Optical label-free detection of SARS-CoV-2: investigating platform spectroscopic properties for oligonucleotide targeting

We characterized a DNA/gold interface designed for the detection of the SARS-CoV-2 RNA-dependent RNA polymerase/Helicase (RdRp/Hel) sequence. Using broadband spectroscopic ellipsometry (SE) and a difference spectra approach, we monitored molecular modifications at the interface, from probe sequence deposition to the insertion of a molecular spacer and subsequent hybridization with the target. The UV region revealed the characteristic DNA absorption peak around 260 nm, while changes in δΔ in the NIR correlated with increased optical thickness following each deposition step. The optical response was analyzed as a function of target concentration, and the binding affinity curve, derived from δΔ values at 800 nm, was fitted using a first-order Langmuir model, yielding a dissociation constant K_D = (70 ± 10) nM, consistent with literature values. Selectivity studies demonstrated that the interface effectively discriminates the SARS-CoV-2 sequence from the SARS-CoV HKU variant, even in a crowded environment. A complementary platform targeting the SARS-CoV HKU sequence confirmed selective detection of HKU over SARS-CoV-2. These findings highlight the potential for parallel detection of different viral sequences.