Abstract: Provided is a method for modification of a working electrode for a sensor which is able to detect at least one dissolved trace metallic element in a liquid, such an electrode, which is obtained by this method, a sensor incorporating it and the use thereof for detecting and possibly quantifying a metallic element in liquids. This method includes a step of depositing a polymeric film based on a poly(catechol) on the surface by electropolymerisation of pyrocatechol in a first environment with a pH between 6 and 8, the method subsequently includes conditioning the working electrode having a succession of reduction-oxidation cycles in a second environment with a pH of at least 10, in order for the modified working electrode to be electroactive in a field of electroactivity, measured by cyclic voltammetry, ranging from ?1 V to +1 V while being able to detect copper and/or lead as the metallic element.