Figure 1.

(a) Sketch of the SARS-CoV-2 and functionalized AuNPs. SARS-CoV-2 proteins (spike, membrane, and envelope) and their corresponding antibody (S, E, and M) are highlighted in dark red, light violet, and gray, respectively. The inset shows the pink colloidal solution containing the anti-SARS-CoV-2 functionalized AuNPs (f-AuNPs). (b) The f-AuNPs surround the virion forming a nanoparticle layer on its surface. Their interaction leads to a shift of the resonance peak in the extinction spectrum and, hence, to a color change visible in the inset. (c) Extinction spectra reporting the OD of f-AuNP colloidal solution mixed with samples from patients with different viral load. At very low virion concentration (curve C_t32) the extinction spectrum is not distinguishable from the spectrum of f-AuNPs (black continuous line). At intermediate virion concentration (curve C_t15) the extinction spectrum is slightly red-shifted and its difference from the “control” (f-AuNPs) produces the curve C_t15-(f-AuNPs) that evidences the contribution entailed by the virion. At high virion concentration (curve C_t7), the extinction spectrum peaks at 560 nm as for C_t15-(f-AuNPs). The agreement between the curve C₇ and the simulated spectrum (gold continuous line, scaled to the experimental one) from a dielectric sphere (100 nm diameter) surrounded by smaller AuNPs (20 nm diameter) confirms the interpretation of the extinction spectra as due to nanoparticle aggregation.