Heteroatom-doped carbon nanoparticles have drawn considerable attention due to their promising applications extending from optoelectronics to biotechnology. Here, we prepared both nitrogen and sulfur co-doped carbon nanoparticles (NS-CNPs) from a single precursor, thioacetamide, using a hydrothermal method and showed that an ultra-low amount of copper ions can be detected through steady state photoluminescence of the NS-CNPs. The fluorescence intensity varies with the concentration of copper ions, and thus can be potentially used for quantification. The limit of detection (LOD) for copper ions is found to be 14 nM in the linear dynamic range from 0.02 to 0.1 μM. Finally, we have employed NS-CNPs for the detection of Cu²⁺ ions in yeast cells.