# Surface Plasmon Resonance-Based Fiber Optic Sensors Utilizing Molecular Imprinting
## 1 Introduction  
During the last three decades, a tremendous amount of work has been carried out on fiber optic sensors (FOS) for applications in energy, the environment, biomedicine, agriculture, the food industry, buildings and many others due to their unique abilities of supporting biocompatibility, remote sensing and online monitoring with the possibility of miniaturized probes for point of care possibilities [1; 2]. The main advantages of using optical fiber for sensing are ease of handling, low weight, low cost, immunity to electromagnetic interference, low power operation, withstanding harsh environment, etc. The optical fiber, in many cases, has replaced traditional substrates such as prisms, electrodes, etc. for the measurement of electrical and mechanical parameters such as strain, stress, magnetic field, current, acceleration, vibration [3], chemical parameters such as pH, glucose, and pesticides, gases, biological samples, refractive index and temperature [4]. In fiber optic sensors, the role of fiber can be either intrinsic or extrinsic, depending on whether the optical fiber is involved in the sensing purpose or not. In the case of extrinsic sensors, the fiber is simply used to guide/carry light from a source to the sensor module whereas in the case of intrinsic sensors, the light guiding properties of the optical fiber are modulated by the sensing medium. The modulation can be in terms of intensity, phase, wavelength, polarization, etc. [5]. For sensing various kinds of spectroscopic techniques/methods such as absorptiometry, refracometry, reflectometry, total internal reflection,interferometry, evanescent field, Doppler effect, Faraday effect, luminescence and surface plasmon resonance (SPR) have all been used. In recent years the SPR technique has become very popular for sensing of various chemical and biological analytes in addition to refractive index of an aqueous medium. This is due to its label free sensing, fast response and high sensitivity. The SPR-based fiber optic sensors rely on the refractive index variation of the sensing medium. To sense, cladding is removed from a small portion of the fiber and is replaced by a thin film of metal. The sensing medium is kept around the metal layer. The properties of the light transmitting through one end of the fiber are affected by the refractive index variation of the sensing medium. The intensity/spectrum of the output/transmitted light is analyzed to determine the change in the refractive index of the medium. SPR-based sensors have been commercialized due to their fast-label free detection with minimum requirement of analyte medium for sensing. The SPR technique utilizes evanescent waves generated at the core-metal interface to excite surface plasmons at the metal-dielectric (sensing medium) interface. The ability to immobilize bioreceptors easily and with much better stability over metal film facilitates its application in the biomedical field. The sensitivity of SPR-based fiber optic sensors can be improved by modifications in their geometry, doping in layers and addition of high index layers [6; 7; 8; 9; 10]. The performance parameter of the SPR sensor that needs improvement is the selectivity of the probe for a particular analyte, which can be improved by tailoring the SPR mechanism with molecular imprinting. Molecular imprinting technology (MIT) is the technique which is used for the construction of template-specific recognition sites in a polymerized medium. In this process the template (an atom, molecule, ion, complex or molecular, macromolecular, or ionic assembly) is frozen in a covalent/non-covalent polymeric phase and the space occupied by the template is made vacant by removing the template with subsequent removal from the polymer [11; 12; 13]. Thus the vacant space acts as the recognition site towards the template because the vacancy created by the template has the complementary shape and size of the template structure. The polymer thus created is called a molecularly imprinted polymer (MIP).