Nanoparticle-Supported, Rapid, and Electronic Detection of SARS-CoV-2 Antibodies and Antigens at the Subfemtomolar Level

Major challenges remain to precisely detect low-abundance proteins rapidly and cost-effectively from diverse biofluids. Here, we present a gold nanoparticle (AuNP)-supported, rapid electronic detection (NasRED) platform with subfemtomolar sensitivity and high specificity. Surface-functionalized AuNPs act as multivalent detectors to recognize target antigens and antibodies through high-affinity binding, subsequently forming aggregates precipitated in a microcentrifuge tube and producing a solution color change. The residual floating AuNPs' optical extinction is digitized using customized circuitry incorporating inexpensive optoelectronics and feedback mechanisms for stabilized readout. NasRED introduces active fluidic forces through engineered centrifugation and vortex agitation, effectively promoting low-concentration protein detection and accelerating signal transduction. Using SARS-CoV-2 as a demonstration, NasRED enables detection of both antibodies and antigens from a small sample volume (6 μL), distinguishes the viral antigens from those of human coronaviruses, and delivers test results in <15 min. The limits of detection (LoDs) for antibody detection are approximately 49 aM (7 fg/mL) in phosphate-buffered saline (PBS), or >3000 times more sensitive than enzyme-linked immunosorbent assay (ELISA), ∼76 aM (11 fg/mL) in human pooled serum and in the femtomolar range in diluted whole blood. For nucleocapsid protein detection, NasRED LoDs are ∼190 aM (10 fg/mL) in human saliva and ∼2 fM (100 fg/mL) in nasal fluid. Unlike centralized platforms, NasRED is a one-pot, in-solution assay without the needs for washing, labeling, expensive instrumentation, or highly trained operators. With low reagent costs and a compact system footprint, this modular digital platform is well-suited for accurate, near-patient diagnosis and screening of a wide range of infectious and chronic diseases.