# SPR-Optical Fiber-Molecularly Imprinted Polymer Sensor for the Detection of Furfural in Wine +
## 3 Results
### MIP Layer Analysis  
The SEM image reported in Figure 3 shows the gold surface modified with MIP polymerized in situ, where the presence of polymer aggregates of small particles can be seen. This SEM image is relative to a sensing platform in which the MIP layer has been obtained by spinning the prepolymeric mixture at high velocity (about 1000 rpm). It shows that at the micro level, the MIP film does not entirely cover the gold surface, so in some way it reproduces the surface derivatized with a bioreceptor, i.e. a large biomolecule.  
Figure 4 reports the comparison between the SPR spectrum in water of a bare platform (SPR-bare) with that of the same platform modified with multiple layers of MIP (SPR-MIP). The MIP made the RI of the region overlying gold higher than pure water, as proven by the resonance wavelength shift towards higher values (red shift). The SPR spectra in Figure 4 are relative to a set of sensors prepared by depositing successively one, two, or three layers of MIP on equal platforms, as reported in Table 1. Water (\(n=1.332\) RIU) was the liquid over the platform. The example in Figure 4 shows that the resonance wavelength shift (\(\Delta\lambda\)) with respect to that of pure water depends on the amount of MIP deposited. The resonance wavelengths are reported in Table 1.  
Figure 2: (**a**) Transmission spectra of the MIP-bare platform in water–glycerol solutions with different refractive index (RI). (**b**) Variation of the resonance wavelength with the RI of the overlying liquid.  
Figure 4: Transmission spectra of C1 (SPR-MIP) platform, with one, two, or three MIP layers, in water, normalized on the spectrum of the corresponding platforms in air.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Sensor** & **N. of MIP Layers** & \(\mathbf{\lambda_{\text{ris}}}\) **in Water [nm]** & \(\mathbf{\Delta\lambda}\) **(SPR-MIP in Water Minus SPR-Bare in Water) [nm]** \\ \hline C1 (SPR-bare) & 0 & 597.7 & 0 \\ C1-1 (SPR-MIP) & 1 & 663.2 & 65.5 \\ C1-2 (SPR-MIP) & 2 & 672.4 & 74.7 \\ C1-3 (SPR-MIP) & 3 & 718.3 & 120.6 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Resonance wavelength of sensors with multiple MIP layers. Formation of each MIP layer: 40 \(\upmu\)L of prepolymeric mixture spun at 300 rpm. Normalization on the corresponding platform in air.  
Figure 3: SEM images of different points at the surface of the sensing part.  
By increasing the number of MIP layers, the resonance wavelength increases, and a shift to higher wavelengths (red shift) is observed (from 663.0 nm to 718.3 nm). Even with three layers deposited (C1-3), the resonance wavelength (718.3) is lower than the maximum wavelength useful for measurement.  
In order to verify if the sensor with three layers of MIP is still sensitive to the RI of the overlying liquid, the SPR transmission spectra in water-glycerol solutions with different refractive indices, positioned like a drop over the MIP (bulk solution), are reported in Figure 5.  
The resonance in water of the SPR-MIP platform is at 715 nm (see Figure 4), with a shift of 115 nm compared to the resonance of SPR-bare in water, indicating that the amount of MIP layer is relatively large. However, a shift to higher wavelengths is observed when liquids with RI higher than water substitute water. This behavior demonstrates that the MIP layer is not so thick as to completely include the plasmonic wave, as schematically illustrated in Figure 1b,c.