## 1 Introduction  
Chemical sensors are based on the strict integration of a receptor with an instrument able to generate a signal upon the combination of a substrate with the receptor. The characteristics of the binding reaction, in particular the affinity of the receptor for the substrate, are of overwhelming relevance for determining the performance of a sensor. Conversely, sensors can be used to investigate the combination of the receptor with the substrate. For example surface plasmon resonance (SPR) has been proposed as transduction method for a number of sensing applications since more than two decades ago [1], however it has been even more widely applied to evaluate the equilibrium and kinetical characteristics of the ligand-substrate interaction in investigations of biochemical and pharmaceutical interest [2; 3; 4]. Actually SPR is considered as one of the most robust and sensitive biosensing techniques that have been implemented in pharmaceutical research worldwide. The reason is that the detection based on SPR transduction is typically marker-free, thus it makes it possible the determination of the kinetics of molecular interactions, and of the equilibrium constants [5; 6] for a wide variety of systems in real time, without the need for a molecular tag or label [7; 8]. The SPR transduction is widely used for investigating the binding properties of bioreceptors as for example antibodies or aptamers, and for synthetic receptors like molecularly imprinted polymers (MIPs) [4; 9]. Most often expensive dedicated apparatuses are used for binding investigation, based on the Kretschmann configuration [2; 4]. In the present study we examine the possibility of using SPR transduction for the characterization of the binding properties of a receptor layer using a non-conventional SPR sensor based on optical fiber [10]. The SPR platform used in the present investigation has been previously proposed by our research group [11; 12; 13] and is based on a plastic optical fiber (POF) with a characteristic D-shape, i.e., with a planar surface obtained by erasing the cladding and partially the core of the optical fiber, over which a multilayer structure was realized as previously described. It is suitable for investigating the binding properties of the MIP since the signal obtained, i.e., the variation of the resonance wavelength (\(\Delta\lambda\)) is due to the variation of the refractive index of the polymeric layer, which depends on the binding of the substrate. Thus the device can be applied to investigate the receptor layer adsorbing properties, as it is often done by SPR measurement based on prism (Kretschmann configuration). Incorporating optical fiber, the cost of the sensor and the dimension of the apparatus are considerably reduced with respect to prism based configuration. Moreover the optical platform considered is easy to prepare directly on the POF core [12; 13]. As a proof of principle, in the present study we investigated a synthetic receptor for a small molecule, 2-furaldehyde (2-FAL), obtained by molecular imprinting (MIP-2 FAL). Receptors obtained by the molecular imprinting method, have been widely demonstrated to be advantageous with respect to biological receptors, in particular in the field of sensing, in terms of reproducibility, fast and low cost development, stability in time and possibility of application in chemically and biologically aggressive environment [14; 15; 16; 17]. For this reason the investigation of the binding properties of these materials is of wide interest for sensing applications. The evaluation of the binding properties of MIPs can be a challenging task, due to the existence of heterogeneous binding sites in MIPs, particularly those synthesized by a non-covalent bulk procedure [18; 19; 20; 21]. The heterogeneity may diminish the performance of MIPs in some analytical application. For example, it can contribute to broad, asymmetric peaks in chromatographic applications [22; 23] and cross-reactivity in binding assays [23].