Development of a low-cost cotton-tipped electrochemical immunosensor for the detection of SARS-CoV-2

Collection of nasopharyngeal samples using swabs followed by the transfer of the virus into a solution and an RNA extraction step to perform reverse transcription polymerase chain reaction (PCR) is the primary method currently used for the diagnosis of COVID-19. However, the need for several reagents and steps and the high cost of PCR hinder its worldwide implementation to contain the outbreak. Here, we report a cotton-tipped electrochemical immunosensor for the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus antigen. Unlike the reported approaches, we integrated the sample collection and detection tools into a single platform by coating screen-printed electrodes with absorbing cotton padding. The immunosensor was fabricated by immobilizing the virus nucleocapsid (N) protein on carbon nanofiber-modified screen-printed electrodes which were functionalized by diazonium electrografting. The detection of the virus antigen was achieved via swabbing followed by competitive assay using a fixed amount of N protein antibody in the solution. A square wave voltammetric technique was used for the detection. The limit of detection for our electrochemical biosensor was 0.8 pg/mL for SARS-CoV-2, indicating very good sensitivity for the sensor. The biosensor did not show significant cross-reactivity with other virus antigens such as influenza A and HCoV, indicating high selectivity of the method. Moreover, the biosensor was successfully applied for the detection of the virus antigen in spiked nasal samples showing excellent recovery percentages. Thus, our electrochemical immunosensor is a promising diagnostic tool for the direct rapid detection of the COVID-19 virus that requires no sample transfer or pretreatment.