Extraction of Gold Nanoparticles Based on Temperature-Dependent Phase Separation using Zwitterionic 3-(Nonyldimethylammonio)-propyl Sulfate

The zwitterionic surfactant 3-(nonyldimethylammonio)-propyl sulfate (C₉-APSO₄) exhibits unique reversible temperature-dependent phase separation properties, which can be used for the extraction of watermiscible negatively charged gold nanoparticles (AuNPs) without any aggregation at high concentrations using only temperature change. This study investigated the extraction behaviors and properties of AuNPs transferred from the water phase to a small volume of the surfactant-rich phase. The AuNPs retained both their original shapes and sizes after temperature-dependent phase separation step. In addition, the concentration of AuNPs was enriched by a factor of 150 in the surfactant-rich phase without any changes in the sizes and shapes of AuNPs. The capacity was 63.7 μg (Au) present in 10 μL of the surfactant-rich phase (i.e., the saturated concentration was 1.6 × 10¹² particles/mL for 60 nm AuNPs). Furthermore, the extraction percentage was greater than 99% regardless of the pH (in the pH range of 2 – 12).