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SAXS nanostructure analysis

SAXS is a non-destructive method for investigating nanostructures in liquids and solids. In a SAXS experiment the X-ray beam hits a nanostructured sample, for example proteins, macromolecules, or nanoparticle dispersions.

The properties of a material in general are related to the structure and arrangement of domains on the nanoscale. To understand the relation between the size, shape, and arrangement of nanostructures and their macroscopic behavior it is necessary to accurately analyze these structures. There are several classical structural methods (many of them imaging methods), such as microscopy (AFM, TEM), which are used for characterizing nanostructured materials. These methods, however, have the disadvantage that averaged, i.e. representative results of a sample can hardly be obtained. Small-angle X-ray scattering (SAXS) ideally complements microscopic methods since it provides representative structural information about a large sample area.

Basic principles of small-angle X-ray scattering (SAXS)

First uses of the SAXS method date back to the middle of 20th century when its fundamental principles were developed and first experiments were performed. Guinier and Fornet [1] used SAXS for studying the nanostructure of metallic alloys, Kratky [2] applied SAXS for the characterization of (biological) macromolecules in solution.

Today SAXS investigations are performed on large synchrotron radiation beamlines and on modern laboratory SAXS systems.

Each SAXS system consists of an X-ray source, a collimation system, a sample stage, and a detector. Appropriate software is used to process and evaluate the measured scattering data.

How a SAXS experiment works

SAXS experiment graph — Fig. 1: Principle of a SAXS experiment

The X-ray source emits an X-ray beam that interacts with the electrons of the sample and is scattered. The detected scattering pattern is characteristic for the nanostructures of the sample and can be used to determine important structural parameters such as particle size, shape, internal structure, porosity, and arrangement (orientation).

Watch how SAXS works in our one-minute clip “SAXS with an S”.

Small and wide angles: the difference

SAXS and WAXS graph — Fig. 2: SAXS and WAXS

The scattered X-rays can be recorded at different angles. In general it can be said: The larger the particles of a sample, the smaller the scattering angle.

In SAXS, the scattering pattern at small angles is analyzed, typically below 10° 2, to probe nano-sized particles and domains in a size range from approx. D ~ 1 to 100 nm, which scatter towards these small angles.

In WAXS (Wide-angle X-ray scattering), smaller structures are investigated, such as crystal lattices at the atomic level. Here the scattered X-rays are interpreted at wider angles. The obtained WAXS pattern enables the analysis of structures below nanometer size (d < 1 nm) such as atoms and interatomic distances which scatter towards wider angles.

Collimation types

Before scattering, i.e. hitting the sample, X-rays are transformed into a well-defined line-shaped or point-shaped beam. This process is called collimation. Each collimation type in a SAXS system is ideal for different applications.

A line-collimated beam has the advantage that it combines a high photon flux with a high scattering volume – which means measurement times can be dramatically shorter than with point collimation. The drawback of a line-collimated beam is that it can only probe isotropic samples. Therefore, a line-shaped beam is preferable for analyzing weakly scattering samples, such as proteins and other soft matter.

A point-collimated beam can be used to also analyze anisotropic samples, such as fibers or porous solids. Point collimation allows you to probe small sample areas and determine their local nanostructure, with the drawback of longer measuring times.

SAXS parameters and applications

SAXS is used to determine several parameters of nanostructured samples – the most common ones are:

Shape
Size
Internal structure
Crystallinity
Porosity
Orientation

Unique SAXS benefits

Representative results

SAXS results are representative of an entire sample, so SAXS ideally complements methods that provide unique but local information, such as electron microscopy.

Nanostructured thin-film samples can be analyzed by applying an incident X-ray beam which grazes the sample under a very small incident angle: the GISAXS (grazing-incidence SAXS) method ideally complements microscopy techniques by providing representative information which is valid for a large sample volume.

Low sample preparation efforts

With respect to sample treatment, SAXS barely requires any sample preparation. This sets it apart from complementary techniques such as electron microscopy or NMR spectroscopy, which often require extensive sample preparation. And, since SAXS allows in-situ measurements, preparation artifacts are avoided and the sample remains unchanged.

Analyzing biological materials in their native state

SAXS also stands out for the fact that it can be used to investigate biological macromolecules in solution, i.e. under physiological conditions. This increasingly popular application known as BioSAXS is a vital tool in molecular biology, where the analysis of samples in their native state is essential for studying the dynamic processes the sample is involved in.

Conclusion

Today SAXS is a frequently used tool for analyzing the size, shape, and internal structure of nano-sized materials, in fundamental research and increasingly in routine analysis, e.g. environmentally-relevant studies of nanoparticles. The method provides representative results and therefore ideally complements related techniques for analyzing nanostructures such as atomic/electron microscopy or NMR.

SAXS covers many different applications, including biological samples (proteins, lipids), nanoparticle dispersions, emulsions, surfactants, metals, polymers, fibers, catalysts and many others.

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References

Guinier A. and Fournet G. (1955). Small-angle scattering of X-rays. New York: Wiley.
Kratky O. and Glatter O. (1982). Small-angle X-ray scattering. London: Academic.

SAXS systems

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