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 starburst dendrimers (SBDs), members of the family of dendritic macromolecules, have been analyzed in the absence and in the presence of dimyristoyl-phosphatidylcoline (DMPC) liposomes and mixed DMPC/DMPA-Na (the sodium salt of DMP-colate) liposomes at different relative percentages of DMPC and DMPA-Na. Under most conditions with dendrimers present, the dendrimers, rather than the liposomes, are visible in the TEM images, demonstrating that the UO22+ is complexed to the dendrimers and not to the liposomes. Only at high composition of DMPA-Na in the liposomes (>40%) and under the condition of high protonation of the dendrimer surface are the liposomes imaged by TEM. Mixed liposomes 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 the nitrogen ligand sites at both the external and the internal dendrimer surfaces. The saturation of the dendrimer by Cu(II) occurs at about 33% complexation of the nitrogen groups. The stability constant of the Cu2+-SBD complex was evaluated, along with an indirect estimation of the stability of the UO22+-SBD complex. The results demonstrate that starburst dendrimers selectively bind to uranyl ions and that the latter compete effectively for these sites, even with Cu(II) ions, which are well-known for forming stable complexes with nitrogen ligands. These results show that dendrimers have the potential for storing uranium derivatives, a process of great importance in the fields of energy production and environmental cleanup.