These thoughts lead us to propose the following mechanism for AFFF-based separation of C/S particles and AuNP (Figure ). Conceptually, AFFF can separate both fractions since they elute at distinct retention times. For optimization purposes, we follow the scheme published by Gigault et al. and aim for improved recovery and resolution. For PEGylated gold nanorods, a 350 µm spacer, a long channel (26.5 cm length and 0.6 to 2.1 cm width), and a 10 kDa PES membrane were already successfully used and serve as a starting point for our optimization. The polyethersulfone membrane meets the need for a negatively charged membrane to reduce losses in the channel since the negative charge remains in a considerable pH region. As an eluent, a highly diluted salt solution with an inert salt and a defined ionic strength is needed (e.g., ammonium nitrate or sodium azide ). Here, we decided to use 200 ppm sodium azide in deionized water, which makes PEGylation necessary to avoid salt-and i.e. Na + -induced aggregation and improve the recovery. Optimized AFFF separation method After fixing these components, the cross-flow profile remains tunable to optimize the resolution (Eq.