Abstract: A device for detecting a target molecule and/or measuring the concentration of a target molecule, which includes: a substrate at the surface of which is covalently attached a grafting molecule; at least one fluorescent probe including at least one receptor bonded to a polypeptide via a covalent bond; two fluorochromes Fa and Fb, in which the fluorochrome Fa is bonded to the receptor and fluorochrome Fb is bonded to the polypeptide; and the fluorochromes Fa and Fb form a FRET donor/acceptor pair, in which the polypeptide is bonded to the grafting molecule via a covalent bond. Also, a fluorescent probe and a method for detecting a target molecule and/or measuring the concentration of a target molecule.