On Size Fractionation of Iron Oxide Nanoclusters by Low Magnetic Field Gradient

Particle size is one of the most important requirements for the successful implementation of magnetic nanoparticles in numerous scientific and engineering applications. Here we proposed the use of low-gradient magnetic separation (LGMS) technique for size fractionation of magnetic nanoclusters (NCs) due to its ease of implementation. Poly(sodium 4-styrenesulfonate) (PSS)-grafted iron oxide NCs, with excellent colloidal stability needed for most environmental engineering applications, were employed as a model system to test the effectiveness of our separation strategy. Results showed that successive reduction in average hydrodynamic size and sample polydispersity was achieved after going through LGMS. This size-selection process is independent of initial particle concentration and gives consistent results over a broad range of particle concentration (from 0.025 g/L to 10 g/L). By fine-tuning the externally applied field strength from similar to 4600 to similar to 1100 gauss, we observed a well-characterized, predictable separation behavior which can be rationalized based upon our understanding of low-gradient magnetophoresis. In addition, we found that the time-dependent magnetophoresis profiles of PSS-grafted iron oxide NCs registered dissimilar behavior as compared to its bare counterpart, at which the latter is more concentration-dependent than the previous ones. Occurrence of size fractionation-based magnetophoresis is believed to account for this variation.