It is well known that phosphate anions exhibit high formation constants with Fe3+ [25], so phosphate-containing molecules such as PPi are expected to form stable complexes with Fe3+. Therefore, f luorescence sensors were proposed for the detection of PPi in this workbased onFe3+-decorated Cys-CdS QDs. The formation of a stable Fe3+ complex will produce an efficient quencher, and therefore the level of PPi can be determined by the fluorescence quenching approach.