The ability to construct size- and shape-controllable architectures is essential for the exploration of nanoparticle-structured properties, and it is a good strategy of employing metal nanoparticles embedded in a polymer matrix in order to prepare new materials with particular properties. Herein, we found that adenosine-5'-triphosphate (ATP) could be applied to adjust and control the formation of silver nanocubes (Ag-NCs) on Nafion film. Nafion could be saturated with [Ag(NH3)(2)](+) when incubated in silver ammonia solution, and it was found that the Nafion film saturated with [Ag(NH3)(2)](+) becomes yellow after immerged in a mixture containing NaOH, ATP, and formaldehyde, resulting in monodisperse Ag-NCs on the film. Thus, ATP as a molecular mediator and Nafion film as a polymer matrix are employed toward the preparation of size-controllable and monodisperse Ag-NCs, and a novel visual method for formaldehyde is further developed on the basis of the color change of the Nafion film, which gives sensitive detection of formaldehyde with a limit of determination of 60 ppb (3 sigma).