CSEM Scientific and Technical Report 2008

Nanostructured Surfaces for High Sensitivity Biosensors in the Field of Safety and
Security
B. Wenger, A.-M. Popa, E. Scolan, R. Pugin, G. Voirin
Selective and sensitive monitoring of pesticides for food safety as well as the detection cocaine in the atmosphere of public buildings can both be
achieved with optical biosensors featuring waveguide chips functionalized with specific antibodies. To further improve the sensitivity of these
devices, nanostructuring techniques to increase the surface available for the biorecognition events were applied.
The specificity of biosensors is probably their most
outstanding property. In fact, these devices take advantage of
the extremely optimized recognition mechanisms offered by
biological macromolecules to detect various targets with high
selectivity. The task of the engineer is then to combine this
molecular specificity with highly sensitive transducers in order
to be able to detect the minimum number of recognition
events.
Over the past few years, CSEM has developed a sensitive
optical biosensing platform which measures the changes in
refractive index caused by the interaction of the analytical
targets with receptors immobilised on an integrated optical
chip (Figure 1a). This technology – which has proven to be
capable of detecting minute amounts of small molecule
analytes (10 -12 g/mm 2 ) [ 1] – led to the creation of a start-up
company in 2007 [2] .
In spite of the low detection levels mentioned above,
improvement of the sensitivity is required for bioassays where
the concentration of immobilized receptors on the surface is
not high enough. This is the case for two immunoassays
which were developed in collaboration with partners within the
EU-funded projects. In the first project, our goal was to detect
traces of cocaine in the atmosphere of public buildings (e.g.
airports) for the rapid identification of criminal activities. For
this purpose, a competitive immunoassay featuring antibodies
selective to cocaine and a protein-drug conjugate immobilized
on the waveguide chip was developed. In the second project,
the objective was the detection of atrazine, a toxic herbicide
causing persistent groundwater contamination, often found in
water and agricultural products.
For both applications, to increase the amount of receptors on
the surface different strategies based on the enhancement of
the surface area within the volume probed by the light
travelling in the waveguide were adopted. This is typically a
region with a thickness of 150-200 nm extending above the
waveguide.
The first approach focused on the formation of nanopillar
arrays by a hybrid technique using a self-assembled periodic
polymer template followed by plasma etching [ 3] . The nano-
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pillars produced this way typically have radii of less than
50 nm and heights between 40-100 nm (Figure 1b).
A second method used to improve the area probed by the light
was to deposit porous films consisting of metal oxide
nanoparticles (e.g. AlOOH, TiO2, SiO2). The particles were
deposited from dispersions in water and subsequently
stabilized by cross-linking with a functionalized polymer
(Figure 1c).
Finally, compact arrays of vertically-aligned zinc oxide
nanowires were formed on the waveguide chips using
hydrothermal growth techniques. This approach consisted of
the synthesis of ZnO nanocrystals by spray-coating a
precursor solution onto the heated substrate. These crystals
acted as a seed layer for the directional growth of wires in a
hot aqueous solution at a rate of ~100 nm/hour (Figure 1d).
Figure 1: a) integrated optical chip, b) AFM picture of nanopillars in
SiO2, c) SEM picture of AlOOH mesoporous films, d) SEM picture of
ZnO nanowires grown on glass.
Sensitivity enhancements of up to 500% have been measured
for the non-specific adsorption of model proteins thanks to the
increased surface available for the biorecognition events.
This work was funded by the European Community via the
Integrated Projects NANOSECURE and NAPOLYDE. CSEM
thanks them for their support.
[1] K. Cottier, M. Wiki, G. Voirin, H. Gao. R. E. Kunz, “Label-free
highly sensitive detection of (small) molecules with wavelength
interrogation of integrated optical chips”, Sensors and
Actuators B, 91 (2003), 241
[2] www.dynetix.ch
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[3] A.-M. Popa, B. Wenger, E. Scolan G. Voirin, H. Heinzelmann,
R. Pugin, “Nanostructured waveguides for evanescent wave
biosensors“, Appl. Surf. Sci., submitted