Miniaturized Surface Plasmon Resonance based Sensor System
Peter Hausler
1
, Christa Genslein
2
, Carina Roth
1
, Thomas Vitzthumecker
1
, Thomas Hirsch
2
and Rudolf Bierl
1
1
Sensorik-ApplikationsZentrum,Ostbayerische Technische Hochschule Regensburg,
Franz-Mayer-Str. 1, Regensburg, Germany
2
Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg,
Universitätsstr. 31, Regensburg, Germany
Keywords: Surface Plasmon Resonance Spectroscopy, Sensor, SPR-Imaging, Miniaturization, Micro-Opto-Electro-
Mechanical Systems.
Abstract: We describe the miniaturization of the Surface Plasmon Resonance (SPR) technology which mainly finds its
applications in pharmaceutical screening and biotechnology so far. SPR spectroscopy is a label-free, non-
destructive and highly sensitive measurement principle detecting changes in the refractive index in striking
distance to a gold surface. A transfer of this technology to a miniaturized sensor will broaden the range of
possible applications. A promising feature which is included in the miniaturized system is the angle-dependent
recording of the SPR signals without moving parts. Commercial SPR assays are mainly working with a small
number of sensing spots. In contrast, the SPR imaging system shown here will allow to use an array of many
sensing spots. In combination with chemical receptors designed as an artificial nose, the simultaneous
detection of many analytes is envisioned for future applications.
1 INTRODUCTION
Surface Plasmon Resonance (SPR) technology is
label free, non-destructive and highly sensitive
(Schasfoort, 2017). Due to these properties, SPR is an
attractive measurement principle for chemical
sensors (Homola, 2003). Nevertheless, there are some
drawbacks limiting its applications so far: Classical
measurement setups are mainly designed for being
used in laboratories and therefore they are very
expensive. The high temperature sensitivity and the
need of trained personal for its operation impede
reliable in-field sensing. Motivated by this we
developed a miniaturized sensor system, which is
able to operate automated in a wide range of
temperature.
The system is based on an imaging read-out
principle enabling a large number of sensor spots on
a small area for addressed parallel online detection.
The individual sensing spot can be designed and
chemically functionalized as an artificial nose
enabling the identification of several analytes in a
complex matrix without labelling. In classical SPR
setups a laser is used as light source, which is replaced
by a modified light emitting diode. While using a
semiconductor laser to illuminate a surface one will
get speckles. If it comes to very small sensing spots,
speckles will ruin your sensing signal. Light,
generated by a LED does not exhibit speckles due to
the missing coherence.
2 PRINCIPLE
While a thin gold film is irradiated by light, typically
the entire light will be reflected (Figure 1). However,
if the light is p-polarised and the angle of incidence is
altered, one can see a narrow dip in the intensity of
the reflected light. This dip is indicating that at this
certain angle of incidence (SPR angle) surface
plasmons are excited. The SPR angle mainly depends
on the refractive index in close proximity to the gold
film which is deposited on coupler, usually a glass
prism. Therefore, the refractive index on one side of
the gold is constant, which means that any variations
in the chemical composition and therefore in the
refractive index next to the other side of the gold
film is determining the position of the SPR-angle.
Selectivity to a special molecule of interest is
generated by a chemical functionalization of the gold
Hausler, P., Genslein, C., Roth, C., Vitzthumecker, T., Hirsch, T. and Bierl, R.
Miniaturized Surface Plasmon Resonance based Sensor System.
DOI: 10.5220/0006555400630066
In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 63-66
ISBN: 978-989-758-286-8
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
63
film by recognition elements (Wolfbeis and Homola,
2010).
Figure 1: SPR-Sensor principle; the reflecting gold film
consists of several region of interests (ROI); the diagram
shows the signal response (R) which is caused by a change
of the refractive index in close proximity to each Sensor
Spot.
3 SYSTEM SETUP
Figure 2 shows a sketch of the sensor setup.
Figure 2: System-Setup, consisting of: 660 nm LED based
light source, beam shaping elements, a sensitive area with
receptors as well as microfluidics, a camera detection
system and a data processing unit.
Lasers are known for generating speckles, which
are very annoying in case of SPR imaging as a
homogeneous lighting of the whole sensing area is
desired. The replacement of the laser by a LED for
illumination overcomes this issue. LED light is very
broadband compared to laser light and it is non-
polarised. Therefore, a notch filter and a polarizer was
integrated into the system. Furthermore, the beam is
tailored and collimated. The modified beam
illuminates a thin gold film which is on top of a glass
prism, which consists of 100 sensing spots at an area
of 15 x 15 mm. The reflected light is collected by a
2D-camera system, which records the spatial change
of the intensity of the reflected light beam.
In order to ensure the homogeneous and constant
supply of the analytes in a thin laminar flow over the
entire gold surface, a microfluidic system was
developed (Figure 3). The microfluidics is 3D-printed
with stainless steel. The seal between the microfluidic
unit and the prism is made of silicone and has a high
of 700 µm. The high of the seal decreases depending
on the applied pressure between prism and
microfluidic unit. Micro piezo membrane pumps
generate the analyte flow. A Tygon hose with an inner
diameter of 1 mm connects the individual
components of the microfluidic unit.
The temperature next to the sensing area is
monitored by PT1000 sensing elements which can be
also included cavities which are suitable for.
Figure 3: Top view left and side views right. The inlet (1)
and outlet (2) of the microfluidic chip is designed to provide
a constant flow rate (3) at the entire gold surface of the
sensor array. For precise temperature control, the chip has
cavities (4) between every inlet and outlet for PT1000
sensors.
