## 3 Results and Discussion
### UV-Vis Spectroscopy  
Figure 8 presents the normalized absorption spectra of the six samples of degraded oil. A Gaussian function fit was necessary to obtain the exact LSPR maximum to determine changes in its wavelength caused by different stages of degradation (mileage) in the oil samples. Table 3 shows the results of the Gaussian function fit for all measured samples; the results clearly demonstrate that the LSPR band shifted to higher wavelengths with the increase in mileage. This displacement is associated with changes in the permittivity of the medium as the engine oils degrades [2]. Figure 7: (**a**) EDS spectrum of the covered area on the polystyrene surface. It confirms the presence of AgNP. SEM micrographs of the AgNP deposited on polystyrene cell before (**b**) and after (**c**) measuring all the oil samples, no apparent variation in size or shape caused by interaction with degraded oil can be observed. Additional to the displacement of the maximum position, a random variation of the widths of the LSPR bands can be observed in the normalized spectra that could be related with the different oxidation rates of the oil samples. [PERSON] et al. [28] found similar results when they studied the oxidation of engine oil at different oxidation times by means of UV-Vis spectroscopy. They focused on an absorbance band of the engine oil and their results show that not all the widths of the absorbance bands become wider as oxidation increases. They attributed this, in part, to the \(n-\pi\) * transition of the lubricant base oils and additives caused from a decrease in the pH values of the oxidized samples, according to their results of the total base number (TBN). Furthermore, as oxidation increases, multiple conjugated bonds could be broken down at different rates changing the engine oil properties. However, the undesired broadening effect mentioned above did not affect the results obtained with the proposed sensor, which operating principle is the displacement of the LSPR band with the change of the permittivity of the medium, related to oil degradation, as described by Equation (1). Moreover, the broadening effect cannot be associated with morphological changes of the AgNP either, as shown by electron microscopy images obtained before and after the measurement. Figure 9 displays a correlation between LSPR peak and mileage, a linear fitting with a \(R^{2}=0.9605\) was obtained. This correlation represents an important result to the sensing device due to its capability to detect changes in the oil degradation caused by mileage. Therefore, this facilitates the interpretation of the sensing device to be an option to monitor  
Figure 8: Normalized absorption spectra of AgNP and six degraded oil samples measured in the sensing prototype. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Band Maximum (nm)** & \(R^{2}\) \\ \hline AgNP & 552.5 &  
engine oil condition. Any sudden change in the LSPR peak tendency will involve an abnormal oil degradation process and it would be detected this way. [PERSON] et al. [29] developed an SPR sensor to detect hydrocarbons and gasoline in engine oils, their sensor was composed of various layers of different materials and a complicated experimental setup. However, they can detect the presence of gasoline at low concentrations and they expected to apply it in areas such as lubricant degeneration detection. [PERSON] et al. [22] used an SPR sensor to measure lubricant degradation in real-time. The sensor was elaborated with different materials such as silver, gold, copper, and titanium, and a simulation was performed to optimize and select the proper combination of metal. However, only two oil samples were analyzed, 0 and 5700 km. The results showed a variation in the SPR angle that can be detected in real-time. Nonetheless, the equipment used to carry out the measurement was complex and expensive. [PERSON] et al. [23] studied the capacity of the SPR sensor to detect contaminants in engine lubricants.