The invention relates to biosensors, methods for obtaining them and their use fro detecting, assaying or locating, in direct immunofluorescence, a ligand such as an antigen or hapten, in a heterogeneous population. The biosensor includes (i) at least one fragment of a receptor which is protein in nature, capable of binding to a ligand via an active site, where at least one amino acid residues of the fragment located in the proximity of said active site is naturally present in the form of a cystein Cys residue, or is substituted with a Cys residue, and (ii) a fluorophore coupled to the Cys residue.