Freely-floating two-dimensional (2D) arrays of metal nanoparticles are promising materials for plasmonic applications such as 2D sensing thanks to their distinctive and unprecedented physicochemical properties and ability to enhance electromagnetic fields at their surfaces when excited by light at specific wavelengths. Specifically, these metal nanoparticle arrays are utilized in Surface-enhanced Raman scattering (SERS) techniques, allowing for highly sensitive and specific detection of hydrophobic contaminants in water. The SERS signal can be maximized by precisely tuning the interparticle gap distance within a 2D AuNP array. In our previous work, we successfully synthesized 2D stabilized gold nanoparticle (AuNP) embedded peptoid nanosheets via surface compression, which resulted in the formation of sandwich-like structures through the assembly and collapse of peptoid monolayers at the oil-water interface. Data has shown that the interparticle gap distances within a nanosheet can be as close as 2.9 ± 0.5 nm. In this experiment, we explore methods to further refine the control over interparticle gap distances by investigating the packing trends of AuNPs with different surface ligands. We functionalized 5 nm AuNPs with different hydrophobic ligands by agitating citrate-stabilized AuNPs with toluene and ligands of interest through vortexing. This process yielded thiolated AuNPs, which could later be used in nanosheet synthesis. UV-visible spectroscopy was utilized to obtain the concentration of AuNP samples post-ligand exchange, providing us with insights into the efficacy of this new synthetic approach. AuNPs embedded peptoid nanosheets were synthesized via surface compression and decompression. These nanosheets underwent various characterizations including UV-visible spectroscopy, surface tension and pressure measure, and scanning electron microscopy. We have found that ligand exchange can be a promising method for the production of thiolated AuNPs that perform comparably to commercially available AuNPs in nanosheet synthesis. This approach of making functionalized AuNPs can be generalized and adaptable to a wide range of ligands. Being able to modify the ligands, we will be able to control the properties of AuNPs, which results in better performance of our plasmonic sensors.
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