fM–aM Detection of the SARS-CoV-2 Antigen by Advanced Lateral Flow Immunoassay Based on Gold Nanospheres

The SARS-CoV-2 global pandemic created an unprecedented need for rapid, sensitive, and inexpensive point-of-care (POC) diagnostic tests to treat and control the disease. Many POC SARS-CoV-2 lateral flow immunoassays (LFAs) have been developed and/or commercialized, but with only limited sensitivity (μM-fM). We created an advanced LFA based on gold nanospheres (GNSs) with comprehensive assay redesign for enhanced specific binding and thermal contrast amplification (TCA) on GNSs for signal amplification, which enabled fM-aM detection sensitivity for SARS-CoV-2 spike receptor-binding domain (RBD) proteins within 30 min. The advanced LFA can visually detect RBD proteins down to 3.6 and 28.6 aM in buffer and human nasopharyngeal wash, respectively. This is the first reported LFA achieving sensitivity comparable to that of the PCR (aM-zM) by visual reading, which was much more sensitive than traditional LFAs. We also developed a fast (<1 min) TCA reading algorithm, with results showing that this TCA could distinguish 26-32% visual false negatives for clinical commercial LFAs. When our advanced LFAs were applied with this TCA, the sensitivities were further improved by eightfold to 0.45 aM (in buffer) and 3.6 aM (in the human nasopharyngeal wash) with a semiquantitative readout. Our proposed advanced LFA with a TCA diagnostic platform can help control the current SARS-CoV-2 pandemic. Furthermore, the simplicity and speed with which this assay was assembled may also facilitate preparedness for future pandemics.