- Email: [email protected]
Novel spectral fiber optic sensor based on surface plasmon resonance
Sensors and Actuators B 74 (2001) 106±111
Novel spectral ®ber optic sensor based on surface plasmon resonance Radan SlavõÂka,*, JirÏÂõ Homolaa, JirÏÂõ CÏtyrokyÂa, Eduard Bryndab a
Institute of Radio Engineering and Electronics AS CR, Chaberska 57, 182 51 Prague, Czech Republic Institute of Macromolecular Chemistry AS CR, HeyrovskeÂho naÂm. 2, 162 06 Prague, Czech Republic
b
Abstract A novel ®ber optic surface plasmon resonance (SPR) sensing device based on spectral interrogation of SPR in a miniature ®ber optic sensing element using depolarized light is reported. Optimization analysis of the sensor based on the equivalent planar waveguide approach and the mode expansion and propagation method is presented. A laboratory prototype of the sensor has been proved to be able to measure refractive index variations as small as 5 10ÿ7 . Suitability of the sensor for biosensing has been demonstrated by detecting IgG via respective monoclonal antibodies immobilized on the SPR sensor surface. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Optical sensors; Optical ®bers; Surface plasmon resonance
1. Introduction Sensors capable of on site (bio)chemical analysis are needed in many areas including industrial process control, medical analysis, and environmental monitoring. Surface plasmon resonance (SPR) sensors holds potential for applications in these areas. Miniaturization of SPR sensing devices, as a prerequisite for the development of portable SPR sensor systems, has been receiving growing attention. In particular, waveguide-based SPR sensing structures has been widely studied, as the utilization of optical waveguides offers numerous advantageous features such as small size, robustness, and potential for remote sensing. Various SPR sensors based on integrated optical waveguides [1] and optical ®bers [1±4] have been developed. In SPR sensors based on multimode ®bers [2,3], the sensing element encompasses a multimode optical ®ber with an exposed core coated around with a thin metal layer supporting surface plasma waves (SPW), [2]. These sensors exhibit a rather limited resolution mainly due to the modal noise presented in multimode ®bers causing the strength of the interaction between the ®ber-guided light wave and the SPW to ¯uctuate. To overcome this inherent limitation of SPR sensing devices based on multimode ®bers, SPR sensors based on a single-mode optical ®ber were proposed [4,5]. They include SPR sensors based on tapered [5] and side-polished [4] single-mode optical ®bers. The SPR sensors using tapered ®bers rely either on spectral interrogation * Corresponding author. Tel.: 420-2-688-1804; fax: 420-2-688-0222. E-mail address: [email protected] (R. SlavõÂk).
at rather short wavelengths (and therefore exhibit rather low sensitivity) or on amplitude interrogation (exhibiting also low sensitivity due to rather broad SPR dips caused by variations in the SPR condition along the sensing region). Amplitude SPR sensors based on side-polished single-mode optical ®bers offer superior sensitivity [4], although suffer from adverse sensitivity to ®ber deformations, as any ®ber deformations change the state of polarization of the ®ber mode and consequently also the strength of its interaction with SPW. In this paper, we propose a novel approach to the development of ®ber optic SPR sensing devices based on spectral interrogation of SPR in the side-polished ®ber optic sensing element using depolarized radiation. This approach allows construction of highly sensitive all-®ber optic sensors with reduced adverse sensitivity to ®ber deformations. We demonstrate applicability of the sensor to refractometry and af®nity biosensing. 2. Sensor structure and principle of operation The proposed SPR sensing element, in detail described in [6], consists of a standard single-mode optical ®ber with locally removed cladding and a thin gold ®lm supporting SPW, Fig. 1. The SPW may be excited by a guided mode propagating in the ®ber if the two waves are closely phasematched. As the propagation constant of the SPW depends dramatically on the refractive index of the medium adjacent to the gold ®lm (sample), the strength of the interaction between the SPW and the ®ber mode, and consequently ®ber
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 7 1 8 - 8
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
107
Fig. 1. SPR sensing structure based on a side-polished single-mode fiber.
mode attenuation are strongly dependent on the refractive index of the sample. Therefore, variations in the refractive index of the sample can be determined by measuring changes in the transmitted optical power at a ®xed wavelength (amplitude mode) or by measuring changes in the wavelength at which the resonant attenuation of the ®ber mode occur (spectral mode). As only light polarized with magnetic vector parallel to the metal ®lm may excite SPW, the process of exciting SPW is highly polarization-sensitive. This adverse sensitivity causes blurring of spectral SPR dip in response to variations in polarization of the ®ber mode due to polarization fading [7], and consequently affects both the detected resonant wavelength and the ®ber mode attenuation in spectral and amplitude modes of operation. The polarization fading could be overcome by the use of highly birefringent (HiBi) ®bers which ensures holding of polarization state of the ®ber mode even under ®ber deformations such as bending and twisting [7]. Relatively high price of HiBi ®bers and the need for precise orientation of the HiBi ®bers for side-polishing are serious drawbacks, however. We present another approach based on standard single-mode ®bers. This approach is based on the excitation of SPR with unpolarized radiation and spectral interrogation. The unpolarized light has even distribution of intensity in polarization space resulting in insensitivity of polarization state of the transmitted light to ®ber deformations.
