Dr. Caterina Minelli of the Nanoanalysis Group of the National Physical Laboratory says: “The CPS technique is very precise and produces particle size distributions with extremely high resolution.”
The National Physical Laboratory (NPL), the United Kingdom’s National Measurement Institute, is well equipped with many means to characterize the size of nanoparticles. These are complementary and are used in concert or alone depending on the type of materials being studied. Every particle sizing technique is based on a different physical principle and therefore measures a slightly different “size” of the nanoparticle. Take, for example, a gold nanoparticle stabilized with citrate molecules. Dynamic Light Scattering, DLS, will measure the hydrodynamic radius; that is the gold core, the molecular coating and every molecule of solvent that moves with the core. In contrast, transmission electron microscopy, TEM, measures the size of the metallic core, thus providing a size of the nanoparticle that is smaller than that measured by DLS.
The Group also uses the CPS Disc Centrifuge system. This technique is based on differential centrifugal sedimentation. Describing this technique, Dr Caterina Minelli says: “The CPS technique is very precise and produces particle size distributions with extremely high resolution. This means that when we attach antibodies to the surface of a nanoparticle, for example, we are able to identify and quantify not only the main population of well functionalized particles, but also the fraction of undesired particles that aggregate in doublets, triplets or larger aggregates. The CPS instrument thus provides us with a means to assess the quality of a surface functionalization protocol and supports its improvement and refinement.”
Continuing, Dr Minelli says: “Another advantage of the CPS technique is that in combination with other techniques, or with careful experiments, we can measure both the size and the density of particles. This is because the instrument measures the particle sedimentation time through a medium, which depends on both the particle size and the density. If the size of the particle is known, for example by measuring it with an independent technique, the CPS [Disc Centrifuge] can provide a direct measurement of the particle density. When we modify the surface of a nanoparticle, for example by coating it with molecules such as proteins, we change both its size and its effective average density. The ability to measure the change in the particle density means we have a tool to characterize the molecular coating surrounding the nanoparticles. This approach is useful to study engineered nanoparticles used in biosensing. Another application involves the study of how protein corona forms at the nanoparticles’ surface as they enter a biological fluid such as blood or urine. The entity of the protein corona will determine the fate of the nanoparticles and has strong implications for nanoparticle toxicity, for example.”
With a wide variety of materials and potential uses for making routine analytical particle size measurements, the CPS Disc Centrifuge is particularly popular because of its versatility and ease of use. From a research perspective, the main use of the CPS is to optimize the highly engineered biomolecular coating of nanoparticles used in biosensing systems or other medical applications. This experience enables the NPL’s Nanoanalysis Group to offer contract analysis to external clients. Further details are available at http://www.npl.co.uk/science-technology/surface-and-nanoanalysis/services/nanomaterials-characterisation-techniques-for-powders-and-liquid-dispersions.
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Reference: (1) “Emerging Techniques for Submicrometer Particle Sizing Applied to Stöber Silica” by Nia C. Bell, Caterina Minelli et al., Langmuir 28 (2012) 10860; (2) “Quantitation of IgG protein adsorption to gold nanoparticles using particle size measurement” by Nia C. Bell, Caterina Minelli and Alexander G. Shard, Analytical Methods 5 (2013) 4591.