In order to evaluate the feasibility of the proposed method in real samples detection, the developed CdS QDs probe was applied to the determination of Ca2+ions in fetal bovine serum samples. The commercial fetal calf serum solution was firstly centrifuged at 7500g for 30 min with a centrifugal filter unit (MWCO 3 KD) to remove some macromolecules. And then the obtained serum filtrate was diluted by 10 times with deionzed water. Different amount of Ca2+ were added to diluted serum samples prepare the spiked samples.The results obtained by standard addition method were shown in Table S1, and the accuracy of the proposed method was evaluated by determining the recoveries of Ca2+ in serum samples. It can be seen that the recoveries in the real samples was between 103 and 106% and the relative standard deviation (RSD) was no more than 3.6%. The above results demonstrated the potential applicability ofthe method for the detection of Ca2+ ions in serum samples.