Tuning the Packing Density of Gold Nanoparticles in Peptoid Nanosheets Prepared at the Oil-Water Interface

Two-dimensional (2D) arrays of gold nanoparticles that can freely float in water are promising materials for solution-based plasmonic applications like sensing. To be effective sensors, it is critical to control the interparticle gap distance and thus the plasmonic properties of the 2D arrays. Here, we demonstrate excellent control over the interparticle gap distance in a family of freely floating gold nanoparticle-embedded peptoid nanosheets. Nanosheets are made via monolayer assembly and collapse at the oil-water interface, allowing for fine control over the solution nanoparticle concentration during assembly. We used surface pressure measurements to monitor the assembly of the peptoid, nanoparticle, and combined system at the oil-water interface to determine a workable range of nanosheet assembly conditions suitable for controlling the interparticle gap distances within the nanosheets. These measurements revealed that the extent of nanoparticle adsorption to the peptoid monolayer can be tuned by varying the bulk nanoparticle concentration, but the ability for the monolayer to collapse into nanosheets is compromised at high nanoparticle concentrations. Peptoid nanosheets were synthesized with varying bulk nanoparticle concentrations and analyzed using light microscopy and UV-visible spectroscopy. Based on the spectral shift of the localized surface plasmon resonance peaks for the nanoparticles in the nanosheets relative to those well dispersed in toluene, we estimate that we can access interparticle gap distances within the nanosheet interior between 2.9 ± 0.5 and 9 ± 2 nm. Our results suggest that the minimum interparticle distance achievable by this method is limited by the nanoparticle ligand length, and so has the potential to be further tuned by varying the ligand chemical structure. The ability to quantitatively control and monitor the assembly conditions by this method provide an opportunity to readily tune the optoelectronic properties of this new class of 2D nanomaterial, making it a promising platform for plasmonic-based sensing applications.