The role of particle-to-cell interactions in dictating nanoparticle aided magnetophoretic separation of microalgal cellsElectronic supplementary information (ESI) available: Table S1: Contact angle measurements. Table S2: Surface energy components of the liquids. Fig. S1: Microscopy images of Chlorella sp. cells trapped inside the bare-IONP flocculated matrix. Fig. S2-S3: TEM micrograph shows the relative size of IONPs to Chlorella sp. cells and the internalization of SF-IONPs into Chlorella sp.

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. Successful application of a magnetophoretic separation technique for harvesting biological cells often relies on the need to tag the cells with magnetic nanoparticles.