Miniaturized Surface Plasmon Resonance based Sensor System

Peter Hausler

, Christa Genslein

, Carina Roth

, Thomas Vitzthumecker

, Thomas Hirsch

and Rudolf Bierl

Sensorik-ApplikationsZentrum,Ostbayerische Technische Hochschule Regensburg,

Franz-Mayer-Str. 1, Regensburg, Germany

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

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 510

-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

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 1° 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

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 5·10

-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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PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology