Refractive Index versus Coupling Effects for Naked and Surfactant-Coated Au Nanoparticle Films: Implications for Selective Detection of Vapor–Liquid Analytes

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From a single dispersion of surfactant-coated Au nanoparticles (NPs), we can assemble colorimetric films of different thickness, organic content, and structure. In this work, we demonstrate how these variables play a fundamental role in determining what sensing mechanism predominates when these films are subjected to liquid-phase ethanol (EtOH), acetone (Ace), and toluene (Tol) analytes. Theoretical calculations, performed with the Korringa–Kohn–Rostoker (KKR) model, agree very well with the experiments if we consider that thinner films with lesser organic/inorganic content (or no organic at all) offer more vacancies for the solvents to partition, leading to effective changes in the permittivity. Instead, thicker films with a more compacted structure suffer minimal changes in the effective permittivity (lesser vacancies), causing color changes dominated by changes in the distance between the Au NPs. These films may be incorporated into a sensor array for improving selectivity toward the detection of vapor/liquid-phase analytes.