measurements. The in¯uence of the major parameters of the structure on performance of the sensing element is illustrated in Figs. 2 and 3. The simulated sensor element response is normalized to the response for the refractive index of sample equal to one (air) to allow for direct comparison with experimental results. The normalized sensor transmission S is then given by: I l; nsample I l; nair TE I l; nsample I TM l; nsample I TE l; nair I TM l; nair
S l; nsample
(1)
where I denotes light intensity at the sensor element output, superscripts TE and TM correspond to TE and TM fiber mode polarizations with respect to the gold film. As the resolution of the sensor depends on the accuracy with which the position of the SPR may be measured, narrow and deep SPR dips are preferred. As follows from Fig. 2, the thickness of the gold ®lm larger than 75 nm leads to too shallow SPR dips. On the other side, thicknesses smaller than 55 nm results in too wide SPR dips. It follows from Fig. 3 that an increasing thickness of the residual cladding (represented by the minimal distance between the ®ber core and the polished surface d0, Fig. 1) weakens the interaction between the ®ber mode and the SPW and also reduces the
3. Theory As the geometry of the considered SPR sensor element structure is very complex, rigorous modeling of its waveguiding properties is very dif®cult. Therefore, in our simulations the optical ®ber was substituted by an equivalent planar waveguide. The resulting planar SPR sensing structure was then analyzed using the mode expansion and propagation (MEP) method [8]. The effect of ®ber curvature was accounted for by substituting the bent waveguide by 21 sections of a straight waveguide of different lengths and distances from the metal ®lm [6]. The dielectric constants of gold were taken from [9]. The Ta2O5 refractive index formula n 1:878 178:4 10ÿ4 =l2 52:7 10ÿ5 =l4 used in the simulations was obtained from ellipsometric
Fig. 2. Simulated spectral transmission of the sensor element for different thicknesses of the gold film; amount of the residual fiber cladding corresponds to d0 0, refractive index of sample is 1.3934.
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R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
Fig. 3. Simulated spectral transmission of the sensor element for different amounts of residual cladding. The thickness of the gold film is 65 nm, refractive index of sample is 1.3934.
width of the SPR dip. Proper combination of gold ®lm thickness and d0 has thus to be looked for as the two parameters in¯uence both the width as well as the depth of SPR dips. Generally, for larger d0 the optimal performance is achieved using thinner gold ®lm and vice versa. As follows from the theoretical analysis, the operating range of the SPR sensing element (the refractive index range of sample at which the SPR dip may be observed) is rather limited. As pointed out in [10], the operating range of SPR waveguide-based sensing devices may be adjusted by employing a thin high-refractive index dielectric overlayer. Although the presence of the dielectric overlayer does not compromise the width and depth of the SPR dip, it causes the spectral sensitivity to drop due to reduced concentration of the evanescent ®eld of the SPW in the sample. Fig. 4 shows the simulated response of the sensing element designed for aqueous environments with operating range adjusted using a thin tantalum pentoxide overlayer. Because of the high refractive index of tantalum pentoxide, a very thin overlayer is suf®cient to shift the operating range of the sensor considerably. It has been calculated that a tantalum pentoxide
Fig. 4. Operation range tuning towards aqueous environment using different thicknesses of tantalum pentoxide overlayer. The gold film thickness is 65 nm, the amount of residual cladding corresponds to d0 500 nm, refractive index of sample is 1.329 (water).
Fig. 5. Simulated spectral transmission of the sensor element for three different refractive indices of sample, thicknesses of gold film, tantalum pentoxide overlayer and residual cladding are 65, 19, and d0 500 nm, respectively.
overalyer of the thickness of 10 nm produces a shift of the SPR dip of about 2:5 10ÿ2 RIU (refractive index unit). For sensing in aqueous environments, the optimization analysis carried out yields the following design parameters of the SPR sensing structure: residual amount of ®ber cladding corresponding to d0 500 nm, gold layer thickness, 65 nm, and tantalum pentoxide overlayer thickness, 20 nm. Theoretical analysis also showed that the sensitivity of the resonant wavelength to variations in the refractive index of sample does not depend on the amount of residual ®ber cladding and the gold ®lm thickness if these parameters falls within 0±2 mm, and 55±75 nm, respectively. The average sensitivity of the spectral SPR ®ber optic sensor for aqueous environment was calculated to be 3150 nm/RIU (Fig. 5). It represents a decrease by a factor of 2.5 compared to the SPR sensing structure without the overlayer. 4. Experimental The ®ber optic SPR sensing element was fabricated using a single-mode optical ®ber with the cut-off wavelength of 724 nm (SM800, Fibercore, Ltd., UK). The ®ber was sidepolished to the proximity of the ®ber core in order to attain d0 500 nm. Thickness of the residual cladding layer was measured using the liquid drop method [11]. Then, the ®ber was coated by a gold ®lm (thickness of 65 nm) and a thin tantalum pentoxide overlayer (thickness of 19 nm) by vacuum evaporation. The con®guration of the developed SPR ®ber optic sensor system is shown in Fig. 6. Light from a pigtailed superluminescent diode (SLD) (SLD-371, Superlum, Ltd., Russia) with the power of 0.9 mW in a singlemode ®ber, full width at a half of maximum (FWHM) of 72 nm, the central wavelength at 816 nm, and the degree of coherence of about 60% was chosen as a source of polychromatic light. A Lyot depolarizer [12] was made up of two pieces of Bow-Tie HiBi ®ber (HB800, Fibercore, Ltd., UK) with lengths of its segments 1.3 and 2.7 m. After depolarizing
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
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Fig. 6. Setup of the built fiber optic SPR sensor.
