Colorimetric and Raman dual-mode lateral flow immunoassay detection of SARS-CoV-2 N protein antibody based on Ag nanoparticles with ultrathin Au shell assembled onto Fe(3)O(4) nanoparticles

Serological antibody tests are useful complements of nuclei acid detection for SARS-CoV-2 diagnosis, which can significantly improve diagnostic accuracy. However, antibody detection in serum or plasma remains challenging to do with high sensitivity. In this study, Ag nanoparticles with ultra-thin Au shells embedded with 4-mercaptobenzoic acid (MBA) (Ag(MBA)@Au) were manufactured and then assembled onto Fe(3)O(4) surface by electrostatic interaction to construct the Fe(3)O(4)-Ag(MBA)@Au nanoparticles (NPs) with magnetic-Raman-colorimetric properties. Based on the composite nanoparticles, a colorimetric and Raman dual-mode lateral flow immunoassay (LFIA) for ultrasensitive identification of SARS-CoV-2 nucleocapsid (N) protein antibody was constructed. The magnetic nanoparticles (Fe(3)O(4) NPs) were acted as the core and coated a layer of Ag(MBA)@Au particles on the surface by electrostatic interaction to prepare Fe(3)O(4)-Ag(MBA)@Au NPs, which can amplify the SERS signal due to multiple Ag(MBA)@Au particles concentrated on a single magnetic nanoparticle. Moreover, the Fe(3)O(4)-Ag(MBA)@Au NPs facilitated pre-purifying sample using magnetic separation, and complex matrix interference would be greatly decreased in the detection. The Fe(3)O(4)-Ag(MBA)@Au NPs modified with N protein recognized and bound with N protein antibodies, which were trapped on the T-line, forming color band for observing detection. Under optimal conditions, the N protein antibodies could be qualitatively detected in colorimetric mode with the visual limit of 10(−8) mg/mL and quantitatively detected by SERS signals between 10(−6) and 10(−10) mg /mL with 0.08 pg/mL detection limit. The coefficients variations (CV) of intra-assay was 8.0%, whereas of inter-assay was 11.7%, confirming of good reproducibility. Finally, this approach was able to discriminate between positive, negative, and weakly positive samples when detecting 107 clinical serum samples. The process enables highly sensitive quantitative assays that are valuable for evaluating disease processes and guiding treatment. GRAPHICAL ABSTRACT: Colorimetric and Raman dual-mode LFIA detection of SARS-CoV-2 N protein antibody based on Fe(3)O(4)-Ag(MBA)@Au nanoparticles [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00216-022-04437-1.