Immobilized cellulose nanospheres in lateral flow immunoassay enable rapid nucleocapsid antigen-based diagnosis of SARS-CoV-2 from salivary samples

Rapid and efficient diagnostic systems are essential in controlling the spread of viral pathogens and efficient patient management. The available technologies for low-cost viral antigen testing have several limitations, including lack of accuracy and sensitivity. Here, we develop sensitive antigen tests based on recently introduced, oppositely charged cellulose II nanoparticles (NPan and NPcat) that are effective in controlling surface protein interactions. Passivation against non-specific adsorption and augmented immobilization of sensing antibodies are achieved by adjusting the electrostatic charge of the nanoparticles. The interactions affecting the performance of the system are investigated by microgravimetry and confocal imaging. We further demonstrate SARS-CoV-2 nucleocapsid rapid sensing by saliva-wicking channels stencil-printed on flexible paper supports. Therein, NPcat inkjet printed on the channels elicit distinctive optical signals, visible after only a few minutes, allowing faster diagnosis compared to current microfluidic devices designed for saliva sampling.