A compact surface plasmon resonance and surface-enhanced Raman scattering sensing device

Transcription

1 A compact surface plasmon resonance and surface-enhanced Raman scattering sensing device J. N. Yih a, S.-J. Chen b,*, K. T. Huang a, Y. T. Su c, and G. Y. Lin c a Institute of Optical Sciences, National Central University, Chung-Li 320, Taiwan b Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan c Department of Mechanical Engineering, National Central University, Chung-Li 320, Taiwan ABSTRACT This paper presents an optical device capable of the simultaneous measurement of the surface plasmon resonance (SPR) spectrum, which provides information regarding the change in the dielectric constant of the binding analytes, and the surface-enhanced Raman scattering (SERS) spectrum, which yields analytical data regarding the structural changes of the analytes. SPR sensing is an established technology in the field of direct real-time analysis of biomolecular interactions such as antibodies/antigens, DNA hybridization, receptors/ligands, etc. Meanwhile, SERS sensing techniques represent a powerful means of acquiring and diagnosing structural information relating to analyte binding. This study adopts the attenuated total reflection (ATR) method and an Au nanocluster-embedded dielectric sensing film in developing a biosensor which integrates the SPR and SERS sensing techniques. The results confirm the effectiveness of the proposed multi-functional device in developing a detailed understanding of the mechanisms of biomolecular recognition. Keywords: Surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS), biomolecular recognition, nanoparticle. 1. INTRODUCTION In the post-genomic era, one of the greatest challenges facing the biochemistry community is to develop mechanisms for the complete recognition of proteome interactions. 1-3 Surface plasmon resonance (SPR) sensors are widely applied in the investigation of biomolecular interactions, and possess the fundamental advantage that extrinsic labeling of the analytes is not required. 5-7 In the SPR technique, tiny changes in the refractive index of the biologically recognized molecules or materials adjacent to the noble metal film are recorded and are taken as indications of the biological interactions taking place at the interface. The detection limitations of SPR biosensors generally restrict their application to investigating the binding of high molecular weight analytes (> 500 Da) to an immobilized ligand. However, some researchers have developed enhanced SPR biosensors featuring metal nanoparticles or Au nanoclusters embedded in the dielectric film and have demonstrated that these devices possess an ultrahigh sensitivity. 8 Some researchers have attempted to utilize the Raman scattering spectrometer to investigate the evolution of molecular structures by examining the corresponding variations of the Raman scattering light intensity and frequency However, Raman scattering has the extremely small scattering cross section (typically of cm 2 /molecule), which is 12~14 orders of magnitude below fluorescence cross sections. Therefore, the sensitivity of the Raman spectroscopy technique is insufficient for the precise characterization of the dynamic processes of monolayer adsorption. One potential means of enhancing the sensitivity of this technique is to apply a novel scattering process based on the excitation of both the surface plasmons (SPs) and the particle plasmons (PPs). If a Raman active molecule is attached to a thin metal surface, it is acted upon by the enhanced electromagnetic (EM) field induced by the excitation of SPs and PPs. Hence, the molecule is excited to higher vibrational or rotational states, and its ejected light wave is enhanced as a result. Therefore, the corresponding Raman signal is also greatly enhanced. This phenomenon is referred to as surface-enhanced Raman scattering (SERS) It has been shown that SERS devices using localized SPs (LSPs) on a roughened metal surface or PPs on metal nanoparticles deliver a significant enhancement of It is noted that this degree of enhancement is far greater than that achieved when using SPs on a smooth metal film. SERS enables the Raman scattering technique to detect single molecules adsorbed on a metal surface. The band characteristics (i.e. * sheanjen@mail.ncku.edu.tw; Tel: ext ; Fax:

