Comparison of two nanomaterial labels for detection of SARS-CoV-2 nucleocapsid antigen to improve analytical performance of lateral flow immunoassays

Enhancing the sensitivity and detection limits of lateral flow immunoassays (LFAs) remains critical for broadening their diagnostic value and uptake, particularly given the reduced performance of some SARS-CoV-2 antigen tests at low viral load concentrations. The choice of a detection label such as the widely used 40 nm gold nanoparticles (AuNPs), strongly influences LFA analytical performance. Cellulose nanobeads (CNBs), recently introduced in several colors and diameters ranging from 330-365 nm, represent an emerging alternative, though comprehensive performance data are still limited. In this study, we assessed LFAs incorporating either CNBs or AuNPs, using identical antibody pairs for SARS-CoV-2 antigen detection. By analyzing spiked samples and archived nasopharyngeal specimens characterized by RT-qPCR, we examined how viral load and cycle threshold values correlated with antigen test positivity across the two label types. The visual cut-off limit concentration of CNB-based LFAs was observed to be two orders of magnitude lower than the WHO target product profile (TPP) benchmark of 10⁶ copies per mL, whereas AuNP-based LFAs met the defined TPP threshold. Both CNB- and AuNP-LFA formats also met the WHO TPP acceptable targets for time to result, number of user steps and target use setting. These results indicate that CNBs can provide superior analytical sensitivity relative to 40 nm AuNPs and may support more sensitive, visually interpreted LFAs without the need for specialized test strip readers.