Green synthesis of gold nanoparticles: pros and cons of natural compounds

Abstract Gold nanoparticles (AuNPs) have attracted scientific interest for their unique properties and diverse applications, across biomedical fields, including sensing, diagnostics, and therapeutic interventions. However, standard wet chemistry synthetic methods for particles synthesis bare limitations, particularly in separating AuNPs from the reaction mixture. Therefore, there is a growing demand for environmentally friendly synthesis routes that minimize waste and utilize non-toxic solvents. Despite advances in this field, many current studies lack standardized protocols, and the underlying mechanisms of the reducing reaction remain unclear. Through comprehensive spectroscopic and morphological analyses, we investigated the redox reactions involved in AuNP formation with various natural extracts, aiming to establish an optimized protocol for producing small, spherical, and monodisperse AuNPs. Our findings demonstrate that cocoa powder extract effectively yields reproducible, spherical AuNPs with a diameter of 11 nm. Comparative analysis with single cocoa powder molecular components identified catechins, a class of flavonoids, as the primary reducing agent in green AuNP synthesis. Catechins also form a protective layer around the AuNPs. However, the simultaneous, unavoidable presence of this layer implemented with fatty acids and proteins, although fundamental for colloidal stability, limits further functionalization of the AuNPs, highlighting a trade-off between stability and functional versatility.