Abstract: The invention relates to an electrochemical sensor for the detection of analytes in liquid media which comprises 4 layers, wherein the first layer (1) comprises a carbonaceous material deposited on a substrate, said layer forming the system of electrodes of the electrochemical sensor, formed at least by a pseudo-reference electrode, a working electrode and a counter electrode; and the fourth layer (4) comprises polythiophene deposited only on the lower end of the working electrode selected from (d1), which comprises a layer comprising a polythiophene deposited on the lower end of the working electrode and a layer comprising a non-conductive polymer gel deposited on said layer of polythiophene; (d2), which is a layer of conductive polymer gel comprising a non-conductive polymer gel and a polythiophene; and (d3), which comprises a layer comprising a polythiophene deposited on the lower end of the working electrode and a layer comprising functionalized magnetic nanoparticles deposited on said layer of polythioph