Self-Assembled Ligand-Capped Plasmonic Au Nanoparticle Films in the Kretschmann Configuration for Sensing of Volatile Organic Compounds

Abstract

Films of close-packed Au nanoparticles are coupled electrodynamically through\ntheir collective plasmon resonances. This collective optical response results\nin enhanced light-matter interactions, which can be exploited in various\napplications. Here, we demonstrate their application in sensing volatile\norganic compounds, using methanol as a test-case. Ordered films over several\ncm2 were obtained by interfacial self-assembly of colloidal Au nanoparticles\n(~10 nm diameter) through controlled evaporation of the solvent. Even though\nisolated nanoparticles of this size are inherently non-scattering, when\narranged in a close-packed film the plasmonic coupling results in a strong\nreflectance and absorbance. The in-situ tracking of vapor phase methanol\nconcentration through UV-Vis transmission measurements of the nanoparticle film\nis first demonstrated. Next, in-situ ellipsometry of the self-assembled films\nin the Kretschmann (also known as ATR) configuration is shown to yield enhanced\nsensitivity, especially with phase difference measurements. Our study shows the\nexcellent agreement between theoretical models of the spectral response of\nself-assembled films with experimental in-situ sensing experiments. At the same\ntime, the theoretical framework provides the basis for the interpretation of\nthe various observed experimental trends. Combining periodic nanoparticle films\nwith ellipsometry in the Kretschmann configuration is a promising strategy\ntowards highly sensitive and selective plasmonic thin-film devices based on\ncolloidal fabrication methods for volatile organic compound (VOC) sensing\napplications.\n