Enabling near-atomic-scale analysis of frozen water

TL;DR

This paper demonstrates the use of atom probe tomography for near-atomic-scale analysis of frozen water, enabling detailed 3D reconstructions and interface analysis in biological and chemical systems.

Contribution

It introduces a novel specimen preparation method using nanoporous gold and shows the feasibility of atom probe analysis of frozen aqueous specimens at near-atomic resolution.

Findings

Successful 3D atom-by-atom analysis of frozen water layers

Identification of Na and Cl concentration trends at the gold-water interface

Discussion of experimental challenges and physical processes in atom probe analysis of liquids

Abstract

Transmission electron microscopy has undergone a revolution in recent years with the possibility to perform routine cryo-imaging of biological materials and (bio)chemical systems, as well as the possibility to image liquids via dedicated reaction cells or graphene-sandwiching. These approaches however typically require imaging a large number of specimens and reconstructing an average representation and often lack analytical capabilities. Here, using atom probe tomography we provide atom-by-atom analyses of frozen liquids and analytical sub-nanometre three dimensional reconstructions. The analyzed ice is in contact with, and embedded within, nanoporous gold (NPG). We report the first such data on 2-3 microns thick layers of ice formed from both high purity deuterated water and a solution of 50mM NaCl in high purity deuterated water. We present a specimen preparation strategy that uses a NPG film and, additionally, we report on an analysis of the interface between nanoporous gold and frozen salt water solution with an apparent trend in the Na and Cl concentrations across the interface. We explore a range of experimental parameters to show that the atom probe analyses of bulk aqueous specimens come with their own special challenges and discuss physical processes that may produce the observed phenomena. Our study demonstrates the viability of using frozen water as a carrier for near-atomic scale analysis of objects in solution by atom probe tomography.