Disclosed herein are nanostructures comprising manganese ions incorporated in a polymeric framework comprising at least five geminal bisphosphonate groups, wherein the geminal bisphosphonate groups independently of each other are incorporated as -R3R4C(P=O(OR2)(OR2))2, wherein R1 and R2 are independently selected from the group consisting of a negative charge, H, alkyl and aryl, and wherein at least one of R3 and R4 is a group connected to the polymeric framework with the proviso that when only one of R3 and R4 is such a connected group, the other of R3 and R4 is either a group being able to connect to the polymeric framework, or the residue of such a group, or selected from the group consisting of H, OH, OR5 and R5, wherein R5 is a lower alkyl. Disclosed are also methods for producing such nanostructures, compositions comprising such nanostructures and use of such nanostructures, i.a. as MRI contrasting agents.
