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Surface Plasmon Resonance Optical Sensing for Simultaneous Multi-Channel Biological Monitoring
Wei Peng*a, Soame Banerjib, Yoon-Chang Kimb and Karl S. Bookshb
aSchool of Physics and Optoelectronic Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, China 116024
bDepartment of Chemistry and Biochemistry, University of Delaware, 106 Brownlab, Newark, Delaware, US 19716
ABSTRACT
This paper presents a multi-angle optical analysis device based on surface plasmon resonance (SPR) for simultaneous biological sample detection. Most applications of optical SPR devices were designed to measure refractive index (RI) of liquid or gas sample by measuring the signal wavelength or angle, the sensitivity and stability of which is easily affected by the fluctuation of various interior or exterior test conditions. In this study, we have proposed a multi-channel optical SPR device which can monitor SPR changes at different wavelengths and angles. The preliminary experimental results demonstrate the characteristic responses of SPR signals from different angles changes independently correspond to the refraction index changes of the liquid samples with which they are in contact. The experimental results confirmed that a practicable high-sensitivity multi-channel SPR biological measurement instrument could be completed with further developments.
Keywords: Surface plasmon resonance, image sensing, refractive index, spectroscopy, multiple channels, biological test
1. INTRODUCTION
Optical surface plasmon resonance (SPR) spectroscopy has been employed for quantitative and qualitative analysis in analytical chemistry, biochemistry, physics and engineering applications. The technique of SPR is based on an electromagnetic phenomenon that is capable of monitoring RI changes of 10-5 and 10-6 within 200 nm of the sensing surface. SPR spectroscopy is becoming increasingly popular for monitoring the growth of thin organic films deposited on the sensing layer. As little as 0.01nm of average film thickness change can be detected when RI difference between the film and bulk solution is 0.1RI unit. Thus a submonolayer of adsorbed protein-like substance from an aqueous solution can easily be observed. 1,2
The physics principle of the SPR phenomenon has been extensively described. To exploit the SPR effect for measurement application, two main types of the SPR sensors have been employed commonly: constant-angle SPR (i.e., information is in the spectral shift) and constant-wavelength SPR (i.e., information is in the observed angular shift). In the prism-based SPR sensor, the surface plasmon wave on the metal coated prism can be excited by light with a modulated angle of incidence. Otherwise, the reflected intensity can be measured by wavelength modulation while the incident angle is kept fixed. Therefore the prism-based SPR sensor system can be used either as a spectral or angular SPR sensor. The prism based SPR sensors are not, however, optimal for in situ industrial or environmental process monitoring. The sensing region is by necessity more bulky than a single fiber optic probe because of the rigid geometry of the sensor head. The use of optical fibers for SPR sensing has recently attracted much attentionbecause the optical fiber sensors have several distinct advantages over the prism-based sensor 1, 3-7 . The fiber-optic sensors are fundamentally simple in structure, less costly, require smaller sample volume, and amenable towards remote sensing applications. However, in the traditional fiber-based SPR sensor system, there is not a fixed incident angle but a range of angles that are allowed to propagate in the multimode fiber probe. The incident light consists of a wide range of wavelengths launched into the fiber in order to excite the SPW across a range of angles defined by the numerical aperture of the fiber optic waveguide. Like the prism based SPR sensors, the SPR coupling wavelength can be tuned
* [email protected]; phone 86 411 8470-6693; fax 86 411 8470-9304; www.dlut.edu.cn
2009 International Conference on Optical Instruments and Technology: Optical Systems and ModernOptoelectronic Instruments, edited by Yongtian Wang, Yunlong Sheng, Kimio Tatsuno, Proc. of SPIE Vol. 7506,
750613 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.837291
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selectively by modifying the angle of incidence between the photon and the sensing area; with the fiber optic sensors this is accomplished by modifying the geometry of the probe tip.1, 5, 8 The fiber optic platform allows the sensor to be easily placed into the sample of interest, and the orientation of the sensing region requires a minimal sample volume for the sensor to be employed as a fiber-optic ‘dip probe’.5
In the past decades, optical surface plasmon resonance (SPR) has been widely investigated for chemical and biological sensing applications because of its capability for real-time measurement with high sensitivity and label free quantization of biochemical compounds1, 2. Compared to conventional Kretchman prism-based SPR sensing devices 3-5, fiber optic SPR sensors have recently drawn considerable attention because of the fundamentally simple structure, low cost, small sample volume, and amenability towards remote sensing applications 6-10.
