Real-Time Monitoring Distance Changes in Surfactant-Coated Au Nanoparticle Films upon Volatile Organic Compounds (VOCs)

We explore into the role of the organic ligands composition surrounding Au nanoparticles (NPs) toward internanoparticles distance changes and film structure during vapor sensing by the use of in situ grazing-incidence small-angle X-ray scattering (GISAXS) and other relevant techniques. We observed distinct changes in core-to-core distance and film structure upon measured concentrations of polar ethanol (EtOH) and nonpolar toluene (Tol) vapors. As-formed tetraoctylammonium bromide (TOABr)-coated Au NPs (SNPs) film exhibited ∼0.4 nm core-to-core increase in distance and improved SNPs correlation upon 80% Tol. In the presence of EtOH, SNPs distance seems not to change and correlation critically diminishes, leading to an amorphous film. As-formed films were exchanged with nonanedithiol (NDT) and exposed to 100% Tol vapor. Interestingly, the incorporation of NDT increased distance between NPs and rendered film flexibility. It is determine that just a small number of S, from NDT ligands, binds to Au NPs, and those exchanged alkanedithiol chains adopted a loop conformation around the Au NPs as evidenced by X-ray absorption near-edge structure (XANES) and kink defects by Fourier transformed infrared (FT-IR) experiments, respectively. These findings may solve some fundamental questions about internanoparticle distance-dependent phenomena such as electron transport in chemiresistors and coupling effects in localized surface plasmon resonance (LSPR) solid-state sensors.