X-ray and neutron imaging with colloids

A series of technical advances are helping to revolutionise the possibilities for X-ray and neutron imaging in colloidal science. These include the development of new imaging modalities with high coherence X-rays such as diffractive imaging, ptychography, and femtosecond holography; and Talbot phase contrast tomography with conventional laboratory based low coherence X-ray sources e.g. standard rotating anodes. A crucial insight is that the available phase contrast with synthetic organic and biological colloids can be two orders of magnitude stronger than the absorption contrast with X-rays, providing large improvements in the signal to noise ratio in the resultant images. Furthermore new developments with the sources of X-rays and neutrons are helping to increase the possibilities for this research as the available coherence, flux and collimation are improved e.g. third generation high brilliance synchrotrons, free electron lasers, high flux pulsed neutron sources and table-top X-ray lasers are being developed. Highlights of the application of these techniques and sources to colloids include: the measurement of the internal strains inside individual crystalline colloidal nanoparticles, the imaging of nanoparticles embedded in opaque solid composite materials, images of defects in the growth of colloidal crystals, and the morphology of nanofoams, intact human chromosomes, protein nanocrystals, viruses, bacteria, and blood cells. The resolution of the reconstructed images can be achieved at the 10–50 nm length scale, without the need for the invasive sample preparation techniques required for transmission electron microscopy e.g. microtoming of specimens is not required. Furthermore fluorescent staining is also not required, as with super-resolution microscopies at visible optical wavelengths (e.g. STED, PALM and STORM), and thick opaque samples can be investigated, although some fragile organic and biological materials require freezing to reduce beam damage with X-rays. Neutron imaging has also benefited from the development of analogous Talbot phase contrast techniques to those possible with low coherence X-rays and a number of useful applications in non-invasive imaging at the 100 μm length scale have been demonstrated e.g. the internal structure of live plants, the inner workings of fuel cells and the three-dimensional domain structure of magnetic materials.