Direct imaging of liquid-nanoparticle interfaces with atom probe tomography

Understanding the structure and chemical composition at the liquid-nanoparticle (NP) interface is crucial for a wide range of physical, chemical, and biological processes. In this study, direct imaging of the liquid-NP interface by atom probe tomography (APT) is reported for the first time, which reveals the distributions and the interactions of key atoms and molecules in this critical domain. The APT specimen is prepared by controlled graphene encapsulation of the solution containing nanoparticles on a metal tip, with an end radius in the range of 50 nm to allow field evaporation. Using gold nanoparticles (AuNPs) in suspension as an example, analysis of the mass spectrum and three-dimensional (3D) chemical maps from APT provides a detailed image of the water-gold interface at near-atomic resolution. A locally dense region of Au+ ions has been reconstructed, representing a portion of an individual AuNP. A large number of water-related ions have also been identified, confirming the AuNP in the hydrated state. At the water-gold interface, a trisodium citrate layer has been observed based on Na+ and C-containing ions.