Bio and chemical sensing by a low cost plasmonic platform in plastic optical fibers

Surface plasmon resonance (SPR) is widely used as a detection principle for many sensors operating in different application fields, such as bio and chemical sensing. When artificial receptors are used for bio/chemicals detection, the film on the metal surface (usually a gold surface) selectively recognizes and captures the analyte present in a liquid sample, so producing a local change in the refractive index at the metal surface. The value of the refractive index change depends on the structure of the analyte molecules [1]. SPR biosensors based on Kretschmann and Otto configurations are usually bulky and require expensive optical equipment, it is not easy to miniaturize them and, in addition, their remote interrogation may be difficult to develop. Jorgenson et al. replaced the prism by a multimode optical fiber [2]. The metal was deposited on the bare core of the fiber. The use of an optical fiber allows for remote sensing and may reduce the cost and the dimensions of the device. Due to the propagation of the light in the fiber, the angle of incidence on the metallic layer exceeds the critical angle, which depends on the refractive indices of both core and cladding components. Therefore SPR only exists for surrounding dielectrics whose refractive index lies in a narrow range. To overcome this drawback, Jorgenson et al. used a polychromatic light source and a spectrograph. This approach results in low cost, easy to implement device and can offer some attractive advantages such as the possibility to be used in the presence of flammable substances and hazardous environments, because of its electricity-free and remote sensing capabilities. Furthermore, because of the small size and non-invasive features, it can be used for medical (self-) diagnosis with the possibility to integrate SPR sensor platforms with optoelectronic devices, eventually leading to “lab on a chip”. In the scientific literature, many different configurations based on SPR in silica optical fibers, have been described [3-6]. On the other hand, for SPR sensor platforms, POFs are especially advantageous due to their excellent flexibility, easy manipulation, great numerical aperture, large diameter, and the fact that plastic is able to withstand smaller bend radii than glass. Furthermore, the advantage of the POF sensors is that they are simpler to manufacture than those based on silica optical fibers [7].