Scheme 1. (A) Schematic illustration of the preparation of S, N, O-CDs and their applications in biosensing and bioimaging of Cu²⁺ in cells. (B) Working Principle of S, N, O-CDs/Cu²⁺ system for the determination of PPi and ALP activity. Inset shows the complexation between Cu²⁺ and PPi as well as the hydrolysis of ALP towards PPi.