A paper-based immunoassay with signal amplification for the sensitive detection of nucleocapsid protein toward the diagnosis of long COVID

Long COVID is characterized by persistent symptoms, including fatigue, cognitive impairment, and respiratory issues, affecting a considerable number of individuals post-infection. The underlying mechanism is not fully understood, but it has been proposed to involve the reactivation of virus, which subsequently induces immune dysregulation. In this proof-of-concept study, we developed a paper-based immunoassay for the detection of nucleocapsid (N) protein, which, due to its stability and low mutation rate, is a valuable biomarker for detecting the presence of residual virus. By utilizing reporter antibodies conjugated to cleavable ionic probes through dendrimer chemistry, we were able to analyze the immunoassay results with ambient mass spectrometry using on-chip paper spray ionization. The used dendrimer enhanced mass spectrometry sensitivity by enabling the attachment of multiple ionic probes to a single reporter antibody. The method presented here achieved a limit of detection of 2.4 pM for N protein detection from paper. Unlike traditional sensitive COVID tests that are only accessible to hospitalized individuals, our paper-based assay has potential to enable long COVID to be detected under resource-limited settings. Our method was applied to analyze 20 human plasma samples, including 10 from individuals with long COVID and 10 from healthy controls with no history of SARS-CoV-2 infection. We observed a significantly higher MS signal-by up to two orders of magnitude-for samples collected from long COVID patients compared to controls. The ability to use the paper device in remote locations was tested by evaluating the stability of the assay, which showed that after 30 days of storage at room temperature, the device retained sufficient analytical performance. Given its robustness, we believe that our platform will be suitable for direct-to-consumer testing, enabling individuals with low viral loads to be screened in a timely fashion.