Highly sensitive sunset yellow-based optical and dielectric biosensing of SARS-CoV-2 nucleocapsid protein in saliva

Optical and dielectric immunosensors for the nucleocapsid (N) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were developed using the nematic phase of a lyotropic chromonic liquid crystal (LCLC), sunset yellow (SSY), as the sensing medium. The biosensing platform was constructed as a liquid crystal (LC) cell with a cell gap of 15 μm coated with a homeotropic alignment reagent so that nematic SSY, a 28 wt% aqueous solution of SSY sandwiched between the parallel glass slides of a LC cell, was aligned homeotropically. In the presence of a controlled amount of immobilized anti-N protein antibody, nematic SSY remained homeotropically aligned, yielding a dark optical texture under a polarizing optical microscope with crossed polarizers. In the presence of the analyte, the formation of N protein immunocomplexes disrupted the ordered orientation of nematic SSY, leading to light leakage and a change in both the real and imaginary parts of the dielectric constant, which correlated with N protein concentration, with the limit of detection (LOD) comparable or superior to most electrochemical and label-based SARS-CoV-2 N protein biosensors. Moreover, the hydrophilic nature of LCLC enabled N protein to be solubilized in nematic SSY so that kinetic analysis of specific binding between the antigen and antibody can be performed, which is unachievable with conventional thermotropic liquid crystal-based biosensors. Finally, as SARS-CoV-2 N protein can be detected noninvasively in human saliva, the clinical potential of the nematic SSY-based immunoassay was demonstrated using artificial saliva spiked with N protein.