The SPR-system can detect changes in the range
of 510
-6
refractive index units (RIU). However, the
refractive index of water is very sensitive to
temperature fluctuations (Abbate, 1978). Thus, it is
obvious that the temperature of the setup needs to be
controlled. Here we solved the temperature issue by a
sensor packaging consisting of a temperature
isolation on both, the inside and the outside. The
outside of the housing is covered with a reflective
aluminium layer to prevent heating from ambient
influences, e.g. by sunlight. The overall thickness of
the isolation is 20 mm with a thermal conductivity
coefficient of 0.04 W·m
-1
·K
-1
. To balance the
remaining heat flux and the heat which is generated
by the components inside the housing a 70 W Peltier
element is integrated into the setup.
This configuration was designed for an
application in field, operating at temperature ranges
from 253K to 323 K. A temperature stabilization of at
least +/- 0.25 K for a change of more than 20 K in
temperature can be easily achieved. To save cooling
capacity, electronics, which are generating large
amount of heat, such as thermoelectric driver,
computing unit and LED- driver, are housed
separately from the sensor-unit.
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
64
Figure 4: Temperature measurement over time shows the
engaging of the sensor system at room temperature
(294.6 K) until green line. Between the green and red line
the system was heated to 318 K. The blue curve shows the
temperature inside the system, in yellow the temperature on
the outer side of the housing and in orange the temperature
of the peltier at the inner side of the housing is displayed.
The graph demonstrates that the current configuration is
suitable to avoid disturbance by temperature fluctuations.
4 SYSTEM PERFORMANCE
One should assess the performance of the system
from three different aspects. The first aspect is the
RIU-resolution. The second aspect is the SPR-angle
range, and the third aspect is the spatial resolution and
the number of Regions of Interest (ROI) which comes
with this aspect. Beside these considerations, the
overall recognition sensitivity for a particular
substance is crucial. This sensitivity can be improved
by graphene-nanostructures, which we showed in
(Genslein et al. 2016). For further comparison to
other systems please see Ref. (Schasfoort, 2017).
4.1 Refractive Index Unit Resolution
To proof the resolution in refractive index sensing,
deionized water and water which contains a defined
amount of sodium chloride were applied as analyte.
In this way, the refractive index was raised in two
steps from 1.3330 to 1.3336. We are able to resolve
this step with 140 digits in the SPR signal recorded
by the digital camera.
This leads us to a sensitivity of 5·10
-6
RIU per
digit. While summing up 100 measurements for one
value we get a standard deviation of 1.4·10
-6
RIU.
Since we are using a 12-bit camera, the resolution can
be enhanced by the factor of four by switching to a
14-bit model.
Figure 5: Resolution in refractive index measurements. The
refractive index next to the sensor surface was adjusted by
changing the analyte from deionized water analyte to
aqueous solutions of different concentrations of sodium
chloride. One digit of the intensity read out by the camera
image equals a change in the refractive index of 5·10
-6
.
4.2 SPR-Angle Range
Our setup is able to perform a motionless angle scan.
We have different SPR-angles on one single
sensor chip at the same time.
The SPR-angle incident changes by per 4 mm
in x-direction. The value of °/mm depends on the type
of light source.
If the spreading of the °/mm is getting higher, the
size of the ROI had to be reduced, otherwise the RIU-
resolution will decrease.
Figure 6: SPR- angle vs. position of ROI. The curve in grey
is shifted by 4 mm in x direction on the sensor surface with
respect to the curve in blue, this leads to an SPR-angle shift
of 1°.
The refractive index of different recognition
elements may vary. Therefore, the spreading of the
SPR-angle can be used to place every recognition
element at its optimum SPR- angle. While a reference
Miniaturized Surface Plasmon Resonance based Sensor System
65
area and its recognition elements always had to be
placed in the same position in x-direction.
4.3 Spatial Resolution
The setup contains a 2-megapixel camera. In
combination with the optical lens system we have
depicted 400.000 pixels within an area of 10 mm x 10
mm. Therefore, every pixel depicts an area of 16 µm
x 16 µm at the sensing area. In order to reduce noise
we determined the area of a region of interest to 10
pixel x 10 pixel. Hence the minimum size of a
recognition element is approximately 160 µm x 160
µm. The size of the recognition element can easily be
reduced by changing the lens system or by switching
to a high-resolution camera system. With a chip size
of 2 x 2 cm which is a common size in commercial
devices about 100 sensing spots can be already
achieved with this miniaturized SPR set-up.
5 CONCLUSION
The miniaturized SPR sensor which is described here
consists of core part of about 15 cm x 10 cm x 5 cm
in size. It also includes the computing unit and it can
work in a standard lab environment. The fully
equipped system has a size of 25 cm x 30 cm x 25 cm
which allows in-field applications at temperatures
ranging from 253 K to 318 K. The work is still in
progress; hence the size of the system will still be
decreased and the temperature operation range will
still be expanded. The current RIU resolution is 10
-6
which is already outstanding for miniaturized system.
The RIU resolution will improve significant by
introducing software algorithms and a more efficient
camera system. Because of the spreading of the spr-
angle, the system can be equipped with a wide range
of receptors. The investigated system has a range of
4 °, which can be altered by a change of the light
source. The overall sensitivity towards individual
chemical analytes will be improved using
nanostructures on top of the gold surface (Genslein et
al. 2017).
ACKNOWLEDGEMENTS
We gratefully acknowledge the help provided by the
staff from Sensorik-ApplikationsZentrum and
Institute of Analytical Chemistry, Chemo- and
Biosensors.
This work was carried out within the
STROMNETZE framework of the Bundesregierung
(Förderkennzeichen 03ET7523A).
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