The Response of Citrate Functionalized Gold and Silver Nanoparticles to the Addition of Heavy Metal Ions

Hypothesis Citrate functionalised gold nanoparticles (AuNPs) have been shown to be effective heavy metal sensors, as their optical spectra quantitatively shifts upon metal-citrate interaction. The speciation and molecular orientation of citrate molecules absorbed on the surfaces of bulk materials are found to be dependent on the substrate material used. It is expected that substituting gold for another nanomaterial should give the citrate molecules at the surface different speciation and molecular orientation, thus providing different citrate/metal interactions and responses. Experimental Citrate functionalised AuNP, and AgNP solutions were synthesised for comparison, and characterised before and after the addition of heavy metal ions. This study concentrated on the molecular interactions of the citrate molecules with the nanomaterial surface and the metal ions in solution. Computational simulations into these interactions were also carried out as a comparison. Findings The citrate speciation for the AuNPs and AgNPs was significantly different, showing single and double carboxylate coordinated molecules respectively. There were also differences in the heavy metal/citrate interaction. Notably, Pb2+ ions produced a rapid coagulation of the AuNPs which was not observed when using AgNPs, demonstrating that the noble metal nanomaterial used has an effect on the nature of the heavy metal/citrate bonding interactions