Sensors andand Materials,Materials, Vol.
Vol. 17,17, No.
No. 77 (2005)(2005) 405– 412
Sensors
MYU Tokyo

405

S & M 0618

Odor Sensor Utilizing Surface Plasmon Resonance
Hidehito Nanto*, Yasuto Kitade, Yoshinori Takei and Naotaka Kubota
Advanced Materials Science R&D Center, Kanazawa Institute of Technology
3-1 Yatsukaho, Hakusan, Ishikawa 924-0838, Japan(Received December 21, 2004; accepted May 9, 2005)
Key words: surface plasmon resonance, odor sensor, molecular recognition membrane, ammonia,amine, CVD

An odor sensor based on surface plasmon resonance (SPR) has been studied for thedetection of harmful gases such as ammonia and amines. The sensor was prepared bydepositing a molecular recognition membrane on a substrate coated with Au thin film using
plasma chemical vapour deposition (CVD). The SPR sensor with acrylic acid thin film as
the molecular recognition membrane exhibited excellent selectivity and high sensitivity forammonia and amines.

1.

Introduction

Surface plasmon resonance (SPR) is an optical phenomenon in which incident light
excites a charge-density wave at the interface between a highly conductive metal and adielectric material. The conditions for excitation are determined by the permittivity of themetal and the dielectric material. SPR transduction is widely used as an analytical tool formeasuring small changes in the refractive index of a thin region adjacent to a metal surface.
The optical excitation of surface plasmon on a thin metallic film has, therefore, beenrecognized as a promising technique for sensitive detection of chemical species.(1) Several
methods have been employed to monitor the excitation of SPR by measuring the lightreflected from sensor interface. These include analysis of angle modulation,(2) wavelength
modulation,(3) intensity modulation(4) and phase modulation.(5)It may be possible to apply SPR to a high sensitive odor sensor. Odors, fragrances andaromas have been identified using arrays of gas sensors, such as quartz resonators or oxide
semiconductors, in conjunction with associated pattern recognition analysis.(6–13) Recently,
we have reported that a novel aroma-sensing method, which uses principle component
analysis and neural network pattern recognition of transient responses measured usingsynthetic-polymer-film-coated quartz resonator gas sensors, is useful in distinguishing thearomas of various kinds of coffee, wines and alcohol.(9,12,13)*Corresponding author, e-mail address: hnanto@neptune.kanazawa-it.ac.jp

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In this paper, we propose that harmful gases such as ammonia, trimethylamine andtriethylamine can be detected by measuring SPR using angle modulation. The technique ofusing a synthetic polymer film as molecular recognition membrane is applied to a SPRsensor.

2.

Experimental

Optical SPR sensors are sensitive to change in the refractive index of a sample near asensor surface. The SPR was measured using the Kretschmann configuration (Toa DKK:PR-40), illustrated in Fig. 1, with a prism and a thin, highly conductive gold metal layer
deposited on the prism base. An LED emitting 660 nm light was used as light source inorder to excite the SPR. The SPR reflection spectrum (reflected light intensity versus angleof incidence with respect to the normal of the metal/dielectric interface) is measured bycoupling transverse magnetically polarized monochromatic light into the prism and measuring the reflected light intensity of the ray exciting the prism versus the angle ofincidence. The reflected light was measured using a CCD camera attached to a personal
computer. The coupling of energy occurs between the incident light and the surface
plasmon waves at the resonance angle, which gives the minimum intensity of the reflectedlight. The relationship between the resonance angle θ and the wave number of the surface
plasmon KSP is given by
KP sinθ = KSP(1)ωc(2)

andKSP =εn 2.
ε + n2Fig. 1. Kretschmann configuration for generating surface plasmon waves.

Sensors and Materials, Vol. 17, No. 7 (2005)407

where KP, Ksp, c, ε and n are the wave number of the incident light, the angular frequencyof the incident light, the speed of light in vacuum, the permittivity of the thin metal film andthe refractive index of a sample near the sensor surface, respectively. Thus, an increase inresonance angle corresponds to an increase in refractive index of a sample near the sensor
surface in the SPR reflection spectrum. The schematic configuration of the SPR system isshown in Fig. 2.
In order to utilize this system as a novel odor sensor, a very thin film of acrylic acid asa molecular recognition membrane was deposited on the gold metal thin film using plasma
chemical vapor deposition (CVD). The schematic of the plasma CVD system is shown inFig. 3. A vaporized monomer was introduced into the chamber evacuated at the basepressure of 10 Pa together with Ar as carrier gas. The partial pressures of the monomer
vapor and Ar gas were set to be approximately 20 and 80 Pa, respectively. The Ar based
plasma was produced by the radio frequency (RF) discharge between two electrodes witha supplied power of 100 W, and activated monomers polymerized on the surface of the thingold film. Two kinds of monomers, acrylic acid with a carboxyl group and styrene with aphenyl group were used as materials for the molecular recognition membrane. Bothmonomers have carbon-carbon double bond and are polymerized easily. A typicaldeposited thickness on the gold film was several tens of nm, which was monitored using
quartz resonator.
A test gas at a concentration of 1000 ppm was injected into the 500 ml sensing cell at 20 safter the beginning of the measurement under temperature of 20°C and humidity of 55%.
The response of SPR sensor was then monitored for 5 min. Eleven harmful gases,
ammonia, acetaldehyde, propionaldehyde, xylene, toluene, trimethylamine, triethylamine,
dimethyamine, hormaldehyde, acetic acid and butyl acetate, were used as test gases.