Multimode fiber optic SPR theory has been extensively described 8. With the fiber optic sensors, the cladding and buffer are removed from a small section of the fiber to expose the core. This can be accomplished by mechanical or chemical stripping along the side of the fiber to make a flat tipped sensor where the sensing area is on the side of the fiber optic 6, 7. Alternately a fiber optic SPR probe can be constructed by polishing through the cladding at buffer at a predetermined pair of complementary angles to make a beveled tip sensor where the sensing area is at the probe tip 8-10. In either case approximately 5 nm chromium and 50 nm gold are deposited on the sensing region to yield a SPR reflectance spectrum. Also, a gold mirror is sputtered on the fiber tip to ensure that the light returns up the optical probe to the detector. In contrast to prism based SPR sensors, fiber optic SPR sensors can only function in the ‘constant angle, multiple wavelength’ regime. Hence low numerical aperture fibers are employed to limit the range of angles propagating along the fiber. The multimode fiber optic geometry, however, allows the sensors to be employed as a compact ‘dip probe’ for in-situ analyses.
Currently, most fiber optic SPR devices are used for single chemical parameter detection. As the requirements of high-sensitivity, multi-analyte detection increase for biological and environment monitoring, multi-channel SPR sensors have attracted more and more attention. G. G. Nenninger, et al 11 reported a lightpipe configuration SPR biosensor with two sensor surfaces for dual-channel monitoring of antibody-antigen binding, a second channel is used to compensate changes in the refractive index of the bulk solution caused by analyte concentration or temperature difference; C. E. H. Berger, et al 12 presented a prism-based SPR device containing multiple sensing channels with recognition elements for specific analyte in liquid phase; M. J. O’Brien II, et al 13 also demonstrated a prism-based SPR biosensor that can simultaneously remove thermal and bulk-composition effects. All of these reported SPR sensor devices are based on bulk optical devices and need flow cells for liquid analyses which are not suitable for in situ remote sensing in biological environment like fiber optic SPR sensors. We reported two fiber optic sensor devices for dual channel SPR monitoring. As far as we are aware, there are seldom reports about multi-channel SPR sensor for biological multi-channel measurement.
We will report the experimental investigation of a multi-angle SPR monitoring device in this paper. Demonstrated here is the ability of multiple channels on a single Kretchman Kretchman prism to independently respond to changes in local refractive index. This device is similar to the reported the conventional Kretchman prism SPR device that can be used for both vapor and liquid analyzes14. However, the main difference is that the new design is aimed at multichannel measurements in one medium where bulk refractive index compensation is required or where multiple analytes are to be monitored. This geometry would be useful for chemical, biochemical and biomedical analyses where SPR sensors have been readily employed. 7-9, 15-17
2. OPERATIONAL PRINCIPLE AND EXPERIMENT RESULTS
The basic design and manufacture of prism-based optical SPR probes and the fiber optic SPR sensor system has been described previously.7 Figure 1 illustrate a modified Kretschmann prism based SPR configuration that used in this study. The broad band light from a white LED is collimated and incident with an adjustable angle range to a 70o SF10 prism face. A SF10 glass slide with the sputtered gold and polymer is placed on top of the prism with index matching fluid to couple with the prism. The reflected light is collected by another collimated lens to focus at the entrance slit of a Jobin Yvon iHR320 imaging spectrometer (HORIBA Jobin Yvon Inc) with a 300/mm grating that provide 275nm of the spectral coverage, the dispersed light was collected by a 1024 × 256 pixel Jobin Yvon Symphony CCD camera with a resolution as 0.269nm. A specific lab-made flow cell is attached with the spurted glass slide which allows different solutions to be monitored by contact the gold film directly. The system employed for this study deviates from previous
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SPR setup in using the focus lens to generate broad band whitelight with a adjustable incident angle range, SF10 prism glass slide is used as a SPR sensing chips to achieve SPR signal with large incident angles.
