Directly and ultrasensitivity detecting SARS-CoV-2 spike protein in pharyngeal swab solution by using SERS-based biosensor

•A new SERS-based COVID-19 biosensor was developed to directly and ultrasensitivity detecting SARS-CoV-2 spike protein.•The SERS-based biosensor assembled by monolayer Ag nanoparticles covered with single-layer graphene is applied as a SERS substrate can improve the repeatability.•The SERS-based biosensor can direct detection of the SARS-CoV-2 spike protein ultra-low concentration of 0.1 fg mL−1 in phosphate-buffered saline and 10 fg mL−1 in pharyngeal swabs solution.•The sandwich structure is designed and fabricated for enhancing the detection sensitivity by using ACE2@AgNPs as an enhancement layer.