Afluorescence sensor was fabricated using water-soluble nanocrystalline quantum dots capped with cysteamine for PPi sensing. The sensor can be easily constructed by decorating Cys-CdS QDs with Fe3+. The key sensing mechanism was based on the formation of a strong complex between Fe3+ and PPi, which promoted the electron transfer process between the Fe3+ complex and the QDs. The decrease in the fluorescence intensity was an increasing linear function of the PPi concentration, indicating that this sensor can be used to detect PPi at the micromolar concentration level. The feasibility of the proposed sensor was demonstrated by determining the PPi content in urine samples with satisfactory results.