Fast phase-contrast imaging offered by modern synchrotron facilities opens the possibility of imaging dynamic processes of biological material such as cells. Cells are mainly composed of carbon and hydrogen, which have low X-ray attenuation, making cell studies with X-ray tomography challenging. At specific low energies, cells provide contrast, but cryo-conditions are required to protect the sample from radiation damage. Thus, non-toxic labelling methods are needed to prepare living cells for X-ray tomography at higher energies. We propose using iron oxide nanoparticles due to their proven compatibility in other biomedical applications. We show how to synthesize and attach iron oxide nanoparticles and demonstrate that cell-penetrating peptides facilitate iron oxide nanoparticle uptake into sperm cells. We show results from the TOMCAT Nanoscope (Swiss Light Source), showing that iron oxide nanoparticles allow the heads and midpiece of fixed sperm samples to be reconstructed from X-ray projections taken at 10 keV.