## 3 Results and Discussion  
### Characterization of the POF-MIP Sensor for Furfural (2-FAL) Detection in Fresh Transformer Oil  
Two POF-MIP sensors, namely sensor 1 (P1) and sensor 2 (P2), have been tested for measurements in a drop, as described in the Experimental section. The transmission spectra, obtained with the POF-MIP sensor in the case of a widely used mineral transformer oil (Nytro Libra insulating oil), exhibit several transmission minima in the considered wavelength range (330-1100 nm). Nevertheless, the minimum at around 520 nm for P1 and 530 nm for P2 is much better defined than the other minima, and, moreover, it depends on the 2-FAL concentration. Figure 2 shows the SPR resonances obtained with the particular POF-MIP sensor P1. For better clarity the inset of Figure 2 reports only the resonance wavelengths, obtained for the standard samples at different concentrations of 2-FAL in fresh transformer oil [26]. It is clearly seen that the resonance wavelength is shifted to lower values (blue shift) when the 2-FAL concentration increases, indicating that 2-FAL effectively combines with MIP from the oil matrix here considered. Typical curves reporting the transmission minimum wavelength variation (\(\Delta\lambda_{\text{ex}}\)) _versus_ the concentration of 2-FAL, obtained with two different sensors are shown in Figure 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Chemical Agent** & **Method** & **Unit** & **CT.134** & **CT.729** \\ \hline Total gas concentration & IEC 60567 & \(\upmu\)L/L & 78,579 & 65,611 \\ \hline Furfural (2-FAL) & \multirow{2}{*}{IEC 61189} & ppm & 0.112 & 0.114 \\ \(\Sigma\) furan derivatives & & ppm & \(<\)0.010 & \(<\)0.010 \\ \hline Methanol & \multirow{2}{*}{ppm} & 0.567 & 0.408 \\ Ethanol & & ppm & 0.306 & 0.250 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of two used transformer oil samples from different substations of the Italian Transmission System Operator. In both cases the function is not linear since the slope decreases at increasing concentration, reaching a zero value at sufficiently high 2-FAL concentration, about 0.15 ppm. Since \(\Delta\lambda_{\mathrm{ex}}\) is directly proportional to the concentration [19,24] the deviation from the linearity must be ascribed to the saturation of the combination sites present in the molecularly imprinted polymer. The affinity constant for the sites can be evaluated from the Langmuir model for the adsorption on a limited number of sites on a solid phase which takes place according to the following equilibrium:  
\[A+\overline{P}\leftrightarrows\overline{AP} \tag{1}\]  
where A indicates the analyte, P the interaction site in MIP obtained by the molecular imprinting procedure and AP the adduct. The barred species are those in the solid phase. The corresponding equilibrium constant (affinity constant) is:  
Figure 3: Absolute value of resonance wavelength shift versus the concentration of 2-FAL [ppm], obtained with two different POF-MIP sensors P1 and P2, and the fitting of the data by Hill equation. Figure 2: Normalized SPR spectra acquired by the POF-MIP sensor P1 for different furfural concentrations (in ppm) in fresh oil. Inset: zoom of resonance wavelengths. \[K_{f}=\frac{[\overline{AP}]}{[A][\overline{P}]} \tag{2}\]  
Considering that the total concentration of sites in the polymer phase is:  
\[c_{site}=[\bar{P}]+[\overline{AP}] \tag{3}\]  
and that the instrumental response is directly proportional to the variation of the concentration of A in the polymer, the following relationship holds:  
\[\Delta\lambda_{ex}=k[\overline{AP}] \tag{4}\]  
Thus the analytical response depends on the analyte concentration (c) according to:  
\[\Delta\lambda_{ex}=\frac{k\epsilon_{site}\left(K_{f}c\right)^{h}}{1+\left(K_{ f}c\right)^{h}} \tag{5}\]  
The parameter \(h\) takes into account the possible lack of homogeneity of the interaction of the substrate with the solid. In the original Langmuir isotherm it is equal to 1, _i.e._, not any dishomogeneity is considered.