Iron(II) as a Green Reducing Agent in Gold Nanoparticle Synthesis

Iron is the most abundant element by mass in the Earth. Despite being an inexpensive material used in a wide range of applications, its role as reducing agent for the synthesis of plasmonic nanoparticles has not been explored in detail. In the present work, we have studied the use of iron(II) sulfate as a green reductant for the synthesis of gold nanoparticles (AuNPs) in the absence or presence of different molecules. First, we demonstrate that iron(II), in the absence of any additive or capping agent, is able to reduce Au(III) to generate AuNPs. However, the reduction yield is not 100% and the obtained NPs are coated by an Fe(II)/Fe(III) oxo/hydroxo sulfate shell. Nevertheless, the presence of polystyrenesulfonate (PSS) in the reaction medium prevents the growth of this inorganic shell and leads to total Au salt reduction. Besides, PSS seems to play a role in the nucleation and growth steps since it induces the formation of raspberry-like AuNPs. Additionally, we have investigated the effect of modulating the formal redox potential of Fe(II), through its complexation with citrate, in the synthesis of AuNPs. Interestingly, we have found that the presence of citrate gives rise to uniform pseudospherical AuNPs with well-defined optical properties, indicating that citrate may alter the nucleation step. Finally, to explore the applicability of these green nanomaterials for SERS applications, the SERS performance of the obtained AuNPs has been analyzed using 4-nitrothiophenol (4-NTP) as model analyte.