Abstract: A mercury vapour sensor in which the sensor surface is a gold substrate, and gold nanostructures with controlled crystallographic facets are strongly adhered to the substrate. A substantial increase in response magnitude and stability of a quartz crystal microbalance (QCM) based mercury vapour sensor is achieved using this sensor surface. The method of forming gold nanostructures on a gold substrate includes the steps of electrodepositing gold onto a gold working electrode from a solution of hydrogen or alkali metal tetrahaloaureate (III) and an additive such as lead acetate at an electro-deposition temperature between 20 and 40° C. and a deposition time of at least 15 seconds. The growth is controlled by the composition of the deposition solution, the temperature and the current density. The deposition rates may be varied as will the deposition times which are preferably about 150 seconds but may be as long as 15 minutes. The preferred deposition solution contains 2.