Density Gradient Ultracentrifugation Technique
As a general, non-destructive, and scalable separation method, DGUC has recently been demonstrated as an efficient way of sorting colloidal nanoparticles according to their differences in chemical, structural, size, or morphology. After the introduction o
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Density Gradient Ultracentrifugation Technique Qian Zhang and Xiong Sun
Abstract As a general, non-destructive, and scalable separation method, DGUC has recently been demonstrated as an efficient way of sorting colloidal nanoparticles according to their differences in chemical, structural, size, or morphology. After the introduction of basic concepts of density gradient ultracentrifugation, for the practical applications, there are various parameters to be considered. Nanoparticles will have different movement ways in different separation systems. In principle, particle movement characteristic in liquid media not only depends on the centrifugal force but also relies on the density, size, and shape of particle and the density and viscosity of the liquid medium and so on, while the gravity and intermolecular force can be ignored. In this chapter, typical parameters such as choice of gradient media, density gradient, rotor type, centrifugation speed, and time will be discussed.
Keywords Gradient media Step gradient Continuous gradient Rotor type Centrifugal force x2 t calculation method K′ coefficient estimation method
3.1
Choice of Gradient Media
Up to now, only a few types of liquids or solutions are utilized to form density gradients to separate biomolecules or nanoparticles (NPs). Since NPs are not biologically active, harsher conditions beyond aqueous solutions could also be used to separate NPs.
3.1.1
Gradient Media
Media applied in density gradient centrifugation includes small hydrophilic molecular organic compound (ethanol, ethylene glycol, glycerin, etc.), macromolecule organic compound (polysaccharide, protein, etc.), alkali metal salts (Cs, K, Rb, Na © The Author(s) 2018 X. Sun et al., Nanoseparation Using Density Gradient Ultracentrifugation, SpringerBriefs in Molecular Science, https://doi.org/10.1007/978-981-10-5190-6_3
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3 Density Gradient Ultracentrifugation Technique
salts, etc.), colloidal silica (Percoll, Ludox, etc.), and three organic toluene derivatives (covering Metrizamide, Nycodenz, Iodixanol, etc.) [1]. Clearly, solution prepared from aforementioned media differs in density, viscosity, concentration, chemical composition, and properties. As a result, solution made from one to two kinds of media chosen above can be used to separate colloidal NPs. Before separation, several important points should be taken into consideration when choosing the ideal gradient media: ① For the nanoparticles to be separated, the gradient media should be inert to colloidal samples under ambient condition. Moreover, it had better has no toxicity. ② The gradient media ought to keep the dispersibility of the separation system without causing agglomeration in the suspension. ③ The physical and chemical properties of gradient media should be known, such as the exact density, viscosity, and the specific mixed ratio of the gradient media. ④ The gradient media should be pH neutral, and its ionic strength should be low enough to avoid possible aggregations of the colloidal NPs. ⑤ The gradient media
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