K+-ion sensing using surface plasmon resonance by NIR light source

Sensors and Actuators B 96 (2003) 446–450

K+-ion sensing using surface plasmon resonance by NIR light source Nyeon-Sik Eum a,∗ , Seung-Ha Lee b , Dong-Rok Lee b , Dae-Kyuk Kwon c , Jang-Kyoo Shin b , Jae-Ho Kim d , Shin-Won Kang b a

Department of Sensor Engineering, Kyungpook National University, Sankyuckdong, Daegu 702-701, South Korea School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 702-701, South Korea c Department of Electronic and Information Engineering, Kyungil University, Kyungsan 712-900, South Korea d Department of Molecular Science & Technology, Ajou University, Suwon 442-749, South Korea

b

Received 16 October 2002; received in revised form 8 July 2003; accepted 8 July 2003

Abstract A surface plasmon resonance (SPR) sensor, which detects K+ ion using gold thin film, was constructed at different incident wavelengths, 670 and 830 nm. The properties of gold thin film and wavelength are very important factors in exciting surface plasmon resonance. We investigated the SPR phenomenon at different incident wavelengths when the surface plasmon resonated with evanescent waves. The resonance angle changed about 4.4◦ as the light source changed from 670 to 830 nm wavelengths in pure water. The sensor chip that is coated with a sensing membrane was tested at each wavelength. We found no resonance point at the 670 nm wavelength because the sensing film was too thick when the K+ ion concentration was varied from 10−8 to 1 M. As the light source was changed to the 830 nm wavelength, we could detect the resonance point even though the film was thick. Therefore, we could detect surface plasmon resonance phenomena at the longer incident wavelength and the smaller resonance angle. We could detect the K+ ion concentration from 10−8 to 1 M by using a 830 nm laser diode and sensing membrane. © 2003 Elsevier B.V. All rights reserved. Keywords: Surface plasmon resonance sensor; K+ ion; Optical sensor; NIR; Wavelength; Sensing membrane

1. Introduction For the last decade surface plasmon resonance sensors have been extensively studied in the region of antibody– antigen reaction [1,2], protein–DNA interactions [3], and ion sensing [4]. Because SPR measurement has many merits, e.g. the kinetics of bio-molecular interactions can be measured in real time, the adsorption of unlabeled analyte molecules to the surface can be monitored, and SPR has a high degree of surface sensitivity. SPR sensors have become widely used in the fields of chemistry and biochemistry to characterize biological surfaces and to monitor binding events. SPR ion sensor has not been extensively studied in relation to above fields because of its small variation of dielectric constant and sensing membrane condition. Ion sensors have been developed using ion selective electrode (ISE) [5], ion selective field effect transistor (ISFET) [6], and metal electrode spectroscopy method [7]. These methods are varied nonlinearly because its concentration is very ∗ Corresponding author. Tel.: +82-1193849389; fax: +82-539506827. E-mail addresses: [email protected] (N.-S. Eum), [email protected] (S.-W. Kang).

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00599-9

low and needs labeling. Instead, the SPR sensor has many merits such as high sensitivity, light weight, low cost, and linear characteristics when applied to chemical sensors and bio-sensors [8]. The sensor chip is made of gold thin film (500 Å) deposited on cover glass by a thermal evaporator and a coated sensing membrane using a spin coater or self-assembled monolayer (SAM) method [9,10]. In the spin-coating method, the sensing membrane is coated as thickly as possible to provide more reacting sites. The membrane is not measured in Kretschmann configuration because it is out of the measuring range. The SAM method is used to compensate for this disadvantage, and now it is suitable for bio-sensing. Because this method has limited sensing sites and has little variation of the dielectric constant for low or high ion concentrations, it has many difficulties for ion sensing. In this paper, we first present the SPR ion sensor using an NIR (830 nm) light source to overcome the membrane thickness problem by offering enough sensing sites. Visible LD (670 nm) replaced NIR-LD as the light source to compare SPR phenomena with K+ ion concentration. Additionally, K+ ions were measured using the sensing membrane containing ionophores and chromoionophores, respectively.

