Self‐Assembly of DNA Functionalized Gold Nanoparticles at the Liquid‐Vapor Interface

Abstract

Surface sensitive synchrotron X‐ray scattering and spectroscopy are used to monitor and characterize the spontaneous formation of 2D Gibbs monolayers of thiolated single‐stranded DNA‐functionalized gold nanoparticles (ssDNA‐AuNPs) at the vapor–solution interface by manipulating salt concentrations. Grazing incidence small‐angle X‐ray scattering and X‐ray reflectivity show that the noncomplementary ssDNA‐AuNPs dispersed in aqueous solution spontaneously accumulate at the vapor–liquid interface in the form of a single layer by increasing MgCl2 or CaCl2 concentrations. Furthermore, the monoparticle layer undergoes a transformation from short‐ to long‐range (hexagonal) order above a threshold salt‐concentration. Using various salts at similar ionic strength to those of MgCl2 or CaCl2 such as, NaCl or LaCl3, it is found that surface adsorbed NPs lack any order. X‐ray fluorescence near total reflection of the same samples provides direct evidence of interfacial gold and more importantly a significant surface enrichment of the cations. Quantitative analysis reveals that divalent cations screen the charge of ssDNA, and that the hydrophobic hexyl‐thiol group, commonly used to functionalize the ssDNA (for capping the AuNPs), is likely the driving force for the accumulation of the NPs at the interface.