A method of functionalization of a gate electrode of a field-effect transistor sensor comprising the steps of: - forming a layer of biological recognition elements on a surface of said gate electrode, wherein said layer of biological recognition elements includes a self-assembled structure of one or more specific-binding-pair-forming substances, - treating the layer of biological recognition elements with a solution containing a blocking agent to fill vacancies and prevent nonspecific binding in the self-assembled structure, wherein said one or more specific-binding-pair-forming substances immobilized in said layer of biological recognition elements are packed at a density comprised 0.1 x 10 4 μm -2 and 10 x 10 4 μm -2, preferably between 1 x 10 4 μm -2 and 2 x 10 4 μm -2, wherein said blocking agent comprises a Bovine Serum Albumine solution, wherein said blocking agent comprises a Bovine Serum Albumine solution in a buffer solution, wherein said buffer solution is a Phosphate Buffered Saline solution, and wherein the blocking agent is a 0.05 to 1 mg ml -1 BSA solution, preferably 0.1 mg ml -1, in Phosphate Buffered Saline 5 mM to 20 mM, preferably 10 mM, wherein the layer of biological recognition elements includes a biological self-assembled structure (B_SAM) including said one or more specific-binding-pair-forming substances and a chemical self-assembled structure (C_SAM) whereon said biological self-assembled structure is grafted, wherein each of said chemical self-assembled structure (C_SAM) and biological self-assembled structure (B_SAM) is a self-assembled monolayer structure, wherein the chemical self-assembled monolayer (C_SAM) is added to the gate electrode by means of a precursor consisting of a layer of alkanethiols terminating with carboxylic functionalities, wherein said treating the layer of biological recognition elements with a solution containing a blocking agent to fill vacancies and prevent nonspecific binding in the self-assembled structure further comprises saturation of unreacted activated carboxy groups of said chemical self-assembled monolayer (C_SAM) by means of concentrated solutions of amines for a time long enough to allow the reaction with all the activated carboxylic groups. - forming a layer of biological recognition elements on a surface of said gate electrode, wherein said layer of biological recognition elements includes a self-assembled structure of one or more specific-binding-pair-forming substances, - treating the layer of biological recognition elements with a solution containing a blocking agent to fill vacancies and prevent nonspecific binding in the self-assembled structure, wherein said one or more specific-binding-pair-forming substances immobilized in said layer of biological recognition elements are packed at a density comprised 0.1 x 10 4 μm -2 and 10 x 10 4 μm -2, preferably between 1 x 10 4 μm -2 and 2 x 10 4 μm -2, wherein said blocking agent comprises a Bovine Serum Albumine solution, wherein said blocking agent comprises a Bovine Serum Albumine solution in a buffer solution, wherein said buffer solution is a Phosphate Buffered Saline solution, and wherein the blocking agent is a 0.05 to 1 mg ml -1 BSA solution, preferably 0.1 mg ml -1, in Phosphate Buffered Saline 5 mM to 20 mM, preferably 10 mM, wherein the layer of biological recognition elements includes a biological self-assembled structure (B_SAM) including said one or more specific-binding-pair-forming substances and a chemical self-assembled structure (C_SAM) whereon said biological self-assembled structure is grafted, wherein each of said chemical self-assembled structure (C_SAM) and biological self-assembled structure (B_SAM) is a self-assembled monolayer structure, wherein the chemical self-assembled monolayer (C_SAM) is added to the gate electrode by means of a precursor consisting of a layer of alkanethiols terminating with carboxylic functionalities, wherein said treating the layer of biological recognition elements with a solution containing a blocking agent to fill vacancies and prevent nonspecific binding in the self-assembled structure further comprises saturation of unreacted activated carboxy groups of said chemical self-assembled monolayer (C_SAM) by means of concentrated solutions of amines for a time long enough to allow the reaction with all the activated carboxylic groups.