N.-S. Eum et al. / Sensors and Actuators B 96 (2003) 446–450

2. Theory

where k0 is propagation constant of incident light in the free space (k0 = 2π/λ0 , λ0 is the free space wavelength), εm is the direct constant of the gold, and na is the refractive index of the solution [15]. This condition is met in the infrared (IR)-visible region for air/metal and water/metal interfaces. The reflectance curve can be made with a sharp dip for gold thin film at the visible or IR wavelength [16]. To apply this theory to proposed experiments, we simulated SPR phenomena using MATLAB programming. The reflectivity R is a function of the light wavelength λ, the direct constant εi (i = 0, 1, 2), the medium thickness di (i = 1), and the incident angle θ. According to the Kretschmann geometry shown in Fig. 1, we describe the relationship of R versus θ by R = |r0,2 |2 ri,2

ri,i+1 =

 2π εi − ε0 sin2 θ λ εi ζ= (i = 0, 1, 2) kzi

kzi =

As has been known for several decades, the SPR phenomenon is a surface electromagnetic wave that propagates along the interface between a metal and a dielectric material. A model predicting the resonance condition can be obtained by using plane-wave Fresnel reflection equations for a structure containing multiple planar layers [11]. As is well known, surface plasmons (SPs) are excited by an incident laser beam and resonate with an evanescent wave [12] at a resonance angle or wavelength. SPs resonate with evanescent waves that have their maximum intensity at the interface and decay exponentially away from the phase boundary to a penetration depth on the order of 200 nm [13]. It is the so-called attenuated-total-reflectance (ATR) method proposed by Kretschmann [14]. The propagation constant of a surface plasmon wave (SPW) can be expressed as
εm n2a kSPW = k0 (1) εm + n2a

ri,i+1 + ri+1,2 exp[2jdi+1 kzi+1 ] = , 1 + ri,i+1 ri+1,2 exp[2jdi+1 kzi+1 ]

447

(2) (i = 0, 1, 2)

ζi+1 − ζi ζi+1 + ζi

Fig. 1. Kretschman configuration of the SPR sensor.

where kzi represents the wave vector of the transmission light in the Z direction in medium i (i = 0, 1, 2), and ri,i+1 is the reflectivity of the interface between two adjacent media [17]. Using Eq. (2), we could simulate the SPR phenomena. When we use a NIR light source, we can expect that NIR excitation will produce the practical dynamic range of the technique, allowing for the measurement of thicker films. Additionally, the NIR-LD can be used to measure thick films that contain species that absorb light in the visible region of the spectrum [13].

3. Experimental procedures 3.1. SPR sensor system The SPR measurements are classified by a few methods. The most widely employed and classical method is scanning SPR angle [18] and others are SPR wavelength shift [19] or imaging [20]. We used the SPR system with the Kretschmann configuration, as shown in Fig. 2. The light sources used were 670 nm (Hitachi, HL6720G) and 830 nm (Hitachi, HL8325G) laser diodes. Incident light was TM polarized by a P polarizer (Suruga Seiki Co.) and was passed through a right angle prism (n = 1.515, Sigma, BK7). Immersion oil (n = 1.515, Merck) was used to decrease the difference of the refractive index between the sensor chip and prism. The X–Y–θ stage, which was connected with the flow cell (volume: 560 ␮l) made of Teflon, was rotated to vary the incident angle. Refracted light was measured using a photodetector (ANDO, AQ-1135E) (Fig. 3). 3.2. Fabrication of membrane and K+ solution The K+ ion sensing membrane was made by an inclusion method using valinomycin (Sigma) 2.3 wt.%, anion site 0.7 wt.%, diocty phthalate (DOP, Sigma) 60.8 wt.%, PVC–

Fig. 2. Schematic diagram of the surface plasmon resonance sensor system.

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Fig. 3. Photograph of the surface plasmon resonance sensor system. Fig. 4. Dark band photograph for SPR phenomenon.

PVAc–PVA copolymer (Aldrich) 3.5 wt.%, and chromoionophore ETH5294 (Fluka) 1.2 wt.%. The PVC–PVAc– PVA copolymer used a matrix. In the above membrane, one membrane used chromoionophore, while the other did not. The 0.05 M Tris–HCl buffer solution (pH 7.4) was made of HCl (Sigma) and deionized water by the Milli-Q water system and measured using a pH-meter (Orion). K+ ion solutions were made by dissolving HCl to make nine kinds of solution from 1 to 10−8 M, respectively.