light from the SLD using the Lyot depolarizer, the residual degree of polarization did not exceed 1%. The depolarized light was coupled into the ®ber optic SPR sensing element. A piece of multimode optical ®ber with core diameter of 400 mm wound 10 times around a mandrel of a diameter of 1 cm was employed to scramble polarization at the input of a spectrograph (MSC-501, Carl Zeiss, FRG) to suppress the effect of polarization sensitivity of the spectrograph. The sensor components were joined by ®ber ®nger-splices (Thorlabs, Inc.). Transmitted spectra were acquired by the spectrograph and normalized with respect to the spectrum corresponding to sensor response when no sample was present (sample with refractive index of 1 was used to account for uneven spectral emission of the SLD). The normalized spectra (Fig. 7) were ®tted by the Lorentz function with a linear term, which compensate for slight non-symmetry of SPR dips, and the minimum of the Lorentz function was recorded. ATe¯on ¯ow cell was attached to the sensing element to contain a measured liquid sample during experiments. The ¯ow cell (volume 40 ml) was interfaced with Te¯on tubing (inner diameter, 0.5 mm); the ¯ow rate was induced and regulated (within 0±0.2 ml/min) by a level difference between reservoir and waste. We have examined the in¯uence of residual polarization of the ®ber mode on the performance of the sensor under conditions of temporal ®ber deformations (e.g. ®ber bending and twisting), see Fig. 8. Fiber optic polarization controllers were attached to both input and output ®bers of the sensing element, and the sensor response was measured while changing the polarization state of light on both sides of the sensing element (the steps in Fig. 8 correspond to
Fig. 7. Measured spectral transmission of the sensor element (ticks) and corresponding data fits (lines) for samples with two different refractive indices 1.3291 (squares) and 1.3351 (circles).
changes in position of the polarization controllers). As follows from Fig. 8, residually polarized part of the ®ber mode, which is in¯uenced by ®ber deformations, considerably increases uncertainty in the determination of the resonant wavelength. Upon this fact, two modes of operation of the sensor were proposed. In the static mode of operation the sensor is held in place without movement, suppressing thus effects of incomplete light depolarization, while in the dynamic mode of operation the movement of the sensor element is allowed. In the static mode of operation, the sensor resolution is primarily determined by the noise of the source and the spectrograph, yielding standard deviation of the temporal resonant wavelength about 1:5 10ÿ3 nm (Fig. 8). In the dynamic mode of operation, the uncertainty in the SPR wavelength is almost two orders of magnitude larger Ð 9 10ÿ2 nm (Fig. 8). 4.1. Fiber optic SPR sensor for refractometry To asses sensor's potential as a refractometer, we ¯owed mixtures of diethyleneglycol and water of known refractive indices across the surface of the sensing element and recorded the sensor response, Fig. 9 (refractive indices of samples are given at the wavelength of 800 nm). In the performed refractometric experiment, the sensor was found to exhibit good stability (baseline drift less than 0.02 nm/h) and repeatability (within 1%). The achieved sensor sensitivity was 3100 nm/RIU. It corresponds to the sensor resolution better than 3 10ÿ5 RIU for the dynamic mode of operation and better than 5 10ÿ7 RIU for the static mode of operation.
Fig. 8. Temporal sensor response. The steps correspond to changes in the fiber mode polarization state.
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with 0.2 wt.% glutaraldehyde in CB for 30 min to crosslink the antibodies. DS was washed out of the crosslinked antibody network with PBS [13]. The biosensing experiment consisted of detection of IgG molecules of increasing concentrations dissolved in the PBS containing 1% of BSA. Corresponding concentration isotherm is shown in Fig. 10. Interpolating the concentration isotherm in Fig. 10 and considering the above mentioned sensor resolution ®gures, it can be concluded that the ultimate sensor resolution is about 1 and 90 ng/ml for the static and dynamic mode of operation, respectively.
Fig. 9. Temporal sensor response to variations in the refractive index of sample.
