treatment method, does circumvent some of the side effect issues; however, it is highly invasive and not always possible. In this respect, Photodynamic Therapy (PDT), which localizes the harmful effects of the sensitizer to areas exposed to radiation, has attracted considerable interest as an alternative, minimally invasive treatment, potentially offering no long-term side effects. In search for PDT agents, conjugated polymers nanoparticles (CPNs) have proven themselves to be a versatile class of materials, with many advantageous properties for biomedical applications. Here, we report the development of CPNs with absorption and emission bands in the 1st near-infrared (NIR) biological transparency window, for combined NIR bioimaging and PDT application. We show that the synthesis procedure of the CPNs can be optimised to achieve CPNs of highest possible fluorescence quantum yield and singlet oxygen production, by varying the ratio of the conjugated polymer and stabilizing copolymer in the precursor solution, as well as changing its pH. We further demonstrate the feasibility of our CPNs for the combined NIR bioimaging/PDT applications on a range of different cancerous and normal cell lines. Furthermore, we show that by modifying our CPNs with a tumour-specific ligand, specific cancerous cell lines can be targeted.