l-Cysteine as a reducing/capping/gel-forming agent for the preparation of silver nanoparticle composites with anticancer properties

The present article reports the in situ preparation of silver nanoparticles (AgNPs) homogeneously distributed in the gel matrix formed using only L-cysteine (CYS) as a bio-reducing agent. The physicochemical methods of analysis confirmed the formation of a gel-network from aggregates consisting of spherical/elliptical cystine-stabilized AgNPs (core) and cysteine/Ag+ complexes (shell) regardless of the used silver salt - AgNO3, AgNO2 or AgOOCCH3. CYS/AgNO3 and CYS/AgOOCCH3 aqueous solution systems needed the addition of electrolytes (Cl- and SO42-) for the gelation process, but the gel-formation in CYS/AgNO2 occurred in one stage without any additional components. The AgNP sizes were about 1-5 nm in diameter for CYS/AgNO3, 5-10 nm for CYS/AgOOCCH3 and 20-40 nm for CYS/AgNO2 systems. The zeta-potential values varied from +60 mV for CYS/AgNO3 to +25 mV for the CYS/AgNO2 system. The MTT-test showed that the obtained composites suppressed the MCF-7 breast cancer cells and the CYS/AgNO3 system possessed the highest activity. Flow cytofluorimetry confirmed that the cell death occurred by apoptosis and this effect was the strongest for the CYS/AgNO3 system. All systems were non-toxic to fibroblast cells. The novel simplest "green chemistry" approach, combining the knowledge of organic, inorganic, physical and supramolecular chemistry could open possibilities for the creation of the newest soft gel materials used in various fields of our life.