Gold nanoparticles (Au-NPs) are among the most commonly used materials for increasing the sensitivity of Raman spectroscopy. This is mainly due to the strong and tunable surface plasmon resonances, which can be easily adjusted by choosing the shape and size of individual NPs. However, recent findings indicate that controlling the density of NPs in their assemblies is an equally important parameter. An efficient way to achieve such control is by using the Langmuir–Schaefer technique, which yields single-layer assemblies of NPs on a solid substrate. In this study, we correlate the density and thermodynamic properties of a monolayer of spherical Au-NPs with their performance as Raman substrates. Hence, hydrophobic NPs were synthesized and assembled into films using the Langmuir–Schaefer technique. The surface-enhanced Raman spectroscopy (SERS) showed various degrees of enhancement depending on the different ranges of transfer surface pressures employed, which almost doubled the expected SERS signal intensity at the highest deposition surface pressure. To gain greater insights into this phenomenon, we investigated the NP distribution in the Langmuir and Langmuir–Schaefer films by detailed spectroscopic and electron and optical microscopic analyses. A correlation was observed between NP enhancements on the micro- and nanoscale. The presented results demonstrated the potential of adjusting the Raman responses in ultrasensitive detection by controlling the thermodynamic properties of NP monolayers.