4. Results 4.1. View of SPR phenomenon We could confirm the SPR phenomenon using an incident angle with an objective lens and collimator. When incident light was focused on the metal surface at various incident angles, a certain light resonated with the surface plasmon. We took a picture with a digital camera, as shown in Fig. 4. The reflected light was illuminated on the screen and the central line was a dark band. The dark band was generated because the excited surface plasmon resonated with the incident light at a certain angle. Other lights re-

flected on the gold surface were seen on the screen as red (λ = 670 nm). 4.2. Resonance angle shift using different light source For measuring the resonance angle shift when the light source was varied 670 and 830 nm wavelength in an air and water environment, we experimented on each wavelength with one sensor chip. Fig. 5 presents numerical analysis results that confirmed the resonant angle shift along wavelengths in two different media. Additionally, we could recognize that the resonance angle was varied 0.2 and 4.4◦ compared with air and water environment at the above wavelengths, respectively. The results are shown in Fig. 6. Numerical analysis showed excellent agreements with empirical results on visible and NIR wavelengths. In addition, the effects of various concentrations of K+ ion solutions were measured by exciting two different light sources on the gold thin films on which sensing membranes were not coated. As in Figs. 7 and 8, no significant change of resonant angle was observed at low concentration, except in the 1 M sample. However, the resonance angle tended to decrease as the wavelength of excited light increases.

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Fig. 5. Simulation results using MATLAB. (a) Resonance phenomena in air and (b) in DI water environments (λ = 670, 830, and 1310 nm).

N.-S. Eum et al. / Sensors and Actuators B 96 (2003) 446–450 Air λ=670nm Water λ=670nm Air λ=830nm Water λ=830nm

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Fig. 6. Resonance angle shift for different wavelength (λ = 670, and 830 nm).

Fig. 8. Resonance angle shift for different K+ ion concentration (λ = 830 nm, Au sensor chip).

4.3. K+ ion concentration dependent resonance angles

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In order to measure the concentration of K+ ions, the sensing membrane was coated on the sensor chip by using the spin-coating method. It is important to make a spacious reacting site to achieve maximum variation of the resonance angle since the thickness of the sensing membrane is an important factor affecting the degree of sensing sensitivity. However, a too thick sensing membrane might cause excessive variation of resonance angles due to increased mass of the sensing membrane which becomes a more significant factor compared to the degree of permittivity variation due to ion reaction. Thus, a thicker sensing membrane requires a wider span of incident angles that might not be feasible for measurement. In this experiment, the sample was prepared by using a sequential spin-coating technique such as 3500 rpm 5 s, 4000 rpm 25 s, and 3500 rpm 5 s in this experiment. As seen in Fig. 9, surface plasmon resonance at 56.9◦ was observed using the 670 nm light source. The difference of 13.7◦ shift of resonance angle compared to Fig. 6

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Fig. 9. SPR phenomenon using coated sensing membrane (λ = 670 nm).

was due to the variation of both permittivity and mass which was induced by applying a sensing membrane on the gold thin film. There was no significant resonance observed when the concentration of K+ ions in the specimen cell varied from low to high under the same experimental conditions, as shown in Fig. 9. However, an 830 nm light source instead

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Fig. 7. Resonance angle shift for different K+ ion concentration (λ = 670 nm, Au sensor chip).

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Fig. 10. Resonance angle shift for K+ ion concentration using coated sensing membrane and gold thin film (λ = 670, and 830 nm).

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of 670 nm under the same experimental conditions provided clear resonance and a shift of resonance point, as shown in Fig. 10. The permittivity variation from the direct contact between the gold thin film and K+ ion solution provided a higher resonance angle of about 0.2 and 2.8◦ for low concentration and 1 M concentration, respectively, after applying the sensing membrane. Furthermore, based on our SPR experiment it is feasible to measure the degree of K+ ion concentration as low as 10−8 M.

[6]

[7]

[8]

[9]

5. Conclusions Various concentrations of K+ ion solutions were measured by the surface plasmon resonance technique. A variation of resonance angle was observed depending on excited wavelength of light sources and the properties of resonance after applying a sensing membrane on gold thin film. Resonance angles shifts of 0.2◦ in the air and 4.4◦ in the water were obtained by using a 830 nm light source instead of 670 nm. Resonance at 56.9◦ was observed after applying the sensing membrane in the air using 670 nm light source, which is an additional 13.7◦ shift compared to the resonance angle without a sensing membrane. There was no resonance observed in the K+ ion solution at 670 nm light source; however, a concentration from 10−8 to 1 M was measured after exchanging NIR light source (830 nm). In addition, an enhanced resonance angle of 0.2◦ for the low concentration and 2.8◦ for the 1 M concentration was obtained compared to resonance point using gold thin film.

