Fig. 3. (A) A schematic diagram showing the synthetic path we used for the development of antibody and 4-aminothiophenol coated gold nanoparticles. (B) SERS intensity from anti-spike antibody (10 ng mL⁻¹) attached gold nanoparticles (1.3 nM), 4-aminothiophenol (300 nM) attached gold nanoparticles (1.3 nM), and anti-spike antibody (10 ng mL⁻¹) as well as 4-aminothiophenol (300 nM) attached gold nanoparticles (1.3 nM). (C) Variations in the SERS intensity from anti-spike antibody and 4-aminothiophenol attached gold nanoparticles in the presence of different concentrations of COVID-19 antigen (COVID-19 Spike Recombinant Antigen). (D) Variations in the Raman intensity change at 1078 cm⁻¹ from anti-spike antibody and 4-aminothiophenol attached gold nanoparticles with the concentration of COVID-19 Antigen (0 pg mL⁻¹ to 2 ng mL⁻¹). (E) Variations in the SERS intensity from anti-spike antibody and 4-aminothiophenol attached gold nanoparticles in the presence of different virus antigens. (F) Variations in the SERS intensity from the anti-spike antibody and 4-aminothiophenol attached gold nanoparticles in the presence of different amounts of pseudo SARS-CoV-2 (number of virus particles per mL). (G) Variations in the Raman intensity change at 1078 cm⁻¹ from anti-spike antibody and 4-aminothiophenol attached gold nanoparticles with the concentration of pseudo SARS-CoV-2 (number of virus particles per mL). (H) FDTD simulation data showing how the (|E|²) profile varies with an increase in the number of gold nanoparticles in aggregates.