A photonic crystal film (PCF) which consists of a porous
layered
structure with a highly ordered periodic arrangement of nanopores
has been used to differentiate between various mixtures of water and
ethanol (EtOH). The refractive index difference between the wall (silica)
of the empty nanopore and air which occupies it results in the structural
color of the PCF. This color disappears when the nanopores are infiltrated
by a liquid with a similar refractive index to silica (or silicon
dioxide). The disappearance of the structural color provides a means
to construct a colorimetric sensor to differentiate between various
water/EtOH mixtures based on their wettability of the nanopores in
the PCF. In this study, an array of silica-based PCFs was synthesized
on a silicon substrate with a precise control of nanopore properties
using the co-assembly/sedimentation method. Using this method, we
benefitted from having different PCFs on a single substrate. Chemical
coatings, neck angles, and film thicknesses on each PCF were
the three factors used to adjust the wettability of the pores. Nanopore
wetting by water/EtOH mixtures was studied in a systematic manner
based on the three factors, and the findings were used to develop
a sensor for visual differentiation of various water/EtOH mixtures.
The final developed sensor consisting of an array of six PCFs was
able to differentiate between seven different water/EtOH mixtures:
W10, W20, W30, W40, W50, W60, and W70, in which W10 means 10% of water
in EtOH.
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