Dissolved black carbon mediated photo-reductive formation of colloidal-stable gold nanoparticles

Gold nanoparticles (AuNPs) have garnered attention as an emerging class of nanomaterials due to their wide range of applications across various sectors. This study reported that the dissolved black carbon (DBC) can mediate Au(III) photoreduction and form uniform and colloidal-stable AuNPs in aqueous solutions at room temperature under sunlight irradiation. Transmission electron microscopy images revealed that the average diameter of the photo-formed AuNPs was approximately 16 nm, and these nanoparticles maintained considerable colloidal-stability over time. Notably, no change in average size was observed following a 23-day storage period at 4°C in a light-free environment. The photo-formed superoxide (O2·-) is the crucial reductant in this reaction. O2·-, generated by DBC, reduces Au(III) to Au(I), which is then further reduced to Au(0), culminating in the formation of AuNPs. Furthermore, our findings illustrate that an increase in reaction pH shifts the predominant species of Au(III) in the solution toward forms with a lower redox potential, thereby decreasing the oxidizability of Au(III) and consequently decelerating the photoreduction process. A mildly acidic environment is more favorable for the DBC-mediated photoformation of AuNPs. This study elucidates that DBC can be observed as a novel reductant to complete the photo-reductive formation of AuNPs under low-cost conditions, opening up a new possibility for the green synthesis of AuNPs. [Display omitted] •Dissolved black carbon (DBC) mediates the photo-reductive formation of AuNPs.•The AuNPs generated via DBC mediation exhibit good colloidal stability.•The photo-formed O2·- is the key reductant.•A weakly acidic environment is more conducive to form AuNPs.