Site-selective surface enhanced Raman scattering study of ligand exchange reactions on aggregated Ag nanocubes

Surface modification of ligand protected nanoparticles (NPs) can be driven by ligand exchange reactions (LER). Little is known about the mechanism of this reaction, especially when the particles are not spherical. Here, we use the intrinsic hot spots generated on the corners/edges of 40 nm silver nanocubes (AgNCs) and the extrinsic hot spots generated by the aggregation of AgNCs to obtain strong enhancements in surface enhanced Raman scattering (SERS) to monitor the displacement of poly(vinylpyrrolidone) with either a hydrophilic (sodium 11-mercapto-1-undecanesulfonate, MUS) or a hydrophobic ligand (1-octanethiol, OT). By focusing on the ratio (R) between the trans and gauche bands of the alkyl backbones of the ligands as an indicator of local order, we find that the LER proceeds in two different ways, particularly regarding the arrangement of the incoming ligands. In the case of OT, R becomes large almost immediately and stays high throughout the reaction, while in the case of MUS, R starts low and increases only towards the end of the reaction. We interpret the first behaviour as a LER that proceeds via the formation of densely packed and ordered OT islands, practically from the beginning of the reaction, that gradually grow over time. In the case of MUS, our interpretation is that molecules randomly deposit on the surface and form denser monolayer regions as the LER proceeds. Both monolayers evolved towards a homogeneous topography that corresponds to an ordered state (near all-trans) in the later stages (R≫1). Such mechanisms may be extended to the cube faces (as planar scaffolds) since clear differences in the topographic profile were found by Atomic Force Microscopy (AFM).