• biosensing
• microfluidics
• SAW
• SPR

The aim of this master thesis work was the realization of a biosensor based on
surface plasmon resonance (SPR) which was integrated with surface acoustic
wave (SAW)-driven microfluidics.

Over the last 15 years SAW-induced mixers for microfluidic devices have been
developed owing to their fast mixing capabilities. Meanwhile, SPR sensors have
also been developed because of their high reliability and quantitative real-time
measurements.

Following the Drude model of electrical conduction, surface plasmons (SPs) can
be considered as propagating electron density waves occurring at the interface
between a metal and a dielectric and can alternatively be viewed as electromag-
netic waves that are strongly bound to this interface. The resonance condition
for SP excitation varies with the refractive index of the dielectric in the proximity
(about 200 nm for visible light) of the surface of the metal film supporting the
SP. A change in the resonance condition measured with an optical setup can be
used to detect changes in the refractive index. SPR sensing is particularly useful
for biological applications. By functionalizing the SPR sensor surface it is pos-
sible to detect binding events in real-time and quantify the concentration of the
analyte to be studied with high reliability. SPR biosensors have applications in
numerous important fields including medical diagnostics, environmental mon-
itoring, and food safety and security with resolution as low as 10 − 7 refractive
index units (RIU) (§1).

Lab-on-a-chip (LOC) devices are typically being developed for use in the life
sciences and diagnostics and represent a fast moving field in which efforts are
being made in order to increase portability and efficiency. Microfluidic systems
are characterized by small Reynolds numbers which indicates that fluid flow is
in general laminar. Efficient mixing is a challenge at these scales that can, how-
ever, be overcome with the use of SAW-induced streaming. SAWs are mechanical
oscillations which propagate along the surface of a given crystal. In piezoelectric
materials they can be generated using interdigitated transducers (IDT), which
are fabricated using thin-film metal deposition. When a SAW comes into con-
tact with the edge of a liquid in its path, the acoustic energy diffracts into the
fluid due to the mismatch between the sound velocity in the substrate and the
liquid, causing a longitudinal pressure wave front that gives rise to the acoustic
streaming. This phenomenon can be exploited to efficiently mix solutions with
1times that are significantly shorter than without SAWs (§2).

By using micro- and nano- fabrication techniques (§3.1) a biosensor was de-
veloped where SAW-driven active mixing and SPR sensing were integrated onto
a common substrate. The optical setup (§3.4) was based on wavelength modula-
tion and Kretschmann geometry where a polychromatic light is totally reflected
through a high refractive index prism (on which the chip is placed). A spectrom-
eter was used to analyze the reflected spectra. The SPR surface was functional-
ized in order to study a biotin-streptavidin system (§3.3). SPR was first character-
ized in droplets and then in polydimethylsiloxan (PDMS) microchannels (§3.2).
By adding an IDT onto the chip it was possible to induce acoustic streaming
in the channel while the biotin functionalization or the biotin-streptavidin event
occurred. SAW was characterized by using a laser doppler vibrometer and a
vector network analyzer. Owing to the chip design it was possible to decouple
the two effects induced by SAW in the microchannel: streaming and heating.
A thermocamera was used to study the second effect. The effect of the SAWs
was studied both on the functionalization process and the streptavidin-biotin
binding (§4). SAW streaming resulted in better surface functionalization than
in the case without SAW. The signal due to the functionalization of gold with
biotin was about 4.4 times higher than the signal detected without SAW-assisted
functionalization. It is possible, then, to conclude that SAW streaming increases
the probability that the biotin will be in contact with the gold surface and attach
to it. Preliminary data also suggest better streptavidin biosensing than control
device.

The biosensor made for this master thesis work was the first SAW-driven mi-
crofluidic device with SPR integrated on the same substrate. It showed promis-
ing results that might be exploited for improving sensitivity and limit of detec-
tion of SPR biosensors.