Knowing that our buffer contained a significant amount of Mg 2+ (12.5 mM), we performed the same study in a buffer composed of solely monovalent cations. To get stable nanotubes, we replaced MgCl2 by a high concentration of NaCl (100 mM). Interestingly, we obtained the same types of superstructures by adding increasing amounts of SPM 4+ (Fig. ), including the formation of rings (Fig. ) having dimensions similar to those obtained in the regular Mg 2+ -containing buffer (Fig ). All these results showed that the multivalent counter-ions were the main driving force for the superstructure formation. Note that the frontiers for the transitions were shifted to higher charge ratios ρ when Na + was used (ρ = 21.4 for the nanotube-network and ρ = 89.3 for the network-bundle transition, Fig. ) instead of Mg 2+ (ρ = 7.14 and ρ = 14.3, Fig. ), attributed to the large concentration of monovalent cations competing for counter-ion exchange and DNA neutralization.