Label-free electrochemical immunosensor for highly sensitive COVID-19 spike protein detection

•A new immunosensor was prepared for the rapid and sensitive detection of COVID-19 spike protein.•N-HRGO/Thionine nanocomposite was the first time to fabricate a label-free COVID-19 immunosensor.•The fabricated immunosensor demonstrated good detection performances for COVID-19 spike protein.•The linear range for COVID-19 spike protein was1 pg mL−1−10 ng mL−1, while the detection limit was 0.3 pg mL−1.•This developed immunosensor shown good detection performances in saliva and human serum.