## 1 Introduction  
Lubricating oil is essential in the automotive industry and it is one of the more important liquids that requires monitoring to guarantee the correct operation of the engine, in order to avoid improper friction between moving surfaces of the different mechanical parts [1]. Automotive manufacturers make conservative time or mileage suggestions to replace it, without knowing the real condition of the lubricant and ignoring the operating conditions of the vehicle, causing an early disposal and an environmental problem [2]. Among the different mechanisms of oil degradation, oxidation is the main reaction to cause the loss of properties of the lubricant, given that its base is composed of long hydrocarbon chains that are susceptible to oxidation when exposed to high temperatures [3]. During this process, free radicals are generated, which are undesirable because they damage the chemical structure of the lubricant [4]. These reactive chemical species trigger a series of complex chemical reactions on the oil base. To avoid early degradation or a delay in this process, some additives, like antioxidants, are added into the composition. However, the oxidation products along with the high temperatures of the medium can react with the sulfur containing species that are present in the lubricant composition, producing sulfated byproducts (SO\({}_{\text{x}}\)), a process known as sulfation [5]. In the same way, the gases produced during the combustion process, filtered through the walls of the cylinders, can react with the oil base producing nitrogenated compounds. Both nitration and sulfation products reduce the effectivity of the different additives and, in the presence of water, they acidify the environment, reducing the lubricant properties of oil, which can produce corrosion or another damage to the engine [6]. Additionally, the oxidation reaction also generatesmolecules with increased polarity with respect the original ones, which in turn change the polarization of the lubricant causing an increase of the permittivity of the medium, as the lubricant oxidates [7; 8]. The chemical properties of the lubricants were widely studied, with Fourier-transform infrared spectroscopy (FT-IR) the main used technique. The American Society for Testing and Materials (ASTM) established a standard practice to monitor oil condition through this technique (ASTM E2412) [9]. However, many investigations have opted to develop sensors to monitor the condition of the engine oil by studying its physical and chemical properties, such as viscosity, refractive index, permittivity, oxidation, sulfation, nitration, wear, water content, insoluble particles, etc. [3; 10; 11]. However, the development of sensors based on the properties of nanoparticles was on the rise over the last decades, generating great advances in many areas of science. In particular, advances were made in the detection of the interaction between molecules close to the surface of the nanoparticles through changes in the surface plasmon resonance (SPR) or LSPR, due to the high sensitivity to the refractive index of the medium surrounding the nanoparticle [12; 13]. These types of sensors have several advantages since they could be used for real-time sensing and the wavelength of the SPR or LSPR can be tuned for different applications, such as detectors for drugs, biomolecular interactions, gases, pH, among others [14; 15; 16; 17; 18; 19; 20; 21]. Therefore, they are good candidates to monitor the condition of the lubricant, since its degradation modifies the physicochemical properties, which can be detected by changes in the SPR or LSPR bands. [PERSON] et al. [22] reported a sensor based on the SPR of different nanoparticles (Ag, Au and Ti), analyzing only two lubricant samples, which corresponded to a new lubricant and another lubricant used during a certain mileage, and the sensor response was a variation of the SPR angle with the used lubricant. Similarly, [PERSON] et al.