Novel dual-stage FO system for sustainable protein enrichment using nanoparticles as intermediate draw solutes

We have proposed a novel dual-stage FO system and conceptually demonstrated its applications, for the first time, to enrich proteins without causing protein structural changes. Highly hydrophilic nanoparticles were utilized as the draw solute to dehydrate protein solutions in the up-stage FO, while model RO retentate as the draw solute to regenerate the nanoparticle solution in the down-stage FO. As a result, the protein enrichment process can be sustainable as the draw solute is re-concentrated continuously by the RO retentate. Proteins enriched in the dual-stage FO are kept intact as examined by both gel track and CD spectra analyses. To explore the universal applicability of the dual-stage FO system for protein enrichment, various types of proteins of different sizes and charge characteristics were studied under different membrane orientations and testing modes. Excellent enrichment performance can be achieved by increasing membrane surface area and osmotic pressure of nanoparticles under the pressure retarded mode in both up-stage and down-stage FO systems. It is believed that the dual-stage FO system has great potential in future applications of protein and pharmaceutical enrichments because of its simplicity, practicality, and economy.