The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory coronavirus 2 (SARS-CoV-2) is still raging all over the world. Hence, the rapid and sensitive screening of the suspected population is in high demand. The nucleocapsid protein (NP) of SARS-CoV-2 has been selected as an ideal marker for viral antigen detection. This study describes a lateral flow immunoassay (LFIA) based on colloidal gold nanoparticles for rapid NP antigen detection, in which sensitivity was improved through copper deposition-induced signal amplification. The detection sensitivity of the developed LFIA for NP antigen detection (using certified reference materials) under the optimized parameters was 0.01 μg/mL and was promoted by three orders of magnitude to 10 pg/mL after copper deposition signal amplification. The LFIA coupled with the copper enhancement technique has many merits such as low cost, high efficiency, and high sensitivity. It provides an effective approach to the rapid screening, diagnosis, and monitoring of the suspected population in the COVID-19 outbreak.

Carbofuran is a highly toxic insecticide and has been banned or restricted for pest control in many countries. Being a potent acetylcholinesterase inhibitor, carbofuran and its metabolite 3-hydroxycarbofuran (3-OH-CBF) are required for the analysis of their residues in food. However, an immunochemical method for simultaneous analysis of carbofuran and 3-OH-CBF is still not available. Herein, we report an enzyme-linked immunosorbent assay (ELISA) based on a broad-specific monoclonal antibody (mAb) for simultaneous detection of carbofuran and 3-OH-CBF. The mAb, designated as 2E3, against both carbofuran and 3-OH-CBF was used to develop an indirect competitive ELISA (icELISA) with 50% inhibition concentrations of 0.76 and 0.69 ng/mL, respectively. The developed icELISA was validated by UPLC-MS/MS and is suitable for the rapid and simultaneous detection of carbofuran and 3-OH-CBF in fruits and vegetables.