  Films of close-packed Au nanoparticles are coupled electrodynamically through
their collective plasmon resonances. This collective optical response results
in enhanced light-matter interactions, which can be exploited in various
applications. Here, we demonstrate their application in sensing volatile
organic compounds, using methanol as a test-case. Ordered films over several
cm2 were obtained by interfacial self-assembly of colloidal Au nanoparticles
(~10 nm diameter) through controlled evaporation of the solvent. Even though
isolated nanoparticles of this size are inherently non-scattering, when
arranged in a close-packed film the plasmonic coupling results in a strong
reflectance and absorbance. The in-situ tracking of vapor phase methanol
concentration through UV-Vis transmission measurements of the nanoparticle film
is first demonstrated. Next, in-situ ellipsometry of the self-assembled films
in the Kretschmann (also known as ATR) configuration is shown to yield enhanced
sensitivity, especially with phase difference measurements. Our study shows the
excellent agreement between theoretical models of the spectral response of
self-assembled films with experimental in-situ sensing experiments. At the same
time, the theoretical framework provides the basis for the interpretation of
the various observed experimental trends. Combining periodic nanoparticle films
with ellipsometry in the Kretschmann configuration is a promising strategy
towards highly sensitive and selective plasmonic thin-film devices based on
colloidal fabrication methods for volatile organic compound (VOC) sensing
applications.
