Low-gradient magnetic separation of magnetic nanoparticles under continuous flow: Experimental study, transport mechanism and mathematical modelling.

Low-gradient magnetic separation (LGMS) of magnetic nanoparticles (MNPs) has been proven as one of the techniques with great potential for biomedical and environmental applications. Recently, the underlying principle of particle capture by LGMS, through a process known as magnetophoresis, under the influence of hydrodynamic effect has been widely studied and illustrated. Even though the hydrodynamic effect is very substantial for batch processes, its impact on LGMS operated at continuous flow (CF) condition remained largely unknown. Hence, in this study, the dynamical behaviour of LGMS process operated under CF was being studied. First, the LGMS experiments using poly(sodium 4-styrenesulfonate)-functionalized-MNP as modelled particle system were performed through batchwise (BW) and CF modes at different operating conditions. Here BW operation was used as a comparative study to elucidate the transport mechanism of MNP under the similar environment of CF-LGMS process, and it was found out that the convection induced by magnetophoresis (timescale effective is ∼1200 s) is only significant at far-from-magnet region. Hence, it can be deduced that forced convection is more dominant on influencing the transport behaviour of CF-LGMS (with resident time ≤240 s). Moreover, we found that the separation efficiency of CF-LGMS process can be boosted by the higher number of magnets, the higher MNP concentration and the lower flowrate of MNP solution. To better illustrate the underlying dynamical behaviour of LGMS process, a mathematical model was developed to predict its kinetic profile and separation efficiency (with average error of ∼2.6% compared to the experimental results).

Toxicity of bare and surfaced functionalized iron oxide nanoparticles towards microalgae.

This study investigates the toxicity of bare iron oxide nanoparticles (IONPs) and surface functionalization iron oxide nanoparticles (SF-IONPs) to the growth of freshwater microalgae Chlorella sp. This study is important due to the increased interest on the application of the magnetic responsive IONPs in various fields, such as biomedical, wastewater treatment, and microalgae harvesting. This study demonstrated that the toxicity of IONPs was mainly contributed by the indirect light shading effect from the suspending nanoparticles which is nanoparticles concentration-dependent, direct light shading effect caused by the attachment of IONPs on cell and the cell aggregation, and the oxidative stress from the internalization of IONPs into the cells. The results showed that the layer of poly(diallyldimethylammonium chloride) (PDDA) tended to mask the IONPs and hence eliminated oxidative stress toward the protein yield but it in turn tended to enhance the toxicity of IONPs by enabling the IONPs to attach on cell surfaces and cause cell aggregation. Therefore, the choice of the polymer that used for surface functionalize the IONPs is the key factor to determine the toxicity of the IONPs.

The role of particle-to-cell interactions in dictating nanoparticle aided magnetophoretic separation of microalgal cells.

Successful application of a magnetophoretic separation technique for harvesting biological cells often relies on the need to tag the cells with magnetic nanoparticles. This study investigates the underlying principle behind the attachment of iron oxide nanoparticles (IONPs) onto microalgal cells, Chlorella sp. and Nannochloropsis sp., in both freshwater and seawater, by taking into account the contributions of various colloidal forces involved. The complex interplay between van der Waals (vdW), electrostatic (ES) and Lewis acid-base interactions (AB) in dictating IONP attachment was studied under the framework of extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. Our results showed that ES interaction plays an important role in determining the net interaction between the Chlorella sp. cells and IONPs in freshwater, while the AB and vdW interactions play a more dominant role in dictating the net particle-to-cell interaction in high ionic strength media (≥100 mM NaCl), such as seawater. XDLVO predicted effective attachment between cells and surface functionalized IONPs (SF-IONPs) with an estimated secondary minimum of -3.12 kT in freshwater. This prediction is in accordance with the experimental observation in which 98.89% of cells can be magnetophoretically separated from freshwater with SF-IONPs. We have observed successful magnetophoretic separation of microalgal cells from freshwater and/or seawater for all the cases as long as XDLVO analysis predicts particle attachment. For both the conditions, no pH adjustment is required for particle-to-cell attachment.

Magnetophoresis of iron oxide nanoparticles at low field gradient: the role of shape anisotropy.

Magnetophoresis of iron oxide magnetic nanoparticle (IOMNP) under low magnetic field gradient (<100 T/m) is significantly enhanced by particle shape anisotropy. This unique feature of magnetophoresis is influenced by the particle concentration and applied magnetic field gradient. By comparing the nanosphere and nanorod magnetophoresis at different concentration, we revealed the ability for these two species of particles to achieve the same separation rate by adjusting the field gradient. Under cooperative magnetophoresis, the nanorods would first go through self- and magnetic field induced aggregation followed by the alignment of the particle clusters formed with magnetic field. Time scale associated to these two processes is investigated to understand the kinetic behavior of nanorod separation under low field gradient. Surface functionalization of nanoparticles can be employed as an effective strategy to vary the temporal evolution of these two aggregation processes which subsequently influence the magnetophoretic separation time and rate.

Rapid magnetophoretic separation of microalgae.

Magnetic collection of the microalgae Chlorella sp. from culture media facilitated by low-gradient magnetophoretic separation is achieved in real time. A removal efficiency as high as 99% is accomplished by binding of iron oxide nanoparticles (NPs) to microalgal cells in the presence of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) as a binder and subsequently subjecting the mixture to a NdFeB permanent magnet with surface magnetic field ≈6000 G and magnetic field gradient <80 T m(-1) . Surface functionalization of magnetic NPs with PDDA before exposure to Chlorella sp. is proven to be more effective in promoting higher magnetophoretic removal efficiency than the conventional procedure, in which premixing of microalgal cells with binder is carried out before the addition of NPs. Rodlike NPs are a superior candidate for enhancing the magnetophoretic separation compared to spherical NPs due to their stable magnetic moment that originates from shape anisotropy and the tendency to form large NP aggregates. Cell chaining is observed for nanorod-tagged Chlorella sp. which eventually fosters the formation of elongated cell clusters.