The detection of ions in biological or environmental media has been widely studied. This detection makes it possible to prevent human diseases or to determine whether soil or water has been polluted.[1] Generally, it is much easier to detect cations, using crown ethers, than anions, but obtaining sensitive and selective probes of a single anion is a real challenge. Seeking to detect the fluoride anion, we therefore decided to design fluorescent probes, which unlike NMR detection, will allow us to work at smaller concentrations.[2] As fluoride-receptor we have chosen a N,N’-diarylurea moiety, well known to make specific hydrogen bonds with this anion.[3] In addition to fluorescence, due to their extended pseudo-quadrupolar push-push or pull-pull structures, such molecules could have also interesting 3rd order nonlinear optical properties, such as two-photon absorption (2PA), potentially opening the possibility of using an excitation wavelength in the near-IR range for detection. The synthesis of these new fluoride probes will be described, as well as their photophysical properties in THF revealing their scope and selectivity. As fairly conclusive results were obtained in organic solvents, we decided to extend our investigations in water after encapsulating our ureas in biocompatible nanoparticles. As will be dicussed in this presentation, these simple N,N’-diarylureas allow effective sensing of fluoride under such conditions, the best probe among them working as a ratiometric sensor.