4.2. Fiber optic SPR sensor for biosensing In order to demonstrate ability of the developed ®ber optic SPR sensor's to act as real-time biosensor, we performed detection of IgG proteins. The used reagents were as follows. Human immunoglobulin (IgG), monoclonal antibody against IgG (a-IgG) were obtained from Seva Immuno, Prague; dextran sulfate sodium salt (DS), average molecular weight of approximately 5000, and bovine serum albumin (BSA), purity better than 99% by agrose electrophoresis, were purchased from Sigma. Glutaraldehyde (GA) was freshly vacuum-distilled at 20 kPa under nitrogen into water to form a stock solution containing 20% of GA. Solutions were prepared using citrate buffer (CB), 0.1 M, pH of 3.96 and phosphate buffered saline (PBS), pH of 7.26. Molecular weights of the used reagents were as follows: 150,000 for IgG and a-IgG, and 65,000 for BSA. First, we functionalized the sensor surface with a doublelayer of a-IgG molecules. Assemblies consisting of alternating molecular layers of an antibody (a-IgG) and DS were formed by alternating adsorption from the antibody and DS solutions. The solutions were used in the following order: CB, antibody in CB (100 mg/ml), CB, DS in CB (1 mg/ml), CB, antibody in CB, CB. Then the assembly was incubated
5. Discussion Observed sensor performance agrees well with the performed simulations in terms of position, depth, and width of SPR dips (Figs. 3±5 and 7±9). The residual polarization of the ®ber mode (below 1%) was found to cause the sensor resolution to drop by almost two orders of magnitude Ð from 5 10ÿ7 to 3 10ÿ5 RIU. Lyot depolarizers with slightly better performance, which are now becoming available, are expected to improve resolution of the sensor operating in the dynamic mode. In performed model biodetection experiment, the lowest detected IgG concentration was 40 ng/ml. The experimentally achieved sensor resolution of 1:5 10ÿ3 nm suggests that the ultimate measurable IgG concentration may be considerably lower (up to 1 ng/ml). 6. Conclusions We have reported a new approach to the development of ®ber optic SPR sensing devices based on spectral interrogation of SPR in a ®ber optic sensing element using depolarized light. Sensor sensitivity of 3100 nm/RIU and resolution as high as 5 10ÿ7 RIU were demonstrated. The attained resolution is comparable with the resolution of best bulkoptic table-top SPR devices. We have demonstrated suitability of the developed SPR ®ber optic sensor for biosensing. In model biodetection experiment, the sensor has been able to detect IgG present in concentration as low as 40 ng/ ml. The sensor may be further miniaturized by cutting the ®ber right after the interaction region and by forming a mirror on the end face of the ®ber [14]. In conjunction with biospeci®c coatings for detection of various biomolecular analytes, this approach may provide robust biosensor technology for on site analysis in many areas such as medicine, environmental monitoring, and biotechnology. Acknowledgements
Fig. 10. SPR biosensor-based detection of IgG. Sensor response as a function of concentration of IgG in PBS.
This work has been supported by the Grant Agency of the Czech Republic under the contracts 102/99/0549 and
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
102/00/1536. Authors acknowledge Dr. V. Malina (IREE, Prague, Czech Republic) for thin ®lm deposition, and Dr. W. Ecke and Mrs. Kerstin Schroeder (IPHT, Jena, FRG) for the fabrication of Lyot depolarizer and useful discussions. References [1] R.D. Harris, J.S. Wilkinson, Waveguide surface plasmon resonance sensors, Sens. Actuators B 29 (1995) 261±267. [2] R.C. Jorgenson, S.S. Yee, A fiber-optic chemical sensor based on surface plasmon resonance, Sens. Actuators B 12 (1993) 213±220. [3] A. Trouillet, C. Ronot-Trioli, C. Veillas, H. Gagnaire, Chemical sensing by surface plasmon resonance in a multimode optical fibre, Pure Appl. Opt. 5 (1996) 227±237. [4] J. Homola, R. SlavõÂk, Fibre-optic sensor based on surface plasmon resonance, Electron. Lett. 32 (1996) 480±482. [5] A.J.C. Tubb, F.P. Payne, R.B. Millington, C.R. Lowe, Single-mode optical fiber surface plasma wave chemical sensor, Sens. Actuators B 41 (1997) 71±79.