Fig. 2. Schematic diagram of SPR sensor system.

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Fig. 3. Schematic of the plasma CVD system.

3.

Results and Discussion

Figures 4 and 5 show typical responses of SPR sensors with acrylic acid (46 nm) andstyrene membrane (a few tens of nm) to exposure to each test gas, respectively. The SPR
sensor using the acrylic acid membrane responded to only the basicitic gases, i.e. ammonia,
trimethylamine and triethylamine, with a change in SPR angle on the order of 1°. An
adsorption reaction might occur because of the acidity of the acrylic acid membrane andthe basicity of these gases, which induced the high response. On the other hand, the SPR
sensor using the styrene membrane responded to ammonia, toluene, dimethyamine andbutyl acetate with a change in SPR angle on the order of 0.001°, which is equal to the angle
resolution of this system. This result suggests that an SPR sensor coated with an acrylicacid membrane exhibits good selectivity for ammonia and amines. The sensitivity of theacrylic acid membrane was 200 times higher than that of the styrene membrane.
The SPR sensor responses of the acrylic acid membrane to various concentrations ofammonia are shown in Fig. 6. An acrylic acid membrane, 46 nm thick was used. The risetime to reach a saturation value for the angle change tended to decrease with increasing
ammonia concentration. In addition, each saturation value increased with increasing
ammonia concentration, as shown in Fig. 7. The ammonia molecules adsorbed on themembrane surface may equilibrate with ammonia molecules in the gas phase at eachequilibrium pressure for various concentrations of ammonia, which causes the coverage ofammonia molecule to be less than unity because change in angle may saturate and become
constant for various concentration of ammonia for coverage less than unity. Thus, the SPR
sensor is also applicable in high concentration regions. Therefore, this dependence
indicates the possibility of quantitatively measurement using the SPR sensor with amolecular recognition membrane. At present, the limiting sensitivity of an SPR sensor

Sensors and Materials, Vol. 17, No. 7 (2005)

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Fig. 4. Responses of an SPR sensor coated with acrylic acid to various gases, where Δθ is change
in resonance angle.

Fig. 5. Responses of an SPR sensor coated with styrene to various gases, where Δθ is change inresonance angle.

coated with acrylic acid thin film for ammonia and amines can be estimated to be several
ppm, because the accuracy of the SPR angle change is about 0.001°. The SPR sensor
response for various thicknesses of acrylic acid membrane for ammonia is shown in Fig. 8.
The concentration of ammonia was 200 ppm and the thicknesses of the membranes were31, 40 and 46 nm. Since rise times of a few second were obtained for each thickness, theadsorption for each thickness might occur though the same process for the range of theconditions evaluated. Each saturation value increased as the thickness of the acrylic acidmembrane increased, which is shown in Fig. 9. The sensor did not respond if themembrane were over 50 nm thick because the distance of approach of the evanescent waveof incident light to the interface between the Au film and the membrane is expected to be410

Sensors and Materials, Vol. 17, No. 7 (2005)

Fig. 6. Responses of an SPR sensor coated with acrylic acid for various concentration of ammonia,
where Δθ is change in resonance angle.

Fig. 7. Dependence of maximum resonance angle change, Δθmax, on ammonia concentration ofacrylic acid membrane.

several tens nm. If ammonia molecules are adsorbed only on the surface of the membrane,
the change of refractive index, namely the change of resonance angle, on the thin Au film
may be small.

4.

Summary

An odor sensor using the SPR phenomenon has been demonstrated. An SPR sensor
with an acrylic acid membrane as a molecular recognition has exhibited excellent selectiv-Sensors and Materials, Vol. 17, No. 7 (2005)

411

Fig. 8. Responses of an SPR sensor for various thicknesses of acrylic acid membrane, where Δθ isthe saturated value of resonance angle change.

Fig. 9. Dependence of maximum resonance angle change Δθmax on thickness of the acrylic acidmembrane based on exposure to ammonia at a concentration of 200 ppm.

ity for ammonia and amines, whereas an SPR sensor with a styrene membrane has little
selectivity for gases studied. The output of the SPR sensor increased with increasing
ammonia concentration, which indicates the possibility of measuring gas concentration
quantitatively with high sensitivity. The detection limit for ammonia is estimated to be
several ppm. Moreover, the output of the SPR sensor for ammonia increased as thethickness of the acrylic acid membrane increased. We expect that the sensitivity of SPR
sensor can be improved by optimizing the thickness of the molecular recognition membrane.
An SPR sensor with molecular recognition membranes may becomes a useful tool to412Sensors and Materials, Vol. 17, No. 7 (2005)

identify odors, fragrances and aromas. It is possible to use such a sensor for the detectionof the freshness of fish because ammonia and the amines are given off during thedeterioration of fish after death.(14)

Acknowledgements
The authors wish to thank M. Komura and S. Sanada for their excellent assistance withthe experiments. This work was partly supported by Grants-in-Aid for Scientific Research(No. 15760328, No.16360208 and No.16656031) and the foundation for High-TechResearch Center from the Ministry of Education, Science, Sports and Culture of Japan.

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