A label-free gold nanoparticles functionalized peptide dendrimer biosensor for visual detection of breakthrough infections in COVID-19 vaccinated patients

Given the global implementation of effective COVID-19 vaccines, which do not confer complete immunity, it is crucial to monitor the occurrence of breakthrough infections, particularly against newly emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. Hence, we developed a label-free colorimetric assay using gold nanoparticles (GNPs) functionalized with a peptide dendrimer incorporating highly reactive epitopes of the nucleocapsid (N) protein. This assay relies on the tween-20 induced colorimetric changes caused by the aggregation of peptide dendrimer-coated GNPs in the absence of anti- SARS-CoV-2 N antibodies, and vice versa. Transmission electron microscopy, dynamic light scattering, and circular dichroism spectroscopy analyses all showed the formation of a uniform and highly stable coating of the peptide dendrimer over GNPs. Surface plasmon resonance experiments have demonstrated a strong binding affinity for the peptide dendrimer and anti- SARS-CoV-2 N antibodies, with a K_D value of 525 nM. To validate the proof-of-concept, we have tested this assay on seventy human serum samples, and receiver operating characteristic curve analysis demonstrated high diagnostic sensitivity (88.89 %) and specificity (100 %). This approach opens up new avenues for the development of simple and rapid diagnostic assays for identifying antibodies against viral infections and other pathogens.