Optimal Design of D-type Plastic Fibers for best sensitivity of SPR Sensors

A design method for a SPR (surface plasmon resonance) sensor based on a D-type plastic optical fiber (POF) geometry in a configuration of a multi-layered scheme is proposed in this paper. The numerical simulation for the optimal sensitivity with spectral interogation was performed in order to choose the practical implementation geometry of the SPR sensor. A side-polish to the half of the fiber’s PMMA core, with about 10mm in length is made and a Microposit 1813 buffer and gold layer deposition is used for sample fabrication. The effect of different thickness of the layers in different multi-layered configuration has been investigated. The proposed setup for sensor test, measures the light intensity instead of phase difference. It can be used for refractive index changes (1.33-1.40) in real time detection of the substances for different biosensing applications. The proposed sensor is simple to use, has small size, it works on small analyt-sample size and it is cost effective for specific non-invasive application users. Introduction The free-label detection of chemical analyt medium using light-excited surface plasmon resonance (SPR) is nowadays a broadly used technique. It is further an object of the invention to provide a solution for a SPR-based device that is easy to manufacture, cost-effective and simple to use for the laboratory specialists. A low-cost plastic optical fiber (POF) solution was considered [1] instead of classically most widely used Kretschmann prism-coupling geometry. The POF fiberbased SPR has many advantages over prism-based model such as flexible optical design because of the easy modeled plastic materials, development of remote sensing setup, it may reduce the cost and dimensions of the device and there exists the possibility of integration with optoelectronic devices (eventually leading to “lab on a chip”), to be used in continuous analysis in real-time (not just one sample at a time) and in situ monitoring. In this paper a SPR sensor response with different stack-layered geometry has been theoretically investigated, in order to obtain an optimal geometry with a good sensitivity and resolution ratio. The design method is used in order to select the optimal manufacturing parameters for the sensor. The SPR biosensor was developed and tested at the Department of Information Engineering, Second University of Naples, Italy and the results was published in [1]. It was used in this study for performance comparison between theoretical and experimental results; the results are in close concordance. The numerical results are obtained based on transfer matrix formalism using Matlab simulations. The preferred mode of surface plasmon wave (and hence the marker) detection is based on spectral analysis of insertion loss experienced in the fiber device. SPW (surface plasmon wave) results in a peak of insertion loss at certain wavelength (identifying type of the marker) and amplitude (indicating its concentration). Suitable interrogator is able to detect increase of insertion loss at certain wavelength corresponding to SPW resonance against reference at other wavelength(s) not influenced by the SPW. Advanced Engineering Forum Online: 2013-06-27 ISSN: 2234-991X, Vols. 8-9, pp 563-573 doi:10.4028/www.scientific.net/AEF.8-9.563 © 2013 Trans Tech Publications, Switzerland This is an open access article under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/) The POF has a PMMA core of 980μm and a fluorinated polymer cladding of 20μm. The D-type geometry enabling it to be a candidate for successful biosensors implementation; the planar gold layer that permits easy-binding process of antibody. Theory The propagation of the free electron density oscillation on the surface of a dielectric medium, subjected to an incident light, in contact with a metal gives rise to a surface plasmon wave (SPW). The SPW can only be supported along the boundary of two materials layers with opposite signs of the real parts of the dielectric constants, such as a dielectric and a metal. Due to the intrinsic electron oscillation damping loss in the metal the SPW attenuate rapidly during the layered-path propagation. In a metal electron oscillates with plasma frequency, ωp=4πne 2 /m [2] (where n is the electron number density, e is the electron charge and m is the electron mass). The resonance condition for the excitation of surface plasmon wave (Eq.1) is obtained considering the dispersion relation for surface waves. The output signal demonstrates a dip at a particular wavelength known as resonance wavelength. The propagation constant of the evanescent wave generated as a result of Attenuated Total Reflection (ATR) of the light incident at an angle θincident through a light coupling device (in this case optical fiber) of refractive index ncore, is equal to that of the SPW: KATR=Kplasmon. (1)