[10]

[11]

[12]

[13]

[14]

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[17]

References [18] [1] P.B. Luppa, L.J. Sokoll, D.W. Chan, Immunosensors-principles and applications to clinical chemistry, Clin. Chim. Acta 314 (2001) 1–26. [2] X. Cui, F. Yang, Y. Sha, X. Yang, Real-time immunoassay of ferritin using surface plasmon resonance biosensor, Talanta 60 (2003) 53–61. [3] E. Mariotti, M. Minunni, M. Mascini, Surface plasmon resonance biosensor for genetically modified organisms detection, Anal. Chim. Acta 453 (2002) 165–172. [4] C.B. Brennan, L.S.S.G. Weber, Investigations of prussian blue films using surface plasmon resonance, Sens. Actuators B 72 (2001) 1–10. [5] N. Yoshikuni, S. Nishiumi, K. Oguma, Preparation of a tellurium ion-selective membrane electrode using an anion-exchange resin and

[19]

[20]

its application to chemical analysis of industrial materials, Anal. Chem. 49 (10) (2000) 791–794. J.C. Chou, Y.S. Li, Study on temperature effect of a-Ta2 O5 gate ISFET, in: Proceedings of the 4th East Asian Conference on Chemical Sensors, 1999, pp. 420–423. M.D. DeGrandpre, L.W. Burgess, Long path fiber-optic sensor for evanescent field absorbance measurements, Anal. Chem. 60 (1988) 2582–2586. X.-M. Zhu, P.-H. Lin, P. Ao, L.B. Sorensen, Surface treatments for surface plasmon resonance biosensors, Sens. Actuators B 84 (2002) 106–112. A.J. Williams, V.K. Gupta, Structure and formation of self-assembled monolayers of helical poly(␥-benzyl l-glutamate) by surface plasmon resonance and infrared spectroscopy, Thin Solid Films 423 (2003) 228–234. B.-K. Oh, Y.-K. Kim, W. Lee, Y.M. Bae, Immunosensor for detection of Legionella pneumophila using surface plasmon resonance, Biosens. Bioelectr. 18 (2003) 605–611. A. Ishimaru, Electromagnetic Wave Propagation, Radiation, and Scattering, Prentice Hall, Englewood Cliffs, NJ, 1991, pp. 43–45 (Chapter 3). S.-M. Lee, S.-W. Jang, S.-H. Lee, J.-H. Kim, S.-H. Kim, S.-W. Kang, Measurement of basic taste substances by a fiber optic taste sensor using evanescent field absorption, Sens. Mater. 14 (1) (2002) 11–21. J.M. Brockman, B.P. Nelson, Surface plasmon resonance imaging measurements of ultrathin organic films, Annu. Rev. Phys. Chem. 51 (2000) 41–63. E. Kretschmann, Die Bestimmung optischer Konstanten von Metallen durch Anregung von Oberflachenplasmaschwingungen, Z. Phys. 241 (1971) 313–324. J. Homola, I. Koudela, S.S. Yee, Surface plasmon resonance sensors based on different gratings and prism coupler: sensitivity comparison, Sens. Actuator B 54 (1999) 16–24. H.P. Chiang, Y.C. Wang, P.T. Leung, W.S. Tse, A theoretical model for the temperature-dependent sensitivity of the optical sensor based on surface plasmon resonance, Opt. Commun. 188 (2001) 283– 289. Sen-fang Sui, Xiao Caide, Yue Zhou, Wenzhang Xie, Jungfeng Liang, Principles of Chemical and Biological Sensors, Chapman & Hall, London, 1999, pp. 95–123. T. Abdallah, S. Abdalla, S. Negm, H. Talaat, Surface plasmons resonance technique for the detection of nicotine in cigarette smoke, Sens. Actuators A 3 (2003) 234–239. J. Dostalek, J. Ctyroky, J. Homola, E. Brynda, M. Skalsky, P. Nekvindova, J. Spirkova, J. Skvor, J. Schrofel, Surface plasmon resonance biosensor based on integrated optical waveguide, Sens. Actuators B 76 (2001) 8–12. Yu Xinglong, Wang Dongsheng, Wang Dingxin, Ou yang Jian Hua, Yan Zibo, Dong Yonggui, Liao Wei, Zhao Xin sheng, Micro-array detection system for gene expression products based on surface plasmon resonance imaging, Sens. Actuators B: Chem., vol. 91, 2003, pp. 133–137.