WHITE-LIGHT SPECTRAL INTERFEROMETRY FOR SURFACE PLASMON RESONANCE SENSOR APPLICATIONS

Transcription

1 A WHITE-LIGHT SPECTRAL INTERFEROMETRY FOR SURFACE PLASMON RESONANCE SENSOR APPLICATIONS NG SIU PANG DOCTOR OF PHILOSOPHY CITY UNIVERSITY OF HONG KONG SEPTEMBER 2010 A

2 B CITY UNIVERSITY OF HONG KONG 香港城市大學 White-light Spectral Interferometry for Surface Plasmon Resonance Sensor Applications 白光光譜干涉的表面等離子體共振傳感器應用 Submitted to Department of Physics and Materials Science 物理及材料科學系 In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 哲學博士學位 by NG SIU PANG 吳兆鵬 September 2010 二零一零年九月 B

3 i ABSTRACT With ever demanding sensing technology for life science exploration, biochemistry research, and environmental monitoring, surface plasmon resonance (SPR) sensor, which is intrinsically a label-free sensing technique, has prevailed as a promising alternative to laborious fluorescence dye-based monitoring schemes. SPR sensors are essentially governed by the photon-to-surface plasmon transformation at a metaldielectric interface subjected to certain criteria. The technique is extremely powerful in detecting refractive index unit (RIU) change across the sensing surface. Existing state-of-the-art SPR systems are well-known for their superior detection limit; however they must incorporate two crucial characteristics in order to compete favorably against dye-based methods: (1) extremely high sensitivity, (2) wide dynamic range of measurement. Although existing laser interferometric SPR systems are well-known for their superior phase sensitivity in detecting minuscule RIU change, their dynamic range are much limited due to the inherent bandwidth of the laser source upon a fixed angle of incidence in typical Kretschmann SPR configuration, which means that the system quickly saturates as molecules accumulated on the sensing surface and renders the system irresponsive to further RIU change. Even though such superior sensitivity can be adequate in certain circumstances, the narrow dynamic range deters its capability of monitoring complex reaction which is common in real life applications. Spectral SPR systems, on the other hand, utilizes a broadband of light to excite SPR on the interface, and provide much wider dynamic ranges than laser based i

4 ii configurations. However, existing spectral schemes are limited to collect only spectrum power information via a spectrometer. The SPR signal is typically registered as a spectral dip in the reflected spectrum, and the position of the dip correlates to the RIU of interest. However, the precise determination of the dip position can be difficult due to imperfection of the charge-coupled device (CCD) data acquisition subsystem. As dark current always dominates the signal when the spectral photon count is low, it happens to be the exact position where photon-to-surface plasmon transformation is at its maximum. Therefore the dip position is usually contaminated by the shot-noise, thus the limit of detection of spectral SPR sensor can be two orders of magnitude below its laser interferometric counterpart. In order to overcome the above weaknesses, a novel phase detecting white-light SPR sensor with extended phase measurement range has been proposed and developed. The new system is capable of acquiring both amplitude and phase information of the entire white-light spectrum undergoing SPR transformation at the metal-dielectric interface. Thus our system addresses two of the major limitations of existing SPR sensors i.e. (1) narrow dynamic range of laser interferometric SPR systems, (2) limited sensitivity of spectral dip locating SPR schemes. To evaluate the feasibility of the our system, it is necessary to include theoretical discussion of the following issues, (1) spectral amplitude and phase response of the reflected white-light spectrum upon RIU change across the sensing surface due to SPR effect; (2) amplitude and phase evaluation of the reflected spectrum via whitelight spectral interferometry; (3) signal processing methodologies essential to extract the spectral phase from the spectral interferometric SPR signal with the best accuracy. ii

5 iii In addition, numerical simulations have been included to support the theoretical development. As typical Kretschmann SPR configuration consists of sandwiched dielectric-metal-dielectric structure, its spectral phase response can be modeled via classical Fresnel s formula with the propagation matrix technique. On the other hand, second order correlations in the space-frequency domain and the spectral interference law governing the interference signal have been analyzed numerically. The combination of (1) Windows Fourier Transform, (2) Continuous Wavelet Transform and (3) Hilbert Transform serves as the foundation of evaluating the phase extraction methodologies. Finally, the experimental setup of our white-light spectral interferometric SPR system consisting of a differential Michelson interferometer has been presented. The verification of its extended phase measurement range and high sensitivity was achieved by monitoring the RIU change upon the injection of sodium chloride (NaCl) solutions of known concentrations. The stability issues that deter the limit of detection such as temperature drift and phase fluctuation were evaluated. The strategies to diminish these effects were elaborated. With successful implementation of these enhancement strategies, the limit of detection of the our system (in terms of RIU) was found to be at least two orders of magnitude better than conventional spectral dip locating systems, and it might even achieve ultimate sensitivity comparable to laser interferometric SPR setups, yet the dynamic range was two orders of magnitude wider than laser based systems. Therefore, the performance of the novel white-light SPR system supersedes the spectroscopy and laser schemes and the benefit for real life sensing application would be enormous. iii

6 v TABLE OF CONTENTS ABSTRACT... i ACKNOWLEDGEMENTS... iv TABLE OF CONTENTS... v LIST OF FIGURES... ix LIST OF TABLES... xv LIST OF SYMBOLS... xvi CHAPTER 1 INTRODUCTION Background Laser Interferometry for SPR Sensing Dynamic Range of SPR Sensing Scope and Objectives... 4 CHAPTER 2 LITERATURE REVIEW History of Surface Plasmon Resonance Sensing Angular Measurement Spectral Measurement Phase Measurement Comparison of Detection Limits Laser Interferometry for SPR Phase Measurement Mach-Zehnder SPR Interferometer v

7 vi Michelson SPR Interferometer Fabry-Pérot SPR Interferometer Polarization SPR Interferometer Phase Evaluation Techniques for Optical Interferometry Fourier Transform Method Windowed Fourier Transform and Wavelet Analysis Hilbert Transform Applications of SPR Sensor Environmental Monitoring Food Safety Medical Diagnostics Pressure and gases sensing CHAPTER 3 THEORETICAL ANALYSIS & NUMERICAL SIMULATION Physics of Surface Plasmon Resonance The Evanescent Wave Excitation of Surface Plasmon Wave Fresnel Formulas and the Transmission Matrix Technique Results & Discussion of SPR Simulation Optimization of the Incident Angle Optimization of Gold Thickness Wavelength Specific Phase-shift and Localized Ultimate Sensitivity 80 vi

8 vii 3.2 White-light Spectral Interferometry Second-order Correlation in Space-Frequency Domain Spectral-domain Interference Law Results & Discussion of Spectral Interferometric Simulation Effects of 2L air and t ef Effect of white noise Spectral phase extraction via WFT, CWT, and HT SPR effect on white-light spectral interference CHAPTER 4 EXPERIMENTAL WORK Optical platform Selection of components Experimental parameters Optical alignment Preparation of analytes Sodium chloride concentration by weight percentage Bovine serum albumin (BSA) Experimental procedures CHAPTER 5 RESULTS & DISCUSSION System stability analysis Sodium chloride solutions Determination of ultimate sensitivity vii

9 viii Calculation of localized ultimate sensitivity Influence of the incident angle Real-time monitoring of refractive index change Reconstruction of the spectral reflectance dip Biosensing of BSA-anti-BSA interaction CHAPTER 6 CONCLUSIONS & FUTURE WORK Conclusions Future work REFERENCE APPENDIX A1. List of publications Related to white-light SPR spectral interferometer Related to optics & shearography A2. Specification of warm-white light emitting diode (LED) viii