## 5 Conclusions  
In this research different methods for the investigation of the binding properties of a synthetic receptor, a molecularly imprinted polymer (MIP-2 FAL), have been applied with the aim of determining its binding properties in wide concentration ranges. It has been found that this investigation can be conveniently performed using different sensing devices with different detection ranges. With respect to the batch equilibration methods, based on the determination of the target substance left in solution after equilibration, the methods based on sensing devices are convenient not only in terms of amount of binding material required for the investigation, for example a few mg of MIP instead of tens of mg, and of the time required for obtaining the information, but also because they make it possible to investigate the interactions at lower concentrations due to their high sensitivity. This strongly depends on the transduction method considered. It has been shown that different concentration ranges can be detected by different transduction methods and that when the concentration ranges overlap, sites with similar affinity are detected. This is a strong indication that actually sites with different affinity are present in the considered MIP. The use of SPR transduction allowed lower concentrations to be investigated than those by voltammetric sensor, even if in the present investigation the measurements were performed with a very simple apparatus based on POF instead of Kretschman configuration on prism. It has been found that at least three kinds of sites, with association constant ranging from about 5 \(\times\) 10\({}^{2}\) M\({}^{-1}\) to 3 \(\times\) 10\({}^{7}\) M\({}^{-1}\), are present in the MIP-2 FAL here examined as a proof of principle. The sites distribution can be different in other MIPs, however the existence of heterogeneous sites in the same MIP, particularly those obtained by non-covalent imprinting, agrees well with a plethora of previous findings [19; 20; 21; 29]. The reasons for the heterogeneity have been widely discussed in a number of review papers, for example by [PERSON] and [PERSON] [30]. No information about the structure of the heterogeneous sites can be obtained from the association constants determined by the approaches of the present investigation. It can be guessed for example that sites can be formed by a different number of functional monomers or with different rigidity of the polymer around the site [20; 30]. In the case here considered the functional monomer was present in large excess with respect to the template. Thus it can be expected that a large part of the functional monomer is free, and contribute to the nonspecific binding, as it has been actually found. Both electrochemical and SPR platforms here proposed are easy and fast to prepare, so they can be considered as useful tools for characterization of the binding properties of the receptor in an easy and rapid way, each of them in a particular concentration range. **Supplementary Materials:** The following are available online at [[http://www.mdpic.com/1424-8220/19/6/1344/s1](http://www.mdpic.com/1424-8220/19/6/1344/s1)]([http://www.mdpic.com/1424-8220/19/6/1344/s1](http://www.mdpic.com/1424-8220/19/6/1344/s1)), Figure S1. UV spectra of 2-FAL, in 0.75H\({}_{2}\)O:0.25 EtOH. 3.02 \(\times\) 10\({}^{-5}\) M (light green); 6.04 \(\times\) 10\({}^{-5}\) M (brown); 9.06 \(\times\) 10\({}^{-5}\) M (blue); 12.08 \(\times\) 10\({}^{-5}\) M (green), Figure S2. UV spectra of HMF, in 0.75H\({}_{2}\)O:0.25 EtOH. 1.95 \(\times\) 10\({}^{-5}\) M (light green); 3.91 \(\times\) 10\({}^{-5}\) M (brown); 5.96 \(\times\) 10\({}^{-5}\) M (blue); 7.81 \(\times\) 10\({}^{-5}\) M (green), Figure S3. UV spectra of FA, in 0.75 H\({}_{2}\)O:0.25 EtOH. 2.89 \(\times\) 10\({}^{-5}\) M (light green); 5.79 \(\times\) 10\({}^{-5}\) M (brown); 8.69 \(\times\) 10\({}^{-5}\) M (blue); 11.58 \(\times\) 10\({}^{-5}\) M (green). **Author Contributions:** Conceptualization, M.P., S.M., N.C., L.D.M. and L.Z. ; Methodology, M.P., N.C. and L.Z.