A PLASMONIC LIQUID WAVEGUIDE SENSOR USING NANOPARTICLES FOR LABEL-FREE MEASUREMENT APPLICATIONS H. J. Huang1, D. P. Tsai2 and A. Q. Liu1† 1School of Electrical and Electronic Engineering Nanyang Technological University, Singapore 639798 2 Department of Physics, National Taiwan University, Taipei, Taiwan (†Corresponding author. Tel: +65-6790 4336; Email:
[email protected]) ABSTRACT A plasmonic liquid-liquid (PLL) waveguide using Au nano-particles (NPs) colloidal solution is configured on a micro-optical-fluidic-system (MOFS) chip [1] for the interest extensions of surface plasmon resonance (SPR) assisted measurements. For characterizing the PLL waveguide, the propagating modes are studied with the scattering images and the measurement of the transmission spectrum. Controllability on the modes of SPR propagation in PLL waveguide is demonstrated. KEYWORDS: Surface plasmons, micro-optical liquid-liquid (LL) waveguide, label-free detection.
fluidic
system
(MOFS),
INTRODUCTION Using a micro-optical fluidic system (MOFS) chip for biological and chemical sensing is one of the uprising topics due to its attractive features of small size, highly integration, small volume on sample consumption and ease for mass production [1]. The optical and microfluidic systems are integrated onto a MOFS chip for large-scale SPR assisted measurement and surface enhanced Raman scattering (SERS) [1] that is a new research field. For SPR related measurements, the propagation path of incident light is carefully arranged for the largest exciting efficiency and the smallest delivering loss. The liquid-liquid waveguide is achieved by sandwiching three layers of laminar flow in a square microchannel [2], and is a good candidate on delivering light energy in liquid with low loss. However, the solution CaCl2, which is dissolved in the core layer for increasing the refractive index, makes the suspended gold NPs in water to be precipitated. In this paper, the PLL waveguide is designed for making the plasmon assisted measurements or SERS available in the liquid phase waveguide. Basic concepts and optical characteristics are discussed with experimental measurements. EXPERIMENTAL RESULTS AND DISCUSSIONS By adding the suspended Au NPs in water, the Maxwell-Garnet model [3] suggests that the small embedded particles change the effective dielectric constant of the material depending on the occupied volume fraction (VF). The Au NPs is fabricated with typical wet chemical reduction method, where the Au NPs occupied a VF of 0.038% is prepared. Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences October 12 - 16, 2008, San Diego, California, USA 978-0-9798064-1-4/µTAS2008/$20©2008CBMS
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Figure 1. Schematic of plasmonic liquid-liquid (PLL) waveguide. A PLL waveguide using Au NPs colloidal solution is designed on a MOFS chip as shown in Fig. 1. The laminar flow of the sandwiched layers is formed by injecting liquids into the three inlets at the right of the microchannel. The colloidal solution with suspended Au NPs becomes an effective metallic cladding layer (M), where water is the effective insulator core layer (I) for constructing the plasmonic liquid-liquid (PLL) waveguide. The experiments are conducted with the source light incidents from the fiber at the left end, which is on the same side as the outlet for waste liquid. An inverted system optical microscope incorporated with the charge coupled device camera is used in the experimental setup for light propagation in the microchannel to record the intensity profile of the scattering light from the bottom of the light transparent polydimethylsiloxane (PDMS) chip. When the light is injected from the left end, the coupling mode is transformed to the guiding mode for light propagation in a long distance with the propagation length, i.e. the length for the light intensity to be decreased to 1/e, longer than 4mm. The propagating light is guided inside the core layer and gradually dispensed to the cladding layer. As shown in Fig. 2, the two ot lines_ are located at interfaces of M-I, which is similar to a typical plasmonic waveguide with an air gap in the metal thin film [4]. The transmission spectrum is shown in Fig. 3. The suspended Au NPs causes the colloidal solution to have an absorption peak around 530 nm, which is the natural absorption wavelength of Au [5]. Therefore, the PLL waveguide can effectively deliver light to the Au NPs in the cladding layer for advanced SPR-assisted biological and chemical materials detection. When the VF of Au NPs is increased, the absorption of light for wavelength larger than 530 nm is increased. This increases the contact rate and the energy transfer efficiency of Au NPs to the target samples for advanced material detection. For a VF of 0.038%, the particle-particle distance of suspended Au NP with 30 nm diameter is about 670 nm, closes to the intermediate-field region of visible or infrared incident light. The light scattering spectrum is affected by varying the concentration of Au NPs in the colloidal solution. In addition, the particle to sample distance also affects the scattering of the combined optical system [5]. The sensor of PLL waveguide will be applied for biological or chemical label-free detection applications in future.
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Figure 2.
Micro photography of coupling light into PLL waveguide with MIM configuration.
Figure 3.
Micro photography of coupling light into PLL waveguide with MIM configuration.
CONCLUSIONS An on-chip PLL waveguide is designed, fabricated and experimented. According to the experimental results of the PLL waveguide sensor, the light can be delivered to Au NPs in the colloidal solution with low loss and high sensitivity. The of PLL waveguide will provide broad applications in biomedical and biological material detection. REFERENCES [1] A. Q. Liu, H. J. Huang, L. K. Chin, Y. F. Yu and X. C. Li, Label-free detection with micro optical fluidic systems (MOFS): a review, Anal Bioanal Chem, DOI 10.1007/s00216-008-1878-2, (2008). [2] D. B. Wolfe, R. S. Conroy, P. Garstecki, B. T. Mayers, M. A. Fischbach, K. E. Paul, M. Prentiss, and G. M. Whitesides, Dynamic control of liquid-core/liquid-cladding optical waveguides, Proc Natl Acad Sci U S A, 101, pp. 12434–12438, (2004). [3] J. C. M. Garnett, Colours in metal glasses, in metallic films, and in metallic solutions, Phil. Trans. A, 203, pp. 385-240, (1904). [4] R. Zia, M. D. Selker, P. B. Catrysse, and M. L. Brongersma, Geometries and materials for subwavelength surface plasmon modes, J. Opt. Soc. Am. A, 21, pp. 2442-2446, (2004). [5] T. Okamoto, Near-field spectral analysis of metallic beads, Near-field optics and surface plasmon polaritons, Springer, pp.97-122, (2001).
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