NUCB1 and COVID-19: A quick immunoassay for instantaneous recognition of antibodies against COVID-19 nucleocapsid protein (Nuc) and three COVID-19 spike protein antigens (receptor binding domain, RBD; spike S1 fragment; spike S1S2 extracellular domain) is provided by a multiplexed grating-coupled fluorescent plasmonics (GC-FP) platform.