A TEM and EPR investigation of the competitive binding of uranyl ions to starburst dendrimers and liposomes: Potential use of dendrimers as uranyl ion sponges

Transmission electron micrographs (TEM) of UO22+-negatively stained starbur st dendrimers (SBDs), members of the family of dendritic macromolecules, ha ve been analyzed in the absence and in the presence of dimyristoyl-phosphat idylcoline (DMPC) liposomes and mixed DMPC/DMPA-Na (the sodium salt of DMP- colate) liposomes at different relative percentages of DMPC and DMPA-Na. Un der most conditions with dendrimers present, the dendrimers, rather than th e liposomes, are visible in the TEM images, demonstrating that the UO22+ is complexed to the dendrimers and not to the liposomes. Only at high composi tion of DMPA-Na in the liposomes (>40%) and under the condition of high pro tonation of the dendrimer surface are the liposomes imaged by TEM. Mixed li posomes show a rodlike shape. To confirm the TEM results, an EPR study was performed by adding to the SBD solution various amounts of Cu2+ and UO22+. Uranyl ions compete favorably with copper ions for the complexation with th e nitrogen ligand sites at both the external and the internal dendrimer sur faces. The saturation of the dendrimer by Cu(II) occurs at about 33% comple xation of the nitrogen groups. The stability constant of the Cu2+-SBD compl ex was evaluated, along with an indirect estimation of the stability of the UO22+-SBD complex. The results demonstrate that starburst dendrimers selec tively bind to uranyl ions and that the latter compete effectively for thes e sites, even with Cu(II) ions, which are well-known for forming stable com plexes with nitrogen ligands. These results show that dendrimers have the p otential for storing uranium derivatives, a process of great importance in the fields of energy production and environmental cleanup.