Assembly Dynamics of Plasmonic DNA-Capped Gold Nanoparticle Monolayers

The self-assembly of nanoparticles in aqueous solutions promises wide applications but requires the careful balance of many parameters not present in organic solvents. While the presence of long-range electrostatic interactions in aqueous solutions may complicate such assemblies, they provide additional parameters through which to control self assembly. Here, with DNA-capped gold nanoparticles and through the variation of the ionic strength in aqueous solutions, we explored the influence of electrostatic interactions on the adsorption of negatively charged nanoparticles on a positively charged surface. Specifically, we studied the kinetics of nanoparticle adsorption from solution using the quartz crystal microbalance with dissipation (QCM-D). We also characterized the structure of the adsorbed monolayers employing a combination of grazing incidence small-angle X-ray scattering (GISAXS) and scanning electron microscopy. We discovered that adsorption kinetics and monolayer structure were under the control of the DNA ligand length, solution ionic strength, and salt species. We also precisely fit the kinetics to a modified Langmuir model, which converged to the simple Langmuir model at high ionic strengths of magnesium chloride. We demonstrated that increasing the ionic strength and decreasing the DNA ligand lengths increased the surface coverage while decreasing the nanoparticle-nanoparticle spacing. The DNA-capped nanoparticle system reported here provides a readily applicable platform for controlling nanoparticle self-assembly in aqueous solution. Finally, we employ this tunability to create a system with a tunable plasmonic response. Our kinetics studies of the assembly process and further characterizations undertaken will facilitate the construction of nanoparticle arrays with precise structure, and such control will aid in the design of future plasmonic and optoelectronic devices.