# A Molecularly Imprinted Polymer Based SPR Sensor for 2-Furaldehyde Determination in Oil Matrices
## 3 Results
### Characterization of the Response of the SPR-POF-MIP Sensors  
The response of the sensor to the concentration of the substance of interest, 2-FAL, was investigated by determining the dose-response curve, and by applying the Langmuir adsorption model, assuming that the adsorption takes place through combination at specific sites, which in this case, are the imprinted sites. The model makes it possible to evaluate the affinity constant of the template for the MIP exactly in the form in which it is used in the sensing system, i.e., as a layer. This approach has been previously applied for the adsorption from water [14]. As an example, in Figure 7, the SPR spectra in oil, at different concentrations of 2-FAL, of the two sensors with MIP layers of different thickness, SPR-POF-A1 MIP2 and SPR-POF-A5 MIP1, normalized on the corresponding bare sensor in air, are reported. The washing between successive standards consisted of five repeated flushing with ethanol. Figure 7: SPR spectra in oil, at different concentrations of 2-FAL, of two sensors with MIP layers of different thickness normalized on the corresponding bare sensor in air: (**a**) SPR-POF-A1 MIP2; (**b**) SPR-POF-A5 MIP1. In both cases, multiple resonance dips are present, much better defined in the case of the sensor with a lower amount of MIP deposited. The resonance depending on the 2-FAL concentration is, respectively, 880 nm and 812 nm for the two considered sensors. This indicates that in the case of SPR-POF-A1 MIP2, the MIP layer is sufficiently thick to avoid the effect of the fluid over the MIP (no effect of the sample bulk), while in the case of SPR-POF-A5 MIP1, which contains a lower amount of MIP, there is a strong effect of the RI of the overlying liquid, which is high for the considered oil, 1.47. Nevertheless the resonance wavelength is still useful, being 880 nm. The corresponding two standardization curves are shown in Figure 8.  
The dose-response curves are similar, but the maximum signal is higher when a higher amount of polymer is deposited, possibly corresponding to the presence of a larger number of accessible combination sites when more polymer is deposited. The parameters of the dose-response curve (Equation (1)) are reported in Table 1.  
The error on the resonance wavelength shift, and the detection limit, is lower for SPR-POF-A5 MIP1, for which the resonance dips are better defined, than for SPR-POF-A1 MIP2. The affinity is high enough to make the determination at a low concentration level possible, useful for application in different fields, for example for the determination of 2-FAL in vegetable oils. Nevertheless, it could be too high for other applications. A possible improvement could take place by applying a more extensive washing procedure. For the sensor SPR-POF-A4 MIP EgDMA, the spectra in oil, normalized to those of the bare platform in air, are similar to the spectra in water and in air. The peak at 850 nm is shifted to higher wavelengths when the concentration of 2-FAL increases. By applying the Langmuir model to the dose-response curve (Equation (1)), the parameters could be evaluated, and they are reported for comparison in Table 2. The affinity constant is lower for SPR-POF-A4 MIP EgDMA than for the sensors prepared with DVD as the cross-linking agent, despite the fact that the functional polymer is always MAA at the same concentration. It could be guessed that some interaction between the MAA and EgDMA \begin{table}
\begin{tabular}{c c c c c} \hline  & \(\mathbf{K_{\text{eff}}}\) (L M\({}^{-1}\)) & **Sens (nm mg\({}^{-1}\) L)** & **SEy (nm)** & **LOD (ug L\({}^{-1}\))** \\ \hline SPR-POF-A5 MIP1 (N = 6) & 1.09 \(\times\) 10\({}^{6}\) & 70 & 0.66 & 18 \\ SPR-POF-A1 MIP2 (N = 5) & 3.69 \(\times\) 10\({}^{5}\) & 53 & 1.69 & 64 \\ SPR-POF-A4 MIP EgDMA (N = 7) & 2.60 \(\times\) 10\({}^{5}\) & 70 & 0.14 & 52 \\ \hline \end{tabular} SEy is the error of the response, \(\Delta\lambda\); N indicates the number of standards considered.  
\end{table}