Surface complexation and multilayer formation in the adsorption of NADA and phosphate on magnetic iron oxide...

Recent decades have seen major advances in industrial production using biological systems. However, downstream processing has not kept pace with increasing productivity and remains a bottleneck in biotechnological production. Magnetic separation offers promising opportunities for bioseparation, but broader application requires a deeper understanding of the capture step. Here, we investigate the adsorption of two small, highly soluble molecules onto bare iron oxide nanoparticles (BIONs). The amino acid derivative N-acetyl-l-2,4-diaminobutyric acid (NADA) forms more than one layer on the BION surface with a saturation capacity of 262 mg g^-1 that decreases to about 164 mg g^-1 for less pure samples. The anion phosphate, measured at lower concentrations in solution, remains below monolayer coverage (12 and 14 mg g^-1 in water and artificial sea water, respectively) and at high ionic strength shows a significant decrease in capacity after apparent saturation. Ferromagnetic resonance measurements of the phosphate-BIONs in water indicate shielding of BION-BION interactions due to the phosphate layer. The shape of the adsorption isotherms, together with infrared (IR) spectroscopy, suggest that multiple adsorption mechanisms govern particle loading as solute concentration increases. IR data point to the formation of inner-sphere phosphate-surface complexes, which persist even when a NADA layer is pre-adsorbed onto the BION surface. In contrast, NADA does not adsorb onto the BIONs from a microalgae lysate, where other biomolecules preferentially occupy the nanoparticle surface. Overall, these results contribute to a better understanding of adsorption processes on solid carriers and provide valuable guidance for designing nanoparticle-based systems capable of capturing and releasing (bio)molecules in bioseparation and related applications.