# An Optimized Graphene-Based Surface Plasmon Resonance Biosensor for Detecting SARS-CoV-2
## 3 Results and Discussions  
### Best Sensor Configuration  
In this part, the objective is to determine the optimal SPR sensor configuration by comparing different multilayer designs without the presence of the analyte (SARS-CoV-2). The configurations are evaluated based on key metrics: percentage attenuation, full FWHM, and percentage sensitivity enhancement. These metrics, displayed in Figure 2 and Table S3, allow us to calculate the intrinsic performance of each configuration and identify the most effective design before introducing the target virus. Figure 2: (**a**) SPR curves as a function of the angle of incidence ranging from 60\({}^{\circ}\) to 80\({}^{\circ}\) for the systems \(\text{Sys}_{0}\) to \(\text{Sys}_{9}\). (**b**) Attenuation percentage, (**c**) FWHM, and (**d**) sensitivity enhancement percentage of all systems under consideration. Figure 2a shows the SPR reflectance curves for each configuration (Sys\({}_{0}\) to Sys\({}_{9}\)) as a function of the angle of incidence, ranging from 60\({}^{\circ}\) to 80\({}^{\circ}\). The position and shape of each SPR dip provide insight into the resonance conditions and sensitivity of each configuration. In the basic configuration, Sys\({}_{0}\), a sharp and narrow SPR dip is observed at approximately 67\({}^{\circ}\), reflecting the interaction between the prism-silver interface and the surrounding water medium. This sharp dip in Sys\({}_{0}\) suggests moderate sensitivity, as the simple silver layer structure provides a minimal enhancement to the evanescent field interaction with the medium. In contrast, the addition of silicon nitride and graphene layers shifts the SPR dip to slightly higher angles and broadens the dip width, as observed in Sys\({}_{6}\), Sys\({}_{8}\), and Sys\({}_{9}\), where SPR dips occur around 70\({}^{\circ}\) to 72\({}^{\circ}\). The broader SPR dips in these configurations indicate a greater interaction between the multilayer structure and the evanescent field, enhancing the sensor's sensitivity [27]. Attenuation, defined as the percent decrease in reflectance, indicates the strength of the SPR signal. A higher attenuation percentage corresponds to a stronger interaction between the evanescent field and the medium. As shown in Figure 2b and detailed in Table S3, the basic system Sys\({}_{0}\) (prism/silver/water) exhibits minimal attenuation (0.023%). For consistency, PBS was used as the medium for configurations Sys\({}_{1}\) through Sys\({}_{9}\), as PBS mimics physiological conditions and provides a more realistic refractive index for future experiments involving SARS-CoV-2 [19]. Despite the absence of an analyte, the introduction of silicon nitride and graphene layers significantly increases attenuation. For instance, Sys\({}_{6}\) (prism/silver/silicon nitride/graphene/PBS) achieves an attenuation of 7.81%, while Sys\({}_{8}\) and Sys\({}_{9}\) reach 7.93% and 6.59%, respectively. These results confirm that the multilayer design enhances signal strength through better interaction with the evanescent field, even in the absence of the target analyte. FWHM, shown in Figure 2c and Table S3, is a crucial metric in SPR biosensors, as it indicates the sharpness of the SPR dip; lower FWHM values suggest a more precise resonance, which is typically desirable [28]. The basic configuration Sys\({}_{0}\) exhibits a relatively narrow FWHM (0.88\({}^{\circ}\)), consistent with a simple prism-silver design in water. For the configurations using PBS, the addition of Si\({}_{3}\)N\({}_{4}\) and graphene layers slightly broadens the FWHM due to enhanced interaction with the evanescent field. Sys\({}_{6}\) shows an FWHM of 1.82\({}^{\circ}\), while Sys\({}_{8}\) and Sys\({}_{9}\) exhibit the broadest values, 1.91\({}^{\circ}\) and 1.85\({}^{\circ}\), respectively. Although increased FWHM indicates slightly reduced dip sharpness, it also suggests stronger interactions within the layered structure, which may enhance sensitivity upon analyte introduction.