Small angle X-ray scattering reveals phase transition in sodium oleate-cysteine systems for optimized selenium nanoparticles synthesis

Selenium nanoparticles (SeNPs) can be synthesised via physical, chemical, or biological methods. While physical and chemical approaches offer benefits, they often involve high costs, toxic reagents, and reduced biocompatibility due to stabilizers. Biological synthesis provides a greener, more biocompatible alternative, but faces challenges such as variability in biological sources, slow synthesis, low yields, and scalability issues due to pH sensitivity, toxic precursors, and costly equipment. To address these limitations, we propose a sustainable workflow for SeNPs production using the selenite (SeO32-)-reducing amino acid L-cysteine (Cys) and the eco- and bio-compatible surfactant sodium oleate (NaOl). We report that Cys not only reduces selenite but also induces phase transitions in NaOl, forming a confined environment of vesicles and lamellar structures that facilitate controlled SeNPs nucleation and growth. Through a detailed phase diagram analysis, we elucidate the critical Cys and NaOl concentration regimes and pH conditions thresholds that drive surfactant self-assembly, providing valuable insights into the optimized colloidal environment for SeNPs synthesis. We also report that these confined structures support SeNPs isotropic growth and long-term stability. This method provides a scalable and eco-friendly approach for producing biocompatible SeNPs, addressing some of the challenges of conventional methods.