Peptide-based biosensors for variant-specific detection of SARS-CoV-2 antibodies†

The pandemic has highlighted an urgent demand for reliable methods to track immunological responses against SARS-CoV-2, especially in vaccinated populations and recovered patients. In this study, we developed biosensors based on the immunodominant peptide P44 (sequence: TGKIADYNYKLPDDF), located in a mutation hotspot of the Spike protein's receptor-binding domain (RBD), to enable the ultrasensitive and specific detection of antibodies against SARS-CoV-2. Gold nanoparticles (AuNPs, ∼30 nm) were synthesized via the Turkevich method and functionalized with P44-WT (wild-type) and its mutated forms, P44-T (gamma) and P44-N (beta), using 4-mercaptobenzoic acid (MBA) as a stabilizer. Functionalization was confirmed by UV-vis spectroscopy and dynamic light scattering (DLS), which revealed shifts in the plasmonic band and increases in the hydrodynamic radius. The optical biosensor, based on surface-enhanced Raman spectroscopy (SERS), analyzed convalescent and control sera (n = 104) using partial least squares discriminant analysis (PLS-DA), achieving 100% sensitivity and 76% specificity. Complementary electrochemical impedance spectroscopy (EIS) on glassy carbon electrodes corroborated the peptide–antibody interaction, yielding detection limits of 0.43, 4.85, and 8.04 ng mL⁻¹ for P44-WT, P44-T, and P44-N, respectively. The platform demonstrated high specificity in complex serum matrices and was unaffected by nonspecific biofouling. Our findings underscore the importance of peptide selection and optimization in enhancing biosensor performance and demonstrate the adaptability of this methodology for detecting emerging infectious diseases.