research activities in 2007 - CSEM

Sol-Gel based Nanoporous Layers as New Sensing Interfaces
E. Scolan, V. Monnier, R. Steiger, R. Pugin
Sol-gel processes are very versatile and well-suited for the deposition of layers with controlled homogeneity, thickness, porosity and associated
surface structures. The high surface area generated by the porosity and the pore size and shape are used to improve the sensitivity and the
selectivity of specifically designed sensors, respectively.
Nanotechnology creates functional materials, devices, and
systems by controlling matter at the atomic and molecular
scales, thus resulting in novel exploitable properties and
phenomena. The control at the nanometer scale is especially
important in the sensor world since most chemical and
biological sensors, as well as many physical sensors, depend
on interactions occurring at these scales.
Sol-gel processes [1] can be broadly defined as the preparation
of designed metal oxide materials at the nanometer scale
(including fibers, powders/particles, shaped/molded bulks,
(thin) films). The gentle wet chemistry route to ceramics has
many advantages. One of the most technologically important
aspects of sol-gel processing is that, prior to gelation, sols are
ideal for preparing thin films, using common coating
processes such as dipping, spinning, spraying or spreading
using a blade, a roll or a bar (see Figure 1). The thickness
(from 10 nm to 100 µm) and adhesion to different kinds of
substrates (glasses, plastics, ceramics, metals, and textiles)
can be controlled over a large range of areas (cm2 to m2 ) with
a high degree of homogeneity.
Figure 1: Bar coater device and plastic sheet bar coated with a
nanoporous sol-gel layer
Figure 2: Specifically designed SiO2 (top) and AlOOH (bottom) based
nanoporous sol-gel layers
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These processes are a bottom-up approach from molecular
precursors to high purity particulate nano-building blocks
(NBB): their stacking within the film generates tunable porosity
by modifying NBB size, shape, surface chemistry and
interaction with porogenic species. Highly homogeneous
nanoporous layers of silica or aluminum oxide have been
prepared (see Figure 2).
Finally sol-gel processes are an ambient temperature
deposition technology, allowing interfaces with organic and
biological species. Thus film porosity can be functionalized by
grafted or embedded sensitive entities (e.g. dyes, proteins,
polymers or cells), that makes the loaded layers suitable for
high performance chemical sensing applications (see
Figure 3). Nanoporous sol-gel matrices possess chemical
inertness, physical rigidity, negligible swelling in contact with
liquids, high thermal stability and optical transparency.
Moreover the large surface area generated by the
nanoporosity greatly improves the sensitivity of the sensing
films to detect specific diffusing gases or diluted
(bio-)molecules. For instance, optical CO2 gas sensors have
been developed based on the absorbance quenching due to
the interaction of CO2 with encapsulated chromophores [2] .
Figure 3: Concept of nanoporous film based sensor
Due to their tunable size and shape, nanoporous sol-gel
coatings can be integrated into various kinds of devices.
Nanosensors and nano-enabled sensors have applications in
many industries, among them environmental protection,
transportation, communications, building and facilities,
medicine, safety, textiles and packaging.
CSEM thanks the OFES and EC (www.napolyde.org) for their
financial support.
[ 1 ] J. Brinker, G. Scherer, “Sol-Gel Science, The Physics and
Chemistry of Sol-Gel Processing”, Academic Press, San Diego,
(1990)
[2] J.F. Fernández-Sánchez, R. Cannas, S. Spichiger, R. Steiger,
U.E. Spichiger-Keller, “Optical CO2-sensing layers for clinical
application based on pH-sensitive indicators incorporated into
nanoscopic metal-oxide supports”, Sensors and Actuators,
B 128, (2007) 145–153