2 intensity and frequency) of the Raman shift spectrum are of a very high quality and contain detailed information regarding the protein composition and conformation and about the post-translational modification. SPs are generally excited using a grating or a prism under an attenuated total reflection (ATR) method which increases the wave vector of the incident light such that it matches that of the SPs. Conversely, PPs can simply be excited via the direct irradiation of light, as in the conventional SERS method. Determining the optimum conditions which deliver the greatest enhancement in sensitivity (i.e. single molecule sensitivity) requires the detailed study of SERS and nanoparticle-enhanced SPR devices. Some researchers have proposed ATR-SPs Raman spectroscopy with metal nanoparticle enhancements or with scanning near-field optical microscopy (SNOM) The present study employs the ATR method and considers nanoparticle enhancement of both the SPR and the SERS devices in developing an integrating multi-functional biosensor capable of providing an understanding of the mechanisms of biomolecular recognition. The proposed ATR-SPR/SERS technique provides a kinetic, direct, flexible, and sensitive approach for the in situ biomolecular recognition of analytes dissolved in an aqueous solution as they interact with immobilized ligands on the sensing interfaces. The ATR-SPR/SERS technique involves the collection of the SPR radiation spectrum from the interface between the aqueous solution and a prism. This spectrum is generated as the evanescent wave exits the prism and penetrates into the aqueous solution, hence exciting the SPs on the metal film and the PPs on the nanoparticles. This study describes the characteristics of an ATR-SPR/SERS device which employs SPs and PPs on a smooth metal layer and on a metal nanoparticle layer, respectively. The experimental setup is fully described and the results are then discussed. 2. EXPERIMENTAL SET UP 2.1. SPR & SERS Optical Device The current ATR-SPR/SERS experimental configuration is illustrated in Figure 1. As can be seen, a focused p-polarized light wave from a nm wavelength He-Ne laser passes through a coupled SF-11 prism and then incidents directly upon a thin gold film and a gold nanoparticle-embedded layer. The gold film has a thickness of 30 nm and is coated on the SF-11 slice (n = nm) via a sputtering deposition process. Prior to the sputtering process, the SF-11 slice is pre-coated with a 2 nm chromium layer in order to improve the adhesion of the gold film. The gold nanoparticle-embedded layer is fabricated using the co-sputtering method described in Section 2.2 below. A matching oil with the same refractive index n = is introduced into the gap between the slice and the prism in order to guide the incident light upon the interface between the slice and the gold film. The resulting SPR spectrum is detected by a linear array CCD camera with 3648 pixels. The spectrum is capable of yielding precise analytical data regarding variations in the refractive index and thickness of the binding analytes. Previous researchers have integrated the Kretschmann ATR method and the SERS technique to investigate the EM field effect for benzoic acid (C 6 H 5 COOH). In the present study, the relationship between the SERS activities and the excitations of the SPs and PPs are studied by simultaneously measuring the SPR reflectivity spectrum and the SERS spectrum. In conducting each measurement, the incident laser beam is carefully orientated to the SPR angle, namely the angular position at which the intensity of the SPR reflectivity spectrum is minimized. A solid-state laser at a wavelength of 532 nm or a He-Ne laser at a wavelength of nm is used as the excitation light source, and the SERS is collected Incident light to excite SPR SPR spectrum Prism Au thin film ~ 30 nm Au nanopatricles Collecting lens SERS spectrum Figure 1. ATR-SPR/SERS optical set up with gold nanoparticle enhancement.

3 Spectrometer Data Analyzer Flow System Linear CCD Camera Laser Excitation USB 2.0 Figure 2. Photograph of ATR-SPR/SERS device. through a convergent lens with a focal length of 300 mm. The background scattering signal is eliminated by means of a holographic notch filter, and the Raman spectra are then analyzed with a CVI SM spectrometer or a B&W portable spectrometer. Figure 2 presents a photograph of the current ATR-SPR/SERS device. 2.2 Fabrication The SPR/SERS sensing film comprises a thin gold thin film layer with a thickness of approximately 30 nm and a gold nanocluster-embedded layer. In the current fabrication method, the Au nanoclusters were embedded in the Au dielectric film via a sputtering process. A thin Au layer of thickness 30 nm was initially deposited on the surface of a glass slide using a DC magnetron sputtering machine (Plasma Sciences Inc. Lorton, VA). Subsequently, mixed thin films composed of SiO 2 and Au nanoparticles were deposited via an RF and DC magnetron co-sputtering process. During this sputtering process, the Au and SiO 2 targets were arranged in a co-sputtering chamber which had previously been degassed to vacuum conditions (< torr.) and then refilled with Ar to a pressure of torr. Transmission electron microscopy (TEM) was employed to analyze the surface morphology and the interparticle spacing of the fabricated Au:SiO 2 films. A typical TEM observation reveals that the Au particle size is approximately 4.0 nm and that the spacing between each particle is approximately 6.0 nm. The present co-sputtering technique provides a feasible and reliable method to control the optimal size and volume fraction of the Au nanoclusters embedded in the dielectric film without the need for complex strategies to immobilize the Au particles via specific biological interactions. Finally, a drop of a solution comprising benzoic acid dissolved in ethanol (0.15 mg/ml) was spotted onto the sensing film, which was then spun at 1000 rpm for a minimum of 20 secs. to reduce the accumulation of benzoic-acid molecules and to obtain a better homogeneity over the film. 3. RESULTS Figure 3 shows the SPR reflectivity curves as a function of the angle of incidence for three different nanoparticle sizes and volume fractions. The dashed line denotes the conventional SPR, the broken-solid line corresponds to the nanoparticle-enhanced SPR with the diameter of 2.7 nm, the solid line indicates the nanoparticle-enhanced SPR with 3.5 nm, and the double-dashed line shows the nanoparticle-enhanced SPR 4.1 nm. It can be seen that the conventional SPR biosensor has a full-width of half maximum of approximately 3 degrees. The nanoparticle-enhanced SPR biosensor broadens the reflectivity spectrum since the momentum of the photons readily causes the particle plasmons to resonate. Therefore, the photons can excite the surface plasmon wave over a wider incident angle. The EM field is enhanced by a factor of between 100 and 1,000, and hence increases the sensitivity of the biosensor. Since it is difficult to identify the minimum of the broadened reflectivity spectrum when the background signal is amplified, the resolution of the SPR biosensor deteriorates. Therefore, the optimal sizes and volume concentrations of the Au or Ag nanoparticles

