Cryo-FIB-SEM-CT: a 'three-in-one' imaging facility for opaque soft matter

点击翻译按钮获取中文摘要

The University of Edinburgh (UoE) is an international centre for soft matter research. Soft matter scientists study complex liquids in which there are structures intermediate in size between the small molecules making up the solvent and the macroscopic world of structures visible to the naked eye. The structures may be pigment particles in paint (colloids), self-assembled aggregates of soap molecules in washing-up liquid (surfactant micelles), bubbles (say, in whipped cream) or long-chain molecules (polymers) such as DNA (as used in gene transfection). The field is of fundamental interest because the properties of these materials are controlled by many competing length and time scales, and can change dramatically under everyday conditions. Such materials are also ubiquitous in formulations of all kinds, such as medicines (CALPOL is a suspension of colloidal paracetamol) and personal care products (a shampoo is a surfactant-polymer mixture). They are also key intermediates in many sectors: all ceramics, for example, begin as soft pastes (think potter's clay) that are pumped into moulds or extruded before they are sintered at high temperatures to form the final hard products. A fundamental challenge in soft matter is to figure out the way these intermediate structural elements are organised, and how such organisation changes in response to external forces. Most of such materials are opaque to light, so that optical microscopies of all kinds are not useful except to give surface information. As a result, scientists sometimes resort to various kinds of electron microscopy, which, however, operate in a vacuum, so that wet samples are desiccated and their native structures destroyed. A major development in the last decades is cryogenic scanning electron microscopy (cryo-SEM). A native, wet soft sample is frozen in liquid nitrogen, using special techniques to ensure rapidity of cooling to preserve intricate microstructures. Then these frozen samples are fractured, exposing internal structures to be imaged by SEM. (One disadvantage is that samples fracture more or less randomly, so that what is exposed to view is haphazard.) A single cryo-SEM instrument exists at UoE. This decade-old instrument is limited in both resolution (ten times worse than the best instruments today) and in access for soft matter scientists, as it requires laborious conversion of the system to cryogenic mode. We propose to purchase a state-of-the-art cryo-SEM to enable this cutting-edge technique to become routinely available for day-to-day work. This purchase is timely, because cryo-SEM has recently been revolutionised for soft matter researchers by the addition of focussed ion beam (FIB). This powerful technique uses a focused beam of charged atoms (ions) to cut and section specimens very accurately inside the SEM. This not only allows the user to expose desired sections at will, but also to build up a complete 3D picture (literally) by imaging the sample section by section to a resolution of 10 nm (100 times the size of atoms). We propose to purchase a cryo-FIB SEM. The technique is so new that we know of only two current instruments in the UK, neither of which is dedicated to the study of soft matter. We propose to add X-ray tomography capability to our cryo-FIB SEM. This allows us to build up a 3D picture non-destructively to 350 nm resolution, much like the way X-ray CT scanners build up 3D images of the body in hospitals. The availability of this combined suite of instruments will transform the ability of soft matter scientists to see inside their samples routinely. A host of exciting applications immediately follow. One example is 'designer electrodes' based on novel soft materials that minimise expansion/shrinkage during charge/discharge cycles. A programme of outreach and training will make this facility available to academic and industrial researchers UK-wide.