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[6] R. SlavõÂk, J. Homola, J. CÏtyrokyÂ, Single-mode optical fiber surface plasmon resonance sensor, Sens. Actuators B 54 (1999) 74±79. [7] S.C. Rahleigh, Origins and control of polarization effects in singlemode fibers, J. Lightwave Technol. LT-1 (1983) 312±331. [8] J. CÏtyrokyÂ, J. Homola, M. SkalskyÂ, Modelling of surface plasmon resonance waveguide sensor by complex mode expansion and propagation method, Opt. Quant. Electron. 29 (1997) 301±311. [9] H. Palik, Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985. Ï tyrokyÂ, J. Homola, M. SkalskyÂ, Tuning of spectral operation range [10] J. C of a waveguide surface plasmon resonance sensor, Electron. Lett. 33 (1997) 1246±1248. [11] M.J.F. Digonet, J.R. Feth, L.F. Stokes, H.J. Shaw, Measurement of the core proximity of polished fiber substrates and couplers, Opt. Lett. 10 (1985) 463±465. [12] K. Mochizuki, Degree of polarization in jointed fibers: the Lyot depolarizer, Appl. Opt. 23 (1984) 3284±3288. [13] E. Brynda, M. Houska, J. SÏkvor, J.J. Ramsden, Immobilisation of multilayer bioreceptor assemblies on solid substrates, Biosens. Bioelectron. 13 (1998) 165±172. [14] R. SlavõÂk, J. Homola, J. CÏtyrokyÂ, Miniturization of fiber optic surface plasmon resonance sensor, Sens. Actuators B 51 (1998) 311±315.
Novel spectral ®ber optic sensor based on surface plasmon resonance Radan SlavõÂka,*, JirÏÂõ Homolaa, JirÏÂõ CÏtyrokyÂa, Eduard Bryndab a
Institute of Radio Engineering and Electronics AS CR, Chaberska 57, 182 51 Prague, Czech Republic Institute of Macromolecular Chemistry AS CR, HeyrovskeÂho naÂm. 2, 162 06 Prague, Czech Republic
b
Abstract A novel ®ber optic surface plasmon resonance (SPR) sensing device based on spectral interrogation of SPR in a miniature ®ber optic sensing element using depolarized light is reported. Optimization analysis of the sensor based on the equivalent planar waveguide approach and the mode expansion and propagation method is presented. A laboratory prototype of the sensor has been proved to be able to measure refractive index variations as small as 5 10ÿ7 . Suitability of the sensor for biosensing has been demonstrated by detecting IgG via respective monoclonal antibodies immobilized on the SPR sensor surface. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Optical sensors; Optical ®bers; Surface plasmon resonance
1. Introduction Sensors capable of on site (bio)chemical analysis are needed in many areas including industrial process control, medical analysis, and environmental monitoring. Surface plasmon resonance (SPR) sensors holds potential for applications in these areas. Miniaturization of SPR sensing devices, as a prerequisite for the development of portable SPR sensor systems, has been receiving growing attention. In particular, waveguide-based SPR sensing structures has been widely studied, as the utilization of optical waveguides offers numerous advantageous features such as small size, robustness, and potential for remote sensing. Various SPR sensors based on integrated optical waveguides [1] and optical ®bers [1±4] have been developed. In SPR sensors based on multimode ®bers [2,3], the sensing element encompasses a multimode optical ®ber with an exposed core coated around with a thin metal layer supporting surface plasma waves (SPW), [2]. These sensors exhibit a rather limited resolution mainly due to the modal noise presented in multimode ®bers causing the strength of the interaction between the ®ber-guided light wave and the SPW to ¯uctuate. To overcome this inherent limitation of SPR sensing devices based on multimode ®bers, SPR sensors based on a single-mode optical ®ber were proposed [4,5]. They include SPR sensors based on tapered [5] and side-polished [4] single-mode optical ®bers. The SPR sensors using tapered ®bers rely either on spectral interrogation * Corresponding author. Tel.: 420-2-688-1804; fax: 420-2-688-0222. E-mail address: [email protected] (R. SlavõÂk).
at rather short wavelengths (and therefore exhibit rather low sensitivity) or on amplitude interrogation (exhibiting also low sensitivity due to rather broad SPR dips caused by variations in the SPR condition along the sensing region). Amplitude SPR sensors based on side-polished single-mode optical ®bers offer superior sensitivity [4], although suffer from adverse sensitivity to ®ber deformations, as any ®ber deformations change the state of polarization of the ®ber mode and consequently also the strength of its interaction with SPW. In this paper, we propose a novel approach to the development of ®ber optic SPR sensing devices based on spectral interrogation of SPR in the side-polished ®ber optic sensing element using depolarized radiation. This approach allows construction of highly sensitive all-®ber optic sensors with reduced adverse sensitivity to ®ber deformations. We demonstrate applicability of the sensor to refractometry and af®nity biosensing. 2. Sensor structure and principle of operation The proposed SPR sensing element, in detail described in [6], consists of a standard single-mode optical ®ber with locally removed cladding and a thin gold ®lm supporting SPW, Fig. 1. The SPW may be excited by a guided mode propagating in the ®ber if the two waves are closely phasematched. As the propagation constant of the SPW depends dramatically on the refractive index of the medium adjacent to the gold ®lm (sample), the strength of the interaction between the SPW and the ®ber mode, and consequently ®ber
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 7 1 8 - 8
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
107
Fig. 1. SPR sensing structure based on a side-polished single-mode fiber.
