Small angle X-ray scattering, or SAXS, is an experimental method where the intensity of the scattered X-rays is measured as a function of the scattering angle. The information obtained in a SAXS experiment can be used to recover information about the bulk microstructure within a sample and is commonly used to study condensed matter systems that are only partially ordered.
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One of the advantages of SAXS for the analysis of nanomaterials is that it can be used to analyze samples on a variety of length scales. In a typical SAXS experiment, the length scales measured can vary from the nanoscale to the mesoscale.
While for nanomaterial studies, some of the unique phenomena exhibited at the smallest length scales, such as enhanced thermal properties, are of primary interest, for many materials applications, the behavior across all length scales can be crucial for ensuring proper device performance.
SAXS Technique
In an X-ray scattering experiment, a sample is illuminated with an incident X-ray beam. Due to the interaction between the X-ray photons and atoms in the sample, some of the incident radiation will be scattered at different angles. Elastic scattering occurs when no energy exchange occurs between the incident X-ray photon and sample, so the scattered radiation is equal in energy to the incident radiation.
Some X-ray photons will be deflected or scattered at different angles to the incident X-ray radiation. By measuring the scattering angle and intensity of the X-ray radiation, information on the structural features of the sample can be recovered, such as particle sizes and distributions or the degree of disorder.
SAXS refers specifically to measuring the elastically scattered X-rays at small (~0 – 10°) angles. There is a related technique, wide-angle X-ray scattering (WAXS), that also measures elastically scattered radiation but at much wider scattering angles (> 10°).
Often, SAXS and WAXS experiments are performed together as changing the relative distance between the detector and the sample is sufficient to switch between a collection of wide or small-angle scattered radiation.
Many SAXS experiments are performed at advanced light source facilities such as synchrotrons as the weak nature of the scattering signal means that a high incident photon intensity is beneficial for improved signal-to-noise and reduced acquisition times in the measurement. However, there are a number of laboratory-based SAXS instruments as well, though acquisition times are typically very long.
Applications of SAXS
SAXS has several applications, including in the analysis of biological materials and nanomaterials. For nanoparticle analysis, SAXS is now often used as an in situ technique to monitor nanoparticle growth and formation. Understanding growth processes is an important part of developing synthesis strategies to grow nanoparticles in a controlled fashion and the structure of the nanoparticles determines their overall properties.
Nanoparticle sizing is a common application of SAXS due to the excellent spatial resolution of the technique that is achievable even in a laboratory environment. SAXS can also be used to extract concentration information from such samples.
SAXS is well-suited to in situ measurements of processes such as nanoparticle growth or materials synthesis as it requires minimal sample preparation and is compatible with a variety of sample types, including disordered solids and colloidal dispersions. In situ measurements require relatively short acquisition times to capture multiple images of the process as it occurs to see its evolution.
Biological applications account for a large number of SAXS studies, as SAXS can be used to determine the protein, nucleic acid and biopolymer structures and sizes.
As many biological systems undergo continual structural changes even at room temperature, one of the advantages of SAXS for biological imaging is that time-resolved variants of the technique can be used to capture processes such as protein folding in action. This makes SAXS an invaluable tool for not just understanding single structures in structural biology but the full landscape of how different conformers and structures are interconnected.
Outlook
As SAXS experiments use 2D detectors, a large amount of data is generated with each image recorded. Finding ways to reduce data sizes, speed up data processing and automate large parts of the analysis procedure has been very important in turning SAXS into a routine analytical tool. It is necessary to use some fitting and reconstruction procedures to convert from the recorded 2D image data to structural information such as particle sizes or distances.
Many of the algorithms for such procedures are available as relatively easy-to-use software packages for simple cases.
Brighter synchrotron sources, more efficient and sensitive detectors and greater degrees of automation of the experimental acquisition and data analysis will all help improve the throughput of SAXS measurements. With advances in nanoscale fabrication methods such as focused ion beams, there will continue to be a great demand for techniques such as SAXS that can characterize materials on the nanoscale reliably.
Time-resolved studies with SAXS are also likely to play an important role with the increasing availability of X-ray free-electron laser sources. Achieving very short (< 100 fs) time resolutions with high photon flux at synchrotrons can be very challenging.
Free-electron lasers, with their high peak brightnesses, also offer the ability to perform SAXS measurements on materials under extreme conditions to understand phenomena such as plasma formation and propagation in materials.
References and Further Reading
Giannini, C., Ladisa, M., Altamura, D., Siliqi, D., Sibillano, T., & Caro, L. De. (2016) X-ray Diffraction : A Powerful Technique for the Multiple-Length-Scale Structural Analysis of Nanomaterials. Crystals, 6, p. 87. https://doi.org/10.3390/cryst6080087
Shi, S., & Russell, T. P. (2018). Nanoparticle Assembly at Liquid – Liquid Interfaces : From the Nanoscale to Mesoscale. Advanced Materials, 30, p. 1800714. https://doi.org/10.1002/adma.201800714
Li, T., Senesi, A. J., & Lee, B. (2016). Small Angle X‑ray Scattering for Nanoparticle Research. Chemical Reviews, 116, pp. 11128–11180. https://doi.org/10.1021/acs.chemrev.5b00690
Garcia, P. R. A. F., Prymak, O., Grasmik, V., Pappert, K., Wlysses, W., Otubo, L., Epple, M., & Oliveira, C. L. P. (2020). SAXS investigation of the formation of silver nanoparticles and bimetallic silver – gold nanoparticles in controlled wet-chemical reduction synthesis. Nanoscale Advances, 2, pp. 225–238. https://doi.org/10.1039/c9na00569b
Pauw, B. R., Ka, C., & Thunemann, A. F. (2017). Nanoparticle size distribution quantification : results of a small-angle X-ray scattering inter-laboratory comparison research papers. Journal of Applied Crystallography, 50, pp. 1280–1288. https://doi.org/10.1107/S160057671701010X
Brosey, C. A., & Tainer, J. A. (2019). Evolving SAXS versatility : solution X-ray scattering for macromolecular architecture , functional landscapes , and integrative structural biology. Current Opinion in Structural Biology, 58, pp. 197–213. https://doi.org/10.1016/j.sbi.2019.04.004
Kluge, T., Rödel, M., Metzkes-ng, J., Pelka, A., Garcia, A. L., Prencipe, I., Rehwald, M., Nakatsutsumi, M., Mcbride, E. E., Schönherr, T., Garten, M., Hartley, N. J., Zacharias, M., Grenzer, J., Erbe, A., Georgiev, Y. M., Galtier, E., Nam, I., Lee, H. J., … Landstraße, B. (2018). Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser. Physical Review X, 8(3), p. 31068. https://doi.org/10.1103/PhysRevX.8.031068
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