The present invention relates to a sensor for detecting one or more target analytes, the sensor comprising: at least one polymeric sensing element capable of selectively and reversibly binding to a target analyte； at least one working electrode having the polymeric sensing element disposed thereon； at least one reference electrode that is electrically communicated with said working electrode； and means for measuring a change in an electrical property across said working electrode and said reference electrode. In particular, the target analyte is Na+, urea or creatinine. Also disclosed is a multi-layered sensor, comprising at least one working electrode layer and at least one reference electrode layer, said working electrode layer and said reference electrode layer being separated by at least one electrically insulating layer.