mode attenuation are strongly dependent on the refractive index of the sample. Therefore, variations in the refractive index of the sample can be determined by measuring changes in the transmitted optical power at a ®xed wavelength (amplitude mode) or by measuring changes in the wavelength at which the resonant attenuation of the ®ber mode occur (spectral mode). As only light polarized with magnetic vector parallel to the metal ®lm may excite SPW, the process of exciting SPW is highly polarization-sensitive. This adverse sensitivity causes blurring of spectral SPR dip in response to variations in polarization of the ®ber mode due to polarization fading [7], and consequently affects both the detected resonant wavelength and the ®ber mode attenuation in spectral and amplitude modes of operation. The polarization fading could be overcome by the use of highly birefringent (HiBi) ®bers which ensures holding of polarization state of the ®ber mode even under ®ber deformations such as bending and twisting [7]. Relatively high price of HiBi ®bers and the need for precise orientation of the HiBi ®bers for side-polishing are serious drawbacks, however. We present another approach based on standard single-mode ®bers. This approach is based on the excitation of SPR with unpolarized radiation and spectral interrogation. The unpolarized light has even distribution of intensity in polarization space resulting in insensitivity of polarization state of the transmitted light to ®ber deformations.
measurements. The in¯uence of the major parameters of the structure on performance of the sensing element is illustrated in Figs. 2 and 3. The simulated sensor element response is normalized to the response for the refractive index of sample equal to one (air) to allow for direct comparison with experimental results. The normalized sensor transmission S is then given by: I l; nsample I l; nair TE I l; nsample I TM l; nsample I TE l; nair I TM l; nair
S l; nsample
(1)
where I denotes light intensity at the sensor element output, superscripts TE and TM correspond to TE and TM fiber mode polarizations with respect to the gold film. As the resolution of the sensor depends on the accuracy with which the position of the SPR may be measured, narrow and deep SPR dips are preferred. As follows from Fig. 2, the thickness of the gold ®lm larger than 75 nm leads to too shallow SPR dips. On the other side, thicknesses smaller than 55 nm results in too wide SPR dips. It follows from Fig. 3 that an increasing thickness of the residual cladding (represented by the minimal distance between the ®ber core and the polished surface d0, Fig. 1) weakens the interaction between the ®ber mode and the SPW and also reduces the
3. Theory As the geometry of the considered SPR sensor element structure is very complex, rigorous modeling of its waveguiding properties is very dif®cult. Therefore, in our simulations the optical ®ber was substituted by an equivalent planar waveguide. The resulting planar SPR sensing structure was then analyzed using the mode expansion and propagation (MEP) method [8]. The effect of ®ber curvature was accounted for by substituting the bent waveguide by 21 sections of a straight waveguide of different lengths and distances from the metal ®lm [6]. The dielectric constants of gold were taken from [9]. The Ta2O5 refractive index formula n 1:878 178:4 10ÿ4 =l2 52:7 10ÿ5 =l4 used in the simulations was obtained from ellipsometric
Fig. 2. Simulated spectral transmission of the sensor element for different thicknesses of the gold film; amount of the residual fiber cladding corresponds to d0 0, refractive index of sample is 1.3934.
108
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
Fig. 3. Simulated spectral transmission of the sensor element for different amounts of residual cladding. The thickness of the gold film is 65 nm, refractive index of sample is 1.3934.
width of the SPR dip. Proper combination of gold ®lm thickness and d0 has thus to be looked for as the two parameters in¯uence both the width as well as the depth of SPR dips. Generally, for larger d0 the optimal performance is achieved using thinner gold ®lm and vice versa. As follows from the theoretical analysis, the operating range of the SPR sensing element (the refractive index range of sample at which the SPR dip may be observed) is rather limited. As pointed out in [10], the operating range of SPR waveguide-based sensing devices may be adjusted by employing a thin high-refractive index dielectric overlayer. Although the presence of the dielectric overlayer does not compromise the width and depth of the SPR dip, it causes the spectral sensitivity to drop due to reduced concentration of the evanescent ®eld of the SPW in the sample. Fig. 4 shows the simulated response of the sensing element designed for aqueous environments with operating range adjusted using a thin tantalum pentoxide overlayer. Because of the high refractive index of tantalum pentoxide, a very thin overlayer is suf®cient to shift the operating range of the sensor considerably. It has been calculated that a tantalum pentoxide
Fig. 4. Operation range tuning towards aqueous environment using different thicknesses of tantalum pentoxide overlayer. The gold film thickness is 65 nm, the amount of residual cladding corresponds to d0 500 nm, refractive index of sample is 1.329 (water).
