The present invention relates to an organic electrochemical transistor based on conducting polymers (for example PEDOT:PSS) in which the gate electrode has been integrated on the surface of the channel through the deposition of Ag halide nanoparticles, AgX (X═CI, Br, I). This configuration allows the device to operate without applying a potential difference to the gate electrode (with consequent reduction in operating costs). The device is able to detect the halide ion: by varying the nature of the nanoparticles and in particular by using AgX (X═CI, Br, I) or Ag2S the sensor acquires sensitivity towards anion X or sulfide.