Mechanism of Shape Evolution in Ag Nanoprisms Stabilized by Thiol-Terminated Poly(ethylene glycol): An in Situ Kinetic Study

Silver nanoprisms are found to undergo a shape transformation from triangular to disk-like upon functionalization with thiol-terminated poly(ethylene glycol) (PEG-SH). The shape transformation starts with a rounding of the nanoprism tips followed by a reduction in the overall particle size. The changes in nanoparticle morphology and surface chemistry during the PEGylation process are investigated by a combination of transmission electron microscopy, in situ UV-vis spectroscopy, and in situ solution-phase X-ray absorption near-edge structure at the Ag L-3- and the S K-edge. We found that PEG-SH etches the Ag surface through a catalytic redox process, whereby oxidized surface atoms are first removed from the nanoprism tips (likely as the silver(I) thiolate complex) and then reduced back to elemental silver (in the form of small clusters) in the solution along with oxidation of the thiolate to the disulfide. The effect of PEG-SH concentration and molecular weight are investigated, along with the role of ambient oxygen in the etching process. The results have implications for the behavior of silver nanoprisms in various plasmonic applications (e.g., surface-enhanced Raman scattering and metal-enhanced fluorescence) and also provide insights into the degradation mechanism of silver nanomaterials in the environment.