Fig. 5. Simulated spectral transmission of the sensor element for three different refractive indices of sample, thicknesses of gold film, tantalum pentoxide overlayer and residual cladding are 65, 19, and d0 500 nm, respectively.
overalyer of the thickness of 10 nm produces a shift of the SPR dip of about 2:5 10ÿ2 RIU (refractive index unit). For sensing in aqueous environments, the optimization analysis carried out yields the following design parameters of the SPR sensing structure: residual amount of ®ber cladding corresponding to d0 500 nm, gold layer thickness, 65 nm, and tantalum pentoxide overlayer thickness, 20 nm. Theoretical analysis also showed that the sensitivity of the resonant wavelength to variations in the refractive index of sample does not depend on the amount of residual ®ber cladding and the gold ®lm thickness if these parameters falls within 0±2 mm, and 55±75 nm, respectively. The average sensitivity of the spectral SPR ®ber optic sensor for aqueous environment was calculated to be 3150 nm/RIU (Fig. 5). It represents a decrease by a factor of 2.5 compared to the SPR sensing structure without the overlayer. 4. Experimental The ®ber optic SPR sensing element was fabricated using a single-mode optical ®ber with the cut-off wavelength of 724 nm (SM800, Fibercore, Ltd., UK). The ®ber was sidepolished to the proximity of the ®ber core in order to attain d0 500 nm. Thickness of the residual cladding layer was measured using the liquid drop method [11]. Then, the ®ber was coated by a gold ®lm (thickness of 65 nm) and a thin tantalum pentoxide overlayer (thickness of 19 nm) by vacuum evaporation. The con®guration of the developed SPR ®ber optic sensor system is shown in Fig. 6. Light from a pigtailed superluminescent diode (SLD) (SLD-371, Superlum, Ltd., Russia) with the power of 0.9 mW in a singlemode ®ber, full width at a half of maximum (FWHM) of 72 nm, the central wavelength at 816 nm, and the degree of coherence of about 60% was chosen as a source of polychromatic light. A Lyot depolarizer [12] was made up of two pieces of Bow-Tie HiBi ®ber (HB800, Fibercore, Ltd., UK) with lengths of its segments 1.3 and 2.7 m. After depolarizing
R. SlavõÂk et al. / Sensors and Actuators B 74 (2001) 106±111
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Fig. 6. Setup of the built fiber optic SPR sensor.
light from the SLD using the Lyot depolarizer, the residual degree of polarization did not exceed 1%. The depolarized light was coupled into the ®ber optic SPR sensing element. A piece of multimode optical ®ber with core diameter of 400 mm wound 10 times around a mandrel of a diameter of 1 cm was employed to scramble polarization at the input of a spectrograph (MSC-501, Carl Zeiss, FRG) to suppress the effect of polarization sensitivity of the spectrograph. The sensor components were joined by ®ber ®nger-splices (Thorlabs, Inc.). Transmitted spectra were acquired by the spectrograph and normalized with respect to the spectrum corresponding to sensor response when no sample was present (sample with refractive index of 1 was used to account for uneven spectral emission of the SLD). The normalized spectra (Fig. 7) were ®tted by the Lorentz function with a linear term, which compensate for slight non-symmetry of SPR dips, and the minimum of the Lorentz function was recorded. ATe¯on ¯ow cell was attached to the sensing element to contain a measured liquid sample during experiments. The ¯ow cell (volume 40 ml) was interfaced with Te¯on tubing (inner diameter, 0.5 mm); the ¯ow rate was induced and regulated (within 0±0.2 ml/min) by a level difference between reservoir and waste. We have examined the in¯uence of residual polarization of the ®ber mode on the performance of the sensor under conditions of temporal ®ber deformations (e.g. ®ber bending and twisting), see Fig. 8. Fiber optic polarization controllers were attached to both input and output ®bers of the sensing element, and the sensor response was measured while changing the polarization state of light on both sides of the sensing element (the steps in Fig. 8 correspond to
Fig. 7. Measured spectral transmission of the sensor element (ticks) and corresponding data fits (lines) for samples with two different refractive indices 1.3291 (squares) and 1.3351 (circles).
changes in position of the polarization controllers). As follows from Fig. 8, residually polarized part of the ®ber mode, which is in¯uenced by ®ber deformations, considerably increases uncertainty in the determination of the resonant wavelength. Upon this fact, two modes of operation of the sensor were proposed. In the static mode of operation the sensor is held in place without movement, suppressing thus effects of incomplete light depolarization, while in the dynamic mode of operation the movement of the sensor element is allowed. In the static mode of operation, the sensor resolution is primarily determined by the noise of the source and the spectrograph, yielding standard deviation of the temporal resonant wavelength about 1:5 10ÿ3 nm (Fig. 8). In the dynamic mode of operation, the uncertainty in the SPR wavelength is almost two orders of magnitude larger Ð 9 10ÿ2 nm (Fig. 8). 4.1. Fiber optic SPR sensor for refractometry To asses sensor's potential as a refractometer, we ¯owed mixtures of diethyleneglycol and water of known refractive indices across the surface of the sensing element and recorded the sensor response, Fig. 9 (refractive indices of samples are given at the wavelength of 800 nm). In the performed refractometric experiment, the sensor was found to exhibit good stability (baseline drift less than 0.02 nm/h) and repeatability (within 1%). The achieved sensor sensitivity was 3100 nm/RIU. It corresponds to the sensor resolution better than 3 10ÿ5 RIU for the dynamic mode of operation and better than 5 10ÿ7 RIU for the static mode of operation.
Fig. 8. Temporal sensor response. The steps correspond to changes in the fiber mode polarization state.
