We propose an optical fiber biosensor based on a minimalistic architecture, labelled ‘reflector-less biosensor“. The sensing unit is simply a high-scattering fiber (doped with MgO nanoparticles), wet-etched in a hydrofluoric acid bath. The Rayleigh scattering signatures are detected in reflection, achieving a sensitivity up to over 10 nm/RIU (refractive index units). The system has been functionalized for thrombin biosensing, achieving a detection of tens of nanomolar concentrations.