SARS-CoV-2 Antibodies Detection
Lateral flow assay (LFA) has long been used as a biomarker detection technique. It has advantages such as low cost, rapid readout, portability, and ease of use. However, its qualitative readout process and lack of sensitivity are the limiting factors. Here a photon-counting approach to accurately quantify LFAs while enhancing sensitivity is presented. In particular, we demonstrate that the density of SARS- CoV-2 antibodies can be quantified and measured with an enhanced sensitivity using this simple laser optical analysis.
The commercial COVID-19 test kits that we have had the best results with give a clearly visible reading at 10 ng/mL sensitivity range, corresponding to about 10 IgG/mL antibodies. In order to detect the small amount of antibodies that accompanies an early-stage infection, and to make a meaningful statement about post disease trace antibody density, a quantitative measurement with high sensitivity is required for COVID-19 surveillance and treatment as well as in general diagnostics. In this experiment, we would like to have a sensitivity of 0.1 ng/mL (i.e., we would like to have a test at the level of around 109 IgG/mL).
Fig. 1. Schematic illustration of a LFA showing the wicking membrane with AuNPs coated with SARS-CoV- 2 spike protein.
The fundamental problem with the process is that the nitrocellulose (NC) embeds the AuNPs in a matrix that primarily scatters the light and thus the reflected light is now a combination of light that has been scattered both by the NC and the AuNPs. The problem facing us presently is how to model both the scattering and absorption processes in the system of Fig. 1. To model the light scattering properties of these spherical bub- bles in the NC, we will use the Lorenz–Mie theory, and add AuNP to the fiber matrix. Finally, we find that there is a resonant absorption of green photons incident on a AuNP. Hence, if we use green 532 nm laser radiation, it will be backscattered by the NC fibers but at a lower intensity due to the photons absorbed by the AuNP. This is the basis of our approach to “sense” the AuNPs at a very dilute density.
We present a schematic of the experiment as in Fig. 2.
FIG. 2. Schematic of the experiment. The 532 nm laser is split into two parts for the AuNP detection and laser power monitor. The laser vertically hits the test strip and is then collected by a photon detector through a two-lens image system.
The main results is shown in Fig. 3.
FIG. 3. (a) Measurements on test strips with IgG concen- trations from 1000 to 0.1 ng/mL, bottom to top. Three scans are taken for each sample within 5 min. Dips at ∼ -5 mm correspond to the IgG test lines, at ∼ 0 mm, they are control lines. Figs (b) and (c) correspond to densi- ties 0.1 ng/mL and 1 ng/mL. The solid lines are Gaussian fittings.
To see more detailed algorithms and descriptions, please read our paper attached below. (The paper about this work is published on Applied Physics Letters)
Enhancing sensitivity of lateral flow assay with application to SARS-CoV-2.pdf
Doctor Tao Peng and Doctor Xiangpei Liu made major contributions toward this work.
