Abstract: An electrical sensor arrangement for measuring a property of a chemical species (32) comprises first and second electrodes (72, 74) comprising first and second generally planar molecular layers (76, 78) each consisting of an array of covalently bonded atoms. The first molecular layer (76) is covalently bonded (82) to the second molecular layer (78) to define an aperture (84) through both the first and second molecular layers. The aperture (84) is configured to enable the chemical species (32) to pass through. The sensor arrangement further comprises an electrical power supply (86) connected to the first electrode (76) and the second electrode (78) and configured to apply a voltage across the first electrode (76) and the second electrode (78).