Nanodiamonds are nanoparticles (10 000 to 100 000 times smaller than a millimeter) milled from diamond and have various potential applications in microbiology and cell biology. Especially nanodiamonds that contain a specific fluorescent defect called 'NV center' are of great interest. They can be used to make very small magnetic signals visible with an optical microscope. This property can be used to detect magnetic molecules in cells, which are hard to detect otherwise and play an important role in processes like ageing, stress and signaling between cells. In my Phd research, I investigated the performance of these nanodiamond sensors in relation to their shape and size and how they interact with living bacteria