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Competition between SANS and SAXS: Which technology can better reveal the mystery of matter?
In materials science and biomedical research, analysis of minute structures is crucial to understanding the behavior of matter. Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) are two important techniques that are widely used to probe structures between the microscopic and macroscopic levels. While the two technologies share many similarities, their respective strengths may make them uniquely valuable in specific applications.
Small-angle neutron scattering is an experimental technique that uses elastic neutron scattering at small scattering angles to probe the structure of various materials.
SANS technology focuses on the mesoscopic scale of approximately 1 to 100 nanometers, which enables it to analyze the internal structure of non-ordered systems. SAXS can also achieve this, but there are significant differences between the two in terms of sensitivity and application. Especially in the study of light elements, SANS shows high sensitivity to light elements such as deuterium, which is particularly important for the study of biological samples. This is because hydrogen scattering has unique behavior relative to deuterium, and this contrast change is crucial in the study of many biopolymers.
Unlike X-ray scattering, neutron scattering technology provides structural information that is closely related to nuclear interactions and can effectively reveal the internal structure of molecules.
The main difference between SANS and SAXS is their scattering mechanisms. Neutron scattering involves the interaction of neutrons with the nucleus rather than with the electron cloud. This means that the scattering behavior of different isotopes can vary significantly. For example, the scattering length of hydrogen is negative, giving it a 180° phase difference in neutron scattering, which provides important contrast information in structural analysis.
SANS structural analysis relies specifically on the so-called contrastive variation technique. In this technique, biological samples are typically exchanged with deuterium, resulting in unique structural features that contribute to the scattering. Current research has shown that at a specific ratio of H2O to D2O (called the matching point), the scattering of the molecules is equal to that of the solvent, allowing the scattering of the sample to be eliminated, thereby better analyzing the structure of biomacromolecules.
For some samples, such as proteins, the match is usually made at a deuterium concentration of 40%-45%, which allows the scattering from the protein to compete with the scattering from the solvent.
In biology, the advantages of SANS make it an ideal tool for studying proteins and other biomolecules. At the same time, small-angle X-ray scattering (SAXS) can be more conveniently used for certain specific material studies. In some cases, the combined use of these two techniques provides a more comprehensive perspective on structural analysis. For example, a recent study combined SAXS, SANS and electron microscopy data to construct an atomic model of a large multi-subunit enzyme, showing the importance of combining different techniques.
Unlike SAXS, SANS generally requires a good collimation of the neutron beam to ensure an optimal signal-to-noise ratio. To address the problem of reduced signal over that length scale, some researchers have chosen to increase the brightness of the source. For example, the introduction of ultra-small angle neutron scattering (USANS) and spin echo small angle neutron scattering (SESANS) technologies has expanded the length range of research to more than 10 microns, demonstrating the flexibility and development potential of neutron scattering technology. .
Whether SANS or SAXS, the wide application of these techniques in materials science and biological science provides us with important tools for a deep understanding of the internal structure of matter.
Currently, there are many neutron facilities around the world that offer a variety of SANS instruments, which are becoming more precise and efficient as technology advances. This allows scientists to study materials at a deeper level and develop a wider range of applications.
With the advancement of related technologies, what unknown scientific mysteries can these methods reveal to us in the future?
