Time-Dependent Packing Density of ODT on Gold Nanoparticles: A TGA-FTIR Analysis for Peptoid Nanosheet Assembly

Freely floating peptoid nanosheets are versatile two-dimensional nanomaterials capable of encapsulating hydrophobic cargo, opening the door for multifunctional materials with tunable optical, electronic, and catalytic properties. The successful assembly of these hybrid organic-inorganic nanostructures at the oil-water interface relies heavily on the stable incorporation of alkanethiol-functionalized gold nanoparticles (AuNPs) as that hydrophobic cargo within the nanosheet's interior. This study investigates the optimal soaking time required to ensure an even and maximized packing density of octadecanethiol (ODT) ligands on AuNP surfaces prior to nanosheet encapsulation. To determine the optimal duration, AuNP samples were soaked in an ODT solution for controlled intervals of 0, 1, 3, 5, and 7 days, and the resulting organic mass loss and thermal degradation profiles were quantified using Thermogravimetric Analysis coupled with Fourier Transform Infrared Spectroscopy (TGA-FTIR). While 0 and 1-day samples displayed inconsistent mass loss, TGA curves for the 3-day samples exhibited a uniform, sharp thermal degradation event between 420 °C and 500 °C. Coupled FTIR spectra at peak degradation confirmed this mass loss was due to the volatilization of the ODT monolayer, evidenced by asymmetric and symmetric C-H stretching peaks just below 3000 cm⁻¹, with longer soaking intervals (5 and 7 days) providing no further enhancement to overall packing density. A 3-day incubation period is the ideal threshold for ODT functionalization on AuNPs, establishing a highly efficient and optimized protocol for synthesizing the stable hydrophobic nanoparticle precursors required for complex peptoid nanosheet assembly.