Abstract
Three techniques were compared for lowering the limit of detection (LOD) of the lateral flow immunoassay (LFIA) of the receptor-binding domain of severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) based on the post-assay in situ enlargement of Au nanoparticles (Au NPs) on a test strip. Silver enhancement (growth of a silver layer over Au NPs—Au@Ag NPs) and gold enhancement (growth of a gold layer over Au NPs) techniques and the novel technique of galvanic replacement of Ag by Au in Au@Ag NPs causing the formation of Au@Ag-Au NPs were performed. All the enhancements were performed on-site after completion of the conventional LFIA and maintained equipment-free assay. The assays demonstrated lowering of LODs in the following rows: 488 pg/mL (conventional LFIA with Au NPs), 61 pg/mL (silver enhancement), 8 pg/mL (galvanic replacement), and 1 pg/mL (gold enhancement). Using gold enhancement as the optimal technique, the maximal dilution of inactivated SARS-CoV-2-containing samples increased 500 times. The developed LFIA provided highly sensitive and rapid (8 min) point-of-need testing.
Highlights
Lateral flow immunoassay (LFIA) is the analytical method that combines the interaction of antibodies with antigens and affinity separation of the formed complexes during their migration along porous membranes [1,2]
The synthesized Au nanoparticles (Au NPs) were applicable for conjugation with monoclonal antibodies (mAb) and use in LFIA
The developed approaches include the step of silver/gold salt reduction catalyzed by Au NPs
Summary
Lateral flow immunoassay (LFIA) is the analytical method that combines the interaction of antibodies with antigens and affinity separation of the formed complexes during their migration along porous membranes [1,2]. Khlebtsov et al [12] studied minimal content of Au NPs in lateral flow membranes causing visually detected coloration They showed that larger Au NPs were detected at the lower amounts—6.5 × 107 and 1.4 × 105 nanoparticles per mm of the membrane with pore size 10 μM (conventionally used nitrocellulose membrane for LFIA) for Au NPs with the diameters 16 and 115 nm, respectively. The use of larger Au NPs is desirable, in practice, the use of large Au NPs is limited by poor migration through porous membranes
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