Cooperative Metal Ion-Mediated Adsorption of Spherical Nucleic Acids with a Large Hysteresis

Spherical nucleic acids (SNA) refer to nanoparticles attached with a high density of oligonuleotides. Linear and spherical nucleic acids have many differences such as hybridization affinity, melting transition, and cellular uptake. In this work, these two types of DNA of the same sequence were compared for adsorption on polydopamine (PDA) nanoparticles and graphene oxide (GO). We focused on the effect of metal ions including Na⁺, Ca²⁺, and Zn²⁺ since metal ions are indispensible for DNA adsorption on PDA and GO. Gold nanoparticles (AuNPs) of various sizes were used to prepare the SNAs. For both PDA and GO, a normal binding curve of one metal ion was obtained for adsorbing the linear DNA, while the spherical DNAs larger than 5 nm showed a sigmoidal binding curve requiring multiple metal ions. Urea and EDTA were used to probe DNA adsorption affinity, where the spherical DNA showed stronger adsorption in general. In the presence of 300 mM Na⁺, 4 M urea or 4 mM EDTA failed to desorb the 13 nm spherical DNA. The spherical DNA showed a very large hysteresis of metal-dependent adsorption. This study demonstrates another unique property of SNA compared to linear DNA, revealing interesting orientation and packing of DNA on AuNPs, which has deepened our understanding of DNA interface chemistry.