Cd(OAc)2 reacted with S and Se at 150°C to form CdSeS core, followed by addition of Zn(OAc)2 and S, and heated at 270°C to form the ZnS shell to produce the CdSeS@ZnS QDs. Product was stored in hexane. One milliliter of QD solution was added to 1 ml of HS-NTA in methane solution (pH was tuned to 12) and then vortexed for 5 min. The precipitate was dispersed in 10 ml of PBS solution after centrifugation. The aqueous layer was filtered with 0.2-μm Acrodisc syringe filters (Pall). Twenty microliters of 50 mM CoCl2 solution was added to 10 ml of HS-NTA decorated QDs in PBS solution. Then, the solution was vortexed, centrifuged at 5000 rpm for 3 min, and filtered with 0.2-μm Acrodisc syringe filters (Pall). The samples thusly prepared were stored in dark place at 4°C.