The present invention concerns a biosensor (1) for sensing stress hormone cortisol concentration in a biofluid (27). The biosensor (1) comprises: an electrical transistor transducer (3) comprising a transistor gate electrode (17); a sensing electrode element (21, 23) comprising a metal element having a biofluid facing surface, and a graphene layer (23) on the biofluid facing surface of the metal element (21), the sensing electrode element (21, 23) being connected to the transistor gate electrode (17) by an electrical connector (33) to form an extended gate configuration with the transistor gate electrode (17); and a reference electrode (29) configured to be in contact with the biofluid (27), and configured to electrically bias the transistor gate electrode (17) through the biofluid (27). The sensing electrode element (21, 23) is functionalised by at least a layer of aptamers (25) placed indirectly or directly on the graphene layer (23), and configured to catch cortisol hormones in the biofluid (27) to thereby change a surface potential of the sensing electrode element (21, 23).