# An Optimized Graphene-Based Surface Plasmon Resonance Biosensor for Detecting SARS-CoV-2
## 3 Results and Discussions
### Application of the Optimized SPR Biosensor for SARS-CoV-2 Sensing
This plateau suggests that the sensor's  
Figure 7: (**a**) SPR curves as a function of the angle of incidence ranging from 65\({}^{\circ}\) to 90\({}^{\circ}\), considering different virus concentrations from 150 to 525 mM. The black curve (n\({}_{1,334}\)) denotes the optimized Sys\({}_{8}\) in PBS solution before virus adsorption. (**b**) Attenuation percentage, (**c**) FWHM, and (**d**) sensitivity enhancement percentage for all virus concentrations tested. responsiveness diminishes at higher concentrations, potentially due to near-complete saturation of the ssDNA functional layer. This behavior implies that while the sensor remains sensitive at low to moderate concentrations, its effectiveness decreases as it approaches higher viral loads. Furthermore, the linear sensitivity trend up to 400 mM indicates that \(\text{Sys}_{8}\) operates effectively within this range, providing reliable measurements and clear differentiation between concentrations. However, the observed plateau and high attenuation beyond 400 mM suggest that the sensor's operational range may be limited, as increased virus concentrations lead to diminishing returns in sensitivity, broad FWHM, and excessive attenuation. Therefore, the optimized \(\text{Sys}_{8}\) configuration is best suited for low to moderate SARS-CoV-2 concentrations, where it achieves a balance between sensitivity, sharpness, and manageable attenuation. To further emphasize the potential of the optimized \(\text{Sys}_{8}\) configuration, we report different performance metrics in Figure 8 and Table S10, including variation angle (\(\Delta\theta\)), sensitivity (S), detection accuracy (DA), and figure of merit (FoM). Actually, the SPR angle shift is a critical indicator of responsiveness to refractive index changes upon SARS-CoV-2 adsorption [25]. As shown in Figure 8a and Table S10, \(\Delta\theta\) increases progressively from 1.85\({}^{\circ}\) at 150 mM to 4.2\({}^{\circ}\) at 400 mM, reflecting the biosensor's strong sensitivity to changes in viral concentration. This significant angle shift, particularly at 400 mM, implies a high level of viral particle interaction with the functionalized ssDNA layer. However, the reduction in \(\Delta\theta\) to 3.9\({}^{\circ}\) at 525 mM suggests the onset of surface saturation, where the sensor's available binding sites reach capacity, limiting additional angle shifts. This plateau effect is typical in biosensors and highlights \(\text{Sys}_{8}\)'s optimal concentration range for quantitative analysis, ideally up to 400 mM. Sensitivity, displayed in Figure 8b and Table S10 as the angular shift per refractive index unit (\({}^{\circ}\)/RIU), is highest at 315.91\({}^{\circ}\)/RIU at 275 mM, demonstrating \(\text{Sys}_{8}\)'s high responsiveness to moderate concentrations. This sensitivity level is advantageous in diagnostic settings where precise measurement of viral load is necessary, especially in early-stage infections or low-viral-load scenarios [31; 32]. Sensitivity decreases to 185.71\({}^{\circ}\)/RIU at 525 mM, reflecting the sensor's diminishing capacity to detect finer concentration changes at high viral loads. This decrease suggests that \(\text{Sys}_{8}\) is best suited for detecting SARS-CoV-2 in the 150-275 mM range, where it achieves sharp and clear angular shifts. This characteristic makes \(\text{Sys}_{8}\) valuable for early diagnostics, allowing healthcare providers to detect lower virus concentrations with high accuracy. Detection accuracy, which represents the sensor's ability to differentiate between varying viral concentrations, peaks at 0.60 at 400 mM, as shown in Figure 8c and Table S10. The increased DA from 0.33 at 150 mM to 0.60 at 400 mM suggests that \(\text{Sys}_{8}\) is highly effective in quantifying SARS-CoV-2 at these concentrations. However, DA declines to 0.45 at 525 mM, representing the trends in \(\Delta\theta\) and S, and indicating potential saturation.