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This brief introduces the classification and mechanism of density gradient ultracentrifugation (DGUC) method with rich examples showing the versatility of such an efficient separation technique. It also gives a strict mathematical description and a computational optimization model to predict the best separation parameters for a given colloidal system. The concept of “Lab in a tube” is proposed in the last chapter, which allows the size-property relationship investigation, synthetic optimization and reaction/assembly mechanism exploration etc.



Chapter 1. Introduction to Nanoseparation

Nanomaterials have been attracted tremendous attentions for decades, due to their unique properties on nanoscale. As well known, the properties, such as chemical, thermal, mechanical, optical, electrical, and magnetic properties, are highly dependent on the size of nanomaterials, as so-called size-dependent quantum effect. Thus, to obtain monodisperse nanostructures is of great significance. With the help of various ligands, solution-phase synthesis could produce colloidal nanostructures with relatively homogeneous morphology and narrow size distribution for some nanosystems. However, owing to the synthetic difficulties, fine control of uniform nanostructures still remains a big challenge. Besides, nanoseparation, as a “post-synthesis” method, is a powerful tool to sort and achieve monodispersity and to avoid possible aggregation of the colloids. In this chapter, the basic principles of nanoseparation and a brief introduction of common techniques used for the separation of nanostructures, including membrane filtration, chromatograph, electrophoresis, magnetic field and centrifugation, will be discussed.

Yun Kuang, Ming Jiang, Kai Sun

Chapter 2. Basic Concepts of Density Gradient Ultracentrifugation

This chapter reviews the development of centrifugal equipment and its achievements in various fields, especially in the field of life sciences. Based on the development of the equipment, centrifugal technology also plays a very crucial role in scientific research. Afterward, we introduce three common types of separation techniques: differential centrifugation, rate-zonal centrifugation, and isopycnic separation. For each type, we describe the basic principles, characteristics, and scope of application in detail.

Jindi Wang, Jun Ma, Xuemei Wen

Chapter 3. 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 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.

Qian Zhang, Xiong Sun

Chapter 4. Particle Sedimentation Behaviors in a Density Gradient

Density gradient centrifugation, as an efficient separation method, is widely used in the purification of nanomaterials including zero, one-, and two-dimensional nanomaterials, such as FeCo@C nanoparticles, gold nanoparticles, gold nanobar, graphene, carbon nanotubes, hydrotalcite, zeolite nanometer sheet (the examples can be found in Chap. 5). Each system needs separation parameter optimization, which comes from tremendous research experiments. When particles are put on the top of density gradient medium, they will have a definite settling rate under centrifugal force (Fc) [1], which is influenced by their net density, size, and shape. In a sufficiently intense centrifugal field, the particle motion held quietly free from gravity and vibration [2]. This is the principle of the density gradient ultracentrifuge. Based on the above principle, we discussed the particle sedimentation behaviors and built the kinetic equation in a density gradient media. The kinetic equation could apply to zero, one-, and two-dimensional nanomaterials, within its variation form accordingly. We found that the separation parameters could be optimized based on the kinetic equation. A MATLAB program was further developed to simulate and optimize the separation parameters. The calculated best parameters could be deployed in practice to separate given nanoparticles successfully.

Pengsong Li

Chapter 5. Density Gradient Ultracentrifugation of Colloidal Nanostructures

According to the centrifugation theory, various factors, such as the media density (ρm), radius (r) and thickness (h) of nanostructures, and solvation shell thickness (t) in different media, will directly influence the particle behavior during the density gradient centrifugation process. Density gradient centrifugation has become a promising tool to purify nanomaterials, such as metal nanostructures, carbon materials (carbon nanotubes and graphene), non-metal nanostructures (e.g., rare-earth nanostructures and oxide nanostructures). For the practical separation, as demonstrated in previous chapters, on the basis of the theoretical analysis of the target nanostructures and the preliminary separation, one can optimize the centrifugation according to the comprehensive consideration. While after all, the optimization direction of nanoseparation should be mainly focused on the net density of nanostructures and media. In this chapter, we will discuss the separation examples according to the dimensional difference of colloidal nanostructures, including 0D, 1D, 2D nanostructures, and assemblies/clusters.

Liang Luo, Qixian Xie, Yinglan Liu

Chapter 6. Application of Nanoseparation in Reaction Mechanism Analysis

Density gradient centrifugation has been established to obtain monodisperse nanoparticles with strictly uniform size and morphology, which are usually hard to be obtained by synthetic optimization. Previous chapters have demonstrated the versatility and universality of such separation method, by which nearly all kinds of nanostructures can be separated, including particles, clusters, and assemblies. Further, reaction mechanism, as well as structure–property relationship, can also be investigated based on the separated fractions. The focus of this chapter is the reaction mechanism analysis using density gradient centrifugation, namely by introducing a distinctive functional gradient layer, such as reaction zone and assembly zone, reaction mechanisms can be therefore studied since the reaction time can be pre-designed and the reaction environment can be switched extremely fast in a centrifugal force field. In a word, “lab in a tube” based on nanoseparation opens a new door for the investigation of synthetic optimization, assembly behavior, and surface reaction of various nanostructures.

Zhao Cai, Xiaohan Qi, Yun Kuang, Qian Zhang
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