An assay method includes providing a luminescent nanocrystal; combining a solution having an adenosine triphosphate molecule; and displaying a light emission by the luminescent nanocrystal and the solution combined. Large applications are accommodated by providing a mechanism for secure program overlays, in which a single large application is broken into two or more smaller applications that can be executed from the same memory space by overwriting one of the smaller applications with another of the smaller applications when the latter needs to be executed. The interactive display includes a plurality of buttons, each of which changes from a normal state to a selected state, and from the selected state to an active state, according to a user operation.