Bovine Serum Albumin-Protected Gold Nanoclusters for Sensing of SARS-CoV-2 Antibodies and Virus

An approach to assess severe acute respiratory syndromecoronavirus2 (SARS-CoV-2) infection (and past infection) was developed. For virusdetection, the SARS-CoV-2 virus nucleocapsid protein (NP) was targeted.To detect the NP, antibodies were immobilized on magnetic beads tocapture the NPs, which were subsequently detected using rabbit anti-SARS-CoV-2nucleocapsid antibodies and alkaline phosphatase (AP)-conjugated anti-rabbitantibodies. A similar approach was used to assess SARS-CoV-2-neutralizingantibody levels by capturing spike receptor-binding domain (RBD)-specificantibodies utilizing RBD protein-modified magnetic beads and detectingthem using AP-conjugated anti-human IgG antibodies. The sensing mechanismfor both assays is based on cysteamine etching-induced fluorescencequenching of bovine serum albumin-protected gold nanoclusters wherecysteamine is generated in proportion to the amount of either SARS-CoV-2virus or anti-SARS-CoV-2 receptor-binding domain-specific immunoglobulinantibodies (anti-RBD IgG antibodies). High sensitivity can be achievedin 5 h 15 min for the anti-RBD IgG antibody detection and 6 h 15 minfor virus detection, although the assay can be run in "rapid"mode, which takes 1 h 45 min for the anti-RBD IgG antibody detectionand 3 h 15 min for the virus. By spiking the anti-RBD IgG antibodiesand virus in serum and saliva, we demonstrate that the assay can detectthe anti-RBD IgG antibodies with a limit of detection (LOD) of 4.0and 2.0 ng/mL in serum and saliva, respectively. For the virus, wecan achieve an LOD of 8.5 x 10(5) RNA copies/mL and8.8 x 10(5) RNA copies/mL in serum and saliva, respectively.Interestingly, this assay can be easily modified to detect myriadanalytes of interest.