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with 0.2 wt.% glutaraldehyde in CB for 30 min to crosslink the antibodies. DS was washed out of the crosslinked antibody network with PBS [13]. The biosensing experiment consisted of detection of IgG molecules of increasing concentrations dissolved in the PBS containing 1% of BSA. Corresponding concentration isotherm is shown in Fig. 10. Interpolating the concentration isotherm in Fig. 10 and considering the above mentioned sensor resolution ®gures, it can be concluded that the ultimate sensor resolution is about 1 and 90 ng/ml for the static and dynamic mode of operation, respectively.
Fig. 9. Temporal sensor response to variations in the refractive index of sample.
4.2. Fiber optic SPR sensor for biosensing In order to demonstrate ability of the developed ®ber optic SPR sensor's to act as real-time biosensor, we performed detection of IgG proteins. The used reagents were as follows. Human immunoglobulin (IgG), monoclonal antibody against IgG (a-IgG) were obtained from Seva Immuno, Prague; dextran sulfate sodium salt (DS), average molecular weight of approximately 5000, and bovine serum albumin (BSA), purity better than 99% by agrose electrophoresis, were purchased from Sigma. Glutaraldehyde (GA) was freshly vacuum-distilled at 20 kPa under nitrogen into water to form a stock solution containing 20% of GA. Solutions were prepared using citrate buffer (CB), 0.1 M, pH of 3.96 and phosphate buffered saline (PBS), pH of 7.26. Molecular weights of the used reagents were as follows: 150,000 for IgG and a-IgG, and 65,000 for BSA. First, we functionalized the sensor surface with a doublelayer of a-IgG molecules. Assemblies consisting of alternating molecular layers of an antibody (a-IgG) and DS were formed by alternating adsorption from the antibody and DS solutions. The solutions were used in the following order: CB, antibody in CB (100 mg/ml), CB, DS in CB (1 mg/ml), CB, antibody in CB, CB. Then the assembly was incubated
5. Discussion Observed sensor performance agrees well with the performed simulations in terms of position, depth, and width of SPR dips (Figs. 3±5 and 7±9). The residual polarization of the ®ber mode (below 1%) was found to cause the sensor resolution to drop by almost two orders of magnitude Ð from 5 10ÿ7 to 3 10ÿ5 RIU. Lyot depolarizers with slightly better performance, which are now becoming available, are expected to improve resolution of the sensor operating in the dynamic mode. In performed model biodetection experiment, the lowest detected IgG concentration was 40 ng/ml. The experimentally achieved sensor resolution of 1:5 10ÿ3 nm suggests that the ultimate measurable IgG concentration may be considerably lower (up to 1 ng/ml). 6. Conclusions We have reported a new approach to the development of ®ber optic SPR sensing devices based on spectral interrogation of SPR in a ®ber optic sensing element using depolarized light. Sensor sensitivity of 3100 nm/RIU and resolution as high as 5 10ÿ7 RIU were demonstrated. The attained resolution is comparable with the resolution of best bulkoptic table-top SPR devices. We have demonstrated suitability of the developed SPR ®ber optic sensor for biosensing. In model biodetection experiment, the sensor has been able to detect IgG present in concentration as low as 40 ng/ ml. The sensor may be further miniaturized by cutting the ®ber right after the interaction region and by forming a mirror on the end face of the ®ber [14]. In conjunction with biospeci®c coatings for detection of various biomolecular analytes, this approach may provide robust biosensor technology for on site analysis in many areas such as medicine, environmental monitoring, and biotechnology. Acknowledgements
Fig. 10. SPR biosensor-based detection of IgG. Sensor response as a function of concentration of IgG in PBS.
This work has been supported by the Grant Agency of the Czech Republic under the contracts 102/99/0549 and
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102/00/1536. Authors acknowledge Dr. V. Malina (IREE, Prague, Czech Republic) for thin ®lm deposition, and Dr. W. Ecke and Mrs. Kerstin Schroeder (IPHT, Jena, FRG) for the fabrication of Lyot depolarizer and useful discussions. References [1] R.D. Harris, J.S. Wilkinson, Waveguide surface plasmon resonance sensors, Sens. Actuators B 29 (1995) 261±267. [2] R.C. Jorgenson, S.S. Yee, A fiber-optic chemical sensor based on surface plasmon resonance, Sens. Actuators B 12 (1993) 213±220. [3] A. Trouillet, C. Ronot-Trioli, C. Veillas, H. Gagnaire, Chemical sensing by surface plasmon resonance in a multimode optical fibre, Pure Appl. Opt. 5 (1996) 227±237. [4] J. Homola, R. SlavõÂk, Fibre-optic sensor based on surface plasmon resonance, Electron. Lett. 32 (1996) 480±482. [5] A.J.C. Tubb, F.P. Payne, R.B. Millington, C.R. Lowe, Single-mode optical fiber surface plasma wave chemical sensor, Sens. Actuators B 41 (1997) 71±79.
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