The application relates to an electrode for use in the electrochemical detection of target species, and in particular for the detection of insulin. The application enables insulin to be detected with high selectivity and with a detection limit in the picomolar range or lower. The electrodes of the invention can easily be reused and are ideally suited for use in point-of-care diagnostics. In a preferred embodiment, the electrode comprises an antibody modified, polyethylene glycol (PEG) monolayer assembled on a gold surface. In a second embodiment, the electrode comprises a chemisorbed zwitterionic carboxybetaine polymer obtained by photopolymerisation of carboxybetaine methacrylate (CBMA). This allows the use of non-Faradaic analysis to determine the insulin concentration even in undiluted blood serum.
