Aggregation mechanism in DNA functionalized gold nanoparticles in multivalent and ionic liquid environment

The stability of colloidal nanoparticle systems is highly sensitive to both the type and concentration of salt, influencing their electrostatic interactions and surface properties. As ionic strength increases, the electrical double layer around the nanoparticle’s compresses, diminishing electrostatic repulsion. This screening effect allows dominance of attractive van der Waals forces, often resulting in nanoparticle aggregation. The degree of this destabilization depends on the specific salt, its concentration, and the nanoparticles' surface ligands.



In this study, we show that increasing ionic strengths of divalent salts and ionic liquids can induce the aggregation of DNA-coated gold nanoparticles in aqueous solution. Furthermore, we demonstrate that this aggregation behavior is strongly influenced by both the salt type and the DNA structure. Through systematic investigations using dynamic light scattering, UV-Visible absorption spectroscopy, transmission electron microscopy, small-angle X-ray scattering, and fluorescence spectroscopy, we have characterized the underlying aggregation mechanism. In the presence of divalent salts, we observed that aggregation is primarily driven by electrostatic screening and ionic bridging, leading to the formation of dense, three-dimensional aggregates at higher ionic strengths, whereas in Ionic Liquid environments, groove binding interactions play a dominant role. For double-stranded DNA, groove binding of the alkyl chains in the ionic liquid promoted robust, multilayered aggregation, whereas for single-stranded DNA, only weak interparticle aggregation was observed. Our study highlights the importance of salt ionic strength and surface ligands on colloidal stability, offering valuable insights for tuning nanoparticle behavior in applications such as biomedicine, catalysis, and materials science.