Figure 1. Schematic of multi-channel optical surface plasma resonance system
The proposed SPR setup was tested by detect water solution. The characteristic SPR spectra for aqueous samples were obtained by normalizing the spectra to an air reference spectrum collected prior to analyses for the sensor as shown in Figure 2.
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Figure 3 shows that the SPR sensing area on the same glass slide has 256 SPR channel in angle dimension that distributed around the whole chips. Figure 3(a) shows the water SPR detected by this device. A series SPR curve at different wavelengths and incident angles can be achieved by processing this SPR image as shown in Figure 3(b). Figure 4(a) illustrates some selected SPR curves from the Jobin Yvon 1024 × 256 CCD chip from row 70 to 130 increased by 10, the enlarged SPR curves are shown in Figure 4(b). Each SPR curve could be used as a independent SPR channel for RI sensing.
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Figure 4. Selected surface plasma resonance spectrum and enlarged SPR dips (a) Selected SPR spectra; (b) Enlarged SPR dips
A series refractive index standard is used to demonstrate the ability of multiple channels in this SPR device to track RI changes across a wide range of solutions and experimental conditions. The standards employed included glucose solution whose RI range from 1.3305 to 1.3412. The minima of each spectrum (λR) was determined by least-squares fit of a second order polynomial about each localized resonance. Compare to Figure 2 to 4 for pure water SPR, when a glucose solution (0.05mol) with refractive index as 1.335 was flown in the flow cell, the SPR image shift to long wavelength direction as shown in Figure 5.
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Figure 5. Shifts of selected surface plasma resonance sensing channels and the Enlargements of their SPR dips
Figure 6 illustrate the SPR wavelength changes of selected Channel 110 as the refractive index changes in the glucose solution of different concentration. As shown in Figure 7, plotting the calculated λR for selected sensing channels from 10 to 256 with 40 separation yield calibration curves separately that are approximately linear across the span of RIs tested.
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Figure 6. Surface plasma resonance wavelength changes with the RI changes in the glucose solution This multi-channel SPR setup is also used to monitoring the refractive changes during polymer growth. Polyallylamine hydrochloride is used as a coating material because its good water solubility and low toxicity. 18 Using the flow cell, the gold-coated surface of SPF10 slide is kept in a 50mM solution of Dithio-bis-succinimidyl-propionate (DSP) in Dimethyl Sulfoxide (DMSO) for 30 minutes and then flow in a solution of 10mM polyallylamine hydrochloride and
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10mM sodium hydroxide in water with epichlorohydrin added as a crosslinker which will make a polymer layer on the sensing surface. The SPR images from SF10 slide are acquired continuously by Jobin Yvon imaging spectrometer. Figure 8 shows is the SPR wavelengths changes as the polymer growth from different channel of CCD chip.
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3. CONCLUSION In summary, we have demonstrated a multi-angle SPR device which can detect the refractive index change simultaneously in multiple channels. This primary experimental investigation make us to better understanding thin-film changes by monitoring the real affected RI part, also if combing with other fiber optic SPR sensor, it is possible to quickly select optimal taper angle and wavelength range for tapered fiber-optic sensors. More investigations are under going to achieve better SPR signal from both two dimensions. Further investigation is in process to develop this sensor as a practicable self-compensating SPR system for biological monitoring.
ACKNOWLEDGEMENT The authors would like thank the joint supports from The National Natural Science Foundation of China (NSFC) under a project number 60977055, Dalian University of Technology, China (Program No. 893344) and University of Delaware, United State.
REFERENCES
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