SERS-based lateral flow immunoassay for sensitive and simultaneous detection of anti-SARS-CoV-2 IgM and IgG antibodies by using gap-enhanced Raman nanotags

Abstract

The lateral flow immunoassay (LFIA) has played a crucial role in early diagnosis during the current COVID-19 pandemic owing to its simplicity, speed and affordability for coronavirus antibody detection. However, the sensitivity of the commercially available LFIAs needs to be improved to better prevent the spread of the infection. Here, we developed an ultra-sensitive surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-based LFIA) strip for simultaneous detection of anti-SARS-CoV-2 IgM and IgG by using gap-enhanced Raman nanotags (GERTs). The GERTs with a 1 nm gap between the core and shell were used to produce the "hot spots", which provided about 30-fold enhancement as compared to conventional nanotags. The COVID-19 recombinant antigens were conjugated on GERTs surfaces and replaced the traditional colloidal gold for the Raman sensitive detection of human IgM and IgG. The LODs of IgM and IgG were found to be 1 ng/mL and 0.1 ng/mL (about 100 times decrease was observed as compared to commercially available LFIA strips), respectively. Moreover, under the condition of common nano-surface antigen, precise SERS signals proved the unreliability of quantitation because of the interference effect of IgM on IgG.

Keywords: COVID-19; Gap-enhanced Raman nanotags (GERTs); IgM/IgG; Lateral flow immunoassay; SARS-CoV-2; Surface-enhanced Raman scattering (SERS).

Graphical abstract

Fig. 1

Schematic diagram of the SERS-based strip for the detection of IgM and IgG.

Fig. 2

(a) Scheme of GERTs and immune-GERTs synthesis. (b) TEM image of 55 ± 5 nm Au cores. (c, d) TEM image of GERTs and an enlarged image with a 1 nm gap between the Au core and shell. (e) SERS spectra of NBT-Au and GERTs. (f) UV–vis extinction spectra and photographs of the four steps of colloid synthesis.

Fig. 3

(a) Photographs and corresponding SERS spectra of the test lines of the SERS-based strips in the presence of IgM and IgG. (b, c) Typical SEM images of the test zone for positive samples; the red arrows point to the nanoparticles of the positive sample in the enlarged image. (d, e) Typical SEM images of the test zone for negative samples.

Fig. 4

Images of colorimetric results of different antibodies concentrations under the interference of BIL (a), HB (c) and TG (e). Corresponding Raman intensity of T1/T2 at different antibodies concentrations under the interference of BIL (b), HB (d) and TG (f). All error bars represent standard deviation (n = 3).

Fig. 5

(a) Photographs of SERS-based LFIA strips for IgM and IgG detection. (b) Photographs of commercial conventional LFIA strips for IgM and IgG detection. (c) SERS mapping images of the typical test zones of SERS-based LFIA strips with different concentrations of IgM and IgG. (d, e) Averaged SERS spectrum of T1 and T2 for different concentrations of IgM and IgG. (f, g) The logarithmic curve of the corresponding calibration lines for IgM and IgG. Error bars represent standard deviation (n = 3). All the insets show the enlarged portion of the figures.

Fig. 6

(a) Schematic diagram of IgM and IgG binding to immune-GERTs in high concentration IgM/IgG mixture, and the immunobinding on T1 and T2. (b) Schematic diagram of IgM and IgG binding to immune-GERTs in low concentration IgM/IgG mixture, and the immunobinding on T1 and T2.