Fig nbsp xA Relative proportion of Eu III

As shown in Fig. 6, the Eu(III) in solution is predominated by three positively charged species (i.e., Eu3 +, Eu(NO3)2 + and Eu(OH)2+) in the whole pH range. The surface of Fe3O4@CD MCs becomes negatively charged at pH > pHzpc. The successive deprotonation of hydroxyl groups enhances the surface electronegativity with increasing pH values, which favors the formation of strong inner-sphere complexes.