Viscosity Gradient as a Novel Mechanism for the Centrifugation‐Based Separation of Nanoparticles

Non-uniform size distribution can be observed in the chemical synthesis of many types of nanoparticles. To obtain uniformly distributed nanoparticles, it is necessary to separate them by size. Several size sorting methods have been developed, including size exclusion chromatography,[1] filtration,[2,3] electrophoresis,[4,5] and solvent/antisolvent selective precipitation.[6] Recently, direct size separation of nanoparticles in the complete liquid phase through centrifugation has been proven to be a more effective method due to its high efficiency, capability of scalable production, and free of nanoparticle aggregation.[7–9] Both density-based isopycnic centrifugation and velocity-related rate zonal centrifugation have been reported. For small nanoparticles (less than 10 nm), their overall density after solvation falls into the density range of gradient medium, thus isopycnic centrifugation can be used to achieve separation.[7,9] Larger nanoparticles are generally denser than the liquid medium, so they could only be separated by the difference of sedimentation velocity.[8–11] Previously, the density gradient has been employed to differentiate the sedimentation velocity of small and large inorganic nanoparticles as well as biological particles.[8–9,12] In these works, it is widely recognized that variation in the medium density can effectively generate a velocity difference for differently sized nanoparticles and thus achieve separation. However, since the density gradient is created by stacking solutions of a compound of different concentrations, in which not only the density but also the viscosity will be different, the hidden contribution of the viscosity change in the density gradient to separation has not been well realized to date. Here, we used a pure viscosity gradient built from aqueous polyvinylpyrrolidone (PVP) solutions, which have dramatically different viscosities but nearly the same densities among different concentrations, to achieve separation of nanoparticles by size. Our work demonstrates conceptually that the viscosity gradient can be employed as an alternative to the conventional density gradient to achieve highly efficient separation of nanoparticles through rate zonal centrifugation. In methodology, previously only separation of small nanoparticles (less than 25 nm) has been achieved through density gradient, while size nonuniformity also appears in the synthesis of large nanoparticles,[13–16] thus a size selection process for these nanoparticles is in demand. Here we demonstrate both experimentally and mathematically that the viscosity gradient is more effective in the separation of larger nanoparticles than the density gradient. This is the first report of a size sorting method for larger nanoparticles (up to 50 nm) with relatively small size difference. We also conducted experiments of separating randomly and widely distributed iron oxide nanoparticle clusters in the PVP viscosity gradient and obtained fairly good purification results based on a single centrifuge run. Extended studies on building a viscosity gradient from PVP of a different molecular weight as well as separation of nanoparticles of different surface capping molecules were also carried out in this work.