Two-dimensional nanostructures with atomically precise building blocks have potential applications in catalysis and sensing. However, structural instability and surface reactivity limit their practical use. In this work, we demonstrate the formation of vertically aligned nanoplates of the [Co6S8DPPE6Cl6] cluster (Co6 in short), protected by 1,2-bis(diphenylphosphino)ethane, using ambient electrospray deposition (ESD). Charged microdroplets of Co6 formed by ESD on a water surface created such nanostructures. Preferential arrangement of clusters in the nanoplates with enhanced surface area results in sensitive and selective electrochemical response toward arsenite down to 5 parts per billion, in tap water. Density functional theory calculations reveal the preferential binding of arsenite with Co6. Our work points to a practical application of atomically precise clusters of large societal relevance. © 2023 American Chemical Society.