4 Reflectivity Incident angle (deg) Figure 3. SPR reflectivity curves as a function of angle of incidence for three different nanoparticle sizes and volume fractions. Dashed line: conventional SPR, broken-solid line: nanoparticle-enhanced SPR with 2.7 nm, solid line: nanoparticle-enhanced SPR with 3.5 nm, and double-dashed line with 4.1 nm. are very important factors when fabricating ultrahigh-resolution SPR biosensors. However, there are significant difficulties involved in utilizing SPR or SERS with PPs or LSPs as an analytical tool because the detailed relationships between the shape and size of each colloidal particle and the corresponding enhancement in SERS are yet to be fully established. SERS analysis of the samples is performed by focusing the collecting lens on a sample area of diameter approximately 25 µm. Figure 4 presents a typical Raman spectrum of benzoic acid generated using a silver sensing film and an excitation wavelength of 532 nm. The Raman-active vibrations of benzoic acid are clearly identifiable Intensity (A.U.) Raman shift (cm -1 ) Figure 4. SERS spectrum of benzoic acid excited at 532 nm.

5 4. CONCULSIONS This paper presents an optical device based on the ATR method capable of the simultaneous measurement of the SPR spectrum, which provides information regarding the change in the dielectric constant of the binding analytes, and the SERS spectrum, which yields analytical data regarding the structural changes of the analytes. This study also employs SPs and PPs on a smooth metal layer and on a metal nanoparticle layer in developing a biosensor which integrates the SPR and SERS sensing techniques. Therefore, the results confirm the effectiveness of the proposed ATR-SPR/SERS device in developing a detailed understanding of the mechanisms of biomolecular recognition. ACKNOWLEDGEMENTS The current authors wish to express their gratitude to the National Science Council, Taiwan for the financial support provided to this study under Grant No. NSC E REFERENCES 1. Y. Jin, Y. Shao, and S. Dong, Direct electrochemistry and surface plasmon resonance characterization of alternate layer-by layer self-assembled DNA-myoglobin thin films on chemically modified gold surfaces, Langmuir, 19 (2003) Y. Iwasaki, T. Horiuchi, and O. Niwa, Detection of electrochemical enzymatic reactions by surface plasmon resonance measurement, Anal. Chem. 73 (2001) J.M. McDonnell, Surface plasmon resonance: towards an understanding of the mechanisms of biological molecular recognition, Current Opinion in Chemical Biology 5, , J. Homola and S.S. Yee, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) H. Sota, Y. Hasegawa, and M. Iwakura, Detection of conformational changes in an immobilized protein using surface plasmon resonance, Anal. Chem. 70 (1998) J.E. Gestwicki, H.V. Hsieh, and J.B. Pitner, Using receptor conformational change to detect low molecular weight analytes by surface plasmon resonance, Anal. Chem. 73 (2001) C. Williams and T.A. Addona, The integration of SPR biosensors with mass spectrometry: possible applications for peoteome analysis, Trends Biotechnol 18 (2000) W.P. Hu, S.-J. Chen, K.-T. Huang, J.H. Hsu, W.Y. Chen, G.L. Chang, and K.-A. Lai, A novel ultrahigh-resolution surface plasmon resonance biosensor with an Au nanocluster-embedded dielectric film, Biosens. Bioelectron. (2004) to appear. 9. K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, and I. Itzkan, Single molecule detection using surface-enhanced Raman scattering (SERS), Phys. Rev. Lett. 78:9 (1997) A.D. Dayan and A.J. Paine, Mechanism of chromium toxicity, carcinogenicity and allergenicity: review of the literature from 1985 to 2000, Hum. Exp. Toxicol. 20 (2001) B. Hecht, H. Bielefeldt, L. Novotny, Y. Inouye, and D. W. Pohl, Local Excitation, Scattering, and Interference of Surface Plasmons, Phys. Rev. Lett. 77, 9, (1996) M.C. Chen, S.D. Tsai, M.R. Chen, S.Y. Ou, W.-H. Li, and K.C. Lee, Effect of silver-nanoparticle aggregation on surface-enhanced Raman scattering from benzoic acid, Phys. Rev. B. 51:7 (1995) T. Tanaka, S. Naga, and H. Ogawa, Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Spectroscopy of Functional Groups of Humic Acid Dissolving in Aqueous Solution, Anal. Sci. 17 suppl., (2001) i1081-i D. Zerulla, G. Isfort, M. Kölbach, A. Otto, and K. Schierbaum, Sensing molecular properties by ATR-SPP Raman spectroscopy on electrochemically structured sensor chips, Electrochim. Acta 48 (2003) M. Futamata, Highly-sensitive ATR Raman spectroscopy using Surface-Plasmon-Polariton, Int. J. Vibrational Spectrosc. 